Formally modeling security protocols

Problem

The problem is to develop a formal model of a security protocol using the ACL2 theorem proving system.

The protocol we have in mind is the Needham-Schroeder Public-Key protocol. It is a very simple security protocol that consists of the following three messages.

• Message 1: a -> b : {a.n_a}_PKb
• Message 2: b -> a : {n_a.n_b}_PKa
• Message 3: a -> b : {n_b}_PKb

We will give you a more detailed description if you choose to work on the project.

In order to make explicit our assumptions about the actions an adversary can perform and to increase our confidence in the correctness of the protocol under those assumptions, we use formal techniques. This includes the following steps.

• Modeling the problem formally. This means using a system with an unambiguous semantics and a proof theory. The system we will use is ACL2. ACL2 consists of a programming language based on applicative Common Lisp, a logic, and a theorem prover. The model will be executable and you will therefore be able to simulate various scenarios.
• Defining what it means for the protocol to be correct. For example, someone can guess a private key, but the chances of that happening are low. How do we model this? We will give you informal requirements.
• Proving that the protocol satisfies its specification or exhibiting a counterexample. You can choose to give informal, but rigorous, arguments or you can use ACL2 to give formal, mechanically checked arguments.

To undertake this project, it would help if one has some understanding of security and has some experience with Lisp. Here are some relevant pointers.

• Common Lisp
• ACL2
• Verifying Security Protocols
• Peter Ryan and Steve Schneider. Modelling and Analysis of Security Protocols. Addison Wesley, 2001.

• Protocol model.
• Short report documenting the work, including the proof of correctness.

Evaluation is based on the quality of the model and report.