Peter,
The 1988 X.411 includes a specific message sequence number
field too, but the PEM developers never felt it that sort of future
was especially useful. I cannot imagine a human user tracking the message
counts for every person he send s message to, or from whom he receives
messages. However, one could imagien using this sort of facility for
interprocess messaging, where people would not keep track of the
sequence numbers. This can be accommodated using PEM by including the
requisite sequence number as part of a structured message format.
Also, as Steve Crocker points out, use of any sort of message
ID, covered by the PEM protection, will suffice to detect replays. The
use of sequence numbers also allows a recipient to detect deletions
as well.
Steve