[PATCH] Use preferred hashing algorithm when updating signature packets

[David Shaw]

On Jan 28, 2012, at 11:43 PM, David Shaw wrote:

> On Jan 28, 2012, at 4:46 PM, Christian Aistleitner wrote:
> 
>> Hello,
>> 
>> when updating a signature packet, GnuPG reuses the hashing algorithm of the
>> original signature packet.
>> Hence, if the preferred hashing algorithm changed since the first
>> signature, the updated signature does not use the currently preferred
>> hashing algorithm.
>> 
>> Kind regards,
>> Christian
>> 
>> ---
>> g10/sign.c |    2 +-
>> 1 files changed, 1 insertions(+), 1 deletions(-)
>> 
>> diff --git a/g10/sign.c b/g10/sign.c
>> index 4cc813c..b7b4c49 100644
>> --- a/g10/sign.c
>> +++ b/g10/sign.c
>> @@ -1584,7 +1584,7 @@ update_keysig_packet( PKT_signature **ret_sig,
>> 	|| (orig_sig->sig_class == 0x18 && !subpk))
>>      return G10ERR_GENERAL;
>> 
>> -    if ( gcry_md_open (&md, orig_sig->digest_algo, 0 ) )
>> +    if ( gcry_md_open (&md, hash_for( pksk ), 0 ) )
> 
> This is not quite correct.  hash_for() returns the appropriate digest for data, not for certification.  If the intent is to have update_keysig_packet() use --cert-digest-algo rather than basing the signature on the existing digest, you want something like this:
> 
> 	if ( opt.cert_digest_algo )
> 	  digest_algo = opt.cert_digest_algo;
> 	else
> 	  digest_algo = orig_sig->digest_algo;

Although, let me add - I think you're right.  The updated certification should use an updated digest, if the user has selected one.  It just needs to be the cert-digest-algo, rather than the digest-algo.

David