Signature Packet Format

[Brian M. Carlson]

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Justin Wienckowski wrote:
> 
> Hey folks,
> 
> I'm writing a little application to generate an OpenPGP message with
a user's RSA public/private key for import into gpg.  I noticed that
GPG won't import the key without a valid self-signature.
> 
> However, the RFC sections regarding the signature packet format are
a little unclear to me.
> 
> My understanding of Version 4 signatures of RSA keys is as follows -
any clarifications/corrections are appreciated.
> 
> 1)  Hash the key packet you are protecting.
> 2)  Hash the User ID packet
> 3)  Hash the Signature packet through the hashed subpacket data, but
no further
> 4)  Hash the 6-byte trailer
> 5)  Insert the 160-bit hash value into an ASN.1 DigestInfo structure
> 
> ....now what?
> 
> The spec indicates that the "signature" portion of the packet for
the RSA algorithm is an MPI of m**d (which I assume is the modulus
raised to the private exponent).  But there's no indication of where
the m**d value and the ASN.1 DigestInfo structure meet in order to
form a secure signature.
> 
> I *know* I"m missing something because this just doesn't make sense
as a cryptographically secure signature.  Anyone able to clarify for
me?


I think, somebody correct me if I'm wrong, that the ASN.1 structure is
m and well, I'm sure you know what d and n are. I suggest you use
RFC2440-bis02 to implement this, as this is likely to become the new
standard and it contains the new MDC packets.

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