# Research Activities

## Provable Security

Since the beginning of public-key cryptography, with the seminal Diffie-Hellman paper, many suitable algorithmic problems for cryptography have been proposed and many cryptographic schemes have been designed, together with more or less heuristic proofs of their security relative to the intractability of the underlying problems. However, many of those schemes have thereafter been broken. The simple fact that a cryptographic algorithm withstood cryptanalytic attacks for several years has often been considered as a kind of validation procedure, but schemes may take a long time before being broken. An example is the Chor-Rivest cryptosystem, based on the knapsack problem, which took more than 10 years to be totally broken, whereas before this attack it was believed to be strongly secure. As a consequence, the lack of attacks at some time should never be considered as a full security validation of the proposal.

A completely different paradigm is provided by the concept of “provable” security. A significant line of research has tried to provide proofs in the framework of complexity theory (a.k.a. “reductionist” security proofs): the proofs provide reductions from a well-studied problem (factoring, RSA or the discrete logarithm) to an attack against a cryptographic protocol. At the beginning, researchers just tried to define the security notions required by actual cryptographic schemes, and then to design protocols which could achieve these notions. The techniques were directly derived from complexity theory, providing polynomial reductions. However, their aim was essentially theoretical. They were indeed trying to minimize the required assumptions on the primitives (one-way functions or permutations, possibly trapdoor, etc), without considering practicality. Therefore, they just needed to design a scheme with polynomial-time algorithms, and to exhibit polynomial reductions from the basic mathematical assumption on the hardness of the underlying problem into an attack of the security notion, in an asymptotic way. However, such a result has no practical impact on actual security. Indeed, even with a polynomial reduction, one may be able to break the cryptographic protocol within a few hours, whereas the reduction just leads to an algorithm against the underlying problem which requires many years. Therefore, those reductions only prove the security when very huge (and thus maybe unpractical) parameters are in use, under the assumption that no polynomial time algorithm exists to solve the underlying problem.

For a few years, more efficient reductions have been expected, under the denomination of either “exact security” or “concrete security”, which provide more practical security results. The perfect situation is reached when one is able to prove that, from an attack, one can describe an algorithm against the underlying problem, with almost the same success probability within almost the same amount of time: “tight reductions”. We have then achieved “practical security”. Unfortunately, in many cases, even just provable security is at the cost of an important loss in terms of efficiency for the cryptographic protocol. Thus, some models have been proposed, trying to deal with the security of efficient schemes: some concrete objects are identified with ideal (or black-box) ones. For example, it is by now usual to identify hash functions with ideal random functions, in the so-called “random-oracle model”, informally introduced by Fiat and Shamir, and later formalized by Bellare and Rogaway. Similarly, block ciphers are identified with families of truly random permutations in the “ideal cipher model”. A few years ago, another kind of idealization was introduced in cryptography, the black-box group, where the group operation, in any algebraic group, is defined by a black-box: a new element necessarily comes from the addition (or the subtraction) of two already known elements. It is by now called the “generic model”. Some works even require several ideal models together to provide some new validations.

More recently, the new trend is to get provable security, without such ideal assumptions (there are currently a long list of publications showing “without random oracles” in their title), but under new and possibly stronger computational assumptions. As a consequence, a cryptographer has to deal with the three following important steps:
• computational assumptions, which are the foundations of the security. We thus need to have a strong evidence that the computational problems are reasonably hard to solve. We study several assumptions, by improving algorithms (attacks), and notably using lattice reductions. We furthermore contribute to the list of “potential” hard problems.
• security model, which makes precise the security notions one wants to achieve, as well as the means the adversary may be given. We contribute to this point, in several ways:
• by providing a security model for many primitives and protocols, and namely group-oriented protocols, which involve many parties, but also many communications (group key exchange, group signatures, etc);
• by enhancing some classical security models;
• by considering new means for the adversary, such as side-channel information.
• design of new schemes/protocols, or more efficient, with additional features, etc.
• security proof, which consists in exhibiting a reduction.

For a long time, the security proofs by reduction used classical techniques from complexity theory, with a direct description of the reduction, and then a long and quite technical analysis for providing the probabilistic estimates. Such analysis is unfortunately error-prone. Victor Shoup proposed a nice way to organize the proofs, and eventually obtain the probabilities, using a sequence of games which highlights the computational assumptions, and splits the analysis in small independent problems. We early adopted and developed this technique. We applied this methodology to various kinds of systems, in order to achieve the highest security properties: authenticity, integrity, confidentiality, privacy, anonymity. Nevertheless, efficiency was also a basic requirement.

However, such reductions are notoriously error-prone: errors have been found in many published protocols. Security errors can have serious consequences, such as loss of money in the case of electronic commerce. Moreover, security errors cannot be detected by testing, because they appear only in the presence of a malicious adversary. Security protocols are therefore an important area for formal verification. We thus worked on the development of two successful automatic protocol verifiers, ProVerif in the formal model and CryptoVerif in the computational model, and we plan to pursue research on this topic.

## Cryptanalysis

Because there is no absolute proof of security, it is essential to study cryptanalysis, which is roughly speaking the science of code-breaking. As a result, key-sizes are usually selected based on the state-of-the-art in cryptanalysis. The previous section emphasized that public-key cryptography required hard computational problems: if there is no hard problem, there cannot be any public-key cryptography either. If any of the computational problems mentioned above turns out to be easy to solve, then the corresponding cryptosystems can be broken, as the public key would actually disclose the private key. This means that one obvious way to cryptanalyze is to solve the underlying algorithmic problems, such as integer factorization, discrete logarithm, lattice reduction, Gröbner bases, etc. Here, we mean a study of the computational problem in its full generality. The project-team has a strong expertise (both in design and analysis) on the best algorithms for lattice reduction, which are also very useful to attack classical schemes based on factorization or discrete logarithm.

Alternatively, one may try to exploit the special properties of the cryptographic instances of the computational problem. Even if the underlying general problem is NP-hard, its cryptographic instances may be much easier, because the cryptographic functionalities typically require a specific mathematical structure. In particular, this means that there might be an attack which can only be used to break the scheme, but not to solve the underlying problem in general. This happened many times in knapsack cryptography and multivariate cryptography. Interestingly, generic tools to solve the general problem perform sometimes even much better on cryptographic instances (this happened for Gröbner bases and lattice reduction).

However, if the underlying computational problem turns out to be really hard both in general and for instances of cryptographic interest, this will not necessarily imply that the cryptosystem is secure. First of all, it is not even clear what is meant exactly by the term secure or insecure . Should an encryption scheme which leaks the first bit of the plaintext be considered secure? Is the secret key really necessary to decrypt ciphertexts or to sign messages? If a cryptosystem is theoretically secure, could there be potential security flaws for its implementation? For instance, if some of the temporary variables (such as pseudo-random numbers) used during the cryptographic operations are partially leaked, could it have an impact on the security of the cryptosystem? This means that there is much more into cryptanalysis than just trying to solve the main algorithmic problems. In particular, cryptanalysts are interested in defining and studying realistic environments for attacks (adaptive chosen-ciphertext attacks, side-channel attacks, etc. ), as well as goals of attacks (key recovery, partial information, existential forgery, distinguishability, etc. ). As such, there are obvious connections with provable security. It is perhaps worth noting that cryptanalysis also proved to be a good incentive for the introduction of new techniques in cryptology. Indeed, several mathematical objects now considered invaluable in cryptographic design were first introduced in cryptology as cryptanalytic tools, including lattices and pairings. The project-team has a strong expertise in cryptanalysis: many schemes have been broken, and new techniques have been developed.

## Symmetric Cryptography

Even if asymmetric cryptography has been a major breakthrough in cryptography, and a key element in its recent development, conventional cryptography (a.k.a. symmetric, or secret key cryptography) is still required in any application: asymmetric cryptography is much more powerful and convenient, since it allows signatures, key exchange, etc. However, it is not well-suited for high-rate communication links, such as video or audio streaming. Therefore, block-ciphers remain a fundamental primitive. However, since the AES Competition (which started in January 1997, and eventually selected the Rijndael algorithm in October 2000), this domain has become less active, even though some researchers are still trying to develop new attacks. On the opposite, because of the lack of widely admitted stream ciphers (able to encrypt high-speed streams of data), ECRYPT (the European Network of Excellence in Cryptology) launched the eSTREAM project, which investigated research on this topic, at the international level: many teams proposed candidates that have been analyzed by the entire cryptographic community. Similarly, in the last few years, hash functions, which are an essential primitive in many protocols, received a lot of attention: they were initially used for improving efficiency in signature schemes, hence the requirement of collision-resistance. But afterwards, hash functions have been used for many purposes, such as key derivation, random generation, and random functions (random oracles). Recently, a bunch of attacks have shown several drastic weaknesses on all known hash functions. Knowing more (how weak they are) about them, but also building new hash functions are major challenges. For the latter goal, the first task is to formally define a security model for hash functions, since no realistic formal model exists at the moment: in a way, we expect too much from hash functions, and it is therefore impossible to design such “ideal” functions. Because of the high priority of this goal (the design of a new hash function), the NIST has launched an international competition, called SHA-3 (similar to the AES competition 10 years ago), in order to select and standardize a hash function in 2012.

One way to design new hash functions may be a new mode of operation, which would involve a block cipher, iterated in a specific manner. This is already used to build stream ciphers and message authentication codes (symmetric authentication). Under some assumptions on the block cipher, it might be possible to apply the above methodology of provable security in order to prove the validity of the new design, according to a specific security model.

# Activity Reports

If you want a precise description of our activity, you may read the activity reports for the years

# Software Developpement

• ProVerif : automatic protocol verifier in the formal model.
• CryptoVerif : automatic protocol verifier in the computational model.

# International Collaborations

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