On the Security of Cryptographic Protocols Using the Little Theorem of Witness Functions

In this paper, we show how practical the little theorem of witness functions is in detecting security flaws in some categories of cryptographic protocols. We convey a formal analysis of the Needham-Schroeder symmetric-key protocol in the theory of witness functions. We show how it helps to warn about a security vulnerability in a given step of this protocol where the value of security of a sensitive ticket in a sent message unexpectedly decreases compared with its value when received. This vulnerability may be exploited by an intruder to mount a replay attack as described by Denning and Sacco.