Abstract:

It is important to evaluate the security of symmetric encryption algorithms used in various application scenarios for protecting data securely.Simpira v2 is a family of cryptographic permutations with a high throughput proposed in ASIACRYPT 2016.It is very suitable for protecting the confidentiality of data in the information system.Simpira-6 is the case of 6 branches in the Simpira v2 encryption algorithm family,and its block length supports bits.This paper studies the security analysis of Simpira-6 as the permutation algorithm of Even-Mansour structure against impossible differential attacks.First,we propose the longest 9-round impossible differential for Simpira-6 currently,on the basis of which the adversary executes the impossible differential attack,whose time complexity is higher than that of the exhaustive search.Second,under the security claim of Simpira v2,we present a 7-round impossible differential attack on Simpira-6 to recover the 384-bit master key.The data and time complexities of this attack are 2^57.07 chosen plaintexts and 2^57.07 7-round Simpira-6 encryptions,respectively.Third,under the security claim of Even-Mansour,we present an 8-round impossible differential attack on Simpira-6 to recover all 768 bits keys.The data and time complexities are 2¹⁶⁸ chosen plaintexts and 2¹⁶⁸ 8-round Simpira-6 encryptions.Those attacks are the first analytical result on Simpira-6 against the impossible differential attack.These results provide an important theoretical foundation for the application of Simpira v2 in future.

Key words: block cipher cryptographic systems, impossible differential attacks, security analysis, Simpira v2, Generalized Feistel Structure, Even-Mansour structure, security claim