Cryptanalysis Of MISTY1Block Cipher

The block cipher is one of the most important basic to ensure modern information security.The block cipher algorithms have been widely used in commercial environment such as AES、MISTY1and KASUMI. Most of the algorithms have withstood all kinds of cryptographicalattacks, and no efficient attack exists so far. Consequently, as an important block cipher,MISTY1block cipher has been used in commercial secure products as well as a recommendede-government cipher, so the study on cryptanalysis of MISTY1is very significant for academicresearch and practical application.This dissertation focuses on studying of the key schedule algorithm and the non-linearfunction FI of MISTY1and looks for the weak-key classes. Three results are presented: therelated-key differential attacks, a related-key amplified boomerang attack on MISTY1and theslide attacks on MISTY1variants. The main works are as follows.1. Utilizing the characteristic of the key schedule algorithm of MISTY1and anlysizing thenon-linear function FI, a weak-key class of MISTY1is found. Based on the weak-key class,6-round and7-round related-key differential characteristics are presented, which lead torelated-key differential attacks on7-round MISTY1and8-round MISTY1(without the first FLlayer). The two attacks requires238and261chosen ciphertexts respectively, and have thecomplexity of238.5and284.6encryptions respectively.2. Analysing the key schedule algorithm of MISTY1, a weak-key class of MISTY1is found,which encompasses290weak keys. Based on the weak-key class, we present a7-roundrelated-key amplified boomerang distinguisher of MISTY1, which leads to an attack on the8-round MISTY1without the last FL layer. The attack requires263chosen plaintexts, and thetime complexity of the attack is270.3. Observing and analysing the distribution of subkeys of MISTY1, slide attack on MISTY1variants is presented. On the one hand, with ignoring the FL layers, there are related keys ofMISTY1for the slide attack, which requires222chosen plaintexts and the time complexity is235.5encryptions. On the other hand, by resetting the subkeys in the FL layers, the slide attackalso can be used to attack on MISTY1variant with the FL layers, which requires242chosenplaintexts and the time complexity is251.3encryptions. The analysis shows that the FL layersplay an important role in MISTY1.