[Rpm-maint] [rpm-software-management/rpm] Ignore most unhashed subpackets in OpenPGP signatures (Issue #1886)

[Demi Marie Obenour]

> > Therefore, it is only safe to process subpackets that restrict the validity of the signature.
> 
> Not quite. It is only safe to process subpackets that are self-authenticating. For example, the issuer subpacket is self-authenticating: It is used as a hint as to which key made the signature. If that key is found, the signature can be verified with it. If successful, this authenticates the issuer subpacket.

Agreed.  I would go further and reject a signature that has a subpacket which cannot validly be in the unhashed section.

> > I recommend going further and ignoring everything except for primary key binding signatures, key ID subpackets, and fingerprint subpackets.
> 
> I'm assuming you mean embedded signature, issuer, and issuer fingerprint subpackets. That list looks good, yes.

Is there ever a reason to have an embedded signature in a type 0 or type 1 signature?  The only use-case I can think of is timestamping.  At the very least, an embedded signature must not also have an embedded signature to avoid recursion.

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