Re: Clarification needed on compressed messages

[Derek Atkins <warlord@xxxxxxx>]

David Shaw <dshaw@xxxxxxxxxxxxxxx> writes:

> I was sent an interesting interoperability problem today with a signed
> message that wouldn't verify in GnuPG.  After some examination, and
> once the encryption was stripped off, it seemed that it was a message
> of the form:
> 
>    signature packet + compressed packet (literal packet)
> 
> That is, a signature packet, followed by a compressed packet which
> contained a literal packet.

This should be legal.  Strange, but legal..

> In the grammar, the latest draft (and 2440 also) say that a "Signed
> Message" is:
> 
> Signed Message :- Signature Packet, OpenPGP Message |
>                One-Pass Signed Message

That's sounds right...

> GnuPG (and it seems the new PGP) generate the One-Pass method, but
> still accept the common SIG+LITERAL construction.  No problems there.
> 
> However, since a valid "OpenPGP Message" may be a "Compressed
> Message", that would also make the message I received a legal
> construction.
> 
> Is this the intent?  And if so, in a SIG+COMPRESSED(LITERAL) message,
> is the SIG issued over COMPRESSED(LITERAL) or LITERAL ?

I believe it is the intent, and in the SIG+(COMPRESSED(LITERAL) the
SIG should be issued over the COMPRESSED(LITERAL).  The only special
case that I know of is SIG+LITERAL, where the SIG is over the data
inside the literal and doesn't include the literal packet itself.

However, all other constructions should build the SIG over the
underlying PGP message object.

Just my $0.02.

> David

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord@xxxxxxx                        PGP key available