Analyse de protocoles cryptographiques par des méthodes sémantiques

Belief-logic deductions are used in the analysis of cryptographic protocols. We show a new method to decide such logics. In addition to the familiar BAN logic, it is also applicable to the more advanced versions of protocol security logics, and GNY in particular; and it employs an efficient forward-chaining algorithm the completeness and termination of which are proved. Theoretic proofs, implementation decisions and results are discussed.

Cryptographic protocols have so far been analyzed for the most part by means of testing (which does not yield proofs of secrecy) and theorem proving (costly). We propose a new approach, based on abstract interpretation and using regular tree languages. The abstraction we use seems fine-grained enough to be able to certify some protocols. Both the concrete and abstract semantics of the protocol description language and implementation issues are discussed in the paper.

When designing a cryptographic protocol or explaining it, one often uses arguments such as “since this message was signed by machine B, machine A can be sure it came from B” in informal proofs justifying how the protocol works. Since it is, in such informal proofs, often easy to overlook an essential assumption, such as a trust relation or the belief that a message is not a replay from a previous session, it seems desirable to write such proofs in a formal system. While such logics do not replace the recent techniques of automatic proofs of safety properties, they help in pointing the weaknesses of the system. In this paper, we present briefly the BAN (Burrows - Abadi - Needham) formal system [?] as well as some derivative. We show how to prove some properties of a simple protocol, as well as detecting undesirable assumptions. We then explain how the manual search for proofs can be made automatic. Finally, we explain how the lack of proper semantics can be a bit worrying.

Questions of belief are essential in analyzing protocols for authentication in distributed computing systems. In this paper we motivate, set out, and exemplify a logic specifically designed for this analysis; we show how various protocols differ subtly with respect to the required initial assumptions of the participants and their final beliefs. Our formalism has enabled us to isolate and express these differences with a precision that was not previously possible. It has drawn attention to features of protocols of which we and their authors were previously unaware, and allowed us to suggest improvements to the protocols. The reasoning about some protocols has been mechanically verified. This paper starts with an informal account of the problem, goes on to explain the formalism to be used, and gives examples of its application to protocols from the literature, both with conventional shared-key cryptography and with public-key cryptography. Some of the examples are chosen because of their practical importance, while others serve to illustrate subtle points of the logic and to explain how we use it. We discuss extensions of the logic motivated by actual practice - for example, in order to account for the use of hash functions in signatures. The final sections contain a formal semantics of the logic and some conclusions.

Keywords: BAN logic, GNY logic, cryptography, authentication, formal methods