emacs/doc/misc/pgg.texi

\input texinfo                  @c -*-texinfo-*-

@setfilename ../../info/pgg.info

@set VERSION 0.1
@settitle PGG @value{VERSION}
@include docstyle.texi

@copying
This file describes PGG @value{VERSION}, an Emacs interface to various
PGP implementations.

Copyright @copyright{} 2001, 2003--2024 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.  A copy of the license
is included in the section entitled ``GNU Free Documentation License.''

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@dircategory Emacs network features
@direntry
* PGG: (pgg).                   An obsolete Emacs interface to various
                                  PGP implementations.
@end direntry

@titlepage
@ifset WEBHACKDEVEL
@title PGG (DEVELOPMENT VERSION)
@end ifset
@ifclear WEBHACKDEVEL
@title PGG
@end ifclear

@author by Daiki Ueno
@page

@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@node Top
@top PGG

PGG is an interface library between Emacs
and various tools for secure communication.  PGG also provides a simple
user interface to encrypt, decrypt, sign, and verify MIME messages.
This package is obsolete; for new code we recommend EasyPG instead.
@xref{Top,, EasyPG, epa, EasyPG Assistant User's Manual}.

@ifnottex
@insertcopying
@end ifnottex

@menu
* Overview::                    What PGG is.
* Prerequisites::               Complicated stuff you may have to do.
* How to use::                  Getting started quickly.
* Architecture::
* Parsing OpenPGP packets::
* GNU Free Documentation License:: The license for this documentation.
* Function Index::
* Variable Index::
@end menu

@node Overview
@chapter Overview

PGG is an interface library between Emacs and various tools for secure
communication.  Even though Mailcrypt has similar feature, it does not
deal with detached PGP messages, normally used in PGP/MIME
infrastructure.  This was the main reason why I wrote the new library.

Note that the PGG library is now obsolete, replaced by EasyPG@.
@xref{Top,, EasyPG, epa, EasyPG Assistant User's Manual}.

PGP/MIME is an application of MIME Object Security Services (RFC1848).
The standard is documented in RFC2015.

@node Prerequisites
@chapter Prerequisites

PGG requires at least one implementation of privacy guard system.
This document assumes that you have already obtained and installed them
and that you are familiar with its basic functions.

By default, PGG uses GnuPG@.  If you are new to such a system, I
recommend that you should look over the GNU Privacy Handbook (GPH)
which is available at @uref{https://www.gnupg.org/documentation/}.

When using GnuPG, we recommend the use of the @code{gpg-agent}
program, which is distributed with versions 2.0 and later of GnuPG@.
This is a daemon to manage private keys independently from any
protocol, and provides the most secure way to input and cache your
passphrases (@pxref{Caching passphrase}).  By default, PGG will
attempt to use @code{gpg-agent} if it is running.  @xref{Invoking
GPG-AGENT,,,gnupg,Using the GNU Privacy Guard}.

PGG also supports Pretty Good Privacy version 2 or version 5.

@node How to use
@chapter How to use

The toplevel interface of this library is quite simple, and only
intended to use with public-key cryptographic operation.

To use PGG, evaluate following expression at the beginning of your
application program.

@lisp
(require 'pgg)
@end lisp

If you want to check existence of pgg.el at runtime, instead you can
list autoload setting for desired functions as follows.

@lisp
(autoload 'pgg-encrypt-region "pgg"
  "Encrypt the current region." t)
(autoload 'pgg-encrypt-symmetric-region "pgg"
  "Encrypt the current region with symmetric algorithm." t)
(autoload 'pgg-decrypt-region "pgg"
  "Decrypt the current region." t)
(autoload 'pgg-sign-region "pgg"
  "Sign the current region." t)
(autoload 'pgg-verify-region "pgg"
  "Verify the current region." t)
(autoload 'pgg-insert-key "pgg"
  "Insert the ASCII armored public key." t)
(autoload 'pgg-snarf-keys-region "pgg"
  "Import public keys in the current region." t)
@end lisp

@menu
* User Commands::
* Selecting an implementation::
* Caching passphrase::
* Default user identity::
@end menu

@node User Commands
@section User Commands

At this time you can use some cryptographic commands.  The behavior of
these commands relies on a fashion of invocation because they are also
intended to be used as library functions.  In case you don't have the
signer's public key, for example, the function @code{pgg-verify-region}
fails immediately, but if the function had been called interactively, it
would ask you to retrieve the signer's public key from the server.

@deffn Command pgg-encrypt-region start end recipients &optional sign passphrase
Encrypt the current region between @var{start} and @var{end} for
@var{recipients}.  When the function were called interactively, you
would be asked about the recipients.

If encryption is successful, it replaces the current region contents (in
the accessible portion) with the resulting data.

If optional argument @var{sign} is non-@code{nil}, the function is
request to do a combined sign and encrypt.  This currently is
confirmed to work with GnuPG, but might not work with PGP or PGP5.

If optional @var{passphrase} is @code{nil}, the passphrase will be
obtained from the passphrase cache or user.
@end deffn

@deffn Command pgg-encrypt-symmetric-region &optional start end passphrase
Encrypt the current region between @var{start} and @var{end} using a
symmetric cipher.  After invocation you are asked for a passphrase.

If optional @var{passphrase} is @code{nil}, the passphrase will be
obtained from the passphrase cache or user.

symmetric-cipher encryption is currently only implemented for GnuPG.
@end deffn

@deffn Command pgg-decrypt-region start end &optional passphrase
Decrypt the current region between @var{start} and @var{end}.  If
decryption is successful, it replaces the current region contents (in
the accessible portion) with the resulting data.

If optional @var{passphrase} is @code{nil}, the passphrase will be
obtained from the passphrase cache or user.
@end deffn

@deffn Command pgg-sign-region start end &optional cleartext passphrase
Make the signature from text between @var{start} and @var{end}.  If the
optional third argument @var{cleartext} is non-@code{nil}, or the
function is called interactively, it does not create a detached
signature.  In such a case, it replaces the current region contents (in
the accessible portion) with the resulting data.

If optional @var{passphrase} is @code{nil}, the passphrase will be
obtained from the passphrase cache or user.
@end deffn

@deffn Command pgg-verify-region start end &optional signature fetch
Verify the current region between @var{start} and @var{end}.  If the
optional third argument @var{signature} is non-@code{nil}, it is treated
as the detached signature file of the current region.

If the optional 4th argument @var{fetch} is non-@code{nil}, or the
function is called interactively, we attempt to fetch the signer's
public key from the key server.
@end deffn

@deffn Command pgg-insert-key
Retrieve the user's public key and insert it as ASCII-armored format.
@end deffn

@deffn Command pgg-snarf-keys-region start end
Collect public keys in the current region between @var{start} and
@var{end}, and add them into the user's keyring.
@end deffn

@node Selecting an implementation
@section Selecting an implementation

Since PGP has a long history and there are a number of PGP
implementations available today, the function which each one has differs
considerably.  For example, if you are using GnuPG, you know you can
select cipher algorithm from 3DES, CAST5, BLOWFISH, and so on, but on
the other hand the version 2 of PGP only supports IDEA.

Which implementation is used is controlled by the @code{pgg-scheme}
variable.  If it is @code{nil} (the default), the value of the
@code{pgg-default-scheme} variable will be used instead.

@defvar pgg-scheme
Force specify the scheme of PGP implementation.  The value can be set to
@code{gpg}, @code{pgp}, and @code{pgp5}.  The default is @code{nil}.
@end defvar

@defvar pgg-default-scheme
The default scheme of PGP implementation.  The value should be one of
@code{gpg}, @code{pgp}, and @code{pgp5}.  The default is @code{gpg}.
@end defvar

@node Caching passphrase
@section Caching passphrase

When using GnuPG (gpg) as the PGP scheme, we recommend using a program
called @code{gpg-agent} for entering and caching
passphrases@footnote{Actually, @code{gpg-agent} does not cache
passphrases but private keys.  On the other hand, from a user's point
of view, this technical difference isn't visible.}.

@defvar pgg-gpg-use-agent
If non-@code{nil}, attempt to use @code{gpg-agent} whenever possible.
The default is @code{t}.  If @code{gpg-agent} is not running, or GnuPG
is not the current PGP scheme, PGG's own passphrase-caching mechanism
is used (see below).
@end defvar

To use @code{gpg-agent} with PGG, you must first ensure that
@code{gpg-agent} is running.  For example, if you are running in the X
Window System, you can do this by putting the following line in your
@file{.xsession} file:

@smallexample
eval "$(gpg-agent --daemon)"
@end smallexample

For more details on invoking @code{gpg-agent}, @xref{Invoking
GPG-AGENT,,,gnupg,Using the GNU Privacy Guard}.

Whenever you perform a PGG operation that requires a GnuPG passphrase,
GnuPG will contact @code{gpg-agent}, which prompts you for the
passphrase.  Furthermore, @code{gpg-agent} ``caches'' the result, so
that subsequent uses will not require you to enter the passphrase
again.  (This cache usually expires after a certain time has passed;
you can change this using the @code{--default-cache-ttl} option when
invoking @code{gpg-agent}.)

If you are running in a X Window System environment, @code{gpg-agent}
prompts for a passphrase by opening a graphical window.  However, if
you are running Emacs on a text terminal, @code{gpg-agent} has trouble
receiving input from the terminal, since it is being sent to Emacs.
One workaround for this problem is to run @code{gpg-agent} on a
different terminal from Emacs, with the @code{--keep-tty} option; this
tells @code{gpg-agent} use its own terminal to prompt for passphrases.

When @code{gpg-agent} is not being used, PGG prompts for a passphrase
through Emacs.  It also has its own passphrase caching mechanism,
which is controlled by the variable @code{pgg-cache-passphrase} (see
below).

There is a security risk in handling passphrases through PGG rather
than @code{gpg-agent}.  When you enter your passphrase into an Emacs
prompt, it is temporarily stored as a cleartext string in the memory
of the Emacs executable.  If the executable memory is swapped to disk,
the root user can, in theory, extract the passphrase from the
swapfile.  Furthermore, the swapfile containing the cleartext
passphrase might remain on the disk after the system is discarded or
stolen.  @code{gpg-agent} avoids this problem by using certain tricks,
such as memory locking, which have not been implemented in Emacs.

@defvar pgg-cache-passphrase
If non-@code{nil}, store passphrases.  The default value of this
variable is @code{t}.  If you are worried about security issues,
however, you could stop the caching of passphrases by setting this
variable to @code{nil}.
@end defvar

@defvar pgg-passphrase-cache-expiry
Elapsed time for expiration in seconds.
@end defvar

If your passphrase contains non-ASCII characters, you might need to
specify the coding system to be used to encode your passphrases, since
GnuPG treats them as a byte sequence, not as a character sequence.

@defvar pgg-passphrase-coding-system
Coding system used to encode passphrase.
@end defvar

@node Default user identity
@section Default user identity

The PGP implementation is usually able to select the proper key to use
for signing and decryption, but if you have more than one key, you may
need to specify the key id to use.

@defvar pgg-default-user-id
User ID of your default identity.  It defaults to the value returned
by @samp{(user-login-name)}.  You can customize this variable.
@end defvar

@defvar pgg-gpg-user-id
User ID of the GnuPG default identity.  It defaults to @samp{nil}.
This overrides @samp{pgg-default-user-id}.  You can customize this
variable.
@end defvar

@defvar pgg-pgp-user-id
User ID of the PGP 2.x/6.x default identity.  It defaults to
@samp{nil}.  This overrides @samp{pgg-default-user-id}.  You can
customize this variable.
@end defvar

@defvar pgg-pgp5-user-id
User ID of the PGP 5.x default identity.  It defaults to @samp{nil}.
This overrides @samp{pgg-default-user-id}.  You can customize this
variable.
@end defvar

@node Architecture
@chapter Architecture

PGG introduces the notion of a "scheme of PGP implementation" (used
interchangeably with "scheme" in this document).  This term refers to a
singleton object wrapped with the luna object system.

Since PGG was designed for accessing and developing PGP functionality,
the architecture had to be designed not just for interoperability but
also for extensibility.  In this chapter we explore the architecture
while finding out how to write the PGG back end.

@menu
* Initializing::
* Back end methods::
* Getting output::
@end menu

@node Initializing
@section Initializing

A scheme must be initialized before it is used.
It had better guarantee to keep only one instance of a scheme.

The following code is snipped out of @file{pgg-gpg.el}.  Once an
instance of @code{pgg-gpg} scheme is initialized, it's stored to the
variable @code{pgg-scheme-gpg-instance} and will be reused from now on.

@lisp
(defvar pgg-scheme-gpg-instance nil)

(defun pgg-make-scheme-gpg ()
  (or pgg-scheme-gpg-instance
      (setq pgg-scheme-gpg-instance
            (luna-make-entity 'pgg-scheme-gpg))))
@end lisp

The name of the function must follow the
regulation---@code{pgg-make-scheme-} follows the back end name.

@node Back end methods
@section Back end methods

In each back end, these methods must be present.  The output of these
methods is stored in special buffers (@ref{Getting output}), so that
these methods must tell the status of the execution.

@deffn Method pgg-scheme-lookup-key scheme string &optional type
Return keys associated with @var{string}.  If the optional third
argument @var{type} is non-@code{nil}, it searches from the secret
keyrings.
@end deffn

@deffn Method pgg-scheme-encrypt-region scheme start end recipients &optional sign passphrase
Encrypt the current region between @var{start} and @var{end} for
@var{recipients}.  If @var{sign} is non-@code{nil}, do a combined sign
and encrypt.  If encryption is successful, it returns @code{t},
otherwise @code{nil}.
@end deffn

@deffn Method pgg-scheme-encrypt-symmetric-region scheme start end &optional passphrase
Encrypt the current region between @var{start} and @var{end} using a
symmetric cipher and a passphrases.  If encryption is successful, it
returns @code{t}, otherwise @code{nil}.  This function is currently only
implemented for GnuPG.
@end deffn

@deffn Method pgg-scheme-decrypt-region scheme start end &optional passphrase
Decrypt the current region between @var{start} and @var{end}.  If
decryption is successful, it returns @code{t}, otherwise @code{nil}.
@end deffn

@deffn Method pgg-scheme-sign-region scheme start end &optional cleartext passphrase
Make the signature from text between @var{start} and @var{end}.  If the
optional third argument @var{cleartext} is non-@code{nil}, it does not
create a detached signature.  If signing is successful, it returns
@code{t}, otherwise @code{nil}.
@end deffn

@deffn Method pgg-scheme-verify-region scheme start end &optional signature
Verify the current region between @var{start} and @var{end}.  If the
optional third argument @var{signature} is non-@code{nil}, it is treated
as the detached signature of the current region.  If the signature is
successfully verified, it returns @code{t}, otherwise @code{nil}.
@end deffn

@deffn Method pgg-scheme-insert-key scheme
Retrieve the user's public key and insert it as ASCII-armored format.
On success, it returns @code{t}, otherwise @code{nil}.
@end deffn

@deffn Method pgg-scheme-snarf-keys-region scheme start end
Collect public keys in the current region between @var{start} and
@var{end}, and add them into the user's keyring.
On success, it returns @code{t}, otherwise @code{nil}.
@end deffn

@node Getting output
@section Getting output

The output of the back end methods (@ref{Back end methods}) is stored in
special buffers, so that these methods must tell the status of the
execution.

@defvar pgg-errors-buffer
The standard error output of the execution of the PGP command is stored
here.
@end defvar

@defvar pgg-output-buffer
The standard output of the execution of the PGP command is stored here.
@end defvar

@defvar pgg-status-buffer
The rest of status information of the execution of the PGP command is
stored here.
@end defvar

@node Parsing OpenPGP packets
@chapter Parsing OpenPGP packets

The format of OpenPGP messages is maintained in order to publish all
necessary information needed to develop interoperable applications.
The standard is documented in RFC 2440.

PGG has its own parser for the OpenPGP packets.

@defun pgg-parse-armor string
List the sequence of packets in @var{string}.
@end defun

@defun pgg-parse-armor-region start end
List the sequence of packets in the current region between @var{start}
and @var{end}.
@end defun

@defvar pgg-ignore-packet-checksum
If non-@code{nil}, don't check the checksum of the packets.
@end defvar

@node GNU Free Documentation License
@appendix GNU Free Documentation License
@include doclicense.texi

@node Function Index
@unnumbered Function Index
@printindex fn

@node Variable Index
@unnumbered Variable Index
@printindex vr

@bye

@c End: