Series of minor questions about OpenPGP 1

[Peter Thomas]

On Mon, Jan 26, 2009 at 11:31 PM, David Shaw <dshaw at jabberwocky.com> wrote:
> 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. :)
Yes, that was clear

> 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.
My intention (and also behind the question whether there is something
like the critical bit for packet types) is this:
Suppose a new packet type (above 15) is added which is VERY critical
for the security, meaning that it would be very very bad if some
implementation isn't able to interpret it.
Is it secured that those applications will blow up, give errors etc.?
If not (and that was my motivation behind the general usage of new
packet headers) it would be better if no packet type (even those below
16) are understood by these legacy applications and thus the whole
key/message would be unusable for them.
See what I mean?