Computationally Sound Compositional Logic for Security Protocols

We have been developing a cryptographically sound formal logic for proving protocol security properties without explicitly reasoning about probability, asymptotic complexity, or the actions of a malicious attacker. The approach rests on a probabilistic, polynomial-time semantics for a protocol security logic that was originally developed using nondeterministic symbolic semantics. This workshop presentation will discuss ways in which the computational semantics lead to different reasoning methods and report our progress to date in several directions. One significant difference between the symbolic and computational settings results from the computational difference between efficiently recognizing and efficiently producing a value. Among the more recent developments are a compositional method for proving cryptographically sound properties of key exchange protocols, and some work on secrecy properties that illustrates the computational interpretation of inductive properties of protocol roles

Quantifying pervasive authentication: the case of the Hancke-Kuhn protocol

As mobile devices pervade physical space, the familiar authentication patterns are becoming insufficient: besides entity authentication, many applications require, e.g., location authentication. Many interesting protocols have been proposed and implemented to provide such strengthened forms of authentication, but there are very few proofs that such protocols satisfy the required security properties. The logical formalisms, devised for reasoning about security protocols on standard computer networks, turn out to be difficult to adapt for reasoning about hybrid protocols, used in pervasive and heterogenous networks. We refine the Dolev-Yao-style algebraic method for protocol analysis by a probabilistic model of guessing, needed to analyze protocols that mix weak cryptography with physical properties of nonstandard communication channels. Applying this model, we provide a precise security proof for a proximity authentication protocol, due to Hancke and Kuhn, that uses a subtle form of probabilistic reasoning to achieve its goals.Comment: 31 pages, 2 figures; short version of this paper appeared in the Proceedings of MFPS 201

Modeling Adversaries in a Logic for Security Protocol Analysis

Logics for security protocol analysis require the formalization of an adversary model that specifies the capabilities of adversaries. A common model is the Dolev-Yao model, which considers only adversaries that can compose and replay messages, and decipher them with known keys. The Dolev-Yao model is a useful abstraction, but it suffers from some drawbacks: it cannot handle the adversary knowing protocol-specific information, and it cannot handle probabilistic notions, such as the adversary attempting to guess the keys. We show how we can analyze security protocols under different adversary models by using a logic with a notion of algorithmic knowledge. Roughly speaking, adversaries are assumed to use algorithms to compute their knowledge; adversary capabilities are captured by suitable restrictions on the algorithms used. We show how we can model the standard Dolev-Yao adversary in this setting, and how we can capture more general capabilities including protocol-specific knowledge and guesses.Comment: 23 pages. A preliminary version appeared in the proceedings of FaSec'0

Modular code-based cryptographic verification

International audienceType systems are effective tools for verifying the security of cryptographic programs. They provide automation, modularity and scalability, and have been applied to large security protocols. However, they traditionally rely on abstract assumptions on the underlying cryptographic primitives, expressed in symbolic models. Cryptographers usually reason on security assumptions using lower level, computational models that precisely account for the complexity and success probability of attacks. These models are more realistic, but they are harder to formalize and automate. We present the first modular automated program verification method based on standard cryptographic assumptions. We show how to verify ideal functionalities and protocols written in ML by typing them against new cryptographic interfaces using F7, a refinement type checker coupled with an SMT-solver. We develop a probabilistic core calculus for F7 and formalize its type safety in Coq. We build typed module and interfaces for MACs, signatures, and encryptions, and establish their authenticity and secrecy properties. We relate their ideal functionalities and concrete implementations, using game-based program transformations behind typed interfaces. We illustrate our method on a series of protocol implementations

CryptoVerif: a Computationally-Sound Security Protocol Verifier (Initial Version with Communications on Channels)

This document presents the security protocol verifier CryptoVerif.CryptoVerif does not rely on the symbolic, Dolev-Yao model, but on the computational model. It can verify secrecy, correspondence (which include authentication), and indistinguishability properties. It produces proofs presented as sequences of games, like those manually written by cryptographers; these games are formalized in aprobabilistic process calculus. CryptoVerif provides a generic method for specifying security properties of the cryptographic primitives.It produces proofs valid for any number of sessions of the protocol, and provides an upper bound on the probability of success of an attack against the protocol as a function of the probability of breaking each primitive and of the number of sessions. It can work automatically, or the user can guide it with manual proof indications

Probabilistic Polynomial-time Semantics for a Protocol Security Logic

We describe a cryptographically sound formal logic for proving protocol security properties without explicitly reasoning about probability, asymptotic complexity, or the actions of a malicious attacker. The approach rests on a new probabilistic, polynomial-time semantics for an existing protocol security logic, replacing an earlier semantics that uses nondeterministic symbolic evaluation. While the basic form of the protocol logic remains unchanged from previous work, there are some interesting technical problems involving the difference between efficiently recognizing and efficiently producing a value, and involving a reinterpretation of standard logical connectives that seems necessary to support certain forms of reasoning

Logical concepts in cryptography

This thesis is about a breadth-first exploration of logical concepts in cryptography and their linguistic abstraction and model-theoretic combination in a comprehensive logical system, called CPL (for Cryptographic Protocol Logic). We focus on two fundamental aspects of cryptography. Namely, the security of communication (as opposed to security of storage) and cryptographic protocols (as opposed to cryptographic operators). The primary logical concepts explored are the following: the modal concepts of belief, knowledge, norms, provability, space, and time. The distinguishing feature of CPL is that it unifies and refines a variety of existing approaches. This feature is the result of our wholistic conception of property-based (modal logics) and model-based (process algebra) formalisms