In this article we present a technique for static analysis, correct with respect to complexity-theoretic definitions of security, of cryptographic protocols for checking whether these protocols satisfy confidentiality properties. The approach is similar to Abadi and Rogaway - we define patterns for cryptographic protocols (they did it for formal expressions), such that the protocol is secure iff the patterns are. We then statically analyse the patterns, they should be easier to analyse than the protocols themselves. We consider symmetric encryption as the cryptographic primitive in protocols. Handling this primitive has so far received comparatively less attention in approaches striving to unite the formal and computational models of cryptography.