gpgme/doc/gpgme.texi

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

@c TODO:
@c GpgmeError gpgme_op_import ( GpgmeCtx c, GpgmeData keydata );
@c GpgmeError gpgme_op_export ( GpgmeCtx c, GpgmeRecipients recp,
@c                              GpgmeData keydata );
@c GpgmeError gpgme_op_genkey ( GpgmeCtx c, const char *parms,
@c                              GpgmeData pubkey, GpgmeData seckey );
@c GpgmeError gpgme_op_delete ( GpgmeCtx c, const GpgmeKey key, int allow_secret);
@c char *gpgme_get_op_info (GpgmeCtx c, int reserved);
@c void       gpgme_cancel (GpgmeCtx c);
@c GpgmeCtx   gpgme_wait (GpgmeCtx c, int hang);
@c char *gpgme_get_notation (GpgmeCtx c);
@c void        gpgme_register_idle (void (*fnc)(void));

@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 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{} 2001 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.
* Preperation::                   What you should do before using the library.
* Protocols and Engines::         Supported crypto protocols.
* 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.

Preperation

* Header::                        What header file you need to include.
* Building the source::           Compiler options to be used.
* Library version check::         Getting and verifying the library version.

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.

Error Handling

* Error values::                  A list of all error values used.
* 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.

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.

Context attributes

* Protocol selection::            Selecting the protocol used by a context.
* @acronym{ASCII} armor::                   Requesting @acronym{ASCII} armored output.
* Text mode::                     Choosing canonical text mode.
* Key listing mode::              Selecting key listing mode.
* Passphrase callback::           Getting the passphrase from the user.
* Progress meter callback::       Being informed about the progress.

Key Management

* Listing keys::                  Browsing the list of available keys.
* Information about keys::        Requesting detailed information about keys.
* Manipulating keys::             Operations on keys.

Trust Item Management

* Listing trust items::           Browsing the list of available trust items.
* Information about trust items:: Requesting detailed 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

* Selecting recipients::          How to choose the recipients.
* Encrypting a plaintext::        How to encrypt a plaintext.


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

The reader is assumed to posess 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.

@strong{Caution:} The @acronym{GPGME} library is not thread-safe.  It
will be to some extent in the future, but currently great care has to
be taken if @acronym{GPGME} is used in a multi-threaded environment.


@node Preperation
@chapter Preperation

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.
* Library version check::         Getting and verifying the library version.
@end menu


@node Header
@section Header

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, @code{Gpgme*} for data types and @code{GPGME_*} for other
symbols.


@node Building the source
@section Building the source

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 compilers
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


@node Library version check
@section Library version check

@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


@node Protocols and Engines
@chapter Protocols and Engines

@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 backends 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 GpgmeProtocol}
The @code{GpgmeProtocol} 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

@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

@deftypefun GpgmeError gpgme_engine_check_version (@w{GpgmeProtocol @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 @code{GPGME_No_Error} if the engine is available
and @code{GPGME_Invalid_Engine} if it is not.
@end deftypefun

@deftypefun GpgmeError gpgme_check_engine (void)
The function @code{gpgme_check_engine} is equivalent to

@example
gpgme_engine_check_version (GPGME_PROTOCOL_OpenPGP);
@end example

This function is deprecated and provided for backwards compatibility
only.  It is obsoleted by @code{gpgme_engine_check_version}.
@end deftypefun


@node Engine information
@section Engine information

@deftypefun {const char *} gpgme_get_engine_info (void)
The function @code{gpgme_get_engine_info} returns an @acronym{XML}
string containing information about the available protocols and the
engine which implement them.  The following information is returned
for each engine:

@table @samp
@item <protocol>
The name of the protocol.
@item <version>
The version of the engine.
@item <path>
The path to the engine binary.
@end table

A string is always returned.  If an error occurs, the string will
contain an @samp{<error>} tag with a description of the failure.
@end deftypefun

Here is the example output of what @code{gpgme_get_engine_info} might
return on your system:

@example
<EngineInfo>
 <engine>
  <protocol>OpenPGP</protocol>
  <version>1.0.6</version>
  <path>/usr/bin/gpg</path>
 </engine>
 <engine>
  <protocol>CMS</protocol>
  <version>0.0.0</version>
  <path>/usr/bin/gpgsm</path>
 </engine>
</EngineInfo>
@end example


@node OpenPGP
@section OpenPGP

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

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

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


@node Error Handling
@chapter 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
what each error message means in general.  Some error values have
specific meanings if returned by a specific function.  Such cases are
described in the documentation of those functions.

@menu
* Error values::                  A list of all error values used.
* Error strings::                 How to get a descriptive string from a value.
@end menu


@node Error values
@section Error values

@deftp {Data type} {enum GpgmeError}
The @code{GpgmeError} type specifies the set of all error values that
are used by @acronym{GPGME}.  Possible values are:

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

@item GPGME_No_Error
This value indicates success.  The value of this error is @code{0}.

@item GPGME_General_Error
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 seperate error value for this type of problem.

@item GPGME_Out_Of_Core
This value means that an out-of-memory condition occured.

@item GPGME_Invalid_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{GpgmeData}
object was expected, but one containing data was provided, this error
value is returned.

@item GPGME_Busy
This value is returned if you try to start a new operation in a
context that is already busy with some earlier operation which was not
canceled or finished yet.

@item GPGME_No_Request
This value is in some sense the opposite of @code{GPGME_Busy}.  There
is no pending operation, but it is required for the function to
succeed.

@item GPGME_Exec_Error
This value means that an error occured when trying to spawn a child
process.

@item GPGME_Too_Many_Procs
This value means that there are too many active backend processes.

@item GPGME_Pipe_Error
This value means that the creation of a pipe failed.

@item GPGME_No_Recipients 
This value means that no recipients for a message have been set.

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

@item GPGME_Conflict
This value means that a conflict of some sort occured.

@item GPGME_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 GPGME_Read_Error
This value means that an I/O read operation failed.

@item GPGME_Write_Error
This value means that an I/O write operation failed.

@item GPGME_Invalid_Type
This value means that a user provided object was of a wrong or
incompatible type.  Usually this refers to the type of a
@code{GpgmeData} object.

@item GPGME_Invalid_Mode
This value means that a @code{GpgmeData} object has an incorrect mode
of operation (for example, doesn't support output although it is
attempted to use it as an output buffer).

@item GPGME_File_Error
This value means that a file I/O operation failed.  The value of
@code{errno} contains the system error value.

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

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

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

@item GPGME_Invalid_Key
This value means that a key was invalid.

@item GPGME_Invalid_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.
@end table
@end deftp


@node Error strings
@section Error strings

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


@node Exchanging Data
@chapter Exchanging Data

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{GpgmeData} objects,
regardless of the communication protocol between @acronym{GPGME} and
the crypto engine in use.

@deftp {Data type} {GpgmeData}
The @code{GpgmeData} 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

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

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

@deftypefun GpgmeError gpgme_data_new_from_mem (@w{GpgmeData *@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{GpgmeData} 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 @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} or
@var{buffer} is not a valid pointer, and @code{GPGME_Out_Of_Core} if
not enough memory is available.
@end deftypefun

@deftypefun GpgmeError gpgme_data_new_from_file (@w{GpgmeData *@var{dh}}, @w{const char *@var{filename}}, @w{int @var{copy}})
The function @code{gpgme_data_new_from_file} creates a new
@code{GpgmeData} 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 @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} or
@var{filename} is not a valid pointer, @code{GPGME_File_Error} if an
I/O operation fails, @code{GPGME_Not_Implemented} if @var{code} is
zero, and @code{GPGME_Out_Of_Core} if not enough memory is available.
@end deftypefun

@deftypefun GpgmeError gpgme_data_new_from_filepart (@w{GpgmeData *@var{dh}}, @w{const char *@var{filename}}, @w{FILE *@var{fp}}, @w{off_t @var{offset}}, @w{off_t @var{length}})
The function @code{gpgme_data_new_from_filepart} creates a new
@code{GpgmeData} 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 @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} and
exactly one of @var{filename} and @var{fp} is not a valid pointer,
@code{GPGME_File_Error} if an I/O operation fails, and
@code{GPGME_Out_Of_Core} if not enough memory is available.
@end deftypefun


@deftypefun GpgmeError gpgme_data_new_with_read_cb (@w{GpgmeData *@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{GpgmeData} 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 @code{GPGME_No_Error} if the data object was
successfully created, @code{GPGME_Invalid_Value} if @var{dh} or
@var{readfunc} is not a valid pointer, and @code{GPGME_Out_Of_Core} if
not enough memory is available.
@end deftypefun


@node Destroying data buffers
@section Destroying data buffers

@deftypefun void gpgme_data_release (@w{GpgmeData @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{GpgmeData @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

@deftypefun GpgmeError gpgme_data_read (@w{GpgmeData @var{dh}}, @w{char *@var{buffer}}, @w{size_t @var{length}}, @w{size_t *@var{nread}})
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}.  The actual amount read is returned in @var{nread}.

If @var{buffer} is @code{NULL}, the function returns the amount of
bytes available in @var{nread} without changing the read pointer.
This is not supported by all types of data objects.  If this function
is not supported, @code{GPGME_Invalid_Type} is returned.

If the end of the data object is reached, the function returns
@code{GPGME_EOF} and sets @var{nread} to zero.

In all other cases, the function returns @code{GPGME_No_Error} if the
operation was successfully performed and @code{GPGME_Invalid_Value} if
@var{dh} is not a valid pointer.
@end deftypefun

@deftypefun GpgmeError gpgme_data_rewind (@w{GpgmeData @var{dh}})
The function @code{gpgme_data_rewind} resets the read pointer of the
data object with the handle @var{dh}, so that a subsequent
@code{gpgme_data_read} operation starts at the beginning of the data.

The function returns @code{GPGME_No_Error} if the operation was
successfully performed, @code{GPGME_Not_Implemented} if the operation
is not supported (for example, by a read callback function supplied by
the user) and @code{GPGME_Invalid_Value} if @var{dh} is not a valid
pointer.
@end deftypefun

@deftypefun GpgmeError gpgme_data_write (@w{GpgmeData @var{dh}}, @w{const char *@var{buffer}}, @w{size_t @var{length}})
The function @code{gpgme_data_write} writes @var{length} bytes
starting from @var{buffer} into the data object with the handle
@var{dh} at the current write position.

The function returns @code{GPGME_No_Error} if the operation was
successfully performed, @code{GPGME_Invalid_Value} if @var{dh} or
@var{buffer} is not a valid pointer, @code{GPGME_Invalid_Type} or
@code{GPGME_Invalid_Mode} if the data object type does not support
writing, and @code{GPGME_Out_Of_Core} if not enough memory is
available.
@end deftypefun

@deftp {Data type} {enum GpgmeDataType}
The @code{GpgmeDataType} type specifies the type of a @code{GpgmeData} object.
The following data types are available:

@table @code
@item GPGME_DATA_TYPE_NONE
This specifies that the type is not yet determined.

@item GPGME_DATA_TYPE_MEM
This specifies that the data is stored in memory.

@item GPGME_DATA_TYPE_FD
This type is not implemented.

@item GPGME_DATA_TYPE_FILE
This type is not implemented.

@item GPGME_DATA_TYPE_CB
This type specifies that the data is provided by a callback function
implemented by the user.
@end table
@end deftp

@deftypefun GpgmeDataType gpgme_data_get_type (@w{GpgmeData @var{dh}})
The function @code{gpgme_data_get_type} returns the type of the data
object with the handle @var{dh}.  If @var{dh} is not a valid pointer,
@code{GPGME_DATA_TYPE_NONE} is returned.
@end deftypefun


@node Contexts
@chapter Contexts

All cryptograhic 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} {GpgmeCtx}
The @code{GpgmeCtx} 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.
@end menu


@node Creating contexts
@section Creating contexts

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

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



@node Destroying contexts
@section Destroying contexts

@deftypefun void gpgme_release (@w{GpgmeCtx @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

@menu
* Protocol selection::            Selecting the protocol used by a context.
* @acronym{ASCII} armor::                   Requesting @acronym{ASCII} armored output.
* Text mode::                     Choosing canonical text mode.
* Key listing mode::              Selecting key listing mode.
* Passphrase callback::           Getting the passphrase from the user.
* Progress meter callback::       Being informed about the progress.
@end menu


@node Protocol selection
@subsection Protocol selection

@deftypefun GpgmeError gpgme_set_protocol (@w{GpgmeCtx @var{ctx}}, @w{GpgmeProtocol @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 @code{GPGME_No_Error} if the protocol could be
set successfully, and @code{GPGME_Invalid_Value} if @var{protocol} is
not a valid protocol.
@end deftypefun


@node @acronym{ASCII} armor
@subsection @acronym{ASCII} armor

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

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

@deftypefun int gpgme_get_armor (@w{GpgmeCtx @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

@deftypefun void gpgme_set_textmode (@w{GpgmeCtx @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.

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

@deftypefun int gpgme_get_textmode (@w{GpgmeCtx @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 Key listing mode
@subsection Key listing mode

@deftypefun void gpgme_set_keylist_mode (@w{GpgmeCtx @var{ctx}}, @w{int @var{mode}})
The function @code{gpgme_set_keylist_mode} changes the default
behaviour of the key listing functions.  Defined values for @var{mode}
are:

@table @code
@item 0
Normal listing.
@item 1
Fast listing without information about the key validity.
@end table
@end deftypefun


@node Passphrase callback
@subsection Passphrase callback

@deftp {Data type} {const char *(*GpgmePassphraseCb)(void *@var{hook}, const char *@var{desc}, void *@var{r_hd})}
The @code{GpgmePasshraseCb} type is the type of functions usable as
passphrase callback function.

The string @var{desc} contains a test usable to be displayed to the
user of the application.  The function should return a passphrase for
the context when invoked with @var{desc} not being @code{NULL}.

The user may store information about the resources associated with the
returned passphrase in @var{*r_hd}.  When the passphrase is no longer
needed by @acronym{GPGME}, the passphrase callback function will be
called with @var{desc} being @var{NULL}, and @var{r_hd} being the same
as at the first invocation.
@end deftp

@deftypefun void gpgme_set_passphrase_cb (@w{GpgmeCtx @var{ctx}}, @w{GpgmePassphraseCb @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


@node Progress meter callback
@subsection Progress meter callback

@deftp {Data type} {const char *(*GpgmeProgressCb)(void *@var{hook}, const char *@var{what}, int @var{type}, int @var{current}, int @var{total})}
The @code{GpgmeProgressCb} 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{GpgmeCtx @var{ctx}}, @w{GpgmeProgressCb @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


@node Key Management
@section 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} GpgmeKey
The @code{GpgmeKey} type is a handle for a public or secret key, and
is used to select the key for operations involving it.

A key can contain several user IDs and sub keys.
@end deftp

@menu
* Listing keys::                  Browsing the list of available keys.
* Information about keys::        Requesting detailed information about keys.
* Manipulating keys::             Operations on keys.
@end menu


@node Listing keys
@subsection Listing keys

@deftypefun GpgmeError gpgme_op_keylist_start (@w{GpgmeCtx @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.

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{GPGME_EOF}), or
@code{gpgme_op_keylist_end} is called to finish the operation.

The function returns @code{GPGME_Invalid_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 GpgmeError gpgme_op_keylist_next (@w{GpgmeCtx @var{ctx}}, @w{GpgmeKey *@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{GpgmeKey} objects in
@acronym{GPGME}.

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

The function returns @code{GPGME_Invalid_Value} if @var{ctx} or
@var{r_key} is not a valid pointer, @code{GPGME_No_Request} if there
is no pending operation, @code{GPGME_Out_Of_Core} if there is not
enough memory for the operation.
@end deftypefun

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

The function returns @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, @code{GPGME_No_Request} if there is no pending
operation, @code{GPGME_Out_Of_Core} if at some time during the
operation there was not enough memory available.
@end deftypefun


@node Information about keys
@subsection Information about keys

@deftypefun {char *} gpgme_key_get_as_xml (@w{GpgmeKey @var{key}})
The function @code{gpgme_key_get_as_xml} returns a string in
@acronym{XML} format describing the key @var{key}.  The user has to
release the string with @code{free}.

The function returns @code{NULL} if @var{key} is not a valid pointer,
or there is not enough memory available.
@end deftypefun

@deftp {Data type} GpgmeAttr
The @code{GpgmeAttr} 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
XXX FIXME

@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{GpgmeKey} 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 is 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
string or a number.  If the key is a secret key, the representation is
``1'' or @code{1}, otherwise it is NULL or @code{0}.

@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 for 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 for certifications.  It is
representable as a number, and is @code{1} if the sub key can be used
for certifications, and @code{0} otherwise.
@end table
@end deftp

@deftp {Data type} GpgmeValidity
The @code{GpgmeValidity} 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

@deftypefun {const char *} gpgme_key_get_string_attr (@w{GpgmeKey @var{key}}, @w{GpgmeAttr @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 occurs more than once in the key, the index is specified by
@var{idx}.  This applies to attributes of sub keys and user IDs.  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{GpgmeKey @var{key}}, @w{GpgmeAttr @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 occurs more than once in the key, the index is specified by
@var{idx}.  This applies to attributes of sub keys and user IDs.  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 keys
@subsection Manipulating keys

@deftypefun void gpgme_key_ref (@w{GpgmeKey @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{GpgmeKey @var{key}})
@deftypefunx void gpgme_key_release (@w{GpgmeKey @var{key}})
The function @code{gpgme_key_ref} 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.

The function @code{gpgme_key_release} is an alias for
@code{gpgme_key_unref}.
@end deftypefun


@node Trust Item Management
@section Trust Item Management

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

@deftp {Data type} GpgmeTrustItem
The @code{GpgmeTrustItem} type is a handle for a trust item.
@end deftp

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


@node Listing trust items
@subsection Listing trust items

@deftypefun GpgmeError gpgme_op_trustlist_start (@w{GpgmeCtx @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 trsut 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{GPGME_EOF}), or
@code{gpgme_op_trustlist_end} is called to finish the operation.

The function returns @code{GPGME_Invalid_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 GpgmeError gpgme_op_trustlist_next (@w{GpgmeCtx @var{ctx}}, @w{GpgmeTrustItem *@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{GpgmeTrustItem} objects in
@acronym{GPGME}.

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

The function returns @code{GPGME_Invalid_Value} if @var{ctx} or
@var{r_item} is not a valid pointer, @code{GPGME_No_Request} if there
is no pending operation, @code{GPGME_Out_Of_Core} if there is not
enough memory for the operation.
@end deftypefun

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

The function returns @code{GPGME_Invalid_Value} if @var{ctx} is not a
valid pointer, @code{GPGME_No_Request} if there is no pending
operation, @code{GPGME_Out_Of_Core} 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

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{GpgmeTrustItem @var{item}}, @w{GpgmeAttr @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}.  If the attribute occurs more than once in the trust
items, the index is specified by @var{idx}.  However, currently no
such attributes exists, so @var{idx} should be @code{0}.  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 int gpgme_trust_item_get_int_attr (@w{GpgmeTrustItem @var{item}}, @w{GpgmeAttr @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

@deftypefun void gpgme_trust_item_release (@w{GpgmeTrustItem @var{item}})
The function @code{gpgme_trust_item_release} destroys a
@code{GpgmeTrustItem} object and releases all associated resources.
@end deftypefun

@node Crypto Operations
@section Crypto Operations

@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

@deftypefun GpgmeError gpgme_op_decrypt (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{cipher}}, @w{GpgmeData @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 @code{GPGME_No_Error} if the ciphertext could be
decrypted successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{cipher} or @var{plain} is not a valid pointer,
@code{GPGME_No_Data} if @var{cipher} does not contain any data to
decrypt, @code{GPGME_Decryption_Failed} if @var{cipher} is not a valid
cipher text, @code{GPGME_No_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

@c @deftypefun GpgmeError gpgme_op_decrypt_start (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{cipher}}, @w{GpgmeData @var{plain}})
@c The function @code{gpgme_op_decrypt_start} initiates a
@c @code{gpgme_op_decrypt} operation.  It can be completed by calling
@c @code{gpgme_wait} on the context.

@c The function returns @code{GPGME_No_Error} if the operation could be
@c started, @code{GPGME_Invalid_Value} if @var{cipher} or @var{plain} is
@c not a valid pointer, and passes through any errors that are reported
@c by the crypto engine support routines.
@c @end deftypefun


@node Verify
@subsection Verify

@deftp {Data type} {enum GpgmeSigStat}
The @code{GpgmeSigStat} 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_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 GpgmeError gpgme_op_verify (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{sig}}, @w{GpgmeData @var{plain}}, @w{GpgmeSigStat *@var{r_stat}})
The function @code{gpgme_op_verify} verifies that the detached
signature in the data object @var{sig} is a valid signature for the
plaintext in the data object @var{plain}.

The combined status of all signatures is returned in @var{r_stat}.
The results of the individual signature verifications can be retrieved
with @code{gpgme_get_sig_status} and @code{gpgme_get_sig_key}.

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

@c GpgmeError gpgme_op_verify_start (GpgmeCtx ctx, GpgmeData sig, GpgmeData plain);

@deftypefun {const char *} gpgme_get_sig_status (@w{GpgmeCtx @var{ctx}}, @w{int @var{idx}}, @w{GpgmeSigStat *@var{r_stat}}, @w{time_t *@var{r_created}})
The function @code{gpgme_get_sig_status} receives information about a
signature after the @code{gpgme_op_verify} or
@code{gpgme_op_verify_decrypt} operation.  A single detached signature
can contain signatures by more than one key.  The @var{idx} specifies
which signature's information should be retrieved, starting from
@var{0}.

The status of the signature will be returned in @var{r_stat} if it is
not @code{NULL}.  The creation time stamp of the signature will be
returned in @var{r_created} if it is not @var{NULL}.

The function returns a statically allocated string that contains the
fingerprint of the key which signed the plaintext, or @code{NULL} if
@var{ctx} is not a valid pointer, the operation is still pending, or
no verification could be performed.
@end deftypefun

@deftypefun {const char *} gpgme_get_sig_key (@w{GpgmeCtx @var{ctx}}, @w{int @var{idx}}, @w{GpgmeSigKey *@var{r_stat}})
The function @code{gpgme_get_sig_status} receives a @code{GpgmeKey}
object for the key which was used to verify the signature after the
@code{gpgme_op_verify} or @code{gpgme_op_verify_decrypt} operation.  A
single detached signature can contain signatures by more than one key.
The @var{idx} specifies which signature's information should be
retrieved, starting from @var{0}.  The key will have on reference for
the user.

The function is a convenient way to retrieve the keys belonging to the
fingerprints returned by @code{gpgme_get_sig_status}.

The function returns @code{GPGME_No_Error} if the key could be
returned, @code{GPGME_Invalid_Value} if @var{r_key} is not a valid
pointer, @code{GPGME_Invalid_Key} if the fingerprint is not valid,
@code{GPGME_EOF} if @var{idx} is too large, or some other error value
if a problem occured requesting the key.
@end deftypefun


@node Decrypt and verify
@subsection Decrypt and verify

@deftypefun GpgmeError gpgme_op_decrypt_verify (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{cipher}}, @w{GpgmeData @var{plain}}, @w{GpgmeSigStat *@var{r_stat}})
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 and their combined status will be returned in @var{r_stat}.

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

The function returns @code{GPGME_No_Error} if the ciphertext could be
decrypted successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{cipher}, @var{plain} or @var{r_stat} is not a valid pointer,
@code{GPGME_No_Data} if @var{cipher} does not contain any data to
decrypt, @code{GPGME_Decryption_Failed} if @var{cipher} is not a valid
cipher text, @code{GPGME_No_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

@c GpgmeError gpgme_op_decrypt_verify (GpgmeCtx c, GpgmeData in, GpgmeData out, GpgmeSigStat *r_status);


@node Sign
@subsection Sign

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

@deftypefun void gpgme_signers_clear (@w{GpgmeCtx @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 GpgmeError gpgme_signers_add (@w{GpgmeCtx @var{ctx}}, @w{const GpgmeKey @var{key}})
The function @code{gpgme_signers_add} adds the key @var{key} to the
list of signers in the context @var{ctx}.

One reference for the key is consumed.
@end deftypefun

@deftypefun GpgmeKey gpgme_signers_enum (@w{const GpgmeCtx @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 GpgmeSigMode}
The @code{GpgmeSigMode} 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 GpgmeError gpgme_op_sign (@w{GpgmeCtx @var{ctx}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{sig}}, @w{GpgmeSigMode @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} and text mode attributes set for the context @var{ctx}
and the requested signature mode @var{mode}.

The function returns @code{GPGME_No_Error} if the signature could be
created successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{plain} or @var{sig} is not a valid pointer, @code{GPGME_No_Data}
if the signature could not be created, @code{GPGME_No_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


@node Encrypt
@subsection Encrypt

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
* Selecting recipients::          How to choose the recipients.
* Encrypting a plaintext::        How to encrypt a plaintext.
@end menu


@node Selecting recipients
@subsubsection Selecting recipients

@deftp {Data type} GpgmeRecipients
The @code{GpgmeRecipients} type is a handle for a set of recipients
that can be used in an encryption process.
@end deftp

@deftypefun GpgmeError gpgme_recipients_new (@w{GpgmeRecipients *@var{r_rset}})
The function @code{gpgme_recipients_new} creates a new, empty set of
recipients and returns a handle for it in @var{r_rset}.

The function returns @code{GPGME_No_Error} if the recipient set could
be created successfully, and @code{GPGME_Out_Of_Core} if not enough
memory was available.
@end deftypefun

@deftypefun void gpgme_recipients_release (@w{GpgmeRecipients @var{rset}})
The function @code{gpgme_recipients_release} destroys the set of
recipients @var{rset} and releases all associated resources.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_add_name (@w{GpgmeRecipients @var{rset}}, @w{const char *@var{name}})
The function @code{gpgme_recipients_add_name} adds the recipient
@var{name} to the set of recipients @var{rset}.  This is equivalent to
@code{gpgme_recipients_add_name_with_validity} with a validity of
@code{GPGME_VALIDITY_UNKNOWN}.

The function returns @code{GPGME_No_Error} if the recipient was added
successfully, @code{GPGME_Invalid_Value} if @var{rset} or @var{name}
is not a valid pointer, and @code{GPGME_Out_Of_Core} if not enough
memory is available.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_add_name_with_validity (@w{GpgmeRecipients @var{rset}}, @w{const char *@var{name}}, @w{GpgmeValidity @var{val}})
The function @code{gpgme_recipients_add_name_with_validity} adds the
recipient @var{name} with the validity @var{val} to the set of
recipients @var{rset}.  If the validity is not known, the function
@code{gpgme_recipients_add_name} can be used.
@xref{Information about keys}, for the possible values for @var{val}.

The function returns @code{GPGME_No_Error} if the recipient was added
successfully, @code{GPGME_Invalid_Value} if @var{rset} or @var{name}
is not a valid pointer, and @code{GPGME_Out_Of_Core} if not enough
memory is available.
@end deftypefun

@deftypefun {unsigned int} gpgme_recipients_count (@w{const @var{GpgmeRecipients rset}})
The function @code{gpgme_recipients_count} returns the number of
recipients in the set @var{rset}.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_enum_open (@w{const GpgmeRecipients @var{rset}}, @w{void **@var{iter}})
The function @code{gpgme_recipients_enum_open} creates a new iterator
@var{iter} that can be used to walk through the set of recipients in
@var{rset}, using @code{gpgme_recipients_enum_read}.

If the iterator is not needed anymore, it can be closed with
@code{gpgme_recipients_enum_close}.

The function returns @code{GPGME_No_Error} if the enumerator was
successfully created and @code{GPGME_Invalid_Value} if @var{rset} or
@var{iter} is not a valid pointer.
@end deftypefun

@deftypefun {const char *} gpgme_recipients_enum_read (@w{const GpgmeRecipients @var{rset}}, @w{void **@var{iter}})
The function @code{gpgme_recipients_enum_read} returns a string
containing the name of the next recipient in the set @var{rset} for
the iterator @var{iter}.  The string is valid as long as @var{rset} is
valid or the function is called the next time with the same recipient
set and iterator, whatever is earlier.
@end deftypefun

@deftypefun GpgmeError gpgme_recipients_enum_close (@w{const GpgmeRecipients @var{rset}}, @w{void **@var{iter}})
The function @code{gpgme_recipients_enum_close} releases the iterator
@var{iter} for the recipient set @var{rset}.
@end deftypefun


@node Encrypting a plaintext
@subsubsection Encrypting a plaintext

@deftypefun GpgmeError gpgme_op_encrypt (@w{GpgmeCtx @var{ctx}}, @w{GpgmeRecipients @var{rset}}, @w{GpgmeData @var{plain}}, @w{GpgmeData @var{cipher}})
The function @code{gpgme_op_crypt} encrypts the plaintext in the data
object @var{plain} for the recipients @var{rset} and stores the
ciphertext in the data object @var{cipher}.  The type of the
ciphertext created is determined by the @acronym{ASCII} and text mode
attributes set for the context @var{ctx}.

The function returns @code{GPGME_No_Error} if the ciphertext could be
created successfully, @code{GPGME_Invalid_Value} if @var{ctx},
@var{rset}, @var{plain} or @var{cipher} is not a valid pointer,
@code{GPGME_No_Recipient} if @var{rset} does not contain any
valid recipients, @code{GPGME_No_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


@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