gpgme/doc/gpgme.texi

\input texinfo                  @c -*- Texinfo -*-
@setfilename gpgme.info
@settitle The `GnuPG Made Easy' Reference Manual

@dircategory GNU Libraries
@direntry
* @acronym{GPGME}: (gpgme).          Adding support for cryptography to your program.
@end direntry

@include version.texi

@c Unify some of the indices.
@syncodeindex tp fn
@syncodeindex pg fn

@ifinfo
This file documents the @acronym{GPGME} library.

This is Edition @value{EDITION}, last updated @value{UPDATED}, of
@cite{The `GnuPG Made Easy' Reference Manual}, for Version
@value{VERSION}.

Copyright @copyright{} 2002, 2003, 2004 g10 Code GmbH.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software Needs Free Documentation'' and
``GNU Lesser General Public License'', the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.

@end ifinfo

@iftex
@shorttitlepage The `GnuPG Made Easy' Reference Manual
@end iftex
@titlepage
@center @titlefont{The `GnuPG Made Easy'}
@sp 1
@center @titlefont{Reference Manual}
@sp 6
@center Edition @value{EDITION}
@sp 1
@center last updated @value{UPDATED}
@sp 1
@center for version @value{VERSION}
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 2002, 2003 g10 Code GmbH.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software Needs Free Documentation'' and
``GNU Lesser General Public License'', the Front-Cover texts being (a)
(see below), and with the Back-Cover Texts being (b) (see below).  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.
@end titlepage
@page

@ifnottex
@node Top
@top Main Menu
This is Edition @value{EDITION}, last updated @value{UPDATED}, of
@cite{The `GnuPG Made Easy' Reference Manual}, for Version
@value{VERSION} of the @acronym{GPGME} library.
@end ifnottex

@menu
* Introduction::                  How to use this manual.
* Preparation::                   What you should do before using the library.
* Protocols and Engines::         Supported crypto protocols.
* Algorithms::                    Supported algorithms.
* Error Handling::                Error numbers and their meanings.
* Exchanging Data::               Passing data to and from @acronym{GPGME}.
* Contexts::                      Handling @acronym{GPGME} contexts.

Appendices

* Copying::                       The GNU General Public License says how you
                                  can copy and share `GnuPG Made Easy'.
* Free Documentation License::    This manual is under the GNU Free
                                  Documentation License.

Indices

* Concept Index::                 Index of concepts and programs.
* Function and Data Index::       Index of functions, variables and data types.


@detailmenu
 --- The Detailed Node Listing ---

Introduction

* Getting Started::               Purpose of the manual, and how to use it.
* Features::                      Reasons to install and use @acronym{GPGME}.
* Overview::                      Basic architecture of the @acronym{GPGME} library.

Preparation

* Header::                        What header file you need to include.
* Building the Source::           Compiler options to be used.
* Largefile Support (LFS)::       How to use @acronym{GPGME} with LFS.
* Using Automake::                Compiler options to be used the easy way.
* Using Libtool::                 Avoiding compiler options entirely.
* Library Version Check::         Getting and verifying the library version.
* Signal Handling::               How @acronym{GPGME} affects signal handling.
* Multi Threading::               How @acronym{GPGME} can be used in an MT environment.

Protocols and Engines

* Engine Version Check::          Verifying the engine version.
* Engine Information::            Obtaining more information about the engines.
* OpenPGP::                       Support for the OpenPGP protocol.
* Cryptographic Message Syntax::  Support for the CMS.

Algorithms

* Public Key Algorithms::         A list of all public key algorithms.
* Hash Algorithms::               A list of all hash algorithms.

Error Handling

* Error Values::                  The error value and what it means.
* Error Codes::                   A list of important error codes.
* Error Sources::                 A list of important error sources.
* Error Strings::                 How to get a descriptive string from a value.

Exchanging Data 

* Creating Data Buffers::         Creating new data buffers.
* Destroying Data Buffers::       Releasing data buffers.
* Manipulating Data Buffers::     Operations on data buffers.

Creating Data Buffers

* Memory Based Data Buffers::     Creating memory based data buffers.
* File Based Data Buffers::       Creating file based data buffers.
* Callback Based Data Buffers::   Creating callback based data buffers.

Contexts

* Creating Contexts::             Creating new @acronym{GPGME} contexts.
* Destroying Contexts::           Releasing @acronym{GPGME} contexts.
* Context Attributes::            Setting properties of a context.
* Key Management::                Managing keys with @acronym{GPGME}.
* Trust Item Management::         Managing trust items with @acronym{GPGME}.
* Crypto Operations::             Using a context for cryptography.
* Run Control::                   Controlling how operations are run.

Context Attributes

* Protocol Selection::            Selecting the protocol used by a context.
* ASCII Armor::                   Requesting @acronym{ASCII} armored output.
* Text Mode::                     Choosing canonical text mode.
* Included Certificates::         Including a number of certificates.
* Key Listing Mode::              Selecting key listing mode.
* Passphrase Callback::           Getting the passphrase from the user.
* Progress Meter Callback::       Being informed about the progress.
* Locale::                        Setting the locale of a context.

Key Management

* Listing Keys::                  Browsing the list of available keys.
* Information About Keys::        Requesting detailed information about keys.
* Key Signatures::                Listing the signatures on a key.
* Manipulating Keys::             Operations on keys.
* Generating Keys::               Creating new key pairs.
* Exporting Keys::                Retrieving key data from the key ring.
* Importing Keys::                Adding keys to the key ring.
* Deleting Keys::                 Removing keys from the key ring.

Trust Item Management

* Listing Trust Items::           Browsing the list of available trust items.
* Information About Trust Items:: Requesting information about trust items.
* Manipulating Trust Items::      Operations on trust items.

Crypto Operations

* Decrypt::                       Decrypting a ciphertext.
* Verify::                        Verifying a signature.
* Decrypt and Verify::            Decrypting a signed ciphertext.
* Sign::                          Creating a signature.
* Encrypt::                       Encrypting a plaintext.

Sign

* Selecting Signers::             How to choose the keys to sign with.
* Creating a Signature::          How to create a signature.

Encrypt

* Encrypting a Plaintext::        How to encrypt a plaintext.

Run Control

* Waiting For Completion::        Waiting until an operation is completed.
* Using External Event Loops::    Advanced control over what happens when.
* Cancellation::                  How to end pending operations prematurely.

Using External Event Loops

* I/O Callback Interface::        How I/O callbacks are registered.
* Registering I/O Callbacks::     How to use I/O callbacks for a context.
* I/O Callback Example::          An example how to use I/O callbacks.
* I/O Callback Example GTK+::     How to integrate @acronym{GPGME} in GTK+.
* I/O Callback Example GDK::      How to integrate @acronym{GPGME} in GDK.
* I/O Callback Example Qt::       How to integrate @acronym{GPGME} in Qt.

@end detailmenu
@end menu

@node Introduction
@chapter Introduction

`GnuPG Made Easy' (@acronym{GPGME}) is a C language library that
allows to add support for cryptography to a program.  It is designed
to make access to public key crypto engines like GnuPG or GpgSM easier
for applications.  @acronym{GPGME} provides a high-level crypto API
for encryption, decryption, signing, signature verification and key
management.

@acronym{GPGME} uses GnuPG and GpgSM as its backends to support
OpenPGP and the Cryptographic Message Syntax (CMS).

@menu
* Getting Started::               Purpose of the manual, and how to use it.
* Features::                      Reasons to install and use @acronym{GPGME}.
* Overview::                      Basic architecture of the @acronym{GPGME} library.
@end menu


@node Getting Started
@section Getting Started

This manual documents the @acronym{GPGME} library programming
interface.  All functions and data types provided by the library are
explained.

The reader is assumed to possess basic knowledge about cryptography in
general, and public key cryptography in particular.  The underlying
cryptographic engines that are used by the library are not explained,
but where necessary, special features or requirements by an engine are
mentioned as far as they are relevant to @acronym{GPGME} or its users.

This manual can be used in several ways.  If read from the beginning
to the end, it gives a good introduction into the library and how it
can be used in an application.  Forward references are included where
necessary.  Later on, the manual can be used as a reference manual to
get just the information needed about any particular interface of the
library.  Experienced programmers might want to start looking at the
examples at the end of the manual, and then only read up those parts
of the interface which are unclear.


@node Features
@section Features

@acronym{GPGME} has a couple of advantages over other libraries doing
a similar job, and over implementing support for GnuPG or other crypto
engines into your application directly.

@table @asis
@item it's free software
Anybody can use, modify, and redistribute it under the terms of the GNU
General Public License (@pxref{Copying}).

@item it's flexible
@acronym{GPGME} provides transparent support for several cryptographic
protocols by different engines.  Currently, @acronym{GPGME} supports
the OpenPGP protocol using GnuPG as the backend, and the Cryptographic
Message Syntax using GpgSM as the backend.

@item it's easy
@acronym{GPGME} hides the differences between the protocols and
engines from the programmer behind an easy-to-use interface.  This way
the programmer can focus on the other parts of the program, and still
integrate strong cryptography in his application.  Once support for
@acronym{GPGME} has been added to a program, it is easy to add support
for other crypto protocols once @acronym{GPGME} backends provide them.
@end table


@node Overview
@section Overview

@acronym{GPGME} provides a data abstraction that is used to pass data
to the crypto engine, and receive returned data from it.  Data can be
read from memory or from files, but it can also be provided by a
callback function.

The actual cryptographic operations are always set within a context.
A context provides configuration parameters that define the behaviour
of all operations performed within it.  Only one operation per context
is allowed at any time, but when one operation is finished, you can
run the next operation in the same context.  There can be more than
one context, and all can run different operations at the same time.

Furthermore, @acronym{GPGME} has rich key management facilities
including listing keys, querying their attributes, generating,
importing, exporting and deleting keys, and acquiring information
about the trust path.

With some precautions, @acronym{GPGME} can be used in a multi-threaded
environment, although it is not completely thread safe and thus needs
the support of the application.


@node Preparation
@chapter Preparation

To use @acronym{GPGME}, you have to perform some changes to your
sources and the build system.  The necessary changes are small and
explained in the following sections.  At the end of this chapter, it
is described how the library is initialized, and how the requirements
of the library are verified.

@menu
* Header::                        What header file you need to include.
* Building the Source::           Compiler options to be used.
* Largefile Support (LFS)::       How to use @acronym{GPGME} with LFS.
* Using Automake::                Compiler options to be used the easy way.
* Using Libtool::                 Avoiding compiler options entirely.
* Library Version Check::         Getting and verifying the library version.
* Signal Handling::               How @acronym{GPGME} affects signal handling.
* Multi Threading::               How @acronym{GPGME} can be used in an MT environment.
@end menu


@node Header
@section Header
@cindex header file
@cindex include file

All interfaces (data types and functions) of the library are defined
in the header file `gpgme.h'.  You must include this in all programs
using the library, either directly or through some other header file,
like this:

@example
#include <gpgme.h>
@end example

The name space of @acronym{GPGME} is @code{gpgme_*} for function names
and data types and @code{GPGME_*} for other symbols.  Symbols internal
to @acronym{GPGME} take the form @code{_gpgme_*} and @code{_GPGME_*}.

Because @acronym{GPGME} makes use of the GPG Error library, using
@acronym{GPGME} will also use the @code{GPG_ERR_*} name space
directly, and the @code{gpg_err*} and @code{gpg_str*} name space
indirectly.


@node Building the Source
@section Building the Source
@cindex compiler options
@cindex compiler flags

If you want to compile a source file including the `gpgme.h' header
file, you must make sure that the compiler can find it in the
directory hierarchy.  This is accomplished by adding the path to the
directory in which the header file is located to the compilers include
file search path (via the @option{-I} option).

However, the path to the include file is determined at the time the
source is configured.  To solve this problem, gpgme ships with a small
helper program @command{gpgme-config} that knows about the path to the
include file and other configuration options.  The options that need
to be added to the compiler invocation at compile time are output by
the @option{--cflags} option to @command{gpgme-config}.  The following
example shows how it can be used at the command line:

@example
gcc -c foo.c `gpgme-config --cflags`
@end example

Adding the output of @samp{gpgme-config --cflags} to the compiler
command line will ensure that the compiler can find the
@acronym{GPGME} header file.

A similar problem occurs when linking the program with the library.
Again, the compiler has to find the library files.  For this to work,
the path to the library files has to be added to the library search
path (via the @option{-L} option).  For this, the option
@option{--libs} to @command{gpgme-config} can be used.  For
convenience, this option also outputs all other options that are
required to link the program with @acronym{GPGME} (in particular, the
@samp{-lgpgme} option).  The example shows how to link @file{foo.o}
with the @acronym{GPGME} library to a program @command{foo}.

@example
gcc -o foo foo.o `gpgme-config --libs`
@end example

Of course you can also combine both examples to a single command by
specifying both options to @command{gpgme-config}:

@example
gcc -o foo foo.c `gpgme-config --cflags --libs`
@end example

If you want to link to one of the thread-safe versions of
@acronym{GPGME}, you must specify the @option{--thread} option before
any other option to select the thread package you want to link with.
Supported thread packages are @option{--thread=pth} and
@option{--thread=pthread}.


@node Largefile Support (LFS)
@section Largefile Support (LFS)
@cindex largefile support
@cindex LFS

@acronym{GPGME} is compiled with largefile support by default, if it
is available on the system.  This means that GPGME supports files
larger than two gigabyte in size, if the underlying operating system
can.  On some systems, largefile support is already the default.  On
such systems, nothing special is required.  However, some systems
provide only support for files up to two gigabyte in size by default.
Support for larger file sizes has to be specifically enabled.

To make a difficult situation even more complex, such systems provide
two different types of largefile support.  You can either get all
relevant functions replaced with alternatives that are largefile
capable, or you can get new functions and data types for largefile
support added.  Those new functions have the same name as their
smallfile counterparts, but with a suffix of 64.

An example: The data type @code{off_t} is 32 bit wide on GNU/Linux PC
systems.  To address offsets in large files, you can either enable
largefile support add-on.  Then a new data type @code{off64_t} is
provided, which is 64 bit wide.  Or you can replace the existing
@code{off_t} data type with its 64 bit wide counterpart.  All
occurences of @code{off_t} are then automagically replaced.

As if matters were not complex enough, there are also two different
types of file descriptors in such systems.  This is important because
if file descriptors are exchanged between programs that use a
different maximum file size, certain errors must be produced on some
file descriptors to prevent subtle overflow bugs from occuring.

As you can see, supporting two different maximum file sizes at the
same time is not at all an easy task.  However, the maximum file size
does matter for @acronym{GPGME}, because some data types it uses in
its interfaces are affected by that.  For example, the @code{off_t}
data type is used in the @code{gpgme_data_seek} function, to match its
@acronym{POSIX} counterpart.  This affects the call-frame of the
function, and thus the ABI of the library.  Furthermore, file
descriptors can be exchanged between GPGME and the application.

For you as the user of the library, this means that your program must
be compiled in the same file size mode as the library.  Luckily, there
is absolutely no valid reason for new programs to not enable largefile
support by default and just use that.  The compatibility modes (small
file sizes or dual mode) can be considered an historic artefact, only
useful to allow for a transitional period.

@acronym{GPGME} is compiled using largefile support by default.  This
means that your application must do the same, at least as far as it is
relevant for using the @file{gpgme.h} header file.  All types in this
header files refer to their largefile counterparts, if they are
different from any default types on the system.

You can enable largefile support, if it is different from the default
on the system the application is compiled on, by using the Autoconf
macro @code{AC_SYS_LARGEFILE}.  If you do this, then you don't need to
worry about anything else: It will just work.  In this case you might
also want to use @code{AC_FUNC_FSEEKO} to take advantage of some new
interfaces, and @code{AC_TYPE_OFF_T} (just in case).

If you do not use Autoconf, you can define the preprocessor symbol
@code{_FILE_OFFSET_BITS} to 64 @emph{before} including any header
files, for example by specifying the option
@code{-D_FILE_OFFSET_BITS=64} on the compiler command line.  You will
also want to define the preprocessor symbol @code{LARGEFILE_SOURCE} to
1 in this case, to take advantage of some new interfaces.

If you do not want to do either of the above, you probably know enough
about the issue to invent your own solution.  Just keep in mind that
the @acronym{GPGME} header file expects that largefile support is
enabled, if it is available.  In particular, we do not support dual
mode (@code{_LARGEFILE64_SOURCE}).


@node Using Automake
@section Using Automake
@cindex automake
@cindex autoconf

It is much easier if you use GNU Automake instead of writing your own
Makefiles.  If you do that you do not have to worry about finding and
invoking the @command{gpgme-config} script at all.  @acronym{GPGME}
provides an extension to Automake that does all the work for you.

@c A simple macro for optional variables.
@macro ovar{varname}
@r{[}@var{\varname\}@r{]}
@end macro
@defmac AM_PATH_GPGME (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
@defmacx AM_PATH_GPGME_PTH (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
@defmacx AM_PATH_GPGME_PTHREAD (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
Check whether @acronym{GPGME} (at least version @var{minimum-version},
if given) exists on the host system.  If it is found, execute
@var{action-if-found}, otherwise do @var{action-if-not-found}, if
given.

Additionally, the function defines @code{GPGME_CFLAGS} to the flags
needed for compilation of the program to find the @file{gpgme.h}
header file, and @code{GPGME_LIBS} to the linker flags needed to link
the program to the @acronym{GPGME} library.

@code{AM_PATH_GPGME_PTH} checks for the version of @acronym{GPGME}
that can be used with GNU Pth, and defines @code{GPGME_PTH_CFLAGS} and
@code{GPGME_PTH_LIBS}.

@code{AM_PATH_GPGME_PTHREAD} checks for the version of @acronym{GPGME}
that can be used with the native pthread implementation, and defines
@code{GPGME_PTHREAD_CFLAGS} and @code{GPGME_PTHREAD_LIBS}.
@end defmac

You can use the defined Autoconf variables like this in your
@file{Makefile.am}:

@example
AM_CPPFLAGS = $(GPGME_CFLAGS)
LDADD = $(GPGME_LIBS)
@end example


@node Using Libtool
@section Using Libtool
@cindex libtool

The easiest way is to just use GNU Libtool.  If you use libtool, and
link to @code{libgpgme.la}, @code{libgpgme-pth.la} or
@code{libgpgme-pthread.la} respectively, everything will be done
automatically by Libtool.


@node Library Version Check
@section Library Version Check
@cindex version check, of the library

@deftypefun {const char *} gpgme_check_version (@w{const char *@var{required_version}})
The function @code{gpgme_check_version} has three purposes.  It can be
used to retrieve the version number of the library.  In addition it
can verify that the version number is higher than a certain required
version number.  In either case, the function initializes some
sub-systems, and for this reason alone it must be invoked early in
your program, before you make use of the other functions in
@acronym{GPGME}.

If @var{required_version} is @code{NULL}, the function returns a
pointer to a statically allocated string containing the version number
of the library.

If @var{required_version} is not @code{NULL}, it should point to a
string containing a version number, and the function checks that the
version of the library is at least as high as the version number
provided.  In this case, the function returns a pointer to a
statically allocated string containing the version number of the
library.  If @var{REQUIRED_VERSION} is not a valid version number, or
if the version requirement is not met, the function returns
@code{NULL}.

If you use a version of a library that is backwards compatible with
older releases, but contains additional interfaces which your program
uses, this function provides a run-time check if the necessary
features are provided by the installed version of the library.
@end deftypefun


After initializing @acronym{GPGME}, you should set the locale
information to the locale required for your output terminal.  This
locale information is needed for example for the curses and Gtk
pinentry.  Here is an example of a complete initialization:

@example
#include <locale.h>
#include <gpgme.h>

void
init_program (void)
@{
  /* Initialize the locale environment.  */
  setlocale (LC_ALL, "");
  gpgme_check_version (NULL);
  gpgme_set_locale (NULL, LC_CTYPE, setlocale (LC_CTYPE, NULL));
  gpgme_set_locale (NULL, LC_MESSAGES, setlocale (LC_MESSAGES, NULL));
@}
@end example

Note that you are highly recommended to initialize the locale settings
like this.  @acronym{GPGME} can not do this for you because it would
not be thread safe.


@node Signal Handling
@section Signal Handling
@cindex signals
@cindex signal handling

The @acronym{GPGME} library communicates with child processes (the
crypto engines).  If a child process dies unexpectedly, for example
due to a bug, or system problem, a @code{SIGPIPE} signal will be
delivered to the application.  The default action is to abort the
program.  To protect against this, @code{gpgme_check_version} sets the
@code{SIGPIPE} signal action to @code{SIG_IGN}, which means that the
signal will be ignored.

@acronym{GPGME} will only do that if the signal action for
@code{SIGPIPE} is @code{SIG_DEF} at the time
@code{gpgme_check_version} is called.  If it is something different,
@code{GPGME} will take no action.

This means that if your application does not install any signal
handler for @code{SIGPIPE}, you don't need to take any precautions.
If you do install a signal handler for @code{SIGPIPE}, you must be
prepared to handle any @code{SIGPIPE} events that occur due to
@acronym{GPGME} writing to a defunct pipe.  Furthermore, if your
application is multi-threaded, and you install a signal action for
@code{SIGPIPE}, you must make sure you do this either before
@code{gpgme_check_version} is called or afterwards.


@node Multi Threading
@section Multi Threading
@cindex thread-safeness
@cindex multi-threading

The @acronym{GPGME} library is not entirely thread-safe, but it can
still be used in a multi-threaded environment if some care is taken.
If the following requirements are met, there should be no race
conditions to worry about:

@itemize @bullet
@item
@acronym{GPGME} supports the thread libraries pthread and GNU Pth.
The support for this has to be enabled at compile time.
@acronym{GPGME} will automatically detect the location in which the
thread libraries are installed and activate the support for them at
build time.

Support for other thread libraries is very easy to add.  Please
contact us if you have the need.

@item
If you want to use @acronym{GPGME} with threads, you must link to the
right version of the library.  The name of the right library is
@code{libgpgme-} followed by the name of the thread package you use.
For example, if you use GNU Pth, the right name is
@code{libgpgme-pth}.  Use the Automake macros or
@command{gpgme-config} program for simplicity.


@item
The function @code{gpgme_check_version} must be called before any
other function in the library, because it initializes the thread
support subsystem in @acronym{GPGME}.  To achieve this in
multi-threaded programs, you must synchronize the memory with respect
to other threads that also want to use @acronym{GPGME}.  For this, it
is sufficient to call @code{gpgme_check_version} before creating the
other threads using @acronym{GPGME}@footnote{At least this is true for
POSIX threads, as @code{pthread_create} is a function that
synchronizes memory with respects to other threads.  There are many
functions which have this property, a complete list can be found in
POSIX, IEEE Std 1003.1-2003, Base Definitions, Issue 6, in the
definition of the term ``Memory Synchronization''.  For other thread
packages other, more relaxed or more strict rules may apply.}.

@item
Any @code{gpgme_data_t} and @code{gpgme_ctx_t} object must only be
accessed by one thread at a time.  If multiple threads want to deal
with the same object, the caller has to make sure that operations on
that object are fully synchronized.

@item
Only one thread at any time is allowed to call @code{gpgme_wait}.  If
multiple threads call this function, the caller must make sure that
all invocations are fully synchronized.  It is safe to start
asynchronous operations while a thread is running in gpgme_wait.

@item
The function @code{gpgme_strerror} is not thread safe.  You have to
use @code{gpgme_strerror_r} instead.
@end itemize


@node Protocols and Engines
@chapter Protocols and Engines
@cindex protocol
@cindex engine
@cindex crypto engine
@cindex backend
@cindex crypto backend

@acronym{GPGME} supports several cryptographic protocols, however, it
does not implement them.  Rather it uses backends (also called
engines) which implement the protocol.  @acronym{GPGME} uses
inter-process communication to pass data back and forth between the
application and the backend, but the details of the communication
protocol and invocation of the backend is completely hidden by the
interface.  All complexity is handled by @acronym{GPGME}.  Where an
exchange of information between the application and the backend is
necessary, @acronym{GPGME} provides the necessary callback function
hooks and further interfaces.

@deftp {Data type} {enum gpgme_protocol_t}
@tindex gpgme_protocol_t
The @code{gpgme_protocol_t} type specifies the set of possible protocol
values that are supported by @acronym{GPGME}.  The following protocols
are supported:

@table @code
@item GPGME_PROTOCOL_OpenPGP
This specifies the OpenPGP protocol.

@item GPGME_PROTOCOL_CMS
This specifies the Cryptographic Message Syntax.
@end table
@end deftp


@deftypefun const char *gpgme_get_protocol_name (@w{gpgme_protocol_t @var{protocol}})
The function @code{gpgme_get_protocol_name} returns a statically
allocated string describing the protocol @var{protocol}, or
@code{NULL} if the protocol number is not valid.
@end deftypefun

@menu
* Engine Version Check::          Verifying the engine version.
* Engine Information::            Obtaining more information about the engines.
* OpenPGP::                       Support for the OpenPGP protocol.
* Cryptographic Message Syntax::  Support for the CMS.
@end menu


@node Engine Version Check
@section Engine Version Check
@cindex version check, of the engines

@deftypefun gpgme_error_t gpgme_engine_check_version (@w{gpgme_protocol_t @var{protocol}})
The function @code{gpgme_engine_check_version} verifies that the
engine implementing the protocol @var{PROTOCOL} is installed in the
expected path and meets the version requirement of @acronym{GPGME}.

This function returns the error code @code{GPG_ERR_NO_ERROR} if the
engine is available and @code{GPG_ERR_INV_ENGINE} if it is not.
@end deftypefun


@node Engine Information
@section Engine Information
@cindex engine, information about

@deftp {Data type} {gpgme_engine_info_t}
@tindex gpgme_protocol_t
The @code{gpgme_engine_info_t} type specifies a pointer to a structure
describing a crypto engine.  The structure contains the following
elements:

@table @code
@item gpgme_engine_info_t next
This is a pointer to the next engine info structure in the linked
list, or @code{NULL} if this is the last element.

@item gpgme_protocol_t protocol
This is the protocol for which the crypto engine is used.  You can
convert this to a string with @code{gpgme_get_protocol_name} for
printing.

@item const char *file_name
This is a string holding the file name of the executable of the crypto
engine.  Currently, it is never @code{NULL}, but using @code{NULL} is
reserved for future use, so always check before you use it.

@item const char *version
This is a string containing the version number of the crypto engine.
It might be @code{NULL} if the version number can not be determined,
for example because the executable doesn't exist or is invalid.

@item const char *req_version
This is a string containing the minimum required version number of the
crypto engine for @acronym{GPGME} to work correctly.  This is the
version number that @code{gpgme_engine_check_version} verifies
against.  Currently, it is never @code{NULL}, but using @code{NULL} is
reserved for future use, so always check before you use it.
@end table
@end deftp

@deftypefun gpgme_error_t gpgme_get_engine_info (gpgme_engine_info_t *info)
The function @code{gpgme_get_engine_info} returns a linked list of
engine info structures in @var{info}.  Each info structure describes
one configured backend.

The memory for the info structures is allocated the first time this
function is invoked, and must not be freed by the caller.

This function returns the error code @code{GPG_ERR_NO_ERROR} if
successful, and a system error if the memory could not be allocated.
@end deftypefun

Here is an example how you can provide more diagnostics if you receive
an error message which indicates that the crypto engine is invalid.

@example
gpgme_ctx_t ctx;
gpgme_error_t err;

[...]

if (gpgme_err_code (err) == GPG_ERR_INV_ENGINE)
  @{
    gpgme_engine_info_t info;
    err = gpgme_get_engine_info (&info);
    if (!err)
      @{
        while (info && info->protocol != gpgme_get_protocol (ctx))
          info = info->next;
        if (!info)
          fprintf (stderr, "GPGME compiled without support for protocol %s",
                   gpgme_get_protocol_name (info->protocol));
        else if (info->path && !info->version)
          fprintf (stderr, "Engine %s not installed properly",
                   info->path);
        else if (info->path && info->version && info->req_version)
          fprintf (stderr, "Engine %s version %s installed, "
                   "but at least version %s required", info->path,
                   info->version, info->req_version);
        else
          fprintf (stderr, "Unknown problem with engine for protocol %s",
                   gpgme_get_protocol_name (info->protocol));
      @}
  @}
@end example


@node OpenPGP
@section OpenPGP
@cindex OpenPGP
@cindex GnuPG
@cindex protocol, GnuPG
@cindex engine, GnuPG

OpenPGP is implemented by GnuPG, the @acronym{GNU} Privacy Guard.
This is the first protocol that was supported by @acronym{GPGME}.

The OpenPGP protocol is specified by @code{GPGME_PROTOCOL_OpenPGP}.


@node Cryptographic Message Syntax
@section Cryptographic Message Syntax
@cindex CMS
@cindex cryptographic message syntax
@cindex GpgSM
@cindex protocol, CMS
@cindex engine, GpgSM
@cindex S/MIME
@cindex protocol, S/MIME

@acronym{CMS} is implemented by GpgSM, the S/MIME implementation for
GnuPG.

The @acronym{CMS} protocol is specified by @code{GPGME_PROTOCOL_CMS}.


@node Algorithms
@chapter Algorithms
@cindex algorithms

The crypto backends support a variety of algorithms used in public key
cryptography.  The following sections list the identifiers used to
denote such an algorithm.

@menu
* Public Key Algorithms::         A list of all public key algorithms.
* Hash Algorithms::               A list of all hash algorithms.
@end menu


@node Public Key Algorithms
@section Public Key Algorithms
@cindex algorithms, public key
@cindex public key algorithms

Public key algorithms are used for encryption, decryption, signing and
verification of signatures.

@deftp {Data type} {enum gpgme_pubkey_algo_t}
@tindex gpgme_pubkey_algo_t
The @code{gpgme_pubkey_algo_t} type specifies the set of all public key
algorithms that are supported by @acronym{GPGME}.  Possible values
are:

@table @code
@item GPGME_PK_RSA
This value indicates the RSA (Rivest, Shamir, Adleman) algorithm.

@item GPGME_PK_RSA_E
Deprecated.  This value indicates the RSA (Rivest, Shamir, Adleman)
algorithm for encryption and decryption only.

@item GPGME_PK_RSA_S
Deprecated.  This value indicates the RSA (Rivest, Shamir, Adleman)
algorithm for signing and verification only.

@item GPGME_PK_DSA
This value indicates DSA, the Digital Signature Algorithm.

@item GPGME_PK_ELG
This value indicates ElGamal.

@item GPGME_PK_ELG_E
This value also indicates ElGamal and is used specifically in GnuPG.
@end table
@end deftp

@deftypefun {const char *} gpgme_pubkey_algo_name (@w{gpgme_pubkey_algo_t @var{algo}})
The function @code{gpgme_pubkey_algo_name} returns a pointer to a
statically allocated string containing a description of the public key
algorithm @var{algo}.  This string can be used to output the name of
the public key algorithm to the user.

If @var{algo} is not a valid public key algorithm, @code{NULL} is
returned.
@end deftypefun


@node Hash Algorithms
@section Hash Algorithms
@cindex algorithms, hash
@cindex algorithms, message digest
@cindex hash algorithms
@cindex message digest algorithms

Hash (message digest) algorithms are used to compress a long message
to make it suitable for public key cryptography.

@deftp {Data type} {enum gpgme_hash_algo_t}
@tindex gpgme_hash_algo_t
The @code{gpgme_hash_algo_t} type specifies the set of all hash algorithms
that are supported by @acronym{GPGME}.  Possible values are:

@table @code
@item GPGME_MD_MD5
@item GPGME_MD_SHA1
@item GPGME_MD_RMD160
@item GPGME_MD_MD2
@item GPGME_MD_TIGER
@item GPGME_MD_HAVAL
@item GPGME_MD_SHA256
@item GPGME_MD_SHA384
@item GPGME_MD_SHA512
@item GPGME_MD_MD4
@item GPGME_MD_CRC32
@item GPGME_MD_CRC32_RFC1510
@item GPGME_MD_CRC24_RFC2440
@end table
@end deftp

@deftypefun {const char *} gpgme_hash_algo_name (@w{gpgme_hash_algo_t @var{algo}})
The function @code{gpgme_hash_algo_name} returns a pointer to a
statically allocated string containing a description of the hash
algorithm @var{algo}.  This string can be used to output the name of
the hash algorithm to the user.

If @var{algo} is not a valid hash algorithm, @code{NULL} is returned.
@end deftypefun


@node Error Handling
@chapter Error Handling
@cindex error handling

Many functions in @acronym{GPGME} can return an error if they fail.
For this reason, the application should always catch the error
condition and take appropriate measures, for example by releasing the
resources and passing the error up to the caller, or by displaying a
descriptive message to the user and cancelling the operation.

Some error values do not indicate a system error or an error in the
operation, but the result of an operation that failed properly.  For
example, if you try to decrypt a tempered message, the decryption will
fail.  Another error value actually means that the end of a data
buffer or list has been reached.  The following descriptions explain
for many error codes what they mean usually.  Some error values have
specific meanings if returned by a certain functions.  Such cases are
described in the documentation of those functions.

@acronym{GPGME} uses the @code{libgpg-error} library.  This allows to
share the error codes with other components of the GnuPG system, and
thus pass error values transparently from the crypto engine, or some
helper application of the crypto engine, to the user.  This way no
information is lost.  As a consequence, @acronym{GPGME} does not use
its own identifiers for error codes, but uses those provided by
@code{libgpg-error}.  They usually start with @code{GPG_ERR_}.

However, @acronym{GPGME} does provide aliases for the functions
defined in libgpg-error, which might be preferred for name space
consistency.

@menu
* Error Values::                  The error value and what it means.
* Error Sources::                 A list of important error sources.
* Error Codes::                   A list of important error codes.
* Error Strings::                 How to get a descriptive string from a value.
@end menu


@node Error Values
@section Error Values
@cindex error values
@cindex error codes
@cindex error sources

@deftp {Data type} {gpgme_err_code_t}
The @code{gpgme_err_code_t} type is an alias for the @code{libgpg-error}
type @code{gpg_err_code_t}.  The error code indicates the type of an
error, or the reason why an operation failed.

A list of important error codes can be found in the next section.
@end deftp

@deftp {Data type} {gpgme_err_source_t}
The @code{gpgme_err_source_t} type is an alias for the
@code{libgpg-error} type @code{gpg_err_source_t}.  The error source
has not a precisely defined meaning.  Sometimes it is the place where
the error happened, sometimes it is the place where an error was
encoded into an error value.  Usually the error source will give an
indication to where to look for the problem.  This is not always true,
but it is attempted to achieve this goal.

A list of important error sources can be found in the next section.
@end deftp

@deftp {Data type} {gpgme_error_t}
The @code{gpgme_error_t} type is an alias for the @code{libgpg-error}
type @code{gpg_error_t}.  An error value like this has always two
components, an error code and an error source.  Both together form the
error value.

Thus, the error value can not be directly compared against an error
code, but the accessor functions described below must be used.
However, it is guaranteed that only 0 is used to indicate success
(@code{GPG_ERR_NO_ERROR}), and that in this case all other parts of
the error value are set to 0, too.

Note that in @acronym{GPGME}, the error source is used purely for
diagnostical purposes.  Only the error code should be checked to test
for a certain outcome of a function.  The manual only documents the
error code part of an error value.  The error source is left
unspecified and might be anything.
@end deftp

@deftypefun {static inline gpgme_err_code_t} gpgme_err_code (@w{gpgme_error_t @var{err}})
The static inline function @code{gpgme_err_code} returns the
@code{gpgme_err_code_t} component of the error value @var{err}.  This
function must be used to extract the error code from an error value in
order to compare it with the @code{GPG_ERR_*} error code macros.
@end deftypefun

@deftypefun {static inline gpgme_err_source_t} gpgme_err_source (@w{gpgme_error_t @var{err}})
The static inline function @code{gpgme_err_source} returns the
@code{gpgme_err_source_t} component of the error value @var{err}.  This
function must be used to extract the error source from an error value in
order to compare it with the @code{GPG_ERR_SOURCE_*} error source macros.
@end deftypefun

@deftypefun {static inline gpgme_error_t} gpgme_err_make (@w{gpgme_err_source_t @var{source}}, @w{gpgme_err_code_t @var{code}})
The static inline function @code{gpgme_err_make} returns the error
value consisting of the error source @var{source} and the error code
@var{code}.

This function can be used in callback functions to construct an error
value to return it to the library.
@end deftypefun

@deftypefun {static inline gpgme_error_t} gpgme_error (@w{gpgme_err_code_t @var{code}})
The static inline function @code{gpgme_error} returns the error value
consisting of the default error source and the error code @var{code}.

For @acronym{GPGME} applications, the default error source is
@code{GPG_ERR_SOURCE_USER_1}.  You can define
@code{GPGME_ERR_SOURCE_DEFAULT} before including @file{gpgme.h} to
change this default.

This function can be used in callback functions to construct an error
value to return it to the library.
@end deftypefun

The @code{libgpg-error} library provides error codes for all system
error numbers it knows about.  If @var{err} is an unknown error
number, the error code @code{GPG_ERR_UNKNOWN_ERRNO} is used.  The
following functions can be used to construct error values from system
errnor numbers.

@deftypefun {gpgme_error_t} gpgme_err_make_from_errno (@w{gpgme_err_source_t @var{source}}, @w{int @var{err}})
The function @code{gpgme_err_make_from_errno} is like
@code{gpgme_err_make}, but it takes a system error like @code{errno}
instead of a @code{gpgme_err_code_t} error code.
@end deftypefun

@deftypefun {gpgme_error_t} gpgme_error_from_errno (@w{int @var{err}})
The function @code{gpgme_error_from_errno} is like @code{gpgme_error},
but it takes a system error like @code{errno} instead of a
@code{gpgme_err_code_t} error code.
@end deftypefun

Sometimes you might want to map system error numbers to error codes
directly, or map an error code representing a system error back to the
system error number.  The following functions can be used to do that.

@deftypefun {gpgme_err_code_t} gpgme_err_code_from_errno (@w{int @var{err}})
The function @code{gpgme_err_code_from_errno} returns the error code
for the system error @var{err}.  If @var{err} is not a known system
error, the function returns @code{GPG_ERR_UNKNOWN_ERRNO}.
@end deftypefun

@deftypefun {int} gpgme_err_code_to_errno (@w{gpgme_err_code_t @var{err}})
The function @code{gpgme_err_code_to_errno} returns the system error
for the error code @var{err}.  If @var{err} is not an error code
representing a system error, or if this system error is not defined on
this system, the function returns @code{0}.
@end deftypefun


@node Error Sources
@section Error Sources
@cindex error codes, list of

The library @code{libgpg-error} defines an error source for every
component of the GnuPG system.  The error source part of an error
value is not well defined.  As such it is mainly useful to improve the
diagnostic error message for the user.

If the error code part of an error value is @code{0}, the whole error
value will be @code{0}.  In this case the error source part is of
course @code{GPG_ERR_SOURCE_UNKNOWN}.

The list of error sources that might occur in applications using
@acronym{GPGME} is:

@table @code
@item GPG_ERR_SOURCE_UNKNOWN
The error source is not known.  The value of this error source is
@code{0}.

@item GPG_ERR_SOURCE_GPGME
The error source is @acronym{GPGME} itself.  This is the default for
errors that occur in the @acronym{GPGME} library.

@item GPG_ERR_SOURCE_GPG
The error source is GnuPG, which is the crypto engine used for the
OpenPGP protocol.

@item GPG_ERR_SOURCE_GPGSM
The error source is GPGSM, which is the crypto engine used for the
CMS protocol.

@item GPG_ERR_SOURCE_GCRYPT
The error source is @code{libgcrypt}, which is used by crypto engines
to perform cryptographic operations.

@item GPG_ERR_SOURCE_GPGAGENT
The error source is @command{gpg-agent}, which is used by crypto
engines to perform operations with the secret key.

@item GPG_ERR_SOURCE_PINENTRY
The error source is @command{pinentry}, which is used by
@command{gpg-agent} to query the passphrase to unlock a secret key.

@item GPG_ERR_SOURCE_SCD
The error source is the SmartCard Daemon, which is used by
@command{gpg-agent} to delegate operations with the secret key to a
SmartCard.

@item GPG_ERR_SOURCE_KEYBOX
The error source is @code{libkbx}, a library used by the crypto
engines to manage local keyrings.

@item GPG_ERR_SOURCE_USER_1
@item GPG_ERR_SOURCE_USER_2
@item GPG_ERR_SOURCE_USER_3
@item GPG_ERR_SOURCE_USER_4
These error sources are not used by any GnuPG component and can be
used by other software.  For example, applications using
@acronym{GPGME} can use them to mark error values coming from callback
handlers.  Thus @code{GPG_ERR_SOURCE_USER_1} is the default for errors
created with @code{gpgme_error} and @code{gpgme_error_from_errno},
unless you define @code{GPGME_ERR_SOURCE_DEFAULT} before including
@file{gpgme.h}.
@end table


@node Error Codes
@section Error Codes
@cindex error codes, list of

The library @code{libgpg-error} defines many error values.  Most of
them are not used by @code{GPGME} directly, but might be returned by
@acronym{GPGME} because it received them from the crypto engine.  The
below list only includes such error codes that have a specific meaning
in @code{GPGME}, or which are so common that you should know about
them.

@table @code
@item GPG_ERR_EOF
This value indicates the end of a list, buffer or file.

@item GPG_ERR_NO_ERROR
This value indicates success.  The value of this error code is
@code{0}.  Also, it is guaranteed that an error value made from the
error code @code{0} will be @code{0} itself (as a whole).  This means
that the error source information is lost for this error code,
however, as this error code indicates that no error occured, this is
generally not a problem.

@item GPG_ERR_GENERAL
This value means that something went wrong, but either there is not
enough information about the problem to return a more useful error
value, or there is no separate error value for this type of problem.

@item GPG_ERR_ENOMEM
This value means that an out-of-memory condition occurred.

@item GPG_ERR_E...
System errors are mapped to GPG_ERR_FOO where FOO is the symbol for
the system error.

@item GPG_ERR_INV_VALUE
This value means that some user provided data was out of range.  This
can also refer to objects.  For example, if an empty
@code{gpgme_data_t} object was expected, but one containing data was
provided, this error value is returned.

@item GPG_ERR_UNUSABLE_PUBKEY
This value means that some recipients for a message were invalid.

@item GPG_ERR_UNUSABLE_SECKEY
This value means that some signers were invalid.

@item GPG_ERR_NO_DATA
This value means that a @code{gpgme_data_t} object which was expected
to have content was found empty.

@item GPG_ERR_CONFLICT
This value means that a conflict of some sort occurred.

@item GPG_ERR_NOT_IMPLEMENTED
This value indicates that the specific function (or operation) is not
implemented.  This error should never happen.  It can only occur if
you use certain values or configuration options which do not work,
but for which we think that they should work at some later time.

@item GPG_ERR_DECRYPT_FAILED
This value indicates that a decryption operation was unsuccessful.

@item GPG_ERR_BAD_PASSPHRASE
This value means that the user did not provide a correct passphrase
when requested.

@item GPG_ERR_CANCELED
This value means that the operation was canceled.

@item GPG_ERR_INV_ENGINE
This value means that the engine that implements the desired protocol
is currently not available.  This can either be because the sources
were configured to exclude support for this engine, or because the
engine is not installed properly.

@item GPG_ERR_AMBIGUOUS_NAME
This value indicates that a user ID or other specifier did not specify
a unique key.

@item GPG_ERR_WRONG_KEY_USAGE
This value indicates that a key is not used appropriately.

@item GPG_ERR_CERT_REVOKED
This value indicates that a key signature was revoced.

@item GPG_ERR_CERT_EXPIRED
This value indicates that a key signature expired.

@item GPG_ERR_NO_CRL_KNOWN
This value indicates that no certificate revocation list is known for
the certificate.

@item GPG_ERR_NO_POLICY_MATCH
This value indicates that a policy issue occured.

@item GPG_ERR_NO_SECKEY
This value indicates that no secret key for the user ID is available.

@item GPG_ERR_MISSING_CERT
This value indicates that a key could not be imported because the
issuer certificate is missing.

@item GPG_ERR_BAD_CERT_CHAIN
This value indicates that a key could not be imported because its
certificate chain is not good, for example it could be too long.

@item GPG_ERR_UNSUPPORTED_ALGORITHM
This value means a verification failed because the cryptographic
algorithm is not supported by the crypto backend.

@item GPG_ERR_BAD_SIGNATURE
This value means a verification failed because the signature is bad.

@item GPG_ERR_NO_PUBKEY
This value means a verification failed because the public key is not
available.

@item GPG_ERR_USER_1
@item GPG_ERR_USER_2
@item ...
@item GPG_ERR_USER_16
These error codes are not used by any GnuPG component and can be
freely used by other software.  Applications using @acronym{GPGME}
might use them to mark specific errors returned by callback handlers
if no suitable error codes (including the system errors) for
these errors exist already.
@end table


@node Error Strings
@section Error Strings
@cindex error values, printing of
@cindex error codes, printing of
@cindex error sources, printing of
@cindex error strings

@deftypefun {const char *} gpgme_strerror (@w{gpgme_error_t @var{err}})
The function @code{gpgme_strerror} returns a pointer to a statically
allocated string containing a description of the error code contained
in the error value @var{err}.  This string can be used to output a
diagnostic message to the user.

This function is not thread safe.  Use @code{gpgme_strerror_r} in
multi-threaded programs.
@end deftypefun


@deftypefun {char *} gpgme_strerror_r (@w{gpgme_error_t @var{err}})
The function @code{gpgme_strerror_r} returns a pointer to a
dynamically allocated string containing a description of the error
code contained in the error value @var{err}.  This string can be used
to output a diagnostic message to the user.  When it is not needed
anymore, the user must deallocate it with @code{free}.
@end deftypefun


@deftypefun {const char *} gpgme_strsource (@w{gpgme_error_t @var{err}})
The function @code{gpgme_strerror} returns a pointer to a statically
allocated string containing a description of the error source
contained in the error value @var{err}.  This string can be used to
output a diagnostic message to the user.
@end deftypefun

The following example illustrates the use of @code{gpgme_strerror}:

@example
gpgme_ctx_t ctx;
gpgme_error_t err = gpgme_new (&ctx);
if (err)
  @{
    fprintf (stderr, "%s: creating GpgME context failed: %s: %s\n",
             argv[0], gpgme_strsource (err), gpgme_strerror (err));
    exit (1);
  @}
@end example


@node Exchanging Data
@chapter Exchanging Data
@cindex data, exchanging

A lot of data has to be exchanged between the user and the crypto
engine, like plaintext messages, ciphertext, signatures and
information about the keys.  The technical details about exchanging
the data information are completely abstracted by @acronym{GPGME}.
The user provides and receives the data via @code{gpgme_data_t} objects,
regardless of the communication protocol between @acronym{GPGME} and
the crypto engine in use.

@deftp {Data type} {gpgme_data_t}
The @code{gpgme_data_t} type is a handle for a container for generic
data, which is used by @acronym{GPGME} to exchange data with the user.
@end deftp

@menu
* Creating Data Buffers::         Creating new data buffers.
* Destroying Data Buffers::       Releasing data buffers.
* Manipulating Data Buffers::     Operations on data buffers.
@end menu


@node Creating Data Buffers
@section Creating Data Buffers
@cindex data buffer, creation

Data objects can be based on memory, files, or callback functions
provided by the user.  Not all operations are supported by all
objects.


@menu
* Memory Based Data Buffers::     Creating memory based data buffers.
* File Based Data Buffers::       Creating file based data buffers.
* Callback Based Data Buffers::   Creating callback based data buffers.
@end menu


@node Memory Based Data Buffers
@subsection Memory Based Data Buffers

Memory based data objects store all data in allocated memory.  This is
convenient, but only practical for an amount of data that is a
fraction of the available physical memory.  The data has to be copied
from its source and to its destination, which can often be avoided by
using one of the other data object 

@deftypefun gpgme_error_t gpgme_data_new (@w{gpgme_data_t *@var{dh}})
The function @code{gpgme_data_new} creates a new @code{gpgme_data_t}
object and returns a handle for it in @var{dh}.  The data object is
memory based and initially empty.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_mem (@w{gpgme_data_t *@var{dh}}, @w{const char *@var{buffer}}, @w{size_t @var{size}}, @w{int @var{copy}})
The function @code{gpgme_data_new_from_mem} creates a new
@code{gpgme_data_t} object and fills it with @var{size} bytes starting
from @var{buffer}.

If @var{copy} is not zero, a private copy of the data is made.  If
@var{copy} is zero, the data is taken from the specified buffer as
needed, and the user has to ensure that the buffer remains valid for
the whole life span of the data object.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} or @var{buffer} is not a valid pointer, and
@code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_file (@w{gpgme_data_t *@var{dh}}, @w{const char *@var{filename}}, @w{int @var{copy}})
The function @code{gpgme_data_new_from_file} creates a new
@code{gpgme_data_t} object and fills it with the content of the file
@var{filename}.

If @var{copy} is not zero, the whole file is read in at initialization
time and the file is not used anymore after that.  This is the only
mode supported currently.  Later, a value of zero for @var{copy} might
cause all reads to be delayed until the data is needed, but this is
not yet implemented.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} or @var{filename} is not a valid pointer,
@code{GPG_ERR_NOT_IMPLEMENTED} if @var{code} is zero, and
@code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_filepart (@w{gpgme_data_t *@var{dh}}, @w{const char *@var{filename}}, @w{FILE *@var{fp}}, @w{off_t @var{offset}}, @w{size_t @var{length}})
The function @code{gpgme_data_new_from_filepart} creates a new
@code{gpgme_data_t} object and fills it with a part of the file specified
by @var{filename} or @var{fp}.

Exactly one of @var{filename} and @var{fp} must be non-zero, the other
must be zero.  The argument that is not zero specifies the file from
which @var{length} bytes are read into the data object, starting from
@var{offset}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} and exactly one of @var{filename} and @var{fp} is not a valid
pointer, and @code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun


@node File Based Data Buffers
@subsection File Based Data Buffers

File based data objects operate directly on file descriptors or
streams.  Only a small amount of data is stored in core at any time,
so the size of the data objects is not limited by @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_data_new_from_fd (@w{gpgme_data_t *@var{dh}}, @w{int @var{fd}})
The function @code{gpgme_data_new_from_fd} creates a new
@code{gpgme_data_t} object and uses the file descriptor @var{fd} to read
from (if used as an input data object) and write to (if used as an
output data object).

When using the data object as an input buffer, the function might read
a bit more from the file descriptor than is actually needed by the
crypto engine in the desired operation because of internal buffering.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_stream (@w{gpgme_data_t *@var{dh}}, @w{FILE *@var{stream}})
The function @code{gpgme_data_new_from_stream} creates a new
@code{gpgme_data_t} object and uses the I/O stream @var{stream} to read
from (if used as an input data object) and write to (if used as an
output data object).

When using the data object as an input buffer, the function might read
a bit more from the stream than is actually needed by the crypto
engine in the desired operation because of internal buffering.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun


@node Callback Based Data Buffers
@subsection Callback Based Data Buffers

If neither memory nor file based data objects are a good fit for your
application, you can implement the functions a data object provides
yourself and create a data object from these callback functions.

@deftp {Data type} {ssize_t (*gpgme_data_read_cb_t) (@w{void *@var{handle}}, @w{void @var{*buffer}}, @w{size_t @var{size}})}
@tindex gpgme_data_read_cb_t
The @code{gpgme_data_read_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to read data from a user-implemented
data object.  The function should read up to @var{size} bytes from the
current read position into the space starting at @var{buffer}.  The
@var{handle} is provided by the user at data object creation time.

The function should return the number of bytes read, 0 on EOF, and -1
on error.  If an error occurs, @var{errno} should be set to describe
the type of the error.
@end deftp

@deftp {Data type} {ssize_t (*gpgme_data_write_cb_t) (@w{void *@var{handle}}, @w{const void @var{*buffer}}, @w{size_t @var{size}})}
@tindex gpgme_data_write_cb_t
The @code{gpgme_data_write_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to write data to a user-implemented
data object.  The function should write up to @var{size} bytes to the
current write position from the space starting at @var{buffer}.  The
@var{handle} is provided by the user at data object creation time.

The function should return the number of bytes written, and -1 on
error.  If an error occurs, @var{errno} should be set to describe the
type of the error.
@end deftp

@deftp {Data type} {off_t (*gpgme_data_seek_cb_t) (@w{void *@var{handle}}, @w{off_t @var{offset}}, @w{int @var{whence}})}
@tindex gpgme_data_seek_cb_t
The @code{gpgme_data_seek_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to change the current read/write
position in a user-implemented data object, just like the @code{lseek}
function.

The function should return the new read/write position, and -1 on
error.  If an error occurs, @var{errno} should be set to describe the
type of the error.
@end deftp

@deftp {Data type} {void (*gpgme_data_release_cb_t) (@w{void *@var{handle}})}
@tindex gpgme_data_release_cb_t
The @code{gpgme_data_release_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to destroy a user-implemented data
object.  The @var{handle} is provided by the user at data object
creation time.
@end deftp

@deftp {Data type} {struct gpgme_data_cbs}
This structure is used to store the data callback interface functions
described above.  It has the following members:

@table @code
@item gpgme_data_read_cb_t read
This is the function called by @acronym{GPGME} to read data from the
data object.  It is only required for input data object.

@item gpgme_data_write_cb_t write
This is the function called by @acronym{GPGME} to write data to the
data object.  It is only required for output data object.

@item gpgme_data_seek_cb_t seek
This is the function called by @acronym{GPGME} to change the current
read/write pointer in the data object (if available).  It is optional.

@item gpgme_data_release_cb_t release
This is the function called by @acronym{GPGME} to release a data
object.  It is optional.
@end table
@end deftp

@deftypefun gpgme_error_t gpgme_data_new_from_cbs (@w{gpgme_data_t *@var{dh}}, @w{gpgme_data_cbs_t @var{cbs}}, @w{void *@var{handle}})
The function @code{gpgme_data_new_from_cbs} creates a new
@code{gpgme_data_t} object and uses the user-provided callback functions
to operate on the data object.

The handle @var{handle} is passed as first argument to the callback
functions.  This can be used to identify this data object.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun

The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_data_new_with_read_cb (@w{gpgme_data_t *@var{dh}}, @w{int (*@var{readfunc})} (@w{void *@var{hook}}, @w{char *@var{buffer}}, @w{size_t @var{count}}, @w{size_t *@var{nread}}), @w{void *@var{hook_value}})
The function @code{gpgme_data_new_with_read_cb} creates a new
@code{gpgme_data_t} object and uses the callback function @var{readfunc}
to retrieve the data on demand.  As the callback function can supply
the data in any way it wants, this is the most flexible data type
@acronym{GPGME} provides.  However, it can not be used to write data.

The callback function receives @var{hook_value} as its first argument
whenever it is invoked.  It should return up to @var{count} bytes in
@var{buffer}, and return the number of bytes actually read in
@var{nread}.  It may return @code{0} in @var{nread} if no data is
currently available.  To indicate @code{EOF} the function should
return with an error code of @code{-1} and set @var{nread} to
@code{0}.  The callback function may support to reset its internal
read pointer if it is invoked with @var{buffer} and @var{nread} being
@code{NULL} and @var{count} being @code{0}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} or @var{readfunc} is not a valid pointer, and
@code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun


@node Destroying Data Buffers
@section Destroying Data Buffers
@cindex data buffer, destruction

@deftypefun void gpgme_data_release (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_release} destroys the data object with
the handle @var{dh}.  It releases all associated resources that were
not provided by the user in the first place.
@end deftypefun

@deftypefun {char *} gpgme_data_release_and_get_mem (@w{gpgme_data_t @var{dh}}, @w{size_t *@var{length}})
The function @code{gpgme_data_release_and_get_mem} is like
@code{gpgme_data_release}, except that it returns the data buffer and
its length that was provided by the object.

The user has to release the buffer with @code{free}.  In case the user
provided the data buffer in non-copy mode, a copy will be made for
this purpose.

In case an error returns, or there is no suitable data buffer that can
be returned to the user, the function will return @code{NULL}.
@end deftypefun


@node Manipulating Data Buffers
@section Manipulating Data Buffers
@cindex data buffere, manipulation

@deftypefun ssize_t gpgme_data_read (@w{gpgme_data_t @var{dh}}, @w{void *@var{buffer}}, @w{size_t @var{length}})
The function @code{gpgme_data_read} reads up to @var{length} bytes
from the data object with the handle @var{dh} into the space starting
at @var{buffer}.

If no error occurs, the actual amount read is returned.  If the end of
the data object is reached, the function returns 0.

In all other cases, the function returns -1 and sets @var{errno}.
@end deftypefun

@deftypefun ssize_t gpgme_data_write (@w{gpgme_data_t @var{dh}}, @w{const void *@var{buffer}}, @w{size_t @var{size}})
The function @code{gpgme_data_write} writes up to @var{size} bytes
starting from @var{buffer} into the data object with the handle
@var{dh} at the current write position.

The function returns the number of bytes actually written, or -1 if an
error occurs.  If an error occurs, @var{errno} is set.
@end deftypefun

@deftypefun off_t gpgme_data_seek (@w{gpgme_data_t @var{dh}}, @w{off_t *@var{offset}}, @w{int @var{whence}})
The function @code{gpgme_data_seek} changes the current read/write
position.

The @var{whence} argument specifies how the @var{offset} should be
interpreted.  It must be one of the following symbolic constants:

@table @code
@item SEEK_SET
Specifies that @var{whence} is a count of characters from the
beginning of the data object.

@item SEEK_CUR
Specifies that @var{whence} is a count of characters from the current
file position.  This count may be positive or negative.

@item SEEK_END
Specifies that @var{whence} is a count of characters from the end of
the data object.  A negative count specifies a position within the
current extent of the data object; a positive count specifies a
position past the current end.  If you set the position past the
current end, and actually write data, you will extend the data object
with zeros up to that position.
@end table

If successful, the function returns the resulting file position,
measured in bytes from the beginning of the data object.  You can use
this feature together with @code{SEEK_CUR} to read the current
read/write position.

If the function fails, -1 is returned and @var{errno} is set.
@end deftypefun

The following function is deprecated and should not be used.  It will
be removed in a future version of @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_data_rewind (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_rewind} is equivalent to:

@example
  return (gpgme_data_seek (dh, 0, SEEK_SET) == -1)
    ? gpgme_error_from_errno (errno) : 0;
@end example
@end deftypefun

@c
@c  gpgme_data_encoding_t
@c
@deftp {Data type} {enum gpgme_data_encoding_t}
@tindex gpgme_data_encoding_t
The @code{gpgme_data_encoding_t} type specifies the encoding of a
@code{gpgme_data_t} object.  This encoding is useful to give the backend
a hint on the type of data.  The following data types are available:

@table @code
@item GPGME_DATA_ENCODING_NONE
This specifies that the encoding is not known.  This is the default
for a new data object.  The backend will try its best to detect the
encoding automatically.

@item GPGME_DATA_ENCODING_BINARY
This specifies that the data is encoding in binary form; i.e. there is
no special encoding.

@item GPGME_DATA_ENCODING_BASE64
This specifies that the data is encoded using the Base-64 encoding
scheme as used by @acronym{MIME} and other protocols.

@item GPGME_DATA_ENCODING_ARMOR
This specifies that the data is encoded in an armored form as used by
OpenPGP and PEM.
@end table
@end deftp

@deftypefun gpgme_data_encoding_t gpgme_data_get_encoding (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_get_encoding} returns the encoding of
the data object with the handle @var{dh}.  If @var{dh} is not a valid
pointer (e.g. @code{NULL}) @code{GPGME_DATA_ENCODING_NONE} is
returned.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_set_encoding (@w{gpgme_data_t @var{dh}, gpgme_data_encoding_t @var{enc}})
The function @code{gpgme_data_set_encoding} changes the encoding of
the data object with the handle @var{dh} to @var{enc}.
@end deftypefun


@c
@c    Chapter Contexts
@c 
@node Contexts
@chapter Contexts
@cindex context

All cryptographic operations in @acronym{GPGME} are performed within a
context, which contains the internal state of the operation as well as
configuration parameters.  By using several contexts you can run
several cryptographic operations in parallel, with different
configuration.

@deftp {Data type} {gpgme_ctx_t}
The @code{gpgme_ctx_t} type is a handle for a @acronym{GPGME} context,
which is used to hold the configuration, status and result of
cryptographic operations.
@end deftp

@menu
* Creating Contexts::             Creating new @acronym{GPGME} contexts.
* Destroying Contexts::           Releasing @acronym{GPGME} contexts.
* Context Attributes::            Setting properties of a context.
* Key Management::                Managing keys with @acronym{GPGME}.
* Trust Item Management::         Managing trust items with @acronym{GPGME}.
* Crypto Operations::             Using a context for cryptography.
* Run Control::                   Controlling how operations are run.
@end menu


@node Creating Contexts
@section Creating Contexts
@cindex context, creation

@deftypefun gpgme_error_t gpgme_new (@w{gpgme_ctx_t *@var{ctx}})
The function @code{gpgme_data_new} creates a new @code{gpgme_ctx_t}
object and returns a handle for it in @var{ctx}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
context was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun


@node Destroying Contexts
@section Destroying Contexts
@cindex context, destruction

@deftypefun void gpgme_release (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_release} destroys the context with the handle
@var{ctx} and releases all associated resources.
@end deftypefun


@node Context Attributes
@section Context Attributes
@cindex context, attributes

@menu
* Protocol Selection::            Selecting the protocol used by a context.
* ASCII Armor::                   Requesting @acronym{ASCII} armored output.
* Text Mode::                     Choosing canonical text mode.
* Included Certificates::       Including a number of certificates.
* Key Listing Mode::              Selecting key listing mode.
* Passphrase Callback::           Getting the passphrase from the user.
* Progress Meter Callback::       Being informed about the progress.
* Locale::                        Setting the locale of a context.
@end menu


@node Protocol Selection
@subsection Protocol Selection
@cindex context, selecting protocol
@cindex protocol, selecting

@deftypefun gpgme_error_t gpgme_set_protocol (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_protocol_t @var{proto}})
The function @code{gpgme_set_protocol} sets the protocol used within
the context @var{ctx} to @var{proto}.  All crypto operations will be
performed by the crypto engine configured for that protocol.
@xref{Protocols and Engines}.

Setting the protocol with @code{gpgme_set_protocol} does not check if
the crypto engine for that protocol is available and installed
correctly.  @xref{Engine Version Check}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
protocol could be set successfully, and @code{GPG_ERR_INV_VALUE} if
@var{protocol} is not a valid protocol.
@end deftypefun

@deftypefun gpgme_protocol_t gpgme_get_protocol (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_protocol} retrieves the protocol currently
use with the context @var{ctx}.
@end deftypefun

@c FIXME: Unfortunately, using @acronym here breaks texi2dvi.
@node ASCII Armor
@subsection @acronym{ASCII} Armor
@cindex context, armor mode
@cindex @acronym{ASCII} armor
@cindex armor mode

@deftypefun void gpgme_set_armor (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{yes}})
The function @code{gpgme_set_armor} specifies if the output should be
@acronym{ASCII} armored.  By default, output is not @acronym{ASCII}
armored.

@acronym{ASCII} armored output is disabled if @var{yes} is zero, and
enabled otherwise.
@end deftypefun

@deftypefun int gpgme_get_armor (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_armor} returns 1 if the output is
@acronym{ASCII} armored, and @code{0} if it is not, or if @var{ctx} is
not a valid pointer.
@end deftypefun


@node Text Mode
@subsection Text Mode
@cindex context, text mode
@cindex text mode
@cindex canonical text mode

@deftypefun void gpgme_set_textmode (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{yes}})
The function @code{gpgme_set_textmode} specifies if canonical text mode
should be used.  By default, text mode is not used.

Text mode is for example used for the RFC2015 signatures; note that
the updated RFC 3156 mandates that the mail user agent does some
preparations so that text mode is not needed anymore.

This option is only relevant to the OpenPGP crypto engine, and ignored
by all other engines.

Canonical text mode is disabled if @var{yes} is zero, and enabled
otherwise.
@end deftypefun

@deftypefun int gpgme_get_textmode (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_textmode} returns 1 if canonical text
mode is enabled, and @code{0} if it is not, or if @var{ctx} is not a
valid pointer.
@end deftypefun


@node Included Certificates
@subsection Included Certificates
@cindex certificates, included

@deftypefun void gpgme_set_include_certs (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{nr_of_certs}})
The function @code{gpgme_set_include_certs} specifies how many
certificates should be included in an S/MIME signed message.  By
default, only the sender's certificate is included.  The possible
values of @var{nr_of_certs} are:

@table @code
@item -2
Include all certificates except the root certificate.
@item -1
Include all certificates.
@item 0
Include no certificates.
@item 1
Include the sender's certificate only.
@item n
Include the first n certificates of the certificates path, starting
from the sender's certificate.  The number @code{n} must be positive.
@end table

Values of @var{nr_of_certs} smaller than -2 are undefined.

This option is only relevant to the CMS crypto engine, and ignored by
all other engines.
@end deftypefun

@deftypefun int gpgme_get_include_certs (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_include_certs} returns the number of
certificates to include into an S/MIME signed message.
@end deftypefun


@node Key Listing Mode
@subsection Key Listing Mode
@cindex key listing mode
@cindex key listing, mode of

@deftypefun void gpgme_set_keylist_mode (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_keylist_mode_t @var{mode}})
The function @code{gpgme_set_keylist_mode} changes the default
behaviour of the key listing functions.  The value in @var{mode} is a
bitwise-or combination of one or multiple of the following bit values:

@table @code
@item GPGME_KEYLIST_MODE_LOCAL
The @code{GPGME_KEYLIST_MODE_LOCAL} symbol specifies that the local
keyring should be searched for keys in the keylisting operation.  This
is the default.

@item GPGME_KEYLIST_MODE_EXTERN
The @code{GPGME_KEYLIST_MODE_EXTERN} symbol specifies that an external
source should be searched for keys in the keylisting
operation.  The type of external source is dependant on the crypto
engine used.  For example, it can be a remote keyserver or LDAP
certificate server.

@item GPGME_KEYLIST_MODE_SIGS
The @code{GPGME_KEYLIST_MODE_SIGS} symbol specifies that the key
signatures should be included in the listed keys.

@item GPGME_KEYLIST_MODE_VALIDATE
The @code{GPGME_KEYLIST_MODE_VALIDATE} symbol specifies that the
backend should do key or certificate validation and not just get the
validity information from an internal cache.  This might be an
expensive operation and is in general not usefule.  Currently only
implemented for the S/MIME backend and ignored for other backends.

@end table

At least one of @code{GPGME_KEYLIST_MODE_LOCAL} and
@code{GPGME_KEYLIST_MODE_EXTERN} must be specified.  For future binary
compatibility, you should get the current mode with
@code{gpgme_get_keylist_mode} and modify it by setting or clearing the
appropriate bits, and then using that calulcated value in the
@code{gpgme_set_keylisting_mode} operation.  This will leave all other
bits in the mode value intact (in particular those that are not used
in the current version of the library).

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
mode could be set correctly, and @code{GPG_ERR_INV_VALUE} if @var{ctx}
is not a valid pointer or @var{mode} is not a valid mode.
@end deftypefun


@deftypefun gpgme_keylist_mode_t gpgme_get_keylist_mode (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_keylist_mode} returns the current key
listing mode of the context @var{ctx}.  This value can then be
modified and used in a subsequent @code{gpgme_set_keylist_mode}
operation to only affect the desired bits (and leave all others
intact).

The function returns 0 if @var{ctx} is not a valid pointer, and the
current mode otherwise.  Note that 0 is not a valid mode value.
@end deftypefun


@node Passphrase Callback
@subsection Passphrase Callback
@cindex callback, passphrase
@cindex passphrase callback

@deftp {Data type} {gpgme_error_t (*gpgme_passphrase_cb_t)(void *@var{hook}, const char *@var{uid_hint}, const char *@var{passphrase_info}, @w{int @var{prev_was_bad}}, @w{int @var{fd}})}
@tindex gpgme_passphrase_cb_t
The @code{gpgme_passphrase_cb_t} type is the type of functions usable as
passphrase callback function.

The argument @var{uid_hint} might contain a string that gives an
indication for which user ID the passphrase is required.  If this is
not available, or not applicable (in the case of symmetric encryption,
for example), @var{uid_hint} will be @code{NULL}.

The argument @var{passphrase_info}, if not @code{NULL}, will give
further information about the context in which the passphrase is
required.  This information is engine and operation specific.

If this is the repeated attempt to get the passphrase, because
previous attempts failed, then @var{prev_was_bad} is 1, otherwise it
will be 0.

The user must write the passphrase, followed by a newline character,
to the file descriptor @var{fd}.  If the user does not return 0
indicating success, the user must at least write a newline character
before returning from the callback.

If an error occurs, return the corresponding @code{gpgme_error_t}
value.  You can use the error code @code{GPG_ERR_CANCELED} to abort
the operation.  Otherwise, return @code{0}.
@end deftp

@deftypefun void gpgme_set_passphrase_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_passphrase_cb_t @var{passfunc}}, @w{void *@var{hook_value}})
The function @code{gpgme_set_passphrase_cb} sets the function that is
used when a passphrase needs to be provided by the user to
@var{passfunc}.  The function @var{passfunc} needs to implemented by
the user, and whenever it is called, it is called with its first
argument being @var{hook_value}.  By default, no passphrase callback
function is set.

Not all crypto engines require this callback to retrieve the
passphrase.  It is better if the engine retrieves the passphrase from
a trusted agent (a daemon process), rather than having each user to
implement their own passphrase query.

The user can disable the use of a passphrase callback function by
calling @code{gpgme_set_passphrase_cb} with @var{passfunc} being
@code{NULL}.
@end deftypefun

@deftypefun void gpgme_get_passphrase_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_passphrase_cb_t *@var{passfunc}}, @w{void **@var{hook_value}})
The function @code{gpgme_get_passphrase_cb} returns the function that
is used when a passphrase needs to be provided by the user in
@var{*passfunc}, and the first argument for this function in
@var{*hook_value}.  If no passphrase callback is set, or @var{ctx} is
not a valid pointer, @code{NULL} is returned in both variables.

@var{passfunc} or @var{hook_value} can be @code{NULL}.  In this case,
the corresponding value will not be returned.
@end deftypefun


@node Progress Meter Callback
@subsection Progress Meter Callback
@cindex callback, progress meter
@cindex progress meter callback

@deftp {Data type} {void (*gpgme_progress_cb_t)(void *@var{hook}, const char *@var{what}, int @var{type}, int @var{current}, int @var{total})}
@tindex gpgme_progress_cb_t
The @code{gpgme_progress_cb_t} type is the type of functions usable as
progress callback function.

The arguments are specific to the crypto engine.  More information
about the progress information returned from the GnuPG engine can be
found in the GnuPG source code in the file @file{doc/DETAILS} in the
section PROGRESS.
@end deftp

@deftypefun void gpgme_set_progress_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_progress_cb_t @var{progfunc}}, @w{void *@var{hook_value}})
The function @code{gpgme_set_progress_cb} sets the function that is
used when progress information about a cryptographic operation is
available.  The function @var{progfunc} needs to implemented by the
user, and whenever it is called, it is called with its first argument
being @var{hook_value}.  By default, no progress callback function
is set.

Setting a callback function allows an interactive program to display
progress information about a long operation to the user.

The user can disable the use of a progress callback function by
calling @code{gpgme_set_progress_cb} with @var{progfunc} being
@code{NULL}.
@end deftypefun

@deftypefun void gpgme_get_progress_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_progress_cb_t *@var{progfunc}}, @w{void **@var{hook_value}})
The function @code{gpgme_get_progress_cb} returns the function that is
used to inform the user about the progress made in @var{*progfunc},
and the first argument for this function in @var{*hook_value}.  If no
progress callback is set, or @var{ctx} is not a valid pointer,
@code{NULL} is returned in both variables.

@var{progfunc} or @var{hook_value} can be @code{NULL}.  In this case,
the corresponding value will not be returned.
@end deftypefun


@node Locale
@subsection Locale
@cindex locale, default
@cindex locale, of a context

A locale setting can be associated with a context.  This locale is
passed to the crypto engine, and used for applications like the PIN
entry, which is displayed to the user when entering a passphrase is
required.

The default locale is used to initialize the locale setting of all
contexts created afterwards.

@deftypefun gpgme_error_t gpgme_set_locale (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{category}}, @w{const char *@var{value}})
The function @code{gpgme_set_locale} sets the locale of the context
@var{ctx}, or the default locale if @var{ctx} is a null pointer.

The locale settings that should be changed are specified by
@var{category}.  Supported categories are @code{LC_CTYPE},
@code{LC_MESSAGES}, and @code{LC_ALL}, which is a wildcard you can use
if you want to change all the categories at once.

The value to be used for the locale setting is @var{value}, which will
be copied to @acronym{GPGME}'s internal data structures.  @var{value}
can be a null pointer, which disables setting the locale, and will
make PIN entry and other applications use their default setting, which
is usually not what you want.

Note that the settings are only used if the application runs on a text
terminal, and that the settings should fit the configuration of the
output terminal.  Normally, it is sufficient to initialize the default
value at startup.

The function returns an error if not enough memory is available.
@end deftypefun


@node Key Management
@section Key Management
@cindex key management

Some of the cryptographic operations require that recipients or
signers are specified.  This is always done by specifying the
respective keys that should be used for the operation.  The following
section describes how such keys can be selected and manipulated.

@deftp {Data type} gpgme_sub_key_t
The @code{gpgme_sub_key_t} type is a pointer to a subkey structure.
Sub keys are one component of a @code{gpgme_key_t} object.  In fact,
subkeys are those parts that contains the real information about the
individual cryptographic keys that belong to the same key object.  One
@code{gpgme_key_t} can contain several subkeys.  The first subkey in
the linked list is also called the primary key.

The subkey structure has the following members:

@table @code
@item gpgme_sub_key_t next
This is a pointer to the next subkey structure in the linked list, or
@code{NULL} if this is the last element.

@item unsigned int revoked : 1
This is true if the subkey is revoked.

@item unsigned int expired : 1
This is true if the subkey is expired.

@item unsigned int disabled : 1
This is true if the subkey is disabled.

@item unsigned int invalid : 1
This is true if the subkey is invalid.

@item unsigned int can_encrypt : 1
This is true if the subkey can be used for encryption.

@item unsigned int can_sign : 1
This is true if the subkey can be used to create data signatures.

@item unsigned int can_certify : 1
This is true if the subkey can be used to create key certificates.

@item unsigned int can_authenticate : 1
This is true if the subkey can be used for authentication.

@item unsigned int secret : 1
This is true if the subkey is a secret key.

@item gpgme_pubkey_algo_t pubkey_algo
This is the public key algorithm supported by this subkey.

@item unsigned int length
This is the length of the subkey (in bits).

@item char *keyid
This is the key ID of the subkey in hexadecimal digits.

@item char *fpr
This is the fingerprint of the subkey in hexadecimal digits, if
available.  This is usually only available for the primary key.

@item long int timestamp
This is the creation timestamp of the subkey.  This is -1 if the
timestamp is invalid, and 0 if it is not available.

@item long int expires
This is the expiration timestamp of the subkey, or 0 if the subkey
does not expire.
@end table
@end deftp

@deftp {Data type} gpgme_key_sig_t
The @code{gpgme_key_sig_t} type is a pointer to a key signature structure.
Key signatures are one component of a @code{gpgme_key_t} object, and
validate user IDs on the key.

The signatures on a key are only available if the key was retrieved
via a listing operation with the @code{GPGME_KEYLIST_MODE_SIGS} mode
enabled, because it is expensive to retrieve all signatures of a key.

The key signature structure has the following members:

@table @code
@item gpgme_key_sig_t next
This is a pointer to the next key signature structure in the linked
list, or @code{NULL} if this is the last element.

@item unsigned int revoked : 1
This is true if the key signature is a revocation signature.

@item unsigned int expired : 1
This is true if the key signature is expired.

@item unsigned int invalid : 1
This is true if the key signature is invalid.

@item unsigned int exportable : 1
This is true if the key signature is exportable.

@item gpgme_pubkey_algo_t pubkey_algo
This is the public key algorithm used to create the signature.

@item char *keyid
This is the key ID of the key (in hexadecimal digits) used to create
the signature.

@item long int timestamp
This is the creation timestamp of the key signature.  This is -1 if
the timestamp is invalid, and 0 if it is not available.

@item long int expires
This is the expiration timestamp of the key signature, or 0 if the key
signature does not expire.

@item gpgme_error_t status
This is the status of the signature and has the same meaning as the
member of the same name in a @code{gpgme_signature_t} object.

@item unsigned int sig_class
This specifies the signature class of the key signature.  The meaning
is specific to the crypto engine.

@item char *uid
This is the main user ID of the key used to create the signature.

@item char *name
This is the name component of @code{uid}, if available.

@item char *comment
This is the comment component of @code{uid}, if available.

@item char *email
This is the email component of @code{uid}, if available.
@end table
@end deftp

@deftp {Data type} gpgme_user_id_t
A user ID is a component of a @code{gpgme_key_t} object.  One key can
have many user IDs.  The first one in the list is the main (or
primary) user ID.

The user ID structure has the following members.

@table @code
@item gpgme_user_id_t next
This is a pointer to the next user ID structure in the linked list, or
@code{NULL} if this is the last element.

@item unsigned int revoked : 1
This is true if the user ID is revoked.

@item unsigned int invalid : 1
This is true if the user ID is invalid.

@item gpgme_validity_t validity
This specifies the validity of the user ID.

@item char *uid
This is the user ID string.

@item char *name
This is the name component of @code{uid}, if available.

@item char *comment
This is the comment component of @code{uid}, if available.

@item char *email
This is the email component of @code{uid}, if available.

@item gpgme_key_sig_t signatures
This is a linked list with the signatures on this user ID.
@end table
@end deftp

@deftp {Data type} gpgme_key_t
The @code{gpgme_key_t} type is a pointer to a key object.  It has the
following members:

@table @code
@item unsigned int keylist_mode
The keylist mode that was active when the key was retrieved.

@item unsigned int revoked : 1
This is true if the key is revoked.

@item unsigned int expired : 1
This is true if the key is expired.

@item unsigned int disabled : 1
This is true if the key is disabled.

@item unsigned int invalid : 1
This is true if the key is invalid. This might have several reasons,
for a example for the S/MIME backend, it will be set in during key
listsing if the key could not be validated due to a missing
certificates or unmatched policies.

@item unsigned int can_encrypt : 1
This is true if the key (ie one of its subkeys) can be used for
encryption.

@item unsigned int can_sign : 1
This is true if the key (ie one of its subkeys) can be used to create
data signatures.

@item unsigned int can_certify : 1
This is true if the key (ie one of its subkeys) can be used to create
key certificates.

@item unsigned int can_authenticate : 1
This is true if the key (ie one of its subkeys) can be used for
authentication.

@item unsigned int secret : 1
This is true if the key is a secret key.

@item gpgme_protocol_t protocol
This is the protocol supported by this key.

@item char *issuer_serial
If @code{protocol} is @code{GPGME_PROTOCOL_CMS}, then this is the
issuer serial.

@item char *issuer_name
If @code{protocol} is @code{GPGME_PROTOCOL_CMS}, then this is the
issuer name.

@item char *chain_id
If @code{protocol} is @code{GPGME_PROTOCOL_CMS}, then this is the
chain ID, which can be used to built the certificate chain.
 
@item gpgme_validity_t owner_trust
If @code{protocol} is @code{GPGME_PROTOCOL_OpenPGP}, then this is the
owner trust.

@item gpgme_sub_key_t subkeys
This is a linked list with the subkeys of the key.  The first subkey
in the list is the primary key and usually available.

@item gpgme_user_id_t uids
This is a linked list with the user IDs of the key.  The first user ID
in the list is the main (or primary) user ID.
@end table
@end deftp

@menu
* Listing Keys::                  Browsing the list of available keys.
* Information About Keys::        Requesting detailed information about keys.
* Key Signatures::                Listing the signatures on a key.
* Manipulating Keys::             Operations on keys.
* Generating Keys::               Creating new key pairs.
* Exporting Keys::                Retrieving key data from the key ring.
* Importing Keys::                Adding keys to the key ring.
* Deleting Keys::                 Removing keys from the key ring.
@end menu


@node Listing Keys
@subsection Listing Keys
@cindex listing keys
@cindex key listing
@cindex key listing, start
@cindex key ring, list
@cindex key ring, search

@deftypefun gpgme_error_t gpgme_op_keylist_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{int @var{secret_only}})
The function @code{gpgme_op_keylist_start} initiates a key listing
operation inside the context @var{ctx}.  It sets everything up so that
subsequent invocations of @code{gpgme_op_keylist_next} return the keys
in the list.

If @var{pattern} is @code{NULL}, all available keys are returned.
Otherwise, @var{pattern} contains an engine specific expression that
is used to limit the list to all keys matching the pattern.  Note that
the total length of the pattern is restricted to an engine-specific
maximum (a couple of hundred characters are usually accepted).  The
pattern should be used to restrict the search to a certain common name
or user, not to list many specific keys at once by listing their
fingerprints or key IDs.

If @var{secret_only} is not @code{0}, the list is restricted to secret
keys only.

The context will be busy until either all keys are received (and
@code{gpgme_op_keylist_next} returns @code{GPG_ERR_EOF}), or
@code{gpgme_op_keylist_end} is called to finish the operation.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_keylist_ext_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}[]}, @w{int @var{secret_only}}, @w{int @var{reserved}})
The function @code{gpgme_op_keylist_ext_start} initiates an extended
key listing operation inside the context @var{ctx}.  It sets
everything up so that subsequent invocations of
@code{gpgme_op_keylist_next} return the keys in the list.

If @var{pattern} or @var{*pattern} is @code{NULL}, all available keys
are returned.  Otherwise, @var{pattern} is a @code{NULL} terminated
array of strings that are used to limit the list to all keys matching
at least one of the patterns verbatim.  Note that the total length of
all patterns is restricted to an engine-specific maximum (the exact
limit also depends on the number of patterns and amount of quoting
required, but a couple of hundred characters are usually accepted).
Patterns should be used to restrict the search to a certain common
name or user, not to list many specific keys at once by listing their
fingerprints or key IDs.

If @var{secret_only} is not @code{0}, the list is restricted to secret
keys only.

The value of @var{reserved} must be @code{0}.

The context will be busy until either all keys are received (and
@code{gpgme_op_keylist_next} returns @code{GPG_ERR_EOF}), or
@code{gpgme_op_keylist_end} is called to finish the operation.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_keylist_next (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t *@var{r_key}})
The function @code{gpgme_op_keylist_next} returns the next key in the
list created by a previous @code{gpgme_op_keylist_start} operation in
the context @var{ctx}.  The key will have one reference for the user.
@xref{Manipulating Keys}.

This is the only way to get at @code{gpgme_key_t} objects in
@acronym{GPGME}.

If the last key in the list has already been returned,
@code{gpgme_op_keylist_next} returns @code{GPG_ERR_EOF}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{r_key} is not a valid pointer, and
@code{GPG_ERR_ENOMEM} if there is not enough memory for the operation.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_keylist_end (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_keylist_next} ends a pending key list
operation in the context @var{ctx}.

After the operation completed successfully, the result of the key
listing operation can be retrieved with
@code{gpgme_op_keylist_result}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if at some
time during the operation there was not enough memory available.
@end deftypefun

The following example illustrates how all keys containing a certain
string (@code{g10code}) can be listed with their key ID and the name
and e-mail address of the main user ID:

@example
gpgme_ctx_t ctx;
gpgme_error_t err = gpgme_new (&ctx);

if (!err)
  @{
    err = gpgme_op_keylist_start (ctx, "g10code", 0);
    while (!err)
      @{
        err = gpgme_op_keylist_next (ctx, &key);
        if (err)
          break;
        printf ("%s: %s <%s>\n", key->keyid, key->name, key->email);
        gpgme_key_release (key);
      @}
    gpgme_release (ctx);
  @}
if (gpg_err_code (err) != GPG_ERR_EOF)
  @{
    fprintf (stderr, "%s: can not list keys: %s\n",
             argv[0], gpgme_strerror (err));
    exit (1);
  @}
@end example

@deftp {Data type} {gpgme_keylist_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_keylist_*} operation.  After successfully ending a key
listing operation, you can retrieve the pointer to the result with
@code{gpgme_op_keylist_result}.  The structure contains the following
member:

@table @code
@item unsigned int truncated : 1
This is true if the crypto backend had to truncate the result, and
less than the desired keys could be listed.
@end table
@end deftp

@deftypefun gpgme_keylist_result_t gpgme_op_keylist_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_keylist_result} returns a
@code{gpgme_keylist_result_t} pointer to a structure holding the
result of a @code{gpgme_op_keylist_*} operation.  The pointer is only
valid if the last operation on the context was a key listing
operation, and if this operation finished successfully.  The returned
pointer is only valid until the next operation is started on the
context.
@end deftypefun

In a simple program, for which a blocking operation is acceptable, the
following function can be used to retrieve a single key.

@deftypefun gpgme_error_t gpgme_get_key (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{fpr}}, @w{gpgme_key_t *@var{r_key}}, @w{int @var{secret}})
The function @code{gpgme_get_key} gets the key with the fingerprint
(or key ID) @var{fpr} from the crypto backend and return it in
@var{r_key}.  If @var{secret} is true, get the secret key.  The
currently active keylist mode is used to retrieve the key.  The key
will have one reference for the user.

If the key is not found in the keyring, @code{gpgme_get_key} returns
the error code @code{GPG_ERR_NO_ERROR} and *@var{r_key} will be set to
@code{NULL}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{r_key} is not a valid pointer or @var{fpr} is not a
fingerprint or key ID, @code{GPG_ERR_AMBIGUOUS_NAME} if the key ID was
not a unique specifier for a key, and @code{GPG_ERR_ENOMEM} if at some
time during the operation there was not enough memory available.
@end deftypefun


@node Information About Keys
@subsection Information About Keys
@cindex key, information about
@cindex key, attributes
@cindex attributes, of a key

Please see the beginning of this section for more information about
@code{gpgme_key_t} objects.

@deftp {Data type} gpgme_validity_t
The @code{gpgme_validity_t} type is used to specify the validity of a user ID
in a key.  The following validities are defined:

@table @code
@item GPGME_VALIDITY_UNKNOWN
The user ID is of unknown validity.  The string representation of this
validity is ``?''.

@item GPGME_VALIDITY_UNDEFINED
The validity of the user ID is undefined.  The string representation of this
validity is ``q''.

@item GPGME_VALIDITY_NEVER
The user ID is never valid.  The string representation of this
validity is ``n''.

@item GPGME_VALIDITY_MARGINAL
The user ID is marginally valid.  The string representation of this
validity is ``m''.

@item GPGME_VALIDITY_FULL
The user ID is fully valid.  The string representation of this
validity is ``f''.

@item GPGME_VALIDITY_ULTIMATE
The user ID is ultimately valid.  The string representation of this
validity is ``u''.
@end table
@end deftp


The following interfaces are deprecated and only provided for backward
compatibility.  Don't use them.  They will be removed in a future
version of @acronym{GPGME}.

@deftp {Data type} gpgme_attr_t
The @code{gpgme_attr_t} type is used to specify a key or trust item
attribute.  The following attributes are defined:

@table @code
@item GPGME_ATTR_KEYID
This is the key ID of a sub key.  It is representable as a string.

For trust items, the trust item refers to the key with this ID.

@item GPGME_ATTR_FPR
This is the fingerprint of a sub key.  It is representable as a
string.

@item GPGME_ATTR_ALGO
This is the crypto algorithm for which the sub key can be used.  It
is representable as a string and as a number.  The numbers correspond
to the @code{enum gcry_pk_algos} values in the gcrypt library.

@item GPGME_ATTR_LEN
This is the key length of a sub key.  It is representable as a
number.

@item GPGME_ATTR_CREATED
This is the timestamp at creation time of a sub key.  It is
representable as a number.

@item GPGME_ATTR_EXPIRE
This is the expiration time of a sub key.  It is representable as a
number.

@item GPGME_ATTR_OTRUST
XXX FIXME  (also for trust items)

@item GPGME_ATTR_USERID
This is a user ID.  There can be more than one user IDs in a
@var{gpgme_key_t} object.  The first one (with index 0) is the primary
user ID.  The user ID is representable as a number.

For trust items, this is the user ID associated with this trust item.

@item GPGME_ATTR_NAME
This is the name belonging to a user ID.  It is representable as a string.

@item GPGME_ATTR_EMAIL
This is the email address belonging to a user ID.  It is representable
as a string.

@item GPGME_ATTR_COMMENT
This is the comment belonging to a user ID.  It is representable as a
string.

@item GPGME_ATTR_VALIDITY
This is the validity belonging to a user ID.  It is representable as a
string and as a number.  See below for a list of available validities.

For trust items, this is the validity that is associated with this
trust item.

@item GPGME_ATTR_UID_REVOKED
This specifies if a user ID is revoked.  It is representable as a
number, and is @code{1} if the user ID is revoked, and @code{0}
otherwise.

@item GPGME_ATTR_UID_INVALID
This specifies if a user ID is invalid.  It is representable as a
number, and is @code{1} if the user ID is invalid, and @code{0}
otherwise.

@item GPGME_ATTR_LEVEL
This is the trust level of a trust item.

@item GPGME_ATTR_TYPE
This returns information about the type of key.  For the string function
this will eother be "PGP" or "X.509".  The integer function returns 0
for PGP and 1 for X.509.  It is also used for the type of a trust item.

@item GPGME_ATTR_IS_SECRET
This specifies if the key is a secret key.  It is representable as a
number, and is @code{1} if the key is revoked, and @code{0} otherwise.

@item GPGME_ATTR_KEY_REVOKED
This specifies if a sub key is revoked.  It is representable as a
number, and is @code{1} if the key is revoked, and @code{0} otherwise.

@item GPGME_ATTR_KEY_INVALID
This specifies if a sub key is invalid.  It is representable as a
number, and is @code{1} if the key is invalid, and @code{0} otherwise.

@item GPGME_ATTR_KEY_EXPIRED
This specifies if a sub key is expired.  It is representable as a
number, and is @code{1} if the key is expired, and @code{0} otherwise.

@item GPGME_ATTR_KEY_DISABLED
This specifies if a sub key is disabled.  It is representable as a
number, and is @code{1} if the key is disabled, and @code{0} otherwise.

@item GPGME_ATTR_KEY_CAPS
This is a description of the capabilities of a sub key.  It is
representable as a string.  The string contains the letter ``e'' if
the key can be used for encryption, ``s'' if the key can be used for
signatures, and ``c'' if the key can be used for certifications.

@item GPGME_ATTR_CAN_ENCRYPT
This specifies if a sub key can be used for encryption.  It is
representable as a number, and is @code{1} if the sub key can be used
for encryption, and @code{0} otherwise.

@item GPGME_ATTR_CAN_SIGN
This specifies if a sub key can be used to create data signatures.  It
is representable as a number, and is @code{1} if the sub key can be
used for signatures, and @code{0} otherwise.

@item GPGME_ATTR_CAN_CERTIFY
This specifies if a sub key can be used to create key certificates.
It is representable as a number, and is @code{1} if the sub key can be
used for certifications, and @code{0} otherwise.

@item GPGME_ATTR_SERIAL
The X.509 issuer serial attribute of the key.  It is representable as
a string.

@item GPGME_ATTR_ISSUE
The X.509 issuer name attribute of the key.  It is representable as a
string.

@item GPGME_ATTR_CHAINID
The X.509 chain ID can be used to build the certification chain.  It
is representable as a string.
@end table
@end deftp

@deftypefun {const char *} gpgme_key_get_string_attr (@w{gpgme_key_t @var{key}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_get_string_attr} returns the value of the
string-representable attribute @var{what} of key @var{key}.  If the
attribute is an attribute of a sub key or an user ID, @var{idx}
specifies the sub key or user ID of which the attribute value is
returned.  The argument @var{reserved} is reserved for later use and
should be @code{NULL}.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun {unsigned long} gpgme_key_get_ulong_attr (@w{gpgme_key_t @var{key}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_get_ulong_attr} returns the value of the
number-representable attribute @var{what} of key @var{key}.  If the
attribute is an attribute of a sub key or an user ID, @var{idx}
specifies the sub key or user ID of which the attribute value is
returned.  The argument @var{reserved} is reserved for later use and
should be @code{NULL}.

The function returns @code{0} if the attribute can't be returned as a
number, @var{key} is not a valid pointer, @var{idx} out of range, or
@var{reserved} not @code{NULL}.
@end deftypefun


@node Key Signatures
@subsection Key Signatures
@cindex key, signatures
@cindex signatures, on a key

The following interfaces are deprecated and only provided for backward
compatibility.  Don't use them.  They will be removed in a future
version of @acronym{GPGME}.

The signatures on a key are only available if the key was retrieved
via a listing operation with the @code{GPGME_KEYLIST_MODE_SIGS} mode
enabled, because it is expensive to retrieve all signatures of a key.

So, before using the below interfaces to retrieve the signatures on a
key, you have to make sure that the key was listed with signatures
enabled.  One convenient, but blocking, way to do this is to use the
function @code{gpgme_get_key}.

@deftp {Data type} gpgme_attr_t
The @code{gpgme_attr_t} type is used to specify a key signature
attribute.  The following attributes are defined:

@table @code
@item GPGME_ATTR_KEYID
This is the key ID of the key which was used for the signature.  It is
representable as a string.

@item GPGME_ATTR_ALGO
This is the crypto algorithm used to create the signature.  It is
representable as a string and as a number.  The numbers correspond to
the @code{enum gcry_pk_algos} values in the gcrypt library.

@item GPGME_ATTR_CREATED
This is the timestamp at creation time of the signature.  It is
representable as a number.

@item GPGME_ATTR_EXPIRE
This is the expiration time of the signature.  It is representable as
a number.

@item GPGME_ATTR_USERID
This is the user ID associated with the signing key.  The user ID is
representable as a number.

@item GPGME_ATTR_NAME
This is the name belonging to a user ID.  It is representable as a string.

@item GPGME_ATTR_EMAIL
This is the email address belonging to a user ID.  It is representable
as a string.

@item GPGME_ATTR_COMMENT
This is the comment belonging to a user ID.  It is representable as a
string.

@item GPGME_ATTR_KEY_REVOKED
This specifies if a key signature is a revocation signature.  It is
representable as a number, and is @code{1} if the key is revoked, and
@code{0} otherwise.

@c @item GPGME_ATTR_KEY_EXPIRED
@c This specifies if a key signature is expired.  It is representable as
@c a number, and is @code{1} if the key is revoked, and @code{0}
@c otherwise.
@c
@item GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature.  It is
representable as a number.  The meaning is specific to the crypto
engine.

@item GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature.  It is
representable as a number.  The meaning is specific to the crypto
engine.

@item GPGME_ATTR_SIG_STATUS
This is the same value as returned by @code{gpgme_get_sig_status}.
@end table
@end deftp

@deftypefun {const char *} gpgme_key_sig_get_string_attr (@w{gpgme_key_t @var{key}}, @w{int @var{uid_idx}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_sig_get_string_attr} returns the value of
the string-representable attribute @var{what} of the signature
@var{idx} on the user ID @var{uid_idx} in the key @var{key}.  The
argument @var{reserved} is reserved for later use and should be
@code{NULL}.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{uid_idx} or @var{idx}
out of range, or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun {unsigned long} gpgme_key_sig_get_ulong_attr (@w{gpgme_key_t @var{key}}, @w{int @var{uid_idx}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_sig_get_ulong_attr} returns the value of
the number-representable attribute @var{what} of the signature
@var{idx} on the user ID @var{uid_idx} in the key @var{key}.  The
argument @var{reserved} is reserved for later use and should be
@code{NULL}.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{uid_idx} or @var{idx}
out of range, or @var{reserved} not @code{NULL}.
@end deftypefun


@node Manipulating Keys
@subsection Manipulating Keys
@cindex key, manipulation

@deftypefun void gpgme_key_ref (@w{gpgme_key_t @var{key}})
The function @code{gpgme_key_ref} acquires an additional reference for
the key @var{key}.
@end deftypefun

@deftypefun void gpgme_key_unref (@w{gpgme_key_t @var{key}})
The function @code{gpgme_key_unref} releases a reference for the key
@var{key}.  If this was the last reference, the key will be destroyed
and all resources associated to it will be released.
@end deftypefun


The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun void gpgme_key_release (@w{gpgme_key_t @var{key}})
The function @code{gpgme_key_release} is equivalent to
@code{gpgme_key_unref}.
@end deftypefun


@node Generating Keys
@subsection Generating Keys
@cindex key, creation
@cindex key ring, add

@deftypefun gpgme_error_t gpgme_op_genkey (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{parms}}, @w{gpgme_data_t @var{public}}, @w{gpgme_data_t @var{secret}})
The function @code{gpgme_op_genkey} generates a new key pair in the
context @var{ctx}.  The meaning of @var{public} and @var{secret}
depends on the crypto backend.

GnuPG does not support @var{public} and @var{secret}, they should be
@code{NULL}.  GnuPG will generate a key pair and add it to the
standard key ring.  The fingerprint of the generated key is available
with @code{gpgme_op_genkey_result}.

GpgSM requires @var{public} to be a writable data object.  GpgSM will
generate a secret key (which will be stored by @command{gpg-agent},
and return a certificate request in @var{public}, which then needs to
be signed by the certification authority and imported before it can be
used.  GpgSM does not make the fingerprint available.

The argument @var{parms} specifies parameters for the key in an XML
string.  The details about the format of @var{parms} are specific to
the crypto engine used by @var{ctx}.  Here is an example for GnuPG as
the crypto engine:

@example
<GnupgKeyParms format="internal">
Key-Type: DSA
Key-Length: 1024
Subkey-Type: ELG-E
Subkey-Length: 1024
Name-Real: Joe Tester
Name-Comment: with stupid passphrase
Name-Email: joe@@foo.bar
Expire-Date: 0
Passphrase: abc
</GnupgKeyParms>
@end example

Here is an example for GpgSM as the crypto engine:

@example
<GnupgKeyParms format="internal">
Key-Type: RSA
Key-Length: 1024
Name-DN: C=de,O=g10 code,OU=Testlab,CN=Joe 2 Tester
Name-Email: joe@@foo.bar
</GnupgKeyParms>
@end example

Strings should be given in UTF-8 encoding.  The only format supported
for now is ``internal''.  The content of the @code{GnupgKeyParms}
container is passed verbatim to the crypto backend.  Control
statements are not allowed.

After the operation completed successfully, the result can be
retrieved with @code{gpgme_op_genkey_result}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{parms} is not a valid XML string, @code{GPG_ERR_NOT_SUPPORTED} if
@var{public} or @var{secret} is not valid, and @code{GPG_ERR_GENERAL}
if no key was created by the backend.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_genkey_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{parms}}, @w{gpgme_data_t @var{public}}, @w{gpgme_data_t @var{secret}})
The function @code{gpgme_op_genkey_start} initiates a
@code{gpgme_op_genkey} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{parms} is not a valid XML string, and
@code{GPG_ERR_NOT_SUPPORTED} if @var{public} or @var{secret} is not
@code{NULL}.
@end deftypefun

@deftp {Data type} {gpgme_genkey_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_genkey} operation.  After successfully generating a
key, you can retrieve the pointer to the result with
@code{gpgme_op_genkey_result}.  The structure contains the following
members:

@table @code
@item unsigned int primary : 1
This is a flag that is set to 1 if a primary key was created and to 0
if not.

@item unsigned int sub : 1
This is a flag that is set to 1 if a subkey was created and to 0
if not.

@item char *fpr
This is the fingerprint of the key that was created.  If both a
primary and a sub key were generated, the fingerprint of the primary
key will be returned.  If the crypto engine does not provide the
fingerprint, @code{fpr} will be a null pointer.
@end table
@end deftp

@deftypefun gpgme_genkey_result_t gpgme_op_genkey_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_genkey_result} returns a
@code{gpgme_genkey_result_t} pointer to a structure holding the result of
a @code{gpgme_op_genkey} operation.  The pointer is only valid if the
last operation on the context was a @code{gpgme_op_genkey} or
@code{gpgme_op_genkey_start} operation, and if this operation finished
successfully.  The returned pointer is only valid until the next
operation is started on the context.
@end deftypefun


@node Exporting Keys
@subsection Exporting Keys
@cindex key, export
@cindex key ring, export from

@deftypefun gpgme_error_t gpgme_op_export (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{unsigned int @var{reserved}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export} extracts public keys and returns
them in the data buffer @var{keydata}.  The output format of the key
data returned is determined by the @acronym{ASCII} armor attribute set
for the context @var{ctx}.

If @var{pattern} is @code{NULL}, all available keys are returned.
Otherwise, @var{pattern} contains an engine specific expression that
is used to limit the list to all keys matching the pattern.

@var{reserved} is reserved for future use and must be @code{0}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} is not a valid empty data buffer, and passes through any
errors that are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_export_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{unsigned int @var{reserved}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export_start} initiates a
@code{gpgme_op_export} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{keydata} is not a valid empty data buffer.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_export_ext (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}[]}, @w{unsigned int @var{reserved}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export} extracts public keys and returns
them in the data buffer @var{keydata}.  The output format of the key
data returned is determined by the @acronym{ASCII} armor attribute set
for the context @var{ctx}.

If @var{pattern} or @var{*pattern} is @code{NULL}, all available keys
are returned.  Otherwise, @var{pattern} is a @code{NULL} terminated
array of strings that are used to limit the list to all keys matching
at least one of the patterns verbatim.

@var{reserved} is reserved for future use and must be @code{0}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} is not a valid empty data buffer, and passes through any
errors that are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_export_ext_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}[]}, @w{unsigned int @var{reserved}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export_ext_start} initiates a
@code{gpgme_op_export_ext} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{keydata} is not a valid empty data buffer.
@end deftypefun


@node Importing Keys
@subsection Importing Keys
@cindex key, import
@cindex key ring, import to

@deftypefun gpgme_error_t gpgme_op_import (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_import} adds the keys in the data buffer
@var{keydata} to the key ring of the crypto engine used by @var{ctx}.
The format of @var{keydata} can be @acronym{ASCII} armored, for example,
but the details are specific to the crypto engine.

After the operation completed successfully, the result can be
retrieved with @code{gpgme_op_import_result}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
import was completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} if @var{ctx} or @var{keydata} is not a valid pointer,
and @code{GPG_ERR_NO_DATA} if @var{keydata} is an empty data buffer.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_import_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_import_start} initiates a
@code{gpgme_op_import} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
import could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} if @var{ctx} or @var{keydata} is not a valid pointer,
and @code{GPG_ERR_NO_DATA} if @var{keydata} is an empty data buffer.
@end deftypefun

@deftp {Data type} {gpgme_import_status_t}
This is a pointer to a structure used to store a part of the result of
a @code{gpgme_op_import} operation.  For each considered key one
status is added that contains information about the result of the
import.  The structure contains the following members:

@table @code
@item gpgme_import_status_t next
This is a pointer to the next status structure in the linked list, or
@code{NULL} if this is the last element.

@item char *fpr
This is the fingerprint of the key that was considered.

@item gpgme_error_t result
If the import was not successful, this is the error value that caused
the import to fail.  Otherwise the error code is
@code{GPG_ERR_NO_ERROR}.

@item unsigned int status
This is a bit-wise OR of the following flags that give more
information about what part of the key was imported.  If the key was
already known, this might be 0.

@table @code
@item GPGME_IMPORT_NEW
The key was new.

@item GPGME_IMPORT_UID
The key contained new user IDs.

@item GPGME_IMPORT_SIG
The key contained new signatures.

@item GPGME_IMPORT_SUBKEY
The key contained new sub keys.

@item GPGME_IMPORT_SECRET
The key contained a secret key.
@end table
@end table
@end deftp

@deftp {Data type} {gpgme_import_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_import} operation.  After a successful import
operation, you can retrieve the pointer to the result with
@code{gpgme_op_import_result}.  The structure contains the following
members:

@table @code
@item int considered
The total number of considered keys.

@item int no_user_id
The number of keys without user ID.

@item int imported
The total number of imported keys.

@item imported_rsa
The number of imported RSA keys.

@item unchanged
The number of unchanged keys.

@item new_user_ids
The number of new user IDs.

@item new_sub_keys
The number of new sub keys.

@item new_signatures
The number of new signatures.

@item new_revocations
The number of new revocations.

@item secret_read
The total number of secret keys read.

@item secret_imported
The number of imported secret keys.

@item secret_unchanged
The number of unchanged secret keys.

@item not_imported
The number of keys not imported.

@item gpgme_import_status_t imports
A list of gpgme_import_status_t objects which contain more information
about the keys for which an import was attempted.
@end table
@end deftp

@deftypefun gpgme_import_result_t gpgme_op_import_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_import_result} returns a
@code{gpgme_import_result_t} pointer to a structure holding the result
of a @code{gpgme_op_import} operation.  The pointer is only valid if
the last operation on the context was a @code{gpgme_op_import} or
@code{gpgme_op_import_start} operation, and if this operation finished
successfully.  The returned pointer is only valid until the next
operation is started on the context.
@end deftypefun

The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_op_import_ext (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{keydata}}, @w{int *@var{nr}})
The function @code{gpgme_op_import_ext} is equivalent to:

@example
  gpgme_error_t err = gpgme_op_import (ctx, keydata);
  if (!err)
    @{
      gpgme_import_result_t result = gpgme_op_import_result (ctx);
      *nr = result->considered;
    @}
@end example
@end deftypefun


@node Deleting Keys
@subsection Deleting Keys
@cindex key, delete
@cindex key ring, delete from

@deftypefun gpgme_error_t gpgme_op_delete (@w{gpgme_ctx_t @var{ctx}}, @w{const gpgme_key_t @var{key}}, @w{int @var{allow_secret}})
The function @code{gpgme_op_delete} deletes the key @var{key} from the
key ring of the crypto engine used by @var{ctx}.  If
@var{allow_secret} is @code{0}, only public keys are deleted,
otherwise secret keys are deleted as well, if that is supported.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the key
was deleted successfully, @code{GPG_ERR_INV_VALUE} if @var{ctx} or
@var{key} is not a valid pointer, @code{GPG_ERR_NO_PUBKEY} if
@var{key} could not be found in the keyring,
@code{GPG_ERR_AMBIGUOUS_NAME} if the key was not specified
unambiguously, and @code{GPG_ERR_CONFLICT} if the secret key for
@var{key} is available, but @var{allow_secret} is zero.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_delete_start (@w{gpgme_ctx_t @var{ctx}}, @w{const gpgme_key_t @var{key}}, @w{int @var{allow_secret}})
The function @code{gpgme_op_delete_start} initiates a
@code{gpgme_op_delete} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation was started successfully, and @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{key} is not a valid pointer.
@end deftypefun


@node Trust Item Management
@section Trust Item Management
@cindex trust item

@strong{Caution:} The trust items interface is experimental.

@deftp {Data type} gpgme_trust_item_t
The @code{gpgme_trust_item_t} type is a pointer to a trust item object.
It has the following members:

@table @code
@item char *keyid
This is a string describing the key to which this trust items belongs.

@item int type
This is the type of the trust item.  A value of 1 refers to a key, a
value of 2 refers to a user ID.

@item int level
This is the trust level.

@item char *owner_trust
The owner trust if @code{type} is 1.

@item char *validity
The calculated validity.

@item char *name
The user name if @code{type} is 2.
@end table
@end deftp

@menu
* Listing Trust Items::           Browsing the list of available trust items.
* Information About Trust Items:: Requesting information about trust items.
* Manipulating Trust Items::      Operations on trust items.
@end menu


@node Listing Trust Items
@subsection Listing Trust Items
@cindex trust item list

@deftypefun gpgme_error_t gpgme_op_trustlist_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{int @var{max_level}})
The function @code{gpgme_op_trustlist_start} initiates a trust item
listing operation inside the context @var{ctx}.  It sets everything up
so that subsequent invocations of @code{gpgme_op_trustlist_next} return
the trust items in the list.

The string @var{pattern} contains an engine specific expression that
is used to limit the list to all trust items matching the pattern.  It
can not be the empty string.

The argument @var{max_level} is currently ignored.

The context will be busy until either all trust items are received
(and @code{gpgme_op_trustlist_next} returns @code{GPG_ERR_EOF}), or
@code{gpgme_op_trustlist_end} is called to finish the operation.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_trustlist_next (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_trust_item_t *@var{r_item}})
The function @code{gpgme_op_trustlist_next} returns the next trust
item in the list created by a previous @code{gpgme_op_trustlist_start}
operation in the context @var{ctx}.  The trust item can be destroyed
with @code{gpgme_trust_item_release}.  @xref{Manipulating Trust Items}.

This is the only way to get at @code{gpgme_trust_item_t} objects in
@acronym{GPGME}.

If the last trust item in the list has already been returned,
@code{gpgme_op_trustlist_next} returns @code{GPG_ERR_EOF}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if @var{ctx} or
@var{r_item} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if
there is not enough memory for the operation.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_trustlist_end (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_trustlist_next} ends a pending key list
operation in the context @var{ctx}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if at some
time during the operation there was not enough memory available.
@end deftypefun


@node Information About Trust Items
@subsection Information About Trust Items
@cindex trust item, information about
@cindex trust item, attributes
@cindex attributes, of a trust item

The following interfaces are deprecated and only provided for backward
compatibility.  Don't use them.  They will be removed in a future
version of @acronym{GPGME}.

Trust items have attributes which can be queried using the interfaces
below.  The attribute identifiers are shared with those for key
attributes.  @xref{Information About Keys}.

@deftypefun {const char *} gpgme_trust_item_get_string_attr (@w{gpgme_trust_item_t @var{item}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_trust_item_get_string_attr} returns the value
of the string-representable attribute @var{what} of trust item
@var{item}.  The arguments @var{idx} and @var{reserved} are reserved
for later use and should be @code{0} and @code{NULL} respectively.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun int gpgme_trust_item_get_int_attr (@w{gpgme_trust_item_t @var{item}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_trust_item_get_int_attr} returns the value of
the number-representable attribute @var{what} of trust item
@var{item}.  If the attribute occurs more than once in the trust item,
the index is specified by @var{idx}.  However, currently no such
attribute exists, so @var{idx} should be @code{0}.  The argument
@var{reserved} is reserved for later use and should be @code{NULL}.

The function returns @code{0} if the attribute can't be returned as a
number, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun


@node Manipulating Trust Items
@subsection Manipulating Trust Items
@cindex trust item, manipulation

@deftypefun void gpgme_trust_item_ref (@w{gpgme_trust_item_t @var{item}})
The function @code{gpgme_trust_item_ref} acquires an additional
reference for the trust item @var{item}.
@end deftypefun

@deftypefun void gpgme_trust_item_unref (@w{gpgme_trust_item_t @var{item}})
The function @code{gpgme_trust_item_unref} releases a reference for
the trust item @var{item}.  If this was the last reference, the trust
item will be destroyed and all resources associated to it will be
released.
@end deftypefun


The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun void gpgme_trust_item_release (@w{gpgme_trust_item_t @var{item}})
The function @code{gpgme_trust_item_release} is an alias for
@code{gpgme_trust_item_unref}.
@end deftypefun


@node Crypto Operations
@section Crypto Operations
@cindex cryptographic operation

Sometimes, the result of a crypto operation returns a list of invalid
keys encountered in processing the request.  The following structure
is used to hold information about such a key.

@deftp {Data type} {gpgme_invalid_key_t}
This is a pointer to a structure used to store a part of the result of
a crypto operation which takes user IDs as one input parameter.  The
structure contains the following members:

@table @code
@item gpgme_invalid_key_t next
This is a pointer to the next invalid key structure in the linked
list, or @code{NULL} if this is the last element.

@item char *fpr
The fingerprint or key ID of the invalid key encountered.

@item gpgme_error_t reason
An error code describing the reason why the key was found invalid.
@end table
@end deftp


@menu
* Decrypt::                       Decrypting a ciphertext.
* Verify::                        Verifying a signature.
* Decrypt and Verify::            Decrypting a signed ciphertext.
* Sign::                          Creating a signature.
* Encrypt::                       Encrypting a plaintext.
@end menu


@node Decrypt
@subsection Decrypt
@cindex decryption
@cindex cryptographic operation, decryption

@deftypefun gpgme_error_t gpgme_op_decrypt (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{cipher}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_decrypt} decrypts the ciphertext in the
data object @var{cipher} and stores it into the data object
@var{plain}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
ciphertext could be decrypted successfully, @code{GPG_ERR_INV_VALUE}
if @var{ctx}, @var{cipher} or @var{plain} is not a valid pointer,
@code{GPG_ERR_NO_DATA} if @var{cipher} does not contain any data to
decrypt, @code{GPG_ERR_DECRYPT_FAILED} if @var{cipher} is not a valid
cipher text, @code{GPG_ERR_BAD_PASSPHRASE} if the passphrase for the
secret key could not be retrieved, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_decrypt_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{cipher}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_decrypt_start} initiates a
@code{gpgme_op_decrypt} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{cipher} or @var{plain} is not a valid pointer.
@end deftypefun

@deftp {Data type} {gpgme_decrypt_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_decrypt} operation.  After successfully encrypting
data, you can retrieve the pointer to the result with
@code{gpgme_op_decrypt_result}.  The structure contains the following
members:

@table @code
@item char *unsupported_algorithm
If an unsupported algorithm was encountered, this string describes the
algorithm that is not supported.

@item unsigned int wrong_key_usage : 1
This is true if the key was not used according to its policy.
@end table
@end deftp

@deftypefun gpgme_decrypt_result_t gpgme_op_decrypt_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_decrypt_result} returns a
@code{gpgme_decrypt_result_t} pointer to a structure holding the
result of a @code{gpgme_op_decrypt} operation.  The pointer is only
valid if the last operation on the context was a
@code{gpgme_op_decrypt} or @code{gpgme_op_decrypt_start} operation.
If the operation failed this might be a @code{NULL} pointer.  The
returned pointer is only valid until the next operation is started on
the context.
@end deftypefun


@node Verify
@subsection Verify
@cindex verification
@cindex signature, verification
@cindex cryptographic operation, verification
@cindex cryptographic operation, signature check

@deftypefun gpgme_error_t gpgme_op_verify (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{sig}}, @w{gpgme_data_t @var{signed_text}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_verify} verifies that the signature in the
data object @var{sig} is a valid signature.  If @var{sig} is a
detached signature, then the signed text should be provided in
@var{signed_text} and @var{plain} should be a null pointer.
Otherwise, if @var{sig} is a normal (or cleartext) signature,
@var{signed_text} should be a null pointer and @var{plain} should be a
writable data object that will contain the plaintext after successful
verification.

The results of the individual signature verifications can be retrieved
with @code{gpgme_op_verify_result}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{sig} or @var{plain} is not a valid pointer,
@code{GPG_ERR_NO_DATA} if @var{sig} does not contain any data to
verify, and passes through any errors that are reported by the crypto
engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_verify_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{sig}}, @w{gpgme_data_t @var{signed_text}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_verify_start} initiates a
@code{gpgme_op_verify} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{sig} or @var{plain} is not a valid pointer, and
@code{GPG_ERR_NO_DATA} if @var{sig} or @var{plain} does not contain
any data to verify.
@end deftypefun

@deftp {Data type} {gpgme_sig_notation_t}
This is a pointer to a structure used to store a part of the result of
a @code{gpgme_op_verify} operation.  The structure contains the
following members:

@table @code
@item gpgme_sig_notation_t next
This is a pointer to the next new signature notation structure in the
linked list, or @code{NULL} if this is the last element.

@item char *name
The name of the notation field.  If this is @code{NULL}, then the
member @code{value} will contain a policy URL.

@item char *value
The value of the notation field.  If @code{name} is @code{NULL}, then
this is a policy URL.
@end table
@end deftp

@deftp {Data type} {gpgme_signature_t}
This is a pointer to a structure used to store a part of the result of
a @code{gpgme_op_verify} operation.  The structure contains the
following members:

@table @code
@item gpgme_signature_t next
This is a pointer to the next new signature structure in the linked
list, or @code{NULL} if this is the last element.

@item gpgme_sigsum_t summary
This is a bit vector giving a summary of the signature status.  It
provides an easy interface to a defined semantic of the signature
status.  Checking just one bit is sufficient to see whether a
signature is valid without any restrictions.

The defined bits are:
  @table @code
  @item GPGME_SIGSUM_VALID
  The signature is fully valid.

  @item GPGME_SIGSUM_GREEN
  The signature is good but one might want to display some extra
  information.  Check the other bits.

  @item GPGME_SIGSUM_RED
  The signature is bad. It might be useful to check other bits and
  display more information, i.e. a revoked certificate might not render a
  signature invalid when the message was received prior to the cause for
  the revocation.

  @item GPGME_SIGSUM_KEY_REVOKED
  The key or at least one certificate has been revoked.

  @item GPGME_SIGSUM_KEY_EXPIRED
  The key or one of the certificates has expired. It is probably a good
  idea to display the date of the expiration.

  @item GPGME_SIGSUM_SIG_EXPIRED
  The signature has expired.

  @item GPGME_SIGSUM_KEY_MISSING
  Can't verify due to a missing key or certificate.

  @item GPGME_SIGSUM_CRL_MISSING
  The CRL (or an equivalent mechanism) is not available. 

  @item GPGME_SIGSUM_CRL_TOO_OLD
  Available CRL is too old.

  @item GPGME_SIGSUM_BAD_POLICY
  A policy requirement was not met. 

  @item GPGME_SIGSUM_SYS_ERROR
  A system error occured. 
  @end table

@item char *fpr
This is the fingerprint or key ID of the signature.

@item gpgme_error_t status
This is the status of the signature.  In particular, the following
status codes are of interest:

  @table @code
  @item GPG_ERR_NO_ERROR
  This status indicates that the signature is valid.  For the combined
  result this status means that all signatures are valid.

  @item GPG_ERR_SIG_EXPIRED
  This status indicates that the signature is valid but expired.  For
  the combined result this status means that all signatures are valid
  and expired.

  @item GPG_ERR_KEY_EXPIRED
  This status indicates that the signature is valid but the key used to
  verify the signature has expired.  For the combined result this status
  means that all signatures are valid and all keys are expired.

  @item GPG_ERR_CERT_REVOKED
  This status indicates that the signature is valid but the key used
  to verify the signature has been revoked.  For the combined result
  this status means that all signatures are valid and all keys are
  revoked.

  @item GPG_ERR_BAD_SIGNATURE
  This status indicates that the signature is invalid.  For the combined
  result this status means that all signatures are invalid.

  @item GPG_ERR_NO_PUBKEY
  This status indicates that the signature could not be verified due to
  a missing key.  For the combined result this status means that all
  signatures could not be checked due to missing keys.

  @item GPG_ERR_GENERAL
  This status indicates that there was some other error which prevented
  the signature verification.
  @end table

@item gpgme_sig_notation_t notations
This is a linked list with the notation data and policy URLs.

@item unsigned long timestamp
The creation timestamp of this signature.

@item unsigned long exp_timestamp
The expiration timestamp of this signature, or 0 if the signature does
not expire.

@item unsigned int wrong_key_usage : 1
This is true if the key was not used according to its policy.

@item gpgme_validity_t validity
The validity of the signature.

@item gpgme_error_t validity_reason
If a signature is not valid, this provides a reason why.

@end table
@end deftp

@deftp {Data type} {gpgme_verify_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_verify} operation.  After verifying a signature, you
can retrieve the pointer to the result with
@code{gpgme_op_verify_result}.  If the operation failed this might be
a @code{NULL} pointer.  The structure contains the following member:

@table @code
@item gpgme_signature_t signatures
A linked list with information about all signatures for which a
verification was attempted.
@end table
@end deftp

@deftypefun gpgme_verify_result_t gpgme_op_verify_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_verify_result} returns a
@code{gpgme_verify_result_t} pointer to a structure holding the result
of a @code{gpgme_op_verify} operation.  The pointer is only valid if
the last operation on the context was a @code{gpgme_op_verify},
@code{gpgme_op_verify_start}, @code{gpgme_op_decrypt_verify} or
@code{gpgme_op_decrypt_verify_start} operation, and if this operation
finished successfully (for @code{gpgme_op_decrypt_verify} and
@code{gpgme_op_decrypt_verify_start}, the error code
@code{GPG_ERR_NO_DATA} counts as successful in this context).  The
returned pointer is only valid until the next operation is started on
the context.
@end deftypefun


The following interfaces are deprecated and only provided for backward
compatibility.  Don't use them.  They will be removed in a future
version of @acronym{GPGME}.

@deftp {Data type} {enum gpgme_sig_stat_t}
@tindex gpgme_sig_stat_t
The @code{gpgme_sig_stat_t} type holds the result of a signature check, or
the combined result of all signatures.  The following results are
possible:

@table @code
@item GPGME_SIG_STAT_NONE
This status should not occur in normal operation.

@item GPGME_SIG_STAT_GOOD
This status indicates that the signature is valid.  For the combined
result this status means that all signatures are valid.

@item GPGME_SIG_STAT_GOOD_EXP
This status indicates that the signature is valid but expired.  For
the combined result this status means that all signatures are valid
and expired.

@item GPGME_SIG_STAT_GOOD_EXPKEY
This status indicates that the signature is valid but the key used to
verify the signature has expired.  For the combined result this status
means that all signatures are valid and all keys are expired.

@item GPGME_SIG_STAT_BAD
This status indicates that the signature is invalid.  For the combined
result this status means that all signatures are invalid.

@item GPGME_SIG_STAT_NOKEY
This status indicates that the signature could not be verified due to
a missing key.  For the combined result this status means that all
signatures could not be checked due to missing keys.

@item GPGME_SIG_STAT_NOSIG
This status indicates that the signature data provided was not a real
signature.

@item GPGME_SIG_STAT_ERROR
This status indicates that there was some other error which prevented
the signature verification.

@item GPGME_SIG_STAT_DIFF
For the combined result this status means that at least two signatures
have a different status.  You can get each key's status with
@code{gpgme_get_sig_status}.
@end table
@end deftp

@deftypefun {const char *} gpgme_get_sig_status (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{idx}}, @w{gpgme_sig_stat_t *@var{r_stat}}, @w{time_t *@var{r_created}})
The function @code{gpgme_get_sig_status} is equivalent to:
 
@example
  gpgme_verify_result_t result;
  gpgme_signature_t sig;

  result = gpgme_op_verify_result (ctx);
  sig = result->signatures;

  while (sig && idx)
    @{
      sig = sig->next;
      idx--;
    @}
  if (!sig || idx)
    return NULL;

  if (r_stat)
    @{
      switch (gpg_err_code (sig->status))
	@{
	case GPG_ERR_NO_ERROR:
	  *r_stat = GPGME_SIG_STAT_GOOD;
	  break;
	  
	case GPG_ERR_BAD_SIGNATURE:
	  *r_stat = GPGME_SIG_STAT_BAD;
	  break;
	  
	case GPG_ERR_NO_PUBKEY:
	  *r_stat = GPGME_SIG_STAT_NOKEY;
	  break;
	  
	case GPG_ERR_NO_DATA:
	  *r_stat = GPGME_SIG_STAT_NOSIG;
	  break;
	  
	case GPG_ERR_SIG_EXPIRED:
	  *r_stat = GPGME_SIG_STAT_GOOD_EXP;
	  break;
	  
	case GPG_ERR_KEY_EXPIRED:
	  *r_stat = GPGME_SIG_STAT_GOOD_EXPKEY;
	  break;
	  
	default:
	  *r_stat = GPGME_SIG_STAT_ERROR;
	  break;
	@}
    @}
  if (r_created)
    *r_created = sig->timestamp;
  return sig->fpr;
@end example
@end deftypefun

@deftypefun {const char *} gpgme_get_sig_string_attr (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{idx}}, @w{gpgme_attr_t @var{what}}, @w{int @var{whatidx}})
The function @code{gpgme_get_sig_string_attr} is equivalent to:
 
@example
  gpgme_verify_result_t result;
  gpgme_signature_t sig;

  result = gpgme_op_verify_result (ctx);
  sig = result->signatures;

  while (sig && idx)
    @{
      sig = sig->next;
      idx--;
    @}
  if (!sig || idx)
    return NULL;

  switch (what)
    @{
    case GPGME_ATTR_FPR:
      return sig->fpr;

    case GPGME_ATTR_ERRTOK:
      if (whatidx == 1)
        return sig->wrong_key_usage ? "Wrong_Key_Usage" : "";
      else
	return "";
    default:
      break;
    @}

  return NULL;
@end example
@end deftypefun

@deftypefun {const char *} gpgme_get_sig_ulong_attr (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{idx}}, @w{gpgme_attr_t @var{waht}}, @w{int @var{whatidx}})
The function @code{gpgme_get_sig_ulong_attr} is equivalent to:
 
@example
  gpgme_verify_result_t result;
  gpgme_signature_t sig;

  result = gpgme_op_verify_result (ctx);
  sig = result->signatures;

  while (sig && idx)
    @{
      sig = sig->next;
      idx--;
    @}
  if (!sig || idx)
    return 0;

  switch (what)
    @{
    case GPGME_ATTR_CREATED:
      return sig->timestamp;

    case GPGME_ATTR_EXPIRE:
      return sig->exp_timestamp;

    case GPGME_ATTR_VALIDITY:
      return (unsigned long) sig->validity;

    case GPGME_ATTR_SIG_STATUS:
      switch (sig->status)
	@{
	case GPG_ERR_NO_ERROR:
	  return GPGME_SIG_STAT_GOOD;
	  
	case GPG_ERR_BAD_SIGNATURE:
	  return GPGME_SIG_STAT_BAD;
	  
	case GPG_ERR_NO_PUBKEY:
	  return GPGME_SIG_STAT_NOKEY;
	  
	case GPG_ERR_NO_DATA:
	  return GPGME_SIG_STAT_NOSIG;
	  
	case GPG_ERR_SIG_EXPIRED:
	  return GPGME_SIG_STAT_GOOD_EXP;
	  
	case GPG_ERR_KEY_EXPIRED:
	  return GPGME_SIG_STAT_GOOD_EXPKEY;
	  
	default:
	  return GPGME_SIG_STAT_ERROR;
	@}

    case GPGME_ATTR_SIG_SUMMARY:
      return sig->summary;

    default:
      break;
    @}
  return 0;
@end example
@end deftypefun

@deftypefun {const char *} gpgme_get_sig_key (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{idx}}, @w{gpgme_key_t *@var{r_key}})
The function @code{gpgme_get_sig_key} is equivalent to:

@example
  gpgme_verify_result_t result;
  gpgme_signature_t sig;

  result = gpgme_op_verify_result (ctx);
  sig = result->signatures;

  while (sig && idx)
    @{
      sig = sig->next;
      idx--;
    @}
  if (!sig || idx)
    return gpg_error (GPG_ERR_EOF);

  return gpgme_get_key (ctx, sig->fpr, r_key, 0);
@end example
@end deftypefun


@node Decrypt and Verify
@subsection Decrypt and Verify
@cindex decryption and verification
@cindex verification and decryption
@cindex signature check
@cindex cryptographic operation, decryption and verification

@deftypefun gpgme_error_t gpgme_op_decrypt_verify (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{cipher}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_decrypt_verify} decrypts the ciphertext in
the data object @var{cipher} and stores it into the data object
@var{plain}.  If @var{cipher} contains signatures, they will be
verified.

After the operation completed, @code{gpgme_op_decrypt_result} and
@code{gpgme_op_verify_result} can be used to retrieve more information
about the signatures.

If the error code @code{GPG_ERR_NO_DATA} is returned, @var{cipher}
does not contain any data to decrypt.  However, it might still be
signed.  The information about detected signatures is available with
@code{gpgme_op_verify_result} in this case.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
ciphertext could be decrypted successfully, @code{GPG_ERR_INV_VALUE}
if @var{ctx}, @var{cipher} or @var{plain} is not a valid pointer,
@code{GPG_ERR_NO_DATA} if @var{cipher} does not contain any data to
decrypt, @code{GPG_ERR_DECRYPT_FAILED} if @var{cipher} is not a valid
cipher text, @code{GPG_ERR_BAD_PASSPHRASE} if the passphrase for the
secret key could not be retrieved, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_decrypt_verify (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{cipher}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_decrypt_verify_start} initiates a
@code{gpgme_op_decrypt_verify} operation.  It can be completed by
calling @code{gpgme_wait} on the context.  @xref{Waiting For
Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{cipher}, @var{plain} or @var{r_stat} is not a valid
pointer, and @code{GPG_ERR_NO_DATA} if @var{cipher} does not contain
any data to decrypt.
@end deftypefun


@node Sign
@subsection Sign
@cindex signature, creation
@cindex sign
@cindex cryptographic operation, signing

A signature can contain signatures by one or more keys.  The set of
keys used to create a signatures is contained in a context, and is
applied to all following signing operations in this context (until the
set is changed).

@menu
* Selecting Signers::             How to choose the keys to sign with.
* Creating a Signature::          How to create a signature.
@end menu


@node Selecting Signers
@subsubsection Selecting Signers
@cindex signature, selecting signers
@cindex signers, selecting

@deftypefun void gpgme_signers_clear (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_signers_clear} releases a reference for each
key on the signers list and removes the list of signers from the
context @var{ctx}.

Every context starts with an empty list.
@end deftypefun

@deftypefun gpgme_error_t gpgme_signers_add (@w{gpgme_ctx_t @var{ctx}}, @w{const gpgme_key_t @var{key}})
The function @code{gpgme_signers_add} adds the key @var{key} to the
list of signers in the context @var{ctx}.

Calling this function acquires an additional reference for the key.
@end deftypefun

@deftypefun gpgme_key_t gpgme_signers_enum (@w{const gpgme_ctx_t @var{ctx}}, @w{int @var{seq}})
The function @code{gpgme_signers_enum} returns the @var{seq}th key in
the list of signers in the context @var{ctx}.  An additional reference
is acquired for the user.

If @var{seq} is out of range, @code{NULL} is returned.
@end deftypefun


@node Creating a Signature
@subsubsection Creating a Signature

@deftp {Data type} {enum gpgme_sig_mode_t}
@tindex gpgme_sig_mode_t
The @code{gpgme_sig_mode_t} type is used to specify the desired type of a
signature.  The following modes are available:

@table @code
@item GPGME_SIG_MODE_NORMAL
A normal signature is made, the output includes the plaintext and the
signature.

@item GPGME_SIG_MODE_DETACH
A detached signature is made.

@item GPGME_SIG_MODE_CLEAR
A clear text signature is made.  The @acronym{ASCII} armor and text
mode settings of the context are ignored.
@end table
@end deftp

@deftypefun gpgme_error_t gpgme_op_sign (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{plain}}, @w{gpgme_data_t @var{sig}}, @w{gpgme_sig_mode_t @var{mode}})
The function @code{gpgme_op_sign} creates a signature for the text in
the data object @var{plain} and returns it in the data object
@var{sig}.  The type of the signature created is determined by the
@acronym{ASCII} armor and text mode attributes set for the context
@var{ctx} and the requested signature mode @var{mode}.

After the operation completed successfully, the result can be
retrieved with @code{gpgme_op_sign_result}.

If an S/MIME signed message is created using the CMS crypto engine,
the number of certificates to include in the message can be specified
with @code{gpgme_set_include_certs}.  @xref{Included Certificates}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
signature could be created successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{plain} or @var{sig} is not a valid pointer,
@code{GPG_ERR_NO_DATA} if the signature could not be created,
@code{GPG_ERR_BAD_PASSPHRASE} if the passphrase for the secret key
could not be retrieved, @code{GPG_ERR_UNUSABLE_SECKEY} if there are
invalid signers, and passes through any errors that are reported by the
crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_sign_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{plain}}, @w{gpgme_data_t @var{sig}}, @w{gpgme_sig_mode_t @var{mode}})
The function @code{gpgme_op_sign_start} initiates a
@code{gpgme_op_sign} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the operation could be
started successfully, and @code{GPG_ERR_INV_VALUE} if @var{ctx},
@var{plain} or @var{sig} is not a valid pointer.
@end deftypefun

@deftp {Data type} {gpgme_new_signature_t}
This is a pointer to a structure used to store a part of the result of
a @code{gpgme_op_sign} operation.  The structure contains the
following members:

@table @code
@item gpgme_new_signature_t next
This is a pointer to the next new signature structure in the linked
list, or @code{NULL} if this is the last element.

@item gpgme_sig_mode_t type
The type of this signature.

@item gpgme_pubkey_algo_t
The public key algorithm used to create this signature.

@item gpgme_hash_algo_t
The hash algorithm used to create this signature.

@item unsigned int sig_class
The signature class of this signature.

@item long int timestamp
The creation timestamp of this signature.

@item char *fpr
The fingerprint of the key which was used to create this signature.
@end table
@end deftp

@deftp {Data type} {gpgme_sign_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_sign} operation.  After successfully generating a
signature, you can retrieve the pointer to the result with
@code{gpgme_op_sign_result}.  The structure contains the following
members:

@table @code
@item gpgme_invalid_key_t invalid_signers
A linked list with information about all invalid keys for which a
signature could not be created.

@item gpgme_new_signature_t signatures
A linked list with information about all signatures created.
@end table
@end deftp

@deftypefun gpgme_sign_result_t gpgme_op_sign_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_sign_result} returns a
@code{gpgme_sign_result_t} pointer to a structure holding the result
of a @code{gpgme_op_sign} operation.  The pointer is only valid if the
last operation on the context was a @code{gpgme_op_sign},
@code{gpgme_op_sign_start}, @code{gpgme_op_encrypt_sign} or
@code{gpgme_op_encrypt_sign_start} operation.  If that operation
failed, the function might return a @code{NULL} pointer, The returned
pointer is only valid until the next operation is started on the
context.
@end deftypefun


@node Encrypt
@subsection Encrypt
@cindex encryption
@cindex cryptographic operation, encryption

One plaintext can be encrypted for several recipients at the same
time.  The list of recipients is created independently of any context,
and then passed to the encryption operation.

@menu
* Encrypting a Plaintext::        How to encrypt a plaintext.
@end menu


@node Encrypting a Plaintext
@subsubsection Encrypting a Plaintext

@deftypefun gpgme_error_t gpgme_op_encrypt (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{recp}[]}, @w{gpgme_encrypt_flags_t @var{flags}}, @w{gpgme_data_t @var{plain}}, @w{gpgme_data_t @var{cipher}})
The function @code{gpgme_op_encrypt} encrypts the plaintext in the data
object @var{plain} for the recipients @var{recp} and stores the
ciphertext in the data object @var{cipher}.  The type of the
ciphertext created is determined by the @acronym{ASCII} armor and text
mode attributes set for the context @var{ctx}.

@var{key} must be a @code{NULL}-terminated array of keys.  The user
must keep references for all keys during the whole duration of the
call (but see @code{gpgme_op_encrypt_start} for the requirements with
the asynchronous variant).

The value in @var{flags} is a bitwise-or combination of one or
multiple of the following bit values:

@table @code
@item GPGME_ENCRYPT_ALWAYS_TRUST
The @code{GPGME_ENCRYPT_ALWAYS_TRUST} symbol specifies that all the
recipients in @var{recp} should be trusted, even if the keys do not
have a high enough validity in the keyring.  This flag should be used
with care; in general it is not a good idea to use any untrusted keys.
@end table

If @code{GPG_ERR_UNUSABLE_PUBKEY} is returned, some recipients in
@var{recp} are invalid, but not all.  In this case the plaintext might
be encrypted for all valid recipients and returned in @var{cipher} (if
this happens depends on the crypto engine).  More information about
the invalid recipients is available with
@code{gpgme_op_encrypt_result}.

If @var{recp} is @code{NULL}, symmetric rather than public key
encryption is performed.  Symmetrically encrypted cipher text can be
deciphered with @code{gpgme_op_decrypt}.  Note that in this case the
crypto backend needs to retrieve a passphrase from the user.
Symmetric encryption is currently only supported for the OpenPGP
crypto backend.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
ciphertext could be created successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{recp}, @var{plain} or @var{cipher} is not a valid
pointer, @code{GPG_ERR_UNUSABLE_PUBKEY} if @var{recp} contains some
invalid recipients, @code{GPG_ERR_BAD_PASSPHRASE} if the passphrase
for the symmetric key could not be retrieved, and passes through any
errors that are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_encrypt_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{recp}[]}, @w{gpgme_encrypt_flags_t @var{flags}}, @w{gpgme_data_t @var{plain}}, @w{gpgme_data_t @var{cipher}})
The function @code{gpgme_op_encrypt_start} initiates a
@code{gpgme_op_encrypt} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

References to the keys only need to be held for the duration of this
call.  The user can release its references to the keys after this
function returns, even if the operation is not yet finished.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{rset}, @var{plain} or @var{cipher} is not a valid
pointer, and @code{GPG_ERR_UNUSABLE_PUBKEY} if @var{rset} does not
contain any valid recipients.
@end deftypefun

@deftp {Data type} {gpgme_encrypt_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_encrypt} operation.  After successfully encrypting
data, you can retrieve the pointer to the result with
@code{gpgme_op_encrypt_result}.  The structure contains the following
members:

@table @code
@item gpgme_invalid_key_t invalid_recipients
A linked list with information about all invalid keys for which
the data could not be encrypted.
@end table
@end deftp

@deftypefun gpgme_encrypt_result_t gpgme_op_encrypt_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_encrypt_result} returns a
@code{gpgme_encrypt_result_t} pointer to a structure holding the
result of a @code{gpgme_op_encrypt} operation.  The pointer is only
valid if the last operation on the context was a
@code{gpgme_op_encrypt}, @code{gpgme_op_encrypt_start},
@code{gpgme_op_sign} or @code{gpgme_op_sign_start} operation.  If this
operation failed, this might be a @code{NULL} pointer.  The returned
pointer is only valid until the next operation is started on the
context.
@end deftypefun


@deftypefun gpgme_error_t gpgme_op_encrypt_sign (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{recp}[]}, @w{gpgme_encrypt_flags_t @var{flags}}, @w{gpgme_data_t @var{plain}}, @w{gpgme_data_t @var{cipher}})
The function @code{gpgme_op_encrypt_sign} does a combined encrypt and
sign operation.  It is used like @code{gpgme_op_encrypt}, but the
ciphertext also contains signatures for the signers listed in
@var{ctx}.

The combined encrypt and sign operation is currently only available
for the OpenPGP crypto engine.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_encrypt_sign_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{recp}}, @w{gpgme_encrypt_flags_t @var{flags}}, @w{gpgme_data_t @var{plain}}, @w{gpgme_data_t @var{cipher}})
The function @code{gpgme_op_encrypt_sign_start} initiates a
@code{gpgme_op_encrypt_sign} operation.  It can be completed by
calling @code{gpgme_wait} on the context.  @xref{Waiting For
Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{ctx}, @var{rset}, @var{plain} or @var{cipher} is not a valid
pointer.
@end deftypefun


@node Run Control
@section Run Control
@cindex run control
@cindex cryptographic operation, running

@acronym{GPGME} supports running operations synchronously and
asynchronously.  You can use asynchronous operation to set up a
context up to initiating the desired operation, but delay performing
it to a later point.

Furthermore, you can use an external event loop to control exactly
when @acronym{GPGME} runs.  This ensures that @acronym{GPGME} only
runs when necessary and also prevents it from blocking for a long
time.

@menu
* Waiting For Completion::        Waiting until an operation is completed.
* Using External Event Loops::    Advanced control over what happens when.
* Cancellation::                  How to end pending operations prematurely.
@end menu


@node Waiting For Completion
@subsection Waiting For Completion
@cindex cryptographic operation, wait for
@cindex wait for completion

@deftypefun gpgme_ctx_t gpgme_wait (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_error_t *@var{status}}, @w{int @var{hang}})
The function @code{gpgme_wait} continues the pending operation within
the context @var{ctx}.  In particular, it ensures the data exchange
between @acronym{GPGME} and the crypto backend and watches over the
run time status of the backend process.

If @var{hang} is true, the function does not return until the
operation is completed or cancelled.  Otherwise the function will not
block for a long time.

The error status of the finished operation is returned in @var{status}
if @code{gpgme_wait} does not return @code{NULL}.

The @var{ctx} argument can be @code{NULL}.  In that case,
@code{gpgme_wait} waits for any context to complete its operation.

@code{gpgme_wait} can be used only in conjunction with any context
that has a pending operation initiated with one of the
@code{gpgme_op_*_start} functions except @code{gpgme_op_keylist_start}
and @code{gpgme_op_trustlist_start} (for which you should use the
corresponding @code{gpgme_op_*_next} functions).  If @var{ctx} is
@code{NULL}, all of such contexts are waited upon and possibly
returned.  Synchronous operations running in parallel, as well as key
and trust item list operations, do not affect @code{gpgme_wait}.

In a multi-threaded environment, only one thread should ever call
@code{gpgme_wait} at any time, irregardless if @var{ctx} is specified
or not.  This means that all calls to this function should be fully
synchronized by locking primitives.  It is safe to start asynchronous
operations while a thread is running in @code{gpgme_wait}.

The function returns the @var{ctx} of the context which has finished
the operation.  If @var{hang} is false, and the timeout expires,
@code{NULL} is returned and @code{*status} will be set to 0.  If an
error occurs, @code{NULL} is returned and the error is returned in
@code{*status}.
@end deftypefun


@node Using External Event Loops
@subsection Using External Event Loops
@cindex event loop, external

@acronym{GPGME} hides the complexity of the communication between the
library and the crypto engine.  The price of this convenience is that
the calling thread can block arbitrary long waiting for the data
returned by the crypto engine.  In single-threaded programs, in
particular if they are interactive, this is an unwanted side-effect.
OTOH, if @code{gpgme_wait} is used without the @var{hang} option being
enabled, it might be called unnecessarily often, wasting CPU time that
could be used otherwise.

The I/O callback interface described in this section lets the user
take control over what happens when.  @acronym{GPGME} will provide the
user with the file descriptors that should be monitored, and the
callback functions that should be invoked when a file descriptor is
ready for reading or writing.  It is then the user's responsibility to
decide when to check the file descriptors and when to invoke the
callback functions.  Usually this is done in an event loop, that also
checks for events in other parts of the program.  If the callback
functions are only called when the file descriptors are ready,
@acronym{GPGME} will never block.  This gives the user mroe control
over the program flow, and allows to perform other tasks when
@acronym{GPGME} would block otherwise.

By using this advanced mechanism, @acronym{GPGME} can be integrated
smoothly into GUI toolkits like GTK+ even for single-threaded
programs.

@menu
* I/O Callback Interface::        How I/O callbacks are registered.
* Registering I/O Callbacks::     How to use I/O callbacks for a context.
* I/O Callback Example::          An example how to use I/O callbacks.
* I/O Callback Example GTK+::     How to use @acronym{GPGME} with GTK+.
* I/O Callback Example GDK::      How to use @acronym{GPGME} with GDK.
* I/O Callback Example Qt::       How to use @acronym{GPGME} with Qt.
@end menu


@node I/O Callback Interface
@subsubsection I/O Callback Interface

@deftp {Data type} {gpgme_error_t (*gpgme_io_cb_t) (@w{void *@var{data}}, @w{int @var{fd}})}
@tindex gpgme_io_cb_t
The @code{gpgme_io_cb_t} type is the type of functions which
@acronym{GPGME} wants to register as I/O callback handlers using the
@code{gpgme_register_io_cb_t} functions provided by the user.

@var{data} and @var{fd} are provided by @acronym{GPGME} when the I/O
callback handler is registered, and should be passed through to the
handler when it is invoked by the user because it noticed activity on
the file descriptor @var{fd}.

The callback handler always returns @code{0}, but you should consider
the return value to be reserved for later use.
@end deftp

@deftp {Data type} {gpgme_error_t (*gpgme_register_io_cb_t) (@w{void *@var{data}}, @w{int @var{fd}}, @w{int @var{dir}}, @w{gpgme_io_cb_t @var{fnc}}, @w{void *@var{fnc_data}}, @w{void **@var{tag}})}
@tindex gpgme_register_io_cb_t
The @code{gpgme_register_io_cb_t} type is the type of functions which can
be called by @acronym{GPGME} to register an I/O callback funtion
@var{fnc} for the file descriptor @var{fd} with the user.
@var{fnc_data} should be passed as the first argument to @var{fnc}
when the handler is invoked (the second argument should be @var{fd}).
If @var{dir} is 0, @var{fnc} should be called by the user when
@var{fd} is ready for writing.  If @var{dir} is 1, @var{fnc} should be
called when @var{fd} is ready for reading.

@var{data} was provided by the user when registering the
@code{gpgme_register_io_cb_t} function with @acronym{GPGME} and will always
be passed as the first argument when registering a callback function.
For example, the user can use this to determine the event loop to
which the file descriptor should be added.

@acronym{GPGME} will call this function when a crypto operation is
initiated in a context for which the user has registered I/O callback
handler functions with @code{gpgme_set_io_cbs}.  It can also call this
function when it is in an I/O callback handler for a file descriptor
associated to this context.

The user should return a unique handle in @var{tag} identifying this
I/O callback registration, which will be passed to the
@code{gpgme_register_io_cb_t} function without interpretation when the file
descriptor should not be monitored anymore.
@end deftp

@deftp {Data type} {void (*gpgme_remove_io_cb_t) (@w{void *@var{tag}})}
The @code{gpgme_remove_io_cb_t} type is the type of functions which can be
called by @acronym{GPGME} to remove an I/O callback handler that was
registered before.  @var{tag} is the handle that was returned by the
@code{gpgme_register_io_cb_t} for this I/O callback.

@acronym{GPGME} can call this function when a crypto operation is in
an I/O callback.  It will also call this function when the context is
destroyed while an operation is pending.
@end deftp

@deftp {Data type} {enum gpgme_event_io_t}
@tindex gpgme_event_io_t
The @code{gpgme_event_io_t} type specifies the type of an event that is
reported to the user by @acronym{GPGME} as a consequence of an I/O
operation.  The following events are defined:

@table @code
@item GPGME_EVENT_START
The operation is fully initialized now, and you can start to run the
registered I/O callback handlers now.  Note that registered I/O
callback handlers must not be run before this event is signalled.
@var{type_data} is @code{NULL} and reserved for later use.

@item GPGME_EVENT_DONE
The operation is finished, the last I/O callback for this operation
was removed.  The accompanying @var{type_data} points to a
@code{gpgme_error_t} variable that contains the status of the operation
that finished.  This event is signalled after the last I/O callback
has been removed.

@item GPGME_EVENT_NEXT_KEY
In a @code{gpgme_op_keylist_start} operation, the next key was
received from the crypto engine.  The accompanying @var{type_data} is
a @code{gpgme_key_t} variable that contains the key with one reference
for the user.

@item GPGME_EVENT_NEXT_TRUSTITEM
In a @code{gpgme_op_trustlist_start} operation, the next trust item
was received from the crypto engine.  The accompanying @var{type_data}
is a @code{gpgme_trust_item_t} variable that contains the trust item with
one reference for the user.
@end table
@end deftp

@deftp {Data type} {void (*gpgme_event_io_cb_t) (@w{void *@var{data}}, @w{gpgme_event_io_t @var{type}}, @w{void *@var{type_data}})}
The @code{gpgme_event_io_cb_t} type is the type of functions which can be
called by @acronym{GPGME} to signal an event for an operation running
in a context which has I/O callback functions registered by the user.

@var{data} was provided by the user when registering the
@code{gpgme_event_io_cb_t} function with @acronym{GPGME} and will always be
passed as the first argument when registering a callback function.
For example, the user can use this to determine the context in which
this event has occured.

@var{type} will specify the type of event that has occured.
@var{type_data} specifies the event further, as described in the above
list of possible @code{gpgme_event_io_t} types.

@acronym{GPGME} can call this function in an I/O callback handler.
@end deftp


@node Registering I/O Callbacks
@subsubsection Registering I/O Callbacks

@deftp {Data type} {struct gpgme_io_cb_ts}
@tindex gpgme_event_io_t
This structure is used to store the I/O callback interface functions
described in the previous section.  It has the following members:

@table @code
@item gpgme_register_io_cb_t add
This is the function called by @acronym{GPGME} to register an I/O
callback handler.  It must be specified.

@item void *add_data
This is passed as the first argument to the @code{add} function when
it is called by @acronym{GPGME}.  For example, it can be used to
determine the event loop to which the file descriptor should be added.

@item gpgme_remove_io_cb_t remove
This is the function called by @acronym{GPGME} to remove an I/O
callback handler.  It must be specified.

@item gpgme_event_io_cb_t event
This is the function called by @acronym{GPGME} to signal an event for
an operation.  It is optional, but if you don't specify it, you can
not retrieve the return value of the operation.

@item void *event_data
This is passed as the first argument to the @code{event} function when
it is called by @acronym{GPGME}.  For example, it can be used to
determine the context in which the event has occured.
@end table
@end deftp

@deftypefun void gpgme_set_io_cbs (@w{gpgme_ctx_t @var{ctx}}, @w{struct gpgme_io_cb_ts *@var{io_cbs}})
The function @code{gpgme_set_io_cbs} enables the I/O callback
interface for the context @var{ctx}.  The I/O callback functions are
specified by @var{io_cbs}.

If @var{io_cbs}->@code{add} is @code{NULL}, the I/O callback interface
is disabled for the context, and normal operation is restored.
@end deftypefun

@deftypefun void gpgme_get_io_cbs (@w{gpgme_ctx_t @var{ctx}}, @w{struct gpgme_io_cb_ts *@var{io_cbs}})
The function @code{gpgme_get_io_cbs} returns the I/O callback
functions set with @code{gpgme_set_io_cbs} in @var{io_cbs}.
@end deftypefun


@node I/O Callback Example
@subsubsection I/O Callback Example

To actually use an external event loop, you have to implement the I/O
callback functions that are used by @acronym{GPGME} to register and
unregister file descriptors.  Furthermore, you have to actually
monitor these file descriptors for activity and call the appropriate
I/O callbacks.

The following example illustrates how to do that.  The example uses
locking to show in which way the the callbacks and the event loop can
run concurrently.  For the event loop, we use a fixed array.  For a
real-world implementation, you should use a dynamically sized
structure because the number of file descriptors needed for a crypto
operation in @acronym{GPGME} is not predictable.

@example
#include <pthread.h>
#include <sys/types.h>
#include <gpgme.h>

/* The following structure holds the result of a crypto operation.  */
struct op_result
@{
  int done;
  gpgme_error_t err;
@};

/* The following structure holds the data associated with one I/O
callback.  */
struct one_fd
@{
  int fd;
  int dir;
  gpgme_io_cb_t fnc;
  void *fnc_data;
@};

struct event_loop
@{
  pthread_mutex_t lock;
#define MAX_FDS 32
  /* Unused slots are marked with FD being -1.  */
  struct one_fd fds[MAX_FDS];
@};
@end example

The following functions implement the I/O callback interface.

@example
gpgme_error_t
add_io_cb (void *data, int fd, int dir, gpgme_io_cb_t fnc, void *fnc_data,
	   void **r_tag)
@{
  struct event_loop *loop = data;
  struct one_fd *fds = loop->fds;
  int i;

  pthread_mutex_lock (&loop->lock);
  for (i = 0; i < MAX_FDS; i++)
    @{
      if (fds[i].fd == -1)
	@{
	  fds[i].fd = fd;
	  fds[i].dir = dir;
	  fds[i].fnc = fnc;
	  fds[i].fnc_data = fnc_data;
	  break;
	@}
    @}
  pthread_mutex_unlock (&loop->lock);
  if (i == MAX_FDS)
    return gpg_error (GPG_ERR_GENERAL);
  *r_tag = &fds[i];
  return 0;
@}

void
remove_io_cb (void *tag)
@{
  struct one_fd *fd = tag;

  pthread_mutex_lock (&loop->lock);
  fd->fd = -1;
  pthread_mutex_unlock (&loop->lock);
@}

void
event_io_cb (void *data, gpgme_event_io_t type, void *type_data)
@{
  struct op_result *result = data;

  /* We don't support list operations here.  */
  if (type == GPGME_EVENT_DONE)
    @{
      result->done = 1;
      result->err = *type_data;
    @}
@}
@end example

The final missing piece is the event loop, which will be presented
next.  We only support waiting for the success of a single operation.

@example
int
do_select (struct event_loop *loop)
@{
  fd_set rfds;
  fd_set wfds;
  int i, n;
  int any = 0;

  pthread_mutex_lock (&loop->lock);
  FD_ZERO (&rfds);
  FD_ZERO (&wfds);
  for (i = 0; i < FDLIST_MAX; i++)
    if (fdlist[i].fd != -1)
      FD_SET (fdlist[i].fd, fdlist[i].dir ? &rfds : &wfds);
  pthread_mutex_unlock (&loop->unlock);

  do
    @{
      n = select (FD_SETSIZE, &rfds, &wfds, NULL, 0);
    @}
  while (n < 0 && errno == EINTR);

  if (n < 0)
    return n;	/* Error or timeout.  */

  pthread_mutex_lock (&loop->lock);
  for (i = 0; i < FDLIST_MAX && n; i++)
    @{
      if (fdlist[i].fd != -1)
	@{
	  if (FD_ISSET (fdlist[i].fd, fdlist[i].dir ? &rfds : &wfds))
	    @{
	      assert (n);
	      n--;
	      any = 1;
              /* The I/O callback handler can register/remove callbacks,
                 so we have to unlock the file descriptor list.  */
              pthread_mutex_unlock (&loop->lock);
	      (*fdlist[i].fnc) (fdlist[i].fnc_data, fdlist[i].fd);
              pthread_mutex_lock (&loop->lock);
	    @}
	@}
    @}
  pthread_mutex_unlock (&loop->lock);
  return any;
@}

void
wait_for_op (struct event_loop *loop, struct op_result *result)
@{
  int ret;

  do
    @{
      ret = do_select (loop);
    @}
  while (ret >= 0 && !result->done);
  return ret;
@}
@end example

The main function shows how to put it all together.

@example
int
main (int argc, char *argv[])
@{
  struct event_loop loop;
  struct op_result result;
  gpgme_ctx_t ctx;
  gpgme_error_t err;
  gpgme_data_t sig, text;
  gpgme_sig_stat_t status;
  int i;
  struct gpgme_io_cb_ts io_cbs =
  @{
    add_io_cb,
    &loop,
    remove_io_cb,
    event_io_cb,
    &result
  @};

  /* Initialize the loop structure.  */
  loop.lock = PTHREAD_MUTEX_INITIALIZER;
  for (i = 0; i < MAX_FDS; i++)
    loop->fds[i].fd = -1;

  /* Initialize the result structure.  */
  result.done = 0;

  err = gpgme_data_new_from_file (&sig, "signature", 1);
  if (!err)
    err = gpgme_data_new_from_file (&text, "text", 1);
  if (!err)
    err = gpgme_new (&ctx);
  if (!err)
    @{
       gpgme_set_io_cbs (ctx, &io_cbs);
       err = gpgme_op_verify_start (ctx, sig, text, &status);
    @}
  if (err)
    @{
      fprintf (stderr, "gpgme error: %s: %s\n",
               gpgme_strsource (err), gpgme_strerror (err));
      exit (1);
    @}

  wait_for_op (&loop, &result);
  if (!result.done)
    @{
      fprintf (stderr, "select error\n");
      exit (1);
    @}
  if (!result.err)
    @{
      fprintf (stderr, "verification failed: %s: %s\n",
               gpgme_strsource (result.err), gpgme_strerror (result.err));
      exit (1);
    @}
  /* Evaluate STATUS.  */
  @dots{}
  return 0;
@}
@end example


@node I/O Callback Example GTK+
@subsubsection I/O Callback Example GTK+
@cindex GTK+, using @acronym{GPGME} with

The I/O callback interface can be used to integrate @acronym{GPGME}
with the GTK+ event loop.  The following code snippets shows how this
can be done using the appropriate register and remove I/O callback
functions.  In this example, the private data of the register I/O
callback function is unused.  The event notifications is missing
because it does not require any GTK+ specific setup.

@example
#include <gtk/gtk.h>

struct my_gpgme_io_cb
@{
  gpgme_io_cb_t fnc;
  void *fnc_data;
  guint input_handler_id
@};

void
my_gpgme_io_cb (gpointer data, gint source, GdkInputCondition condition)
@{
  struct my_gpgme_io_cb *iocb = data;
  (*(iocb->fnc)) (iocb->data, source);
@}

void
my_gpgme_remove_io_cb (void *data)
@{
  struct my_gpgme_io_cb *iocb = data;
  gtk_input_remove (data->input_handler_id);
@}

void
my_gpgme_register_io_callback (void *data, int fd, int dir, gpgme_io_cb_t fnc,
                               void *fnc_data, void **tag)
@{
  struct my_gpgme_io_cb *iocb = g_malloc (sizeof (struct my_gpgme_io_cb));
  iocb->fnc = fnc;
  iocb->data = fnc_data;
  iocb->input_handler_id = gtk_input_add_full (fd, dir
                                                   ? GDK_INPUT_READ
                                                   : GDK_INPUT_WRITE,
                                               my_gpgme_io_callback,
                                               0, iocb, NULL);
  *tag = iocb;
  return 0;
@}
@end example


@node I/O Callback Example GDK
@subsubsection I/O Callback Example GDK
@cindex GDK, using @acronym{GPGME} with

The I/O callback interface can also be used to integrate
@acronym{GPGME} with the GDK event loop.  The following code snippets
shows how this can be done using the appropriate register and remove
I/O callback functions.  In this example, the private data of the
register I/O callback function is unused.  The event notifications is
missing because it does not require any GDK specific setup.

It is very similar to the GTK+ example in the previous section.

@example
#include <gdk/gdk.h>

struct my_gpgme_io_cb
@{
  gpgme_io_cb_t fnc;
  void *fnc_data;
  gint tag;
@};

void
my_gpgme_io_cb (gpointer data, gint source, GdkInputCondition condition)
@{
  struct my_gpgme_io_cb *iocb = data;
  (*(iocb->fnc)) (iocb->data, source);
@}

void
my_gpgme_remove_io_cb (void *data)
@{
  struct my_gpgme_io_cb *iocb = data;
  gdk_input_remove (data->tag);
@}

void
my_gpgme_register_io_callback (void *data, int fd, int dir, gpgme_io_cb_t fnc,
                               void *fnc_data, void **tag)
@{
  struct my_gpgme_io_cb *iocb = g_malloc (sizeof (struct my_gpgme_io_cb));
  iocb->fnc = fnc;
  iocb->data = fnc_data;
  iocb->tag = gtk_input_add_full (fd, dir ? GDK_INPUT_READ : GDK_INPUT_WRITE,
                                  my_gpgme_io_callback, iocb, NULL);
  *tag = iocb;
  return 0;
@}
@end example


@node I/O Callback Example Qt
@subsubsection I/O Callback Example Qt
@cindex Qt, using @acronym{GPGME} with

The I/O callback interface can also be used to integrate
@acronym{GPGME} with the Qt event loop.  The following code snippets
show how this can be done using the appropriate register and remove
I/O callback functions.  In this example, the private data of the
register I/O callback function is unused.  The event notifications is
missing because it does not require any Qt specific setup.

@example
#include <qsocketnotifier.h>
#include <qapplication.h>

struct IOCB @{
  IOCB( GpgmeIOCb f, void * d, QSocketNotifier * n )
    : func( f ), data( d ), notifier( n ) @{@}
  GpgmeIOCb func;
  void * data;
  QSocketNotifier * notifier;
@}

class MyApp : public QApplication @{

  // ...
  
  static void registerGpgmeIOCallback( void * data, int fd, int dir,
                                       GpgmeIOCb func, void * func_data,
                                       void ** tag ) @{
    QSocketNotifier * n =
      new QSocketNotifier( fd, dir ? QSocketNotifier::Read
                                   : QSocketNotifier::Write );
    connect( n, SIGNAL(activated(int)),
             qApp, SLOT(slotGpgmeIOCallback(int)) );
    qApp->mIOCBs.push_back( IOCB( func, func_data, n ) );
    *tag = (void*)n;
  @}

  static void removeGpgmeIOCallback( void * tag ) @{
    if ( !tag ) return;
    QSocketNotifier * n = static_cast<QSocketNotifier*>( tag );
    for ( QValueList<IOCB>::iterator it = qApp->mIOCBs.begin() ;
          it != qApp->mIOCBs.end() ; ++it )
      if ( it->notifier == n ) @{
        delete it->notifier;
        qApp->mIOCBs.erase( it );
        return;
      @}
  @}

public slots:
  void slotGpgmeIOCallback( int fd ) @{
    for ( QValueList<IOCB>::const_iterator it = mIOCBs.begin() ;
          it != mIOCBs.end() ; ++it )
      if ( it->notifier && it->notifier->socket() == fd )
        (*(it->func)) ( it->func_data, fd );
  @}

  // ...

private:
  QValueList<IOCB> mIOCBs;
  // ...
@};
@end example


@node Cancellation
@subsection Cancellation
@cindex cryptographic operation, aborting
@cindex cryptographic operation, cancelling
@cindex aborting operations
@cindex cancelling operations

Sometimes you do not want to wait for an operation to finish.  If you
use external I/O callbacks, you can cancel a pending operation.
However, you must ensure that no other thread is currently using the
context in which the operation you want to cancel runs.  This includes
callback handlers.  So your external event loop must either be halted
or otherwise it must be guaranteed that no installed I/O callbacks are
run for this context.

@deftypefun gpgme_ctx_t gpgme_cancel (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_cancel} attempts to cancel a pending
operation in the context @var{ctx}.  This only works if you use the
global event loop or your own event loop.

If you use the global event loop, you must not call @code{gpgme_wait}
or @code{gpgme_wait} during cancellation.  After successful
cancellation, you can call @code{gpgme_wait} (optionally waiting on
@var{ctx}), and the context @var{ctx} will appear as if it had
finished with the error code @code{GPG_ERR_CANCEL}.

If you use your an external event loop, you must ensure that no I/O
callbacks are invoked for this context (for example by halting the
event loop).  On successful cancellation, all registered I/O callbacks
for this context will be unregistered, and a @code{GPGME_EVENT_DONE}
event with the error code @code{GPG_ERR_CANCEL} will be signaled.

The function returns an error code if the cancellation failed (in this
case the state of @var{ctx} is not modified).
@end deftypefun


@include gpl.texi


@include fdl.texi


@node Concept Index
@unnumbered Concept Index

@printindex cp


@node Function and Data Index
@unnumbered Function and Data Index

@printindex fn


@summarycontents
@contents
@bye