Caching symmetric encryption passphrase with gpg-agent

[Daiki Ueno]

Hi,

Perhaps this is a stupid idea, but let me propose a proof of concept.
The attached is a patch which (partially) enables passphrase caching
even if symmetric encryption is used.  It diverts the S2K salt to the
identity of the encrypted data.  Here is the sample session:

$ eval `gpg-agent --daemon`

$ echo aaa | ./g10/gpg2 --status-fd=2 --symmetric > test.gpg
[GNUPG:] S2K 3 2 6BB569FF913024B9 96

pinentry-gtk will prompt a passphrase.  Then,

$ ./g10/gpg2 --status-fd=2 < test.gpg

pinentry-gtk will prompt a passphrase.  Again,

$ ./g10/gpg2 --status-fd=2 < test.gpg

The cached passphrase is used here.

$ echo bbb | ./g10/gpg2 --status-fd=2 --symmetric \
  --s2k-salt 6BB569FF913024B9 > test.gpg

The cached passphrase is used here since the same cache key
6BB569FF913024B9 is specified with --s2k-salt.

What do you think?

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Regards,
-- 
Daiki Ueno