Re: How to Calculate Signatures?

[Ben Laurie <[e-mail]>]

Hal Finney wrote:
> Ben Laurie writes:
> 
> 
>>The sections on calculating signatures are really confusing. I can't 
>>currently suggest alternate text for most of it because its far from 
>>clear to me what the actual algorithms are. If someone explains, I'll do 
>>my best to write clarifying text.
> 
> 
> You're right, this is really messed up.
> 
> The authoritative section on what to hash is 5.2.4.  We should refer
> forward to that section and not include detailed information about
> what is hashed in the sections on V3 and V4 signature packets.
> 
> We should make it clear that the DSA signature algorithm works directly
> on the hash value that results from 5.2.4.
> 
> We should say that RSA signatures use that hash and prepend the sequence
> of bytes identified as the "full hash prefixes".  We could probably remove
> the hexadecimal equivalents to the ASN.1 OIDs; if someone understands
> ASN.1 well then the OIDs are enough, and if not then they can just
> follow the rule to prepend the proper byte sequences and that will work.
> This then gets padded as in PKCS#1 v1.5 signatures.  We should have a
> sentence clarifying that this is what gives us the value "m" used in
> the signature calculation.

We also need to specify emLen, which I presume (by logic and experiment) 
is equal to the RSA key size.

I will send diffs soon. Thanks for the clarification.

Cheers,

Ben.

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