Marking as resolved since this fixes the issue reported in this ticket for me.

Does this fix?

Changes have been made in Git (master):

commit 9281a2cda2ec8ed9ae4b25fdee59460b822581b4
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Apr 9 15:28:09 2013 -0600

     [mms] Fix signature verification for PGP combined signed & 
encrypted messages (Bug #12142).

     Conflicts:
             imp/lib/Mime/Viewer/Pgp.php
             imp/package.xml

  imp/docs/CHANGES            |    2 +
  imp/lib/Crypt/Pgp.php       |   44 ++++++++++++++++++++++++++++++++++--------
  imp/lib/Mime/Viewer/Pgp.php |   39 ++++++++++++++++++++++++++-----------
  imp/package.xml             |    1 +
  4 files changed, 65 insertions(+), 21 deletions(-)

http://git.horde.org/horde-git/-/commit/9281a2cda2ec8ed9ae4b25fdee59460b822581b4

Changes have been made in Git (FRAMEWORK_5_0):

commit 2f4b732a481e1c948221f5e04ef0038904c0fd16
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Apr 9 15:28:09 2013 -0600

     [mms] Fix signature verification for PGP combined signed & 
encrypted messages (Bug #12142).

  imp/docs/CHANGES            |    2 +
  imp/lib/Crypt/Pgp.php       |   44 ++++++++++++++++++++++++++++++++++--------
  imp/lib/Mime/Viewer/Pgp.php |   37 +++++++++++++++++++++++++----------
  imp/package.xml             |    2 +
  4 files changed, 65 insertions(+), 20 deletions(-)

http://git.horde.org/horde-git/-/commit/2f4b732a481e1c948221f5e04ef0038904c0fd16

its about case 3:
* when there is a pgp-encrypted part which contains both an encrypted 
packet and a signature packet, the signature is only verified in 
Horde_Crypt_Pgp::_decryptMessage, which does not fetch the key from a 
keyserver.
* whereas when the encrypted part contains a message with a detached 
signature, it is verified in the viewer, and the key is properly 
fetched.

for the two cases see http://www.ietf.org/rfc/rfc2015.txt  section 6.2 vs. 6.1

I am not following.  Signed messages can be sent in one of three ways:

1) As PGP armored text (handled in Plain viewer).
2) multpart/signed w/ application/pgp-signature (handled in PGP viewer).
3) Encrypted + signed message -- multipart/encrypted (handled in PGP 
viewer).  There are acutally 2 types here (encrypted with embedded 
signed part AND encrypted+signed).  Both are handled by the PGP viewer.

Not sure which one is not being handled properly for you.

now i'm embarassed. i can't either. sorry if i wasted your time!
ehm nothing, i was just guessing, since i didn't really understand the code...

so now there is only one special case, that is somewhat missing:

mails created with enigmail don't have a detached signature. so the 
signature verification is done in Horde_Crypt_Pgp::_decryptMessage. 
since in this method the pubkeyring consists only my own pubkey, this 
will always yield "Can't check signature: No public key" (opposed to 
the detached signatures which are verified in 
IMP_Crypt_Pgp::verifySignature which automatically tries to fetch the 
correct key....)

(see Mime/Viewer/Pgp.php under  /* Check for combined 
encryption/signature data. */ for the beginning of this call path)

but i don't yet see an easy solution to this problem. because all the 
signature logic is in the mime viewer, the key fetching logic is in 
IMP_Crypt_Pgp and the decryption in Horde_Crypt_Pgp. the signature 
keyid is in the encrypted container, so somehow you would need to 
first decrypt it, then fetch the key, then decrypt it again, with the 
fetched key in the keyring...

I can't reproduce.  Signature verification works fine for me.
So?  The one missing case is not used in the PGP code.  So not sure 
what that has to do with this ticket.

Changes have been made in Git (master):

commit f1c84583504f142ca37eae1b9dcda3b99dab801c
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Mar 27 22:18:44 2013 -0600

     Fix returning stream if requested (Bug #12142)

  imp/lib/Contents.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/f1c84583504f142ca37eae1b9dcda3b99dab801c

(line numbers as in origin master)
actually i'm totally unsure about the code there, maybe it should be 
able to provide this, but then its broken.

this piece of code from line 287 to 299 looks really wrong to me, 
since there should be 4 possible cases (mime: yes/no, stream: yes/no) 
but the code has only 3 different execution paths.

gee, i really missed your comment. thanks for saving my time :)

the fix introduced there indeed fixes verification for signed and 
encrypted mails, but breaks verification of signed only.

the problem seems to be with the change introduced on line 443, as
'mimeheaders' => true and 'stream' => true cannot be set both as can 
be seen in

./imp/lib/Contents.php:292

And that's exactly the code that was changed/fixed in Bug #12100.

i traced the problem down quite a bit already:

in ./www/imp/lib/Mime/Viewer/Pgp.php:427

$this->getConfigParam('imp_contents')->getBodyPart($signed_id, 
array('mimeheaders' => true))
returns an empty string

because:

in ./www/imp/lib/Contents.php:245

$part->getContents();

returns an empty string, therefore 
Horde_Mime_Part::getRawPartText(...) three lines below fails, since 
there are no mime parts in the string. (the string contains only the 
mime headers of the base_id container)

why this happens is unclear to me

Duplicate of Bug #12100?

I sign an email with enigmail using PGP/mime
-> Horde states:
gpg: Signature made Tue 26 Mar 2013 07:27:20 PM CET using RSA key ID xxx
gpg: Good signature from xxx
(the key being on a keyserver)

When i sign+encrypt an email with enigmail using PGP/mime (exactly the 
same account and key)
-> After decryption Horde states:
gpg: Signature made Tue 26 Mar 2013 07:27:20 PM CET using RSA key ID xxx
gpg: Can't check signature: No public key