Section: Scientific Foundations

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  [80] , [79] , [74] , [75] , [73] , 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  [64] ). Recently, a bunch of attacks  [66] , [85] , [86] , [87] , [88] , [90] , [89] 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.