Affiliation: Sharif University of Technology
A Bit-Vector Differential Model for the Modular Addition by a Constant 📺
ARX algorithms are a class of symmetric-key algorithms constructed by Addition, Rotation, and XOR, which achieve the best software performances in low-end microcontrollers. To evaluate the resistance of an ARX cipher against differential cryptanalysis and its variants, the recent automated methods employ constraint satisfaction solvers, such as SMT solvers, to search for optimal characteristics. The main difficulty to formulate this search as a constraint satisfaction problem is obtaining the differential models of the non-linear operations, that is, the constraints describing the differential probability of each non-linear operation of the cipher. While an efficient bit-vector differential model was obtained for the modular addition with two variable inputs, no differential model for the modular addition by a constant has been proposed so far, preventing ARX ciphers including this operation from being evaluated with automated methods. In this paper, we present the first bit-vector differential model for the n-bit modular addition by a constant input. Our model contains O(log2(n)) basic bit-vector constraints and describes the binary logarithm of the differential probability. We also represent an SMT-based automated method to look for differential characteristics of ARX, including constant additions, and we provide an open-source tool ArxPy to find ARX differential characteristics in a fully automated way. To provide some examples, we have searched for related-key differential characteristics of TEA, XTEA, HIGHT, and LEA, obtaining better results than previous works. Our differential model and our automated tool allow cipher designers to select the best constant inputs for modular additions and cryptanalysts to evaluate the resistance of ARX ciphers against differential attacks.
Weak Composite Diffie-Hellman is not Weaker than Factoring
In1985, Shmuley proposed a theorem about intractability of Composite Diffie-Hellman [Sh85]. The Theorem of Shmuley may be paraphrased as saying that if there exist a probabilistic poly-time oracle machine which solves the Diffie-Hellman modulo an RSA-number with odd-order base then there exist a probabilistic algorithm which factors the modulo. In the other hand factorization of the module obtained only if we can solve the Diffie-Hellman with odd-order base. In this paper we show that even if there exist a probabilistic poly-time oracle machine which solves the problem only for even-order base and abstain answering the problem for odd-order bases still a probabilistic algorithm can be constructed which factors the modulo in poly-time for more than 98% of RSA-numbers.
On the Statistically Optimal Divide and Conquer Correlation Attack on the Shrinking Generator
The shrinking generator is a well-known key stream generator composed of two LFSR?s, LFSRx and LFSRc, where LFSRx is clock-controlled according to the regularly clocked LFSRc. In this paper we investigate the minimum required length of the output sequence for successful reconstruction of the LFSRx initial state in an optimal probabilistic divide and conquer correlation attack. We extract an exact expression for the joint probability of the prefix of length m of the output sequence of LFSRx and prefix of length n of the output sequence of the generator. Then we use computer simulation to compare our probability measure and two other probability measures, previousely proposed in  and , in the sense of minimum required output length. Our simulation results show that our measure reduces the required output length.