SMT-based Cube Attack on Simeck32/64

Satisfiability modulo theories or SMT can be stated as a generalization of Boolean satisfiability problem or SAT. The core idea behind the introduction of SMT solvers is to reduce the complexity through providing more information about the problem environment. In this paper, we take advantage of a similar idea and feed the SMT solver itself, by extra information provided through middle state Cube characteristics, to introduce a new method which we call SMT-based Cube Attack, and apply it to improve the success of the solver in attacking reduced-round versions of the Simeck32/64 lightweight block cipher. We first propose a new algorithm to find cubes with most number of middle state characteristics. Then, we apply these obtained cubes and their characteristics as extra information in the SMT definition of the cryptanalysis problem, to evaluate its effectiveness. Our cryptanalysis results in a full key recovery attack by 64 plaintext/ciphertext pairs on 12 rounds of the cipher in just 122.17 seconds. This is the first practical attack so far presented against the reduced-round versions of Simeck32/64. We also conduct the cube attack on the Simeck32/64 to compare with the SMT-based cube attack. The results indicate that the proposed attack is more powerful than the cube attack

Symmetric block ciphers with a block length of 32 bit

Subject of the thesis at hand is the analysis of symmetric block ciphers with a block length of 32 bit. It is meant to give a comprising overview over the topic of 32 bit block ciphers. The topic is divided in the examination of three questions. It contains a list of state of the art block ciphers with a block length of 32 bit. The block ciphers are being described, focussing on the encryption function. An SPN-based cipher with 32 bit block length is being proposed by rescaling the AES cipher. The 32 bit block length results in certain security issues. These so called risk factors are analysed and mitigating measures are proposed. The result of the thesis is, that 32 bit block ciphers can be implemented in a secure manner. The use of 32 bit ciphers should be limited to specific use-cases and with a profound risk analysis, to determine the protection class of the data to be encrypted