Moses Liskov

Alan T. Sherman, Erin Lanus, Moses Liskov et al.

We analyze the Secure Remote Password (SRP) protocol for structural weaknesses using the Cryptographic Protocol Shapes Analyzer (CPSA) in the first formal analysis of SRP (specifically, Version 3). SRP is a widely deployed Password Authenticated Key Exchange (PAKE) protocol used in 1Password, iCloud Keychain, and other products. As with many PAKE protocols, two participants use knowledge of a pre-shared password to authenticate each other and establish a session key. SRP aims to resist dictionary attacks, not store plaintext-equivalent passwords on the server, avoid patent infringement, and avoid export controls by not using encryption. Formal analysis of SRP is challenging in part because existing tools provide no simple way to reason about its use of the mathematical expression $v + g^b \mod q$. Modeling $v + g^b$ as encryption, we complete an exhaustive study of all possible execution sequences of SRP. Ignoring possible algebraic attacks, this analysis detects no major structural weakness, and in particular no leakage of any secrets. We do uncover one notable weakness of SRP, which follows from its design constraints. It is possible for a malicious server to fake an authentication session with a client, without the client's participation. This action might facilitate an escalation of privilege attack, if the client has higher privileges than does the server. We conceived of this attack before we used CPSA and confirmed it by generating corresponding execution shapes using CPSA.

Moses D. Liskov, Joshua D. Guttman, John D. Ramsdell et al.

Enrich-by-need protocol analysis is a style of symbolic protocol analysis that characterizes all executions of a protocol that extend a given scenario. In effect, it computes a strongest security goal the protocol achieves in that scenario. CPSA, a Cryptographic Protocol Shapes Analyzer, implements enrich-by-need protocol analysis. In this paper, we describe how to analyze protocols using the Diffie-Hellman mechanism for key agreement (DH) in the enrich-by-need style. DH, while widespread, has been challenging for protocol analysis because of its algebraic structure. DH essentially involves fields and cyclic groups, which do not fit the standard foundational framework of symbolic protocol analysis. By contrast, we justify our analysis via an algebraically natural model. This foundation makes the extended CPSA implementation reliable. Moreover, it provides informative and efficient results. An appendix explains how unification is efficiently done in our framework.

Joshua D. Guttman, Moses D. Liskov, John D. Ramsdell et al.

Many cryptographic protocols are designed to achieve their goals using only messages passed over an open network. Numerous tools, based on well-understood foundations, exist for the design and analysis of protocols that rely purely on message passing. However, these tools encounter difficulties when faced with protocols that rely on non-local, mutable state to coordinate several local sessions. We adapt one of these tools, {\cpsa}, to provide automated support for reasoning about state. We use Ryan's Envelope Protocol as an example to demonstrate how the message-passing reasoning can be integrated with state reasoning to yield interesting and powerful results. Keywords: protocol analysis tools, stateful protocols, TPM, PKCS#11.