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

How to explain security protocols to your children

International audienceSecurity protocols combine two key components: a logical structure (who answers what, under which conditions?) as well as cryptography (encryption, signature, hash,. . .). It is not so easy to explain their principles and weaknesses to a non expert audience. Why is something an attack or not? For which attacker? With what purpose? In this paper, we propose an approach to introduce security protocols to a general audience, including children or even scientists from different fields. Its goal is to convey the implicit assumptions of our community, such as threat models or the participants' behaviour. This all-public introduction can be thought of as a story but, interestingly, can also be implemented physically with boxes and padlocks: manipulation helps to understand how protocols operate, even permitting non-expert participants to design their own-and thus to size the challenges of this task

CRYScanner: Finding cryptographic libraries misuse

Cryptographic libraries have become an integral part of every digital device. Studies have shown that these systems are not only vulnerable due to bugs in cryptographic libraries, but also due to misuse of these libraries. In this paper, we focus on vulnerabilities introduced by the application developer. We performed a survey on the potential misusage of well-known libraries such as PKCS #11. We introduced a generic tool CRYScanner, to identify such misuses during and post-development. It works on the similar philosophy of an intrusion detection system for an internal network. This tool provides verification functions needed to check the safety of the code, such as detecting incorrect call flow and input parameters. We performed a feature-wise comparison with the existing state of the art solutions. CRYScanner includes additional features, preserving the capabilities of both static and dynamic analysis tools. We also show the detection of potential vulnerabilities in the several sample codes found online

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 validate the secret of a Ring Learning with Errors (RLWE) key

We use the signal function from RLWE key exchange to derive an efficient zero knowledge authentication protocol to validate an RLWE key $p=as+e$ with secret $s$ and error $e$ in the Random Oracle Model (ROM). With this protocol, a verifier can validate that a key $p$ presented to him by a prover $P$ is of the form $p=as+e$ with $s,e$ small and that the prover knows $s$. We accompany the description of the protocol with proof to show that it has negligible soundness and completeness error. The soundness of our protocol relies directly on the hardness of the RLWE problem. The protocol is applicable for both LWE and RLWE but we focus on the RLWE based protocol for efficiency and practicality. We also present a variant of the main protocol with a commitment scheme to avoid using the ROM

Efficiently deciding equivalence for standard primitives and phases

International audiencePrivacy properties like anonymity or untraceability are now well identified, desirable goals of many security protocols. Such properties are typically stated as equivalence properties. However, automatically checking equivalence of protocols often yields efficiency issues. We propose an efficient algorithm, based on graph planning and SAT-solving. It can decide equivalence for a bounded number of sessions, for protocols with standard cryptographic primitives and phases (often necessary to specify privacy properties), provided protocols are well-typed, that is encrypted messages cannot be confused. The resulting implementation , SAT-Equiv, demonstrates a significant speed-up w.r.t. other existing tools that decide equivalence, covering typically more than 100 sessions. Combined with a previous result, SAT-Equiv can now be used to prove security, for some protocols, for an unbounded number of sessions

Secure Data Sharing and Collaboration in the Cloud

Cloud technology can be leveraged to enable data-sharing capabilities, which can benefit the user through greater productivity and efficiency. However, the Cloud is susceptible to many privacy and security vulnerabilities, which hinders the progress and widescale adoption of data sharing for the purposes of collaboration. Thus, there is a strong demand for data owners to not only ensure that their data is kept private and secure in the Cloud, but to also have a degree of control over their own data contents once they are shared with data consumers. Specifically, the main issues for data sharing in the Cloud include key management, security attacks, and data-owner access control. In terms of key management, it is vital that data must first be encrypted before storage in the Cloud, to prevent privacy and security breaches. However, the management of encryption keys is a great challenge. The sharing of keys with data consumers has proven to be ineffective, especially when considering data-consumer revocation. Security attacks may also prevent the widescale usage of the Cloud for data-sharing purposes. Common security attacks include insider attacks, collusion attacks, and man-in-the-middle attacks. In terms of access control, authorised data consumers could do anything they wish with an owner's data, including sending it to their peers and colleagues without the data owner's knowledge. Throughout this thesis, we investigate ways in which to address these issues. We first propose a key partitioning technique that aims to address the key management problem. We deploy this technique in a number of scenarios, such as remote healthcare management. We also develop secure data-sharing protocols that aim to mitigate and prevent security attacks on the Cloud. Finally, we focus on giving the data owner greater control, by developing a self-controlled software object called SafeProtect