[openpgp] Definition of Subkey Revocation Signatures

[Paul Schaub <[e-mail]>]

Hey!

While hacking around I discovered, that there was a discrepancy between 
my implementation and rfc4880 that surprised me.

https://www.rfc-editor.org/rfc/rfc4880#section-5.2.1 states for section 
0x28:

>     0x28: Subkey revocation signature
>         The signature is calculated directly on the subkey being revoked.
>         [...]
In my code, I calculated the signature over the primary key and the 
subkey instead.

The descriptions for binding signature types 0x18 and 0x19 explicitly 
stated that the signature would be generated over the primary key and 
the subkey:

>     0x18: Subkey Binding Signature
>         [...] This signature is calculated
>         directly on the primary key and subkey, and not on any User ID or
>         other packets. [...]
>
>     0x19: Primary Key Binding Signature
>         [...] This signature
>         is calculated the same way as a 0x18 signature: directly on the
>         primary key and subkey, and not on any User ID or other packets.
Since I haven't had any interoperability problems with other 
implementations with regards to subkey revocations in the past, I 
suspect that most implementations did not follow the spec to the word 
either and did how I did.

Therefore I proposed a patch that brings the wording of 0x28 signatures 
in line with 0x18 and 0x19: 
https://gitlab.com/openpgp-wg/rfc4880bis/-/merge_requests/307

Paul