Extending Dolev-Yao with Assertions

Cryptographic protocols often require principals to send certifications asserting partial knowledge of terms (for instance, that an encrypted secret is 0 or 1). Such certificates are themselves modelled by cryptographic primitives or sequences of communications. For logical analysis of such protocols based on the Dolev-Yao model [12], we suggest that it is useful to separate terms and assertions about them in communications. We propose a perfect assertion assumption by which the underlying model ensures the correctness of the assertion when it is generated. The recipient may then rely on the certificate but may only forward it as second-hand information. We use a simple propositional modal assertion language involving disjunction (for partial knowledge) and formulas of the form

A

says

α

(for delegation). We study the complexity of the term derivability problem and

safety checking

in the presence of an active intruder (for bounded protocols). We show that assertions add complexity to verification, but when they involve only boundedly many disjunctions, the complexity is the same as that of the standard Dolev-Yao model.