Security Cryptanalysis Of Block Ciphers Based On Related-Key Differential Cryptanalysis And Meet-in-the-Middle Attack

Modern society is rapidly moving towards the fast lane of information technology development.Some emerging digital industries,such as cloud computing and blockchain,are accelerating their landing in many industries.Block ciphers have always undertaken the significant task of protecting information security and maintaining confidentiality,providing essential support for the development of information technology.In various highly-constrained environments,there is an increasing focus on the utilization of lightweight block ciphers,including cryptographic algorithms with Feistel and ARX structures.In order to ensure the security of block ciphers,cryptanalysts must conduct comprehensive research and evaluation on them.Only then can these block ciphers be widely applied with confidence.In this article,the lightweight block cipher LBlock,the upgraded version LBlock-s and the J algorithm are selected for security research.According to their characteristics,this article studies their security against related-key differential cryptanalysis and meet-in-the-middle attack respectively.The main results of this article are as follows:Firstly,this article evaluates the ability of LBlock cipher against related-key differential cryptanalysis more accurately based on the SMT automatic search method.This article proposes tighter lower bounds on the number of active S-boxes for up to 19 rounds of LBlock,which is 8 more rounds than previous works.It also proposes the upper bounds of total probabilities for up to 19 rounds of LBlock for the first time.The security bound obtained by probability is more suitable.Furthermore,this article proposes a method to count the number of active S-boxes in the extended rounds before and after the distinguisher to improve the SMT automatic search model and uses this improved model to find a 17-round related-key differential distinguisher which attacks on 22-and 23-round LBlock.These results are the best related-key differential attack on LBlock so far.Secondly,this article evaluates the ability of LBlock-s cipher against related-key differential cryptanalysis more accurately based on SMT automatic search method.This article uses the improved model to find a 18-round related-key differential distinguisher which attacks on 22-round LBlock-s.This result is the best related-key differential attack on LBlock-s so far.Finally,this article gives the result of the meet-in-the-middle attack for the J algorithm.1～8 rounds of actual key recovery attack have a time complexity with no more than 2.full rounds encryptions and 9～22 rounds of actual key recovery attack have a time complexity with no more than 2 full rounds encryptions.This work obtains the results of the key recovery attack for 1～22 rounds J algorithm,and finally wins the third prize in the 2022 National Cryptomathematics Challenge.