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 This package is obsolete; for new code we recommend EasyPG instead. See EasyPG.

This file describes PGG 0.1, an Emacs interface to various PGP implementations.

Copyright © 2001, 2003–2014 Free Software Foundation, Inc.

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.”

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

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1 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. See EasyPG.

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

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2 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 http://www.gnupg.org/documentation/.

When using GnuPG, we recommend the use of the 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 (see Caching passphrase). By default, PGG will attempt to use gpg-agent if it is running. See Invoking GPG-AGENT.

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

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3 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.

     (require 'pgg)

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

     (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)

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3.1 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 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.

— Command: pgg-encrypt-region start end recipients &optional sign passphrase

Encrypt the current region between start and end for 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 sign is non-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 passphrase is nil, the passphrase will be obtained from the passphrase cache or user.

— Command: pgg-encrypt-symmetric-region &optional start end passphrase

Encrypt the current region between start and end using a symmetric cipher. After invocation you are asked for a passphrase.

If optional passphrase is nil, the passphrase will be obtained from the passphrase cache or user.

symmetric-cipher encryption is currently only implemented for GnuPG.

— Command: pgg-decrypt-region start end &optional passphrase

Decrypt the current region between start and end. If decryption is successful, it replaces the current region contents (in the accessible portion) with the resulting data.

If optional passphrase is nil, the passphrase will be obtained from the passphrase cache or user.

— Command: pgg-sign-region start end &optional cleartext passphrase

Make the signature from text between start and end. If the optional third argument cleartext is non-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 passphrase is nil, the passphrase will be obtained from the passphrase cache or user.

— Command: pgg-verify-region start end &optional signature fetch

Verify the current region between start and end. If the optional third argument signature is non-nil, it is treated as the detached signature file of the current region.

If the optional 4th argument fetch is non-nil, or the function is called interactively, we attempt to fetch the signer's public key from the key server.

— Command: pgg-insert-key

Retrieve the user's public key and insert it as ASCII-armored format.

— Command: pgg-snarf-keys-region start end

Collect public keys in the current region between start and end, and add them into the user's keyring.

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3.2 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 pgg-scheme variable. If it is nil (the default), the value of the pgg-default-scheme variable will be used instead.

— Variable: pgg-scheme

Force specify the scheme of PGP implementation. The value can be set to gpg, pgp, and pgp5. The default is nil.

— Variable: pgg-default-scheme

The default scheme of PGP implementation. The value should be one of gpg, pgp, and pgp5. The default is gpg.

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3.3 Caching passphrase

When using GnuPG (gpg) as the PGP scheme, we recommend using a program called gpg-agent for entering and caching passphrases1.

— Variable: pgg-gpg-use-agent

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

To use gpg-agent with PGG, you must first ensure that 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 .xsession file:

     eval "$(gpg-agent --daemon)"

For more details on invoking gpg-agent, See Invoking GPG-AGENT.

Whenever you perform a PGG operation that requires a GnuPG passphrase, GnuPG will contact gpg-agent, which prompts you for the passphrase. Furthermore, 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 --default-cache-ttl option when invoking gpg-agent.)

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

When 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 pgg-cache-passphrase (see below).

There is a security risk in handling passphrases through PGG rather than 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. gpg-agent avoids this problem by using certain tricks, such as memory locking, which have not been implemented in Emacs.

— Variable: pgg-cache-passphrase

If non-nil, store passphrases. The default value of this variable is t. If you are worried about security issues, however, you could stop the caching of passphrases by setting this variable to nil.

— Variable: pgg-passphrase-cache-expiry

Elapsed time for expiration in seconds.

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.

— Variable: pgg-passphrase-coding-system

Coding system used to encode passphrase.

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3.4 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.

— Variable: pgg-default-user-id

User ID of your default identity. It defaults to the value returned by ‘(user-login-name)’. You can customize this variable.

— Variable: pgg-gpg-user-id

User ID of the GnuPG default identity. It defaults to ‘nil’. This overrides ‘pgg-default-user-id’. You can customize this variable.

— Variable: pgg-pgp-user-id

User ID of the PGP 2.x/6.x default identity. It defaults to ‘nil’. This overrides ‘pgg-default-user-id’. You can customize this variable.

— Variable: pgg-pgp5-user-id

User ID of the PGP 5.x default identity. It defaults to ‘nil’. This overrides ‘pgg-default-user-id’. You can customize this variable.

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4 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.

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4.1 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 pgg-gpg.el. Once an instance of pgg-gpg scheme is initialized, it's stored to the variable pgg-scheme-gpg-instance and will be reused from now on.

     (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))))

The name of the function must follow the regulation—pgg-make-scheme- follows the back end name.

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4.2 Back end methods

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

— Method: pgg-scheme-lookup-key scheme string &optional type

Return keys associated with string. If the optional third argument type is non-nil, it searches from the secret keyrings.

— Method: pgg-scheme-encrypt-region scheme start end recipients &optional sign passphrase

Encrypt the current region between start and end for recipients. If sign is non-nil, do a combined sign and encrypt. If encryption is successful, it returns t, otherwise nil.

— Method: pgg-scheme-encrypt-symmetric-region scheme start end &optional passphrase

Encrypt the current region between start and end using a symmetric cipher and a passphrases. If encryption is successful, it returns t, otherwise nil. This function is currently only implemented for GnuPG.

— Method: pgg-scheme-decrypt-region scheme start end &optional passphrase

Decrypt the current region between start and end. If decryption is successful, it returns t, otherwise nil.

— Method: pgg-scheme-sign-region scheme start end &optional cleartext passphrase

Make the signature from text between start and end. If the optional third argument cleartext is non-nil, it does not create a detached signature. If signing is successful, it returns t, otherwise nil.

— Method: pgg-scheme-verify-region scheme start end &optional signature

Verify the current region between start and end. If the optional third argument signature is non-nil, it is treated as the detached signature of the current region. If the signature is successfully verified, it returns t, otherwise nil.

— Method: pgg-scheme-insert-key scheme

Retrieve the user's public key and insert it as ASCII-armored format. On success, it returns t, otherwise nil.

— Method: pgg-scheme-snarf-keys-region scheme start end

Collect public keys in the current region between start and end, and add them into the user's keyring. On success, it returns t, otherwise nil.

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4.3 Getting output

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

— Variable: pgg-errors-buffer

The standard error output of the execution of the PGP command is stored here.

— Variable: pgg-output-buffer

The standard output of the execution of the PGP command is stored here.

— Variable: pgg-status-buffer

The rest of status information of the execution of the PGP command is stored here.

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5 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.

— Function: pgg-parse-armor string

List the sequence of packets in string.

— Function: pgg-parse-armor-region start end

List the sequence of packets in the current region between start and end.

— Variable: pgg-ignore-packet-checksum

If non-nil, don't check the checksum of the packets.

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Appendix A GNU Free Documentation License

Version 1.3, 3 November 2008
     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     http://fsf.org/
     
     Everyone is permitted to copy and distribute verbatim copies
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  1. PREAMBLE

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  6. COMBINING DOCUMENTS

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  7. COLLECTIONS OF DOCUMENTS

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  10. TERMINATION

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  11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

  12. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

       Copyright (C)  year  your name.
       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, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being list their titles, with
         the Front-Cover Texts being list, and with the Back-Cover Texts
         being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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Footnotes

[1] Actually, 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.