Passphrase caching for GnuPG in Emacs?

[Jens Lechtenboerger <jens.lechtenboerger@fsfe.org>]

Hi there,

I plan to refactor the code used for GnuPG in the Message mode of
Emacs (Gnus) and started a discussion on the Gnus devel mailing list
ding.  An open issue is passphrase caching within Emacs, which might
be removed in the future.  So if you encrypt e-mail and cache
passphrases inside Emacs (instead of gpg-agent), please let me know
why.  Also, do you think that it is necessary to have four different
variables to enable caching and another four to control caching
duration?  (mml-secure-cache-passphrase, mml1991-cache-passphrase,
mml2015-cache-passphrase, mml-smime-cache-passphrase;
mml-secure-passphrase-cache-expiry, mml1991-passphrase-cache-expiry,
mml2015-passphrase-cache-expiry, mml-smime-passphrase-cache-expiry)

As gpg-agent is the daemon to manage secret keys, I recommend to use
that as trusted place to cache passphrases, which works with GnuPG
1.x. as well as 2.x.  Note that GnuPG 2.x is where the development
happens, and according to GnuPG’s README, “2.0 is the current stable
version for general use”, while “1.4 is the old standalone version
which is most suitable for older or embedded platforms.”

If I’m not mistaken (please let me know if I’m wrong), with GnuPG
2.x (and gpgsm) passphrases cannot be cached within Emacs as
gpg-agent is started automatically and tries to invoke a pinentry
program.  So, caching-related variables only apply to GnuPG 1.x and
S/MIME with openssl (I suggest not to advertise the latter any
longer and will send a separate e-mail to discuss that), which
should be documented if we plan to keep that functionality.

What’s your opinion?

Best wishes
Jens

P.S. Encryption is self-defense.  Learn more:
https://emailselfdefense.fsf.org/