Series of minor questions about OpenPGP 1

[David Shaw]

On Mon, Jan 26, 2009 at 10:36:21PM +0100, Peter Thomas wrote:

> On Mon, Jan 26, 2009 at 5:40 PM, David Shaw <dshaw at jabberwocky.com> wrote:
> >> Uhm, I just wanted to follow the recommendation of the RFC ;-)
> > The RFC says "If interoperability is not an issue, the new packet
> > format is RECOMMENDED."  Given that interoperability *is* an issue
> > (GPG works with PGP 2.x), we don't use the new packet format except
> > when necessary.
> Ah,.. ok so to support older applications. Good decision.
> Uhm what happens if one of these older implementations sees a packet
> type above 15? Do they give a warning? Or is there something similar
> to the critical bit?

No, they don't have a concept of a packet type above 15.  There are
only 4 type bits in the old-style packet header. :)

Old programs will basically blow up if they see something they don't
understand.  There is a special packet, the Marker Packet (tag 10)
which basically exists to make PGP 2.x print out "You need a newer
version of PGP" before PGP 2.x would blow up.

David