Deduction Soundness: Prove One, Get Five for Free

Most computational soundness theorems deal with a limited number of primitives, thereby limiting their applicability. The notion of deduction soundness of Cortier and Warinschi (CCS\u2711) aims to facilitate soundness theorems for richer frameworks via composition results: deduction soundness extends, generically, with asymmetric encryption and public data structures. Unfortunately, that paper also hints at rather serious limitations regarding further composition results: composability with digital signatures seems to be precluded. In this paper we provide techniques for bypassing the perceived limitations of deduction soundness and demonstrate that it enjoys vastly improved composition properties. More precisely, we show that a deduction sound implementation can be modularly extended with all of the basic cryptographic primitives (symmetric/asymmetric encryption, message authentication codes, digital signatures, and hash functions). We thus obtain the first soundness framework that allows for the joint use of multiple instances of all of the basic primitives. In addition, we show how to overcome an important restriction of the bare deduction soundness framework which forbids sending encrypted secret keys. In turn, this prevents its use for the analysis of a large class of interesting protocols (e.g. key exchange protocols). We allow for more liberal uses of keys as long as they are hidden in a sense that we also define. All primitives typically used to send secret data (symmetric/asymmetric encryption) satisfy our requirement which we also show to be preserved under composition

A Survey of Symbolic Methods in Computational Analysis of Cryptographic Systems

Since the 1980s, two approaches have been developed for analyzing security protocols. One of the approaches relies on a computational model that considers issues of complexity and probability. This approach captures a strong notion of security, guaranteed against all probabilistic polynomial-time attacks. The other approach relies on a symbolic model of protocol executions in which cryptographic primitives are treated as black boxes. Since the seminal work of Dolev and Yao, it has been realized that this latter approach enables significantly simpler and often automated proofs. However, the guarantees that it offers have been quite unclear. For more than twenty years the two approaches have coexisted but evolved mostly independently. Recently, significant research efforts attempt to develop paradigms for cryptographic systems analysis that combines the best of both worlds. There are two broad directions that have been followed. {\em Computational soundness} aims to establish sufficient conditions under which results obtained using symbolic models imply security under computational models. The {\em direct approach} aims to apply the principles and the techniques developed in the context of symbolic models directly to computational ones. In this paper we survey existing results along both of these directions. Our goal is to provide a rather complete summary that could act as a quick reference for researchers who want to contribute to the field, want to make use of existing results, or just want to get a better picture of what results already exist

How to prove security of communication protocols? A discussion on the soundness of formal models w.r.t. computational ones.

Security protocols are short programs that aim at securing communication over a public network. Their design is known to be error-prone with flaws found years later. That is why they deserve a careful security analysis, with rigorous proofs. Two main lines of research have been (independently) developed to analyse the security of protocols. On the one hand, formal methods provide with symbolic models and often automatic proofs. On the other hand, cryptographic models propose a tighter modeling but proofs are more difficult to write and to check. An approach developed during the last decade consists in bridging the two approaches, showing that symbolic models are sound w.r.t. symbolic ones, yielding strong security guarantees using automatic tools. These results have been developed for several cryptographic primitives (e.g. symmetric and asymmetric encryption, signatures, hash) and security properties. While proving soundness of symbolic models is a very promising approach, several technical details are often not satisfactory. Focusing on symmetric encryption, we describe the difficulties and limitations of the available results

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

Soundness of Symbolic Equivalence for Modular Exponentiation

In this paper, we study the Dynamic Decisional Diffie-Hellman (3DH) problem, a powerful generalization of the Decisional Diffie-Hellman (DDH) problem. Our main result is that DDH implies 3DH. This result leads to significantly simpler proofs for protocols by relying directly on the more general problem. Our second contribution is a computationally sound symbolic technique for reasoning about protocols that use symmetric encryption and modular exponentiation. We show how to apply our results in the case of the Burmester & Desmedt protocol

Formally based semi-automatic implementation of an open security protocol

International audienceThis paper presents an experiment in which an implementation of the client side of the SSH Transport Layer Protocol (SSH-TLP) was semi-automatically derived according to a model-driven development paradigm that leverages formal methods in order to obtain high correctness assurance. The approach used in the experiment starts with the formalization of the protocol at an abstract level. This model is then formally proved to fulfill the desired secrecy and authentication properties by using the ProVerif prover. Finally, a sound Java implementation is semi-automatically derived from the verified model using an enhanced version of the Spi2Java framework. The resulting implementation correctly interoperates with third party servers, and its execution time is comparable with that of other manually developed Java SSH-TLP client implementations. This case study demonstrates that the adopted model-driven approach is viable even for a real security protocol, despite the complexity of the models needed in order to achieve an interoperable implementation