EasyPG

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* What's this?

EasyPG is an all-in-one GnuPG interface for Emacs.  It has two
aspects: convenient tools which allow to use GnuPG from Emacs (EasyPG
Assistant), and a fully functional interface library to GnuPG (EasyPG
Library).

* Features

The EasyPG Assistant provides the following features:

- Keyring browser.
- Cryptographic operations on regions.
- Cryptographic operations on files.
- Dired integration.
- Encryption/decryption of *.gpg files.

The EasyPG Library provides the following features:

- The API covers most functions of GnuPG like GPGME.
- S/MIME support using gpgsm.
- Designed to avoid potential security pitfalls around Emacs

* Requirements

** GNU Emacs 21.4, XEmacs 21.4, or later

** GnuPG 1.4.3 or later
   Earlier versions are supported but not recommended.

* Quick start

** Installation

  $ ./configure
  $ sudo make install

Add the following line to your ~/.emacs

  (require 'epa-setup)

Restart emacs and type `M-x epa- TAB', and you will see a lot of
commands available.  For example,

- To browse your keyring, type `M-x epa-list-keys'
- To create a cleartext signature of the region, type `M-x epa-sign-region'

You can also do some cryptographic operations from dired.

  M-x dired
  (mark some files)
  : e (or M-x epa-dired-do-encrypt)
  (select recipients by 'm' and click [OK])

* MUA Integration

The EasyPG Library can be used in combination with various MUA (Mail
User Agents).

** CVS version of Gnus

CVS version of Gnus uses EasyPG by default.  To make sure of that,
check mml2015-use set to 'epg.  Other options which affect on the
EasyPG are

  mml2015-signers
  mml2015-encrypt-to-self
  mml2015-cache-passphrase
  mml2015-passphrase-cache-expiry
  mml2015-verbose

NOTE: You don't need pgg-epg.el in this case.

** PGG based MUA

PGG is outdated PGP library used by old Gnus (<= v5.11), MH-E, etc.

There is a PGG backend written using EasyPG called pgg-epg.el.
However, EasyPG API is a superset of PGG and pgg-epg.el provides
nothing but compatibility with PGG.

*** Limitations of PGG

As I noted "PGG is outdated PGP library" above, PGG has several
limitations.  For example

- PGG can't handle a message signed with multiple keys.
- PGG can't prompt a user which key is being used.
- PGG can't create a binary PGP messages.
- PGG doesn't provide a way to select keys per cryptographic operation.
- PGG ignores GnuPG's trust metrics.

** SEMI based MUA

SEMI is the MIME library used by Wanderlust, cmail, T-gnus, etc.

There is an EasyPG capable SEMI library called EMIKO-EasyPG.  It can
be downloaded from the same site of the EasyPG distribution point.

* Security

There are security pitfalls around Emacs.  EasyPG is written with
avoiding them.

** Passphrase may leak to a temporary file.

The function call-process-region writes data in region to a temporary
file. If your PGP library used this function, your passphrases would
leak to the filesystem.

The EasyPG Library does not use call-process-region to communicate
with a gpg subprocess.

** Passphrase may be stolen from a core file.

If Emacs crashes and dumps core, Lisp strings in memory are also
dumped within the core file. read-passwd function clears passphrase
strings by (fillarray string 0) to avoid this risk. However, Emacs
performs compaction in gc_sweep phase. If GC happens before fillarray,
passphrase strings may be moved elsewhere in memory. Therefore,
passphrase caching in elisp is generally a bad idea.

The EasyPG Library dares to disable passphrase caching. Fortunately,
there is more secure way to cache passphrases - use gpg-agent.