On Jan 26, 2009, at 6:15 PM, Peter Thomas wrote:
On Mon, Jan 26, 2009 at 11:31 PM, David Shaw <[email protected]>
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.?
They should at least fail - a new style RFC-4880 (or 2440) packet (of
any type) is unreadable by an old RFC-1991 program. It simply won't
be meaningful. At to *how* it will fail, that depends on the program.
The point of the Marker Packet is to force a graceful failure early.
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.
If there was such a situation, then forcing the use of a new packet
header would certainly break old programs, but this isn't sufficient:
most programs understand new packet headers, but they may not
understand your new packet type. Or put another way - you can't solve
that problem with packet headers.
David
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