Hi dkg,

On the mailing list and in T1928, we discussed
why it shouldn't be possible for a program to pass the passphrase to gpg agent.
This feature request is at odds with the conclusion drawn there. Should this
issue be closed as WONTFIX?

Thanks,

:) Neal

Hm, you make a good point about this being undesirable in the general case --
access to a normal gpg-agent shouldn't provide an attacker with a way to guess
passwords silently.

However, consider the mailpile case -- where gpg-agent is running on the
webserver, and the login webpage wants to verify a given user based on the
password for the user's secret key (and wants to avoid keeping some extra
/etc/shadow-equivalent file lying around).

Maybe such an application would start gpg-agent in a different/simpler mode? Or
should we recommend that such an application test the provided passphrase in
some other way, without using gpg-agent at all?

Why not doing a dummy signing then?

We can already do a dummy signature, but it feels sloppy for several reasons:

• it's not clear exactly what to sign with a dummy signature -- should you sign

static text or a dynamic (random) block?

• it's not clear what to do with the signatures after creation. It's

concievable that a dummy signature could be abused/misused if the material being
signed turns out to have some other meaning.

• gpg-agent can be configured to log to an append-only file as a means of

monitoring what signatures have ever been made by a given key. Dummy signatures
introduce spurious signing events that are indistinguishable from real
signatures in this log

• the creation of arbitrary signatures takes more time than testing passphrase

and returning feedback.

Done with commit 9bca96d. Here is how to use it:

  $ gpg-connect-agent 
  > passwd --verify 2C1103C5C84AAD061B5E3221C048A93D878F7EEE
  OK
  > passwd --verify 2C1103C5C84AAD061B5E3221C048A93D878F7EEE
  ERR 83886179 Operation cancelled <Pinentry>
  > passwd --verify 2C1103C5C84AAD061B5E3221C048A93D878F7EEE
  ERR 67108875 Bad passphrase <GPG Agent>

For the OK I entered the correct passpharse, for the bad passpharse I entered a
bad passphrase three times in a row.