I'm having trouble decrypting some mails. I use an encryption sub-key with a
unusual length of 3104 bits. I described my problem in the gnupg-users mailing
list and there the following problem was identified:
<quote>
I think that it is deterministic; The cause is that the RSA keysize is
not the one in the set of: 1024, 1536, 2048, 3072, 4096. When data to
be decrypted is padded, scdaemon can't decrypt, I suppose.
I am not sure the exact reason why scdaemon only supports limited set of
keysize for encryption. But we have this handling of padding in the
current code:
/* We might encounter a couple of leading zeroes in the
cryptogram. Due to internal use of MPIs these leading zeroes
are stripped. However the OpenPGP card expects exactly 128
bytes for the cryptogram (for a 1k key). Thus we need to fix
it up. We do this for up to 16 leading zero bytes; a
cryptogram with more than this is with a very high
probability anyway broken. If a signed conversion was used
we may also encounter one leading zero followed by the correct
length. We fix that as well. */
if (indatalen >= (128-16) && indatalen < 128) /* 1024 bit key. */
fixuplen = 128 - indatalen;
else if (indatalen >= (192-16) && indatalen < 192) /* 1536 bit key. */
fixuplen = 192 - indatalen;
else if (indatalen >= (256-16) && indatalen < 256) /* 2048 bit key. */
fixuplen = 256 - indatalen;
else if (indatalen >= (384-16) && indatalen < 384) /* 3072 bit key. */
fixuplen = 384 - indatalen;
else if (indatalen >= (512-16) && indatalen < 512) /* 4096 bit key. */
fixuplen = 512 - indatalen;
else if (!*(const char *)indata && (indatalen == 129
|| indatalen == 193
|| indatalen == 257
|| indatalen == 385
|| indatalen == 513))
fixuplen = -1;
else
fixuplen = 0;Perhaps, it was due to support all existing OpenPGP card
implementations, I mean, somehow historical, and it was easier to list
up specific keysizes.
This should be fixed.
</quote>
I also attached to log-files of the scdaemon. One for a successful and one for a
failed decryption attempt.
Please let me know if you need any additional information.