Structural Evaluation of AES and Chosen-Key Distinguisher of 9-round AES-128

Pierre-Alain Fouque (1) , Jérémy Jean (2) , Thomas Peyrin (3)

1 ENS-PSL - École normale supérieure - Paris
2 CASCADE - Construction and Analysis of Systems for Confidentiality and Authenticity of Data and Entities
3 NTU - Nanayang Technological University

While the symmetric-key cryptography community has now a good experience on how to build a secure and efficient fixed permutation, it remains an open problem how to design a key-schedule for block ciphers, as shown by the numerous candidates broken in the related-key model or in a hash function setting. Provable security against differential and linear cryptanalysis in the related-key scenario is an important step towards a better understanding of its construction. Using a structural analysis, we show that the full AES-128 cannot be proven secure unless the exact coefficients of the MDS matrix and the S-Box differential properties are taken into account since its structure is vulnerable to a related-key differential attack. We then exhibit a chosen-key distinguisher for AES-128 reduced to 9 rounds, which solves an open problem of the symmetric community. We obtain these results by revisiting algorithmic theory and graph-based ideas to compute all the best differential characteristics in SPN ciphers, with a special focus on AES-like ciphers subject to related-keys. We use a variant of Dijkstra's algorithm to efficiently find the most efficient related-key attacks on SPN ciphers with an algorithm linear in the number of rounds.

Domaines: Cryptographie et sécurité [cs.CR]

Pierre-Alain Fouque, Jérémy Jean, Thomas Peyrin. Structural Evaluation of AES and Chosen-Key Distinguisher of 9-round AES-128. CRYPTO 2013, Aug 2013, Santa Barbara, United States. ⟨hal-00870453⟩