Re: [Opensc-devel] Bad signature generated by pkcs15-crypt ?

[Douglas E E. <dee...@gm...>]

Try the attache patch. It is against http:/github.com/OpenSC/OpenSC

Using the data and key thea produces the short signature you should see in debug log:
Incoming APDU data [  513 bytes] =====================================
...
card-openpgp.c:XXXX:pgp_compute_signature: returning with: 512
where it used to say 511

On 2/21/2016 3:53 AM, Joerg Kesten wrote:
> Thanks for your quick reply!
>
> I would also have thought that if this is a general issue with the card
> it would have shown up a long time ago...
>
> But if I understood your comment correctly it is really "just" a missing
> leading zero, right? In that case it would be relatively straight
> forward and a valid workaround to detect and correct this from opensc
> without introducing some security issue?
>
> I am obviously not familiar with the source code in card-openpgp and not
> an expert in this area, but if someone points me to the right functions
> I'd need to look at I'd be happy to help adding this.
>
>
> On 20.02.2016 16:41, Douglas E Engert wrote:
>> Some one who knows the card-openpgp.c code needs to look at this.
>>
>>
>> One more comment. Mathematically, the RSA signature is a large integer. But when used in certificates it is stored as
>> an ASN.1 BIT STRING with leading 0 (because signature is multiple of 8 bits)  followed by the 512 bytes of the signature for the 4096 bit key.
>> So there is a 1/256 chance the first byte of the actual signature being zero. (I have one such certificate.)
>> Your card is dropping the leading zero.
>>
>> I would have assumed that if all openpgp cards dropped a leading zero byte, that this would have showed up long ago
>> as a bug in the OpenSC openpgp driver. I could be wrong. But code could be added to the driver to handle this.
>> It may be OpenPGP does not care, but when used within OpenSC, the leading byte should be there.
>>
>>
>> On 2/19/2016 9:08 AM, Joe...@we... wrote:
>>> Hi everyone,
>>> I am using the Gemalto Open GPG dongle v2.1 with a RSA 4096 bit key to generate signatures for relatively short ID-strings. In general the process I set up runs fine, but I get a bad signature for
>>> about 2% of my inputs. Bad means the data is 1 byte short and fails verification with openssl. But is not just truncated, comparing to a valid signature generated with openssl it looks completely
>>> different.
>>> I am doing the following:
>>> $ echo -ne "CgABEQS/SUEAAAAAAAAINA==" | openssl dgst -binary -sha256  > dgst.txt
>>> $ tools/pkcs15-crypt.exe -s -k 1 -r 1 -i dgst.txt -o sig.txt  --sha-256 --pkcs1 -p 123456
>>> $ echo -ne "CgABEQS/SUEAAAAAAAAINA=="  | openssl dgst -sha256 -verify pubkey.pem -signature sig.txt
>>> Verification Failure
>>> Doing the same with slightly altered input data runs fine:
>>> $ echo -ne "CgABEQS/SUEAAAAAAAAINB==" | openssl dgst -binary -sha256  > dgst.txt
>>> $ tools/pkcs15-crypt.exe -s -k 1 -r 1 -i dgst.txt -o sig.txt  --sha-256 --pkcs1 -p 123456
>>> $ echo -ne "CgABEQS/SUEAAAAAAAAINB=="  | openssl dgst -sha256 -verify pubkey.pem -signature sig.txt
>>> Verified OK
>>> This is reproducible with different keys (each key fails for different input data though), I saw the same issue when generating rsa signatures with pkcs11-tool (using parameters -s -m RSA-PKCS -i
>>> dgst.txt -o sig.txt) , again the bad signatures happen for different input data, so to me it seems that certain key/data combinations may have an issue, or I am doing something wrong...
>>> Please find attached the logs of both runs I mentioned above with OPENSC_DEBUG=9 set (you can see the good run returned 512 bytes as signature, whereas the bad one only returned 511), and the public
>>> and private key stored on the card  I used for this example (which I explicitly generated for this test ;-) )
>>> I have no idea what I might be doing wrong, any help would be highly appreciated!
>>> Thanks
>>> Jörg
>>>
>>>
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>
>
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-- 

  Douglas E. Engert  <DEE...@gm...>