gpgme/doc/uiserver.texi

@c uiserver.texi                    -*- mode: texinfo; coding: latin-1; -*-
@c Specification of the UI server protocol.
@c To be included by gpgme.texi

@node UI Server Protocol
@appendix The GnuPG UI Server Protocol
@cindex UI server
@cindex user interface server


This section specifies the protocol used between clients and a User
Interface Server (UI server).  This protocol helps to build a system
where all cryptographic operations are done by a server and the server
is responsible for all dialogs.  Although @acronym{GPGME} has no direct
support for this protocol it is believed that servers will utilize the
@acronym{GPGME} library; thus having the specification included in this
manual is an appropriate choice.  This protocol should be referenced as
`The GnuPG UI Server Protocol'.

@noindent
A server needs to implement these commands:@footnote{In all examples we
assume that the connection has already been established; see the Assuan
manual for details.}

@menu
* UI Server Encrypt::                Encrypt a message.
* UI Server Sign::                   Sign a message.
* UI Server Decrypt::                Decrypt a message.
* UI Server Verify::                 Verify a message.
* UI Server Set Input Files::        Specifying the input files to operate on.
* UI Server Sign/Encrypt Files::     Encrypting and signing files.
* UI Server Verify/Decrypt Files::   Decrypting and verifying files.
* UI Server Import/Export Keys::     Managing certificates.
* UI Server Checksum Files::         Create and verify checksums for files.
* Miscellaneous UI Server Commands::   Commands not related to a specific operation.
@end menu



@node UI Server Encrypt
@section UI Server: Encrypt a Message

Before encryption can be done the recipients must be set using the
command:

@deffn Command RECIPIENT @var{string}

Set the recipient for the encryption.  @var{string} is an RFC-2822
recipient name ("mailbox" as per section 3.4).  This command may or may
not check the recipient for validity right away; if it does not all
recipients are expected to be checked at the time of the @code{ENCRYPT}
command.  All @code{RECIPIENT} commands are cumulative until a
successful @code{ENCRYPT} command or until a @code{RESET} command.
Linefeeds are obviously not allowed in @var{string} and should be folded
into spaces (which are equivalent).
@end deffn

@noindent
To tell the server the source and destination of the data, the next two
commands are to be used:

@deffn Command INPUT FD=@var{n}
Set the file descriptor for the message to be encrypted to @var{n}.  The
message send to the server is binary encoded.

GpgOL is a Windows only program, thus @var{n} is not a libc file
descriptor but a regular system handle.  Given that the Assuan
connection works over a socket, it is not possible to use regular
inheritance to make the file descriptor available to the server.
Thus @code{DuplicateHandle} needs to be used to duplicate a handle
to the server process.  This is the reason that the server needs to
implement the @code{GETINFO pid} command.  Sending this command a second
time replaces the file descriptor set by the last one.
@c If @var{n} is not given, this commands uses the
@c %last file descriptor passed to the application.
@c %@xref{fun-assuan_sendfd, ,the assuan_sendfd function,assuan,the
@c %Libassuan manual}, on how to do descriptor passing.
@end deffn

@deffn Command OUTPUT FD=@var{n} [--binary]
Set the file descriptor to be used for the output (i.e. the encrypted
message) to @var{n}.  If the option @code{--binary} is given the
output shall be in binary format; if not given, the output for OpenPGP
needs to be ASCII armored and for CMS Base-64 encoded.  For details on
the file descriptor, see the @code{INPUT} command.
@end deffn

@noindent
The setting of the recipients, the data source and destination may
happen in any order, even intermixed.  If this has been done the actual
encryption operation is called using:

@deffn Command ENCRYPT -@w{}-protocol=@var{name}

This command reads the plaintext from the file descriptor set by the
@code{INPUT} command, encrypts it and writes the ciphertext to the file
descriptor set by the @code{OUTPUT} command.  The server may (and
should) overlap reading and writing.  The recipients used for the
encryption are all the recipients set so far.  If any recipient is not
usable the server should take appropriate measures to notify the user
about the problem and may cancel the operation by returning an error
code.  The used file descriptors are void after this command; the
recipient list is only cleared if the server returns success.

@noindent
Because GpgOL uses a streaming mode of operation the server is not
allowed to auto select the protocol and must obey to the mandatory
@var{protocol} parameter:

@table @code
@item OpenPGP
Use the OpenPGP protocol (RFC-2440).
@item CMS
Use the CMS (PKCS#7) protocol (RFC-3852).
@end table

@end deffn

To support automagically selection of the protocol depending on the
selected keys, the server MAY implement the command:

@deffn Command PREP_ENCRYPT [-@w{}-protocol=@var{name}] [-@w{}-expect-sign]

This commands considers all recipients set so far and decides whether it
is able to take input and start the actual encryption.  This is kind of
a dry-run @command{ENCRYPT} without requiring or using the input and
output file descriptors.  The server shall cache the result of any user
selection to avoid asking this again when the actual @command{ENCRYPT}
command is send.  The @option{--protocol} option is optional; if it is
not given, the server should allow the user to select the protocol to be
used based on the recipients given or by any other means.

If @option{--expect-sign} is given the server should expect that the
message will also be signed and use this hint to present a unified
recipient and signer selection dialog if possible and desired.  A
selected signer should then be cached for the expected SIGN command
(which is expected in the same session but possible on another
connection).

If this command is given again before a successful @command{ENCRYPT}
command, the second one takes effect.

Before sending the OK response the server shall tell the client the
protocol to be used (either the one given by the argument or the one
selected by the user) by means of a status line:
@end deffn

@deffn {Status line} PROTOCOL @var{name}
Advise the client to use the protocol @var{name} for the
@command{ENCRYPT} command.  The valid protocol names are listed under
the description of the @command{ENCRYPT} command.  The server shall emit
exactly one PROTOCOL status line.
@end deffn

@noindent
Here is an example of a complete encryption sequence; client lines are
indicated by a @sc{c:}, server responses by @sc{c:}:

@smallexample
@group
  @clnt{RESET}
  @srvr{OK}
  @clnt{RECIPIENT foo@@example.net}
  @srvr{OK}
  @clnt{RECIPIENT bar@@example.com}
  @srvr{OK}
  @clnt{PREP_ENCRYPT}
  @srvr{S PROTOCOL OpenPGP}
  @srvr{OK}
  @clnt{INPUT FD=17}
  @srvr{OK}
  @clnt{OUTPUT FD=18}
  @srvr{OK}
  @clnt{ENCRYPT}
  @srvr{OK}
@end group
@end smallexample



@node UI Server Sign
@section UI Server: Sign a Message

The server needs to implement opaque signing as well as detached
signing.  Due to the nature of OpenPGP messages it is always required to
send the entire message to the server; sending just the hash is not
possible.  The following two commands are required to set the input and
output file descriptors:

@deffn Command INPUT FD=@var{n}
Set the file descriptor for the message to be signed to @var{n}.  The
message send to the server is binary encoded.  For details on the file
descriptor, see the description of @code{INPUT} in the @code{ENCRYPT}
section.
@end deffn

@deffn Command OUTPUT FD=@var{n} [--binary]
Set the file descriptor to be used for the output.  The output is
either the complete signed message or in case of a detached signature
just that detached signature.  If the option @code{--binary} is given
the output shall be in binary format; if not given, the output for
OpenPGP needs to be ASCII armored and for CMS Base-64 encoded.  For
details on the file descriptor, see the @code{INPUT} command.
@end deffn

@noindent
To allow the server the selection of a non-default signing key the
client may optionally use the @code{SENDER} command, see @ref{command
SENDER}.

@noindent
The signing operation is then initiated by:

@deffn Command SIGN -@w{}-protocol=@var{name} [-@w{}-detached]
Sign the data set with the @code{INPUT} command and write it to the sink
set by OUTPUT.  @var{name} is the signing protocol used for the
message. For a description of the allowed protocols see the
@code{ENCRYPT} command.  With option @code{--detached} given, a detached
signature is created; this is actually the usual way the command is
used.
@end deffn

@noindent
The client expects the server to send at least this status information
before the final OK response:

@deffn {Status line} MICALG @var{string}
The @var{string} represents the hash algorithm used to create the
signature. It is used with RFC-1847 style signature messages and defined by
PGP/MIME (RFC-3156) and S/MIME (RFC-3851).  The GPGME library has a
supporting function @code{gpgme_hash_algo_name} to return the algorithm
name as a string.  This string needs to be lowercased and for OpenPGP
prefixed with "@code{pgp-}".
@end deffn



@node UI Server Decrypt
@section UI Server: Decrypt a Message

Decryption may include the verification of OpenPGP messages.  This is
due to the often used combined signing/encryption modus of OpenPGP.  The
client may pass an option to the server to inhibit the signature
verification.  The following two commands are required to set the input
and output file descriptors:

@deffn Command INPUT FD=@var{n}
Set the file descriptor for the message to be decrypted to @var{n}.  The
message send to the server is either binary encoded or --- in the case
of OpenPGP --- ASCII armored.  For details on the file descriptor, see
the description of @code{INPUT} in the @code{ENCRYPT} section.
@end deffn

@deffn Command OUTPUT FD=@var{n}
Set the file descriptor to be used for the output. The output is binary
encoded. For details on the file descriptor, see the description of
@code{INPUT} in the @code{ENCRYPT} section.
@end deffn

@noindent
The decryption is started with the command:

@deffn Command DECRYPT -@w{}-protocol=@var{name} [-@w{}-no-verify] [-@w{}-export-session-key]
@var{name} is the encryption protocol used for the message. For a
description of the allowed protocols see the @code{ENCRYPT} command.
This argument is mandatory.  If the option @option{--no-verify} is
given, the server should not try to verify a signature, in case the
input data is an OpenPGP combined message. If the option
@option{--export-session-key} is given and the underlying engine knows
how to export the session key, it will appear on a status line
@end deffn


@node UI Server Verify
@section UI Server: Verify a Message

The server needs to support the verification of opaque signatures as
well as detached signatures.  The kind of input sources controls what
kind message is to be verified.

@deffn Command MESSAGE FD=@var{n}
This command is used with detached signatures to set the file descriptor
for the signed data to @var{n}. The data is binary encoded (used
verbatim).  For details on the file descriptor, see the description of
@code{INPUT} in the @code{ENCRYPT} section.
@end deffn

@deffn Command INPUT FD=@var{n}
Set the file descriptor for the opaque message or the signature part of
a detached signature to @var{n}.  The message send to the server is
either binary encoded or -- in the case of OpenPGP -- ASCII armored.
For details on the file descriptor, see the description of @code{INPUT}
in the @code{ENCRYPT} section.
@end deffn

@deffn Command OUTPUT FD=@var{n}
Set the file descriptor to be used for the output. The output is binary
encoded and only used for opaque signatures.  For details on the file
descriptor, see the description of @code{INPUT} in the @code{ENCRYPT}
section.
@end deffn

@noindent
The verification is then started using:

@deffn Command VERIFY -@w{}-protocol=@var{name} [-@w{}-silent]
@var{name} is the signing protocol used for the message. For a
description of the allowed protocols see the @code{ENCRYPT} command.
This argument is mandatory.  Depending on the combination of
@code{MESSAGE} @code{INPUT} and @code{OUTPUT} commands, the server needs
to select the appropriate verification mode:

@table @asis
@item MESSAGE and INPUT
This indicates a detached signature.  Output data is not applicable.
@item INPUT
This indicates an opaque signature.  As no output command has been given,
the server is only required to check the signature.
@item INPUT and OUTPUT
This indicates an opaque signature.  The server shall write the signed
data to the file descriptor set by the output command.  This data shall
even be written if the signatures can't be verified.
@end table
@end deffn

With @option{--silent} the server shall not display any dialog; this is
for example used by the client to get the content of opaque signed
messages. The client expects the server to send at least this status
information before the final OK response:

@deffn {Status line} SIGSTATUS @var{flag} @var{displaystring}
Returns the status for the signature and a short string explaining the
status.  Valid values for @var{flag} are:

@table @code
@item none
The message has a signature but it could not not be verified due to a
missing key.
@item green
The signature is fully valid.
@item yellow
The signature is valid but additional information was shown regarding the
validity of the key.
@item red
The signature is not valid.
@end table

@var{displaystring} is a percent-and-plus-encoded string with a short
human readable description of the status.  For example

@smallexample
S SIGSTATUS green Good+signature+from+Keith+Moon+<keith@@example.net>
@end smallexample

Note that this string needs to fit into an Assuan line and should be
short enough to be displayed as short one-liner on the clients window.
As usual the encoding of this string is UTF-8 and it should be send in
its translated form.

The server shall send one status line for every signature found on the
message.


@end deffn


@node UI Server Set Input Files
@section UI Server: Specifying the input files to operate on.

All file related UI server commands operate on a number of input files
or directories, specified by one or more @code{FILE} commands:

@deffn Command FILE [--clear] @var{name}
Add the file or directory @var{name} to the list of pathnames to be
processed by the server.  The parameter @var{name} must be an absolute
path name (including the drive letter) and is percent espaced (in
particular, the characters %, = and white space characters are always
escaped).  If the option @code{--clear} is given, the list of files is
cleared before adding @var{name}.

Historical note: The original spec did not define @code{--clear} but
the keyword @code{--continued} after the file name to indicate that
more files are to be expected.  However, this has never been used and
thus removed from the specs.
@end deffn


@node UI Server Sign/Encrypt Files
@section UI Server: Encrypting and signing files.

First, the input files need to be specified by one or more
@code{FILE} commands.  Afterwards, the actual operation is requested:

@deffn Command ENCRYPT_FILES --nohup
@deffnx Command SIGN_FILES --nohup
@deffnx Command ENCRYPT_SIGN_FILES --nohup
Request that the files specified by @code{FILE} are encrypted and/or
signed.  The command selects the default action.  The UI server may
allow the user to change this default afterwards interactively, and
even abort the operation or complete it only on some of the selected
files and directories.

What it means to encrypt or sign a file or directory is specific to
the preferences of the user, the functionality the UI server provides,
and the selected protocol.  Typically, for each input file a new file
is created under the original filename plus a protocol specific
extension (like @code{.gpg} or @code{.sig}), which contain the
encrypted/signed file or a detached signature.  For directories, the
server may offer multiple options to the user (for example ignore or
process recursively).

The @code{ENCRYPT_SIGN_FILES} command requests a combined sign and
encrypt operation.  It may not be available for all protocols (for
example, it is available for OpenPGP but not for CMS).

The option @code{--nohup} is mandatory.  It is currently unspecified
what should happen if @code{--nohup} is not present.  Because
@code{--nohup} is present, the server always returns @code{OK}
promptly, and completes the operation asynchronously.
@end deffn


@node UI Server Verify/Decrypt Files
@section UI Server: Decrypting and verifying files.

First, the input files need to be specified by one or more
@code{FILE} commands.  Afterwards, the actual operation is requested:

@deffn Command DECRYPT_FILES --nohup
@deffnx Command VERIFY_FILES --nohup
@deffnx Command DECRYPT_VERIFY_FILES --nohup
Request that the files specified by @code{FILE} are decrypted and/or
verified.  The command selects the default action.  The UI server may
allow the user to change this default afterwards interactively, and
even abort the operation or complete it only on some of the selected
files and directories.

What it means to decrypt or verify a file or directory is specific to
the preferences of the user, the functionality the UI server provides,
and the selected protocol.  Typically, for decryption, a new file is
created for each input file under the original filename minus a
protocol specific extension (like @code{.gpg}) which contains the
original plaintext.  For verification a status is displayed for each
signed input file, indicating if it is signed, and if yes, if the
signature is valid.  For files that are signed and encrypted, the
@code{VERIFY} command transiently decrypts the file to verify the
enclosed signature.  For directories, the server may offer multiple
options to the user (for example ignore or process recursively).

The option @code{--nohup} is mandatory.  It is currently unspecified
what should happen if @code{--nohup} is not present.  Because
@code{--nohup} is present, the server always returns @code{OK}
promptly, and completes the operation asynchronously.
@end deffn


@node UI Server Import/Export Keys
@section UI Server: Managing certificates.

First, the input files need to be specified by one or more
@code{FILE} commands.  Afterwards, the actual operation is requested:

@deffn Command IMPORT_FILES --nohup
Request that the certificates contained in the files specified by
@code{FILE} are imported into the local certificate databases.

For directories, the server may offer multiple options to the user
(for example ignore or process recursively).

The option @code{--nohup} is mandatory.  It is currently unspecified
what should happen if @code{--nohup} is not present.  Because
@code{--nohup} is present, the server always returns @code{OK}
promptly, and completes the operation asynchronously.
@end deffn

FIXME: It may be nice to support an @code{EXPORT} command as well,
which is enabled by the context menu of the background of a directory.


@node UI Server Checksum Files
@section UI Server: Create and verify checksums for files.

First, the input files need to be specified by one or more
@code{FILE} commands.  Afterwards, the actual operation is requested:

@deffn Command CHECKSUM_CREATE_FILES --nohup
Request that checksums are created for the files specified by
@code{FILE}.  The choice of checksum algorithm and the destination
storage and format for the created checksums depend on the preferences
of the user and the functionality provided by the UI server.  For
directories, the server may offer multiple options to the user (for
example ignore or process recursively).

The option @code{--nohup} is mandatory.  It is currently unspecified
what should happen if @code{--nohup} is not present.  Because
@code{--nohup} is present, the server always returns @code{OK}
promptly, and completes the operation asynchronously.
@end deffn


@deffn Command CHECKSUM_VERIFY_FILES --nohup
Request that checksums are created for the files specified by
@code{FILE} and verified against previously created and stored
checksums.  The choice of checksum algorithm and the source storage
and format for previously created checksums depend on the preferences
of the user and the functionality provided by the UI server.  For
directories, the server may offer multiple options to the user (for
example ignore or process recursively).

If the source storage of previously created checksums is available to
the user through the Windows shell, this command may also accept such
checksum files as @code{FILE} arguments.  In this case, the UI server
should instead verify the checksum of the referenced files as if they
were given as INPUT files.

The option @code{--nohup} is mandatory.  It is currently unspecified
what should happen if @code{--nohup} is not present.  Because
@code{--nohup} is present, the server always returns @code{OK}
promptly, and completes the operation asynchronously.
@end deffn




@c
@c M I S C E L L A N E O U S  C O M M A N D S
@c
@node Miscellaneous UI Server Commands
@section Miscellaneous UI Server Commands

The server needs to implement the following commands which are not
related to a specific command:

@deffn Command GETINFO @var{what}
This is a multi purpose command, commonly used to return a variety of
information.  The required subcommands as described by the @var{what}
parameter are:

@table @code
@item pid
Return the process id of the server in decimal notation using an Assuan
data line.
@end table
@end deffn


@noindent
To allow the server to pop up the windows in the correct relation to the
client, the client is advised to tell the server by sending the option:

@deffn {Command option} window-id @var{number}
The @var{number} represents the native window ID of the clients current
window.  On Windows systems this is a windows handle (@code{HWND}) and
on X11 systems it is the @code{X Window ID}.  The number needs to be
given as a hexadecimal value so that it is easier to convey pointer
values (e.g. @code{HWND}).
@end deffn


@noindent
A client may want to fire up the certificate manager of the server.  To
do this it uses the Assuan command:

@deffn Command START_KEYMANAGER
The server shall pop up the main window of the key manager (aka
certificate manager).  The client expects that the key manager is brought
into the foregound and that this command immediately returns (does not
wait until the key manager has been fully brought up).
@end deffn

@noindent
A client may want to fire up the configuration dialog of the server.  To
do this it uses the Assuan command:

@deffn Command START_CONFDIALOG
The server shall pop up its configuration dialog.  The client expects
that this dialog is brought into the foregound and that this command
immediately returns (i.e. it does not wait until the dialog has been
fully brought up).
@end deffn

@anchor{command SENDER}
@noindent
When doing an operation on a mail, it is useful to let the server know
the address of the sender:

@deffn Command SENDER [-@w{}-info] [-@w{}-protocol=@var{name}] @var{email}
@var{email} is the plain ASCII encoded address ("addr-spec" as per
RFC-2822) enclosed in angle brackets.  The address set with this command
is valid until a successful completion of the operation or until a
@code{RESET} command.  A second command overrides the effect of the
first one; if @var{email} is not given and @option{--info} is not used,
the server shall use the default signing key.

If option @option{--info} is not given, the server shall also suggest a
protocol to use for signing.  The client may use this suggested protocol
on its own discretion.  The same status line as with PREP_ENCRYPT is
used for this.

The option @option{--protocol} may be used to give the server a hint on
which signing protocol should be preferred.
@end deffn

@noindent
To allow the UI-server to visually identify a running operation or to
associate operations the server MAY support the command:

@deffn Command SESSION @var{number} [@var{string}]
The @var{number} is an arbitrary value, a server may use to associate
simultaneous running sessions.  It is a 32 bit unsigned integer with
@code{0} as a special value indicating that no session association shall
be done.

If @var{string} is given, the server may use this as the title of a
window or, in the case of an email operation, to extract the sender's
address. The string may contain spaces; thus no plus-escaping is used.

This command may be used at any time and overrides the effect of the
last command.  A @code{RESET} undoes the effect of this command.

@end deffn