  The design and verification of cryptographic protocols is a notoriously
difficult task, even in symbolic models which take an abstract view of
cryptography. This is mainly due to the fact that protocols may interact with
an arbitrary attacker which yields a verification problem that has several
sources of unboundedness (size of messages, number of sessions, etc. In this
paper, we characterize a class of protocols for which deciding security for an
unbounded number of sessions is decidable. More precisely, we present a simple
transformation which maps a protocol that is secure for a bounded number of
protocol sessions (a decidable problem) to a protocol that is secure for an
unbounded number of sessions. The precise number of sessions that need to be
considered is a function of the security property and we show that for several
classical security properties a single session is sufficient. Therefore, in
many cases our results yields a design strategy for security protocols: (i)
design a protocol intended to be secure for a {single session}; and (ii) apply
our transformation to obtain a protocol which is secure for an unbounded number
of sessions.

Cryptography and Security