Agence nationale de la sécurité des systèmes d’information

Defended by Joana Treger of cryptography and components laboratory of FNISA on monday 28^th june 2010.

Jury :
 Jean-Sébastien Coron - Univ. Luxembourg
 Pierre-Alain Fouque - ENS
 Henri Gilbert - Orange Labs (Referee)
 Eliane Jaulmes - ANSSI
 Antoine Joux - UVSQ & DGA
 David Naccache - Univ. Paris II & ENS (Referee)
 Jacques Patarin - UVSQ (Director)

The thesis focuses on the security of block ciphers and multivariate schemes.

The first part deals with the study of block ciphers, Feistel networks with internal permutations and Misty-like schemes. The context is generic, in the sense that the internal permutations are supposed random. This allows to obtain properties that only concern the structure of the scheme and do not depend on any particular application. This part focuses on generic attacks on these two schemes.

The second part is about multivariate cryptosystems. A differential property of the public key of HM is shown, resulting in an efficient distinguisher. Moreover, we can invert the system by using Gröbner bases. We also describe a key-recovery attack on HFE, which works for a specific family of secret keys, now known as ``weak keys’’. The overall complexity of this key-recovery attack comes down to solving one instance of the IP problem.

Étude de la Sécurité de Schémas de Chiffrement par Bloc et de Schémas Multivariés

Joana Treger, thèse de doctorat, 28 juin 2010.