Analysing Password Protocol Security Against Off-line Dictionary Attacks

We study the security of password protocols against off-line dictionary attacks. In addition to the standard adversary abilities, we also consider further cryptographic advantages given to the adversary when considering the password protocol being instantiated with particular encryption schemes. We work with the applied pi-calculus of Abadi and Fournet, in which the (new) adversary abilities are modelled as equations between terms. As case studies, we analyse the Encrypted Password Transmission (EPT) protocol of Halevi and Krawczyk, and the wellknown Encrypted Key (EKE) of Bellovin and Merritt. In the latter, we find an attack that arises when considering the ability of distinguishing ciphertexts from random noise. We propose a modification to EKE that prevents this attack

Crypto-Verifying Protocol Implementations in ML

We intend to narrow the gap between concrete implementations and verified models of cryptographic protocols. We consider protocols implemented in F#, a variant of ML, and verified using CryptoVerif, Blanchet's protocol verifier for computational cryptography. We experiment with compilers from F# code to CryptoVerif processes, and from CryptoVerif declarations to F# code. We present two case studies: an implementation of the Otway-Rees protocol, and an implementation of a simplified password-based authentication protocol. In both cases, we obtain concrete security guarantees for a computational model closely related to executable code

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

YAPA: A generic tool for computing intruder knowledge

Reasoning about the knowledge of an attacker is a necessary step in many formal analyses of security protocols. In the framework of the applied pi calculus, as in similar languages based on equational logics, knowledge is typically expressed by two relations: deducibility and static equivalence. Several decision procedures have been proposed for these relations under a variety of equational theories. However, each theory has its particular algorithm, and none has been implemented so far. We provide a generic procedure for deducibility and static equivalence that takes as input any convergent rewrite system. We show that our algorithm covers most of the existing decision procedures for convergent theories. We also provide an efficient implementation, and compare it briefly with the tools ProVerif and KiSs

Analysing Password Protocol Security Against Off-line Dictionary Attacks

We study the security of password protocols against off-line dictionary attacks. In addition to the standard adversary abilities, we also consider further cryptographic advantages given to the adversary when considering the password protocol being instantiated with particular encryption schemes. We work with the applied pi-calculus of Abadi and Fournet, in which the (new) adversary abilities are modelled as equations between terms. As case studies, we analyse the Encrypted Password Transmission (EPT) protocol of Halevi and Krawczyk, and the wellknown Encrypted Key (EKE) of Bellovin and Merritt. In the latter, we find an attack that arises when considering the ability of distinguishing ciphertexts from random noise. We propose a modification to EKE that prevents this attack

Reducing Equational Theories for the Decision of Static Equivalence

International audienceStatic equivalence is a well established notion of indistinguishability of sequences of terms which is useful in the symbolic analysis of cryptographic protocols. Static equivalence modulo equational theories allows for a more accurate representation of cryptographic primitives by modelling properties of operators by equational axioms. We develop a method that allows us in some cases to simplify the task of deciding static equivalence in a multi-sorted setting, by removing a symbol from the term signature and reducing the problem to several simpler equational theories. We illustrate our technique at hand of bilinear pairings

Data Minimisation in Communication Protocols: A Formal Analysis Framework and Application to Identity Management

With the growing amount of personal information exchanged over the Internet, privacy is becoming more and more a concern for users. One of the key principles in protecting privacy is data minimisation. This principle requires that only the minimum amount of information necessary to accomplish a certain goal is collected and processed. "Privacy-enhancing" communication protocols have been proposed to guarantee data minimisation in a wide range of applications. However, currently there is no satisfactory way to assess and compare the privacy they offer in a precise way: existing analyses are either too informal and high-level, or specific for one particular system. In this work, we propose a general formal framework to analyse and compare communication protocols with respect to privacy by data minimisation. Privacy requirements are formalised independent of a particular protocol in terms of the knowledge of (coalitions of) actors in a three-layer model of personal information. These requirements are then verified automatically for particular protocols by computing this knowledge from a description of their communication. We validate our framework in an identity management (IdM) case study. As IdM systems are used more and more to satisfy the increasing need for reliable on-line identification and authentication, privacy is becoming an increasingly critical issue. We use our framework to analyse and compare four identity management systems. Finally, we discuss the completeness and (re)usability of the proposed framework