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Re: [Opensc-devel] Bad signature generated by pkcs15-crypt ?
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From: Douglas E E. <dee...@gm...> - 2016-02-22 18:17:01
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On 2/22/2016 9:51 AM, Joe...@we... wrote:
> Thanks for providing this patch, with this I got it _almost_ working :-)
> I ran into one real and two minor issues:
> 1) The real issue is that the outlen does not seem to be the expected signature length,
> but the size of the buffer with some extra space. In my case it is 1024 and not the expected
> 512, so this does not work. But I guess it would be possible to compute the expected signature
> length in a general way?
sc_pkcs15_compute_signature set modlen lines 324-336 from the type of key and its size, then tests if outlen is big enough:
339 if (inlen > sizeof(buf) || outlen < modlen)
But then it passes to lower levels, it passes outlen:
434 r = use_key(p15card, obj, &senv, sc_compute_signature, tmp, inlen,
435 out, outlen);
In all cases other then the card you have this is not a problem.
So one possible fix is to set line 435 to:
out, modlen);
then do the memmove stuff if its too short.
BUT THIS IS A GLOBAL CHANGE, and would need testing for other cards. I don't see why it would be an issue,
but you never know...
If you try and do an openpgp only fix, it looks like by the time pgp_set_security_env and pgp_compute_signature
are called, they size of the key is not known, just the outlen. Som info cold be saved in the
Another way: card-openpgp.c only supports RSA. And only 4K, 2K and maybe 1K keys are used.
So if apdu.resplen within 4 bytes of one of these values, assume it is dropped 1, 2, 3 or 4 bytes,
and do the memmove stuff then. (Not perfect, but chance of failure to catch a short signature is 1/2^32)
There may be more info in the OpenPGP documents that would show how to save the key size internally
in one of the card-openpgp.c internal structures.
> 2) Minor techical issues: the apdu was not updated in the end to return the new length,
> and src and dest were mixed up in the memmove
OK, I never tested the code. good to here you got it working.
> With this hacked up version of your patch I was able to get a valid signature :-) , but obviously it works only
> for exactly my usecase with at most one leading zero:
> --- a/src/libopensc/card-openpgp.c
> +++ b/src/libopensc/card-openpgp.c
> @@ -1656,6 +1656,13 @@ pgp_compute_signature(sc_card_t *card, const u8 *data,
> r = sc_check_sw(card, apdu.sw1, apdu.sw2);
> LOG_TEST_RET(card->ctx, r, "Card returned error");
>
> + /* some cards may drop leading 0x00 byte on a signature */
> + if (apdu.resplen < 512) {
> + memmove(out + 1 , out, apdu.resplen); /* overlaping */
> + memset(out, 0, 1);
> + apdu.resplen = 512;
> + }
> +
> *Gesendet:* Sonntag, 21. Februar 2016 um 20:54 Uhr
> *Von:* "Douglas E Engert" <dee...@gm...>
> *An:* ope...@li...
> *Betreff:* Re: [Opensc-devel] Bad signature generated by pkcs15-crypt ?
> The patch I sent you has a bug:
>
> memmove(out, out -(outlen - apdu.resplen), apdu.resplen); /* overlaping */
> should be:
>
> memmove(out, out + (outlen - apdu.resplen), apdu.resplen); /* overlaping */
>
>
> I have not tried the patch.
>
> On 2/21/2016 7:53 AM, Douglas E Engert wrote:
>
> > Try the attache patch. It is against http:/github.com/OpenSC/OpenSC
> >
>
>
> --
>
> Douglas E. Engert <DEE...@gm...>
>
>
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--
Douglas E. Engert <DEE...@gm...>
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