PGP pre-2.3 signatures are not supported

[David Shaw]

On Wed, Nov 19, 2003 at 08:15:33PM +0300, ls+gnupg.devel.gnupg.org at gambit.com.ru wrote:
> >> I just found an article
> >> news:1992Dec20.102732.11494 at extropia.wimsey.bc.ca which has "BAD
> >> signature" in GnuPG 1.2.3 and "Good signature" in PGP 6.5.8.
> >> Can you reproduce this problem?
> 
> David Shaw wrote:
> > Yes. Note that the signature was issued by PGP 2.1. The problem is
> > that PGP 2.3 changed the representation of the signature hash to
> > be PKCS compatible.
> 
> Is is possible to report "signature is not
> PKCS compatible" instead of "BAD signature"?

Not easily.  The best that could happen is a message saying "Bad
signature or old signature representation" - without a good amount of
work, there is no way to tell the two apart.  These are all v2
signatures, but unfortunately, some v2s have the >=2.3 format and some
have the <2.3 format.

> > PGP 6.5.8 has code to do this.  GnuPG doesn't.  There isn't really
> > any major technical reason why GnuPG couldn't do this, but PGP 2.1
> > is almost 11 years old (note the date of the signature: December,
> > 1992) and is no longer used.
> 
> But there are still many documents signed by old versions and many
> people expect that valid signatures will be validated as Good.

Backwards compatibility is always a balancing act between complexity,
effort, and how far back to go.  There are even earlier signature
types than PGP 2.1, but we have to stop somewhere.  Why not here?

David