Abstract. Proofs of security protocols typically employ simple abstractions of cryptographic operations, so that large parts of such proofs pendent of cryptographic details. The typical abstraction is the Dolev-Yao model, which treats cryptographic operations as a specific term algebra. However, there is no cryptographic semantics, i.e., no thet says what a proof with the Dolev-Yao abstraction implies for the real protocol, even if provably secure cryptographic primitives are used. Recently we introduced an extension to the Dolev-Yao model for which such a cryptographic semantics exists, i.e., where security is preserved bstractions are instantiated with provably secure cryptographic primitives. Only asymmetric operations (digital signatures and publickey encryption) are considered so far. Here we extend this model to include a first symmetric primitive, message authentication, and prove that the extended model still has all desired properties. The proof is a combination of a probabi...