Security Analysis On Three Typical Kinds Of Hybrid Symmetric Ciphers

For the recent decade, the research on symmetric ciphers has got rapid improvement andmany novel design principles appeared. Among which a new design rationale that combining theadvantages of stream ciphers and the advantages of block ciphers is becoming an importantdesign rationale and a new tendency evidently. According to the structure, design rationale andfunctions, we divide the hybrid symmetric ciphers into three categories: Dual character hybridsymmetric ciphers, Principle borrowing hybrid symmetric ciphers and Function enhancinghybrid symmetric ciphers. In this thesis, we presented security analysis on the typical algorithmsof these three categories, i.e. SCB, CSA, Hummingbird-2and QUARK family ciphers. The mainworks and creations are as follows:(1) Security analysis on Dual character hybrid symmetric ciphers SCB and CSA algorithm.For SCB algorithm, we first find the linear relationship between the keystream bits andinternal states of the registers, then we use the guess-and-determine method to present an attackon the stream cipher part of SCB, we need totally1832-bit keystream words and thecomputational complexity to recover256-bit key is O(244). Combining with the cryptanalysis ofthe stream cipher part of SCB, we attack the whole algorithm with differential fault analysismethod, when we induce single bit random fault640times, the success rate to recover the keycan reach99.4%.For CSA algorithm, we first proposed a slide resynchronization attack on the stream cipherpart of CSA, when we use215pairs of differential keys, the success rate can reach98%, we canalso recover9bits of the key at the same time. Then we presented an impossible differentialattack on the block cipher part reduced to21rounds, the computational complexity is O(222.7),data complexity is O(244.5) and memory complexity is O(210.5), the attack can recover16bits ofthe key. What’s more, we found a flaw on the structure of CSA which makes it possible topresent an impossible differential cryptanalysis on CSA with block cipher part reduced to21rounds. The computational complexity is O(221.7), data complexity is O(243.5) and memorycomplexity is O(210.5), we can recover8bits of the key accordingly.(2) Security analysis on Principle borrowing hybrid symmetric cipher Hummingbird-2.We proposed a differential-related key attack on Hummingbird-2, under15pairs of relatedkeys, the attack demands229.7chosen IV, the computational complexity to recover the128-bitkey is O(229.7). The success rate of our attack is97%. The experimental result corroborates ourattack and shows that under related key attack model, Hummingbird-2can be broken in real time on a single PC, the process needs about6.3minutes in average.(3) Security analysis on Function enhancing hybrid symmetric ciphers QUARK familyciphers and lately proposed C-QUARK.First of all, we proposed an improved conditional differential cryptanalysis method.Concretely speaking, we proposed a new concept named “Free Diffusion Degree” to realize thefiltration of the interim differential state. Otherwise, according to the “Least propagationcriterion”, we proposed an optimized backward computation algorithm, which can realize morerounds of the backward computation. For these two improvements, we proposed an improvedconditional differential cryptanalysis method and then apply it into analyzing the QUARK familyciphers and C-QUARK. For QUARK family ciphers, under the same advantage and datacomplexity, we attacked10to30more rounds than the specification. For C-QUARK, under thesame advantage and data complexity, we attack52more rounds than the specification.