Secrecy for Cryptographic Protocols is Easy

Abstract:

Cryptographic protocols are commonly used to  guarantee
  security of  transactions over insecure networks.   One of the
  most important goals of security is to preserve secrecy.    We
  present a sufficient condition which ensures secrecy of data for
  cryptographic protocols using symmetric keys.  A lot of
  authentication protocols can be rewritten in order to satisfy this
  condition. Our modelling approach follows more or less Paulson's
  (in Journal of Computer Security, 1999), even if notations are slightly different.