Algebraic attacks have been applied to several types of clock-controlled stream ciphers. However, to date there are no such
attacks in the literature on mutually clock-controlled ciphers. In this paper, we present a preliminary step in this direction
by giving the first algebraic analysis of mutually clock-controlled feedback shift register stream ciphers: the bilateral
stop-and-go generator, A5/1, Alpha 1 and the MICKEY cipher. We show that, if there are no regularly clocked shift registers
included in the system, mutually clock-controlled feedback shift register ciphers appear to be highly resistant to algebraic
attacks. As a demonstration of the weakness inherent in the presence of a regularly clocked shift register, we present a simple
algebraic attack on Alpha 1 based on only 29 keystream bits.
"The majority clocking function used in the A5.1 cipher will be used due to its simplicity and to allow reuse of analyses of A5.1. Moreover, it was extensively analysed algebraically in . Pseudocode for these algorithms can be found in Appendix C. A discussion of alternative design choices for Heraclitus is found in Appendix B "
[Show abstract][Hide abstract] ABSTRACT: Algebraic analysis of A5/2, the weaker version of GSM encryption algorithm, is presented in this article. We have enhanced
existing cryptanalysis of A5/2 in terms of data requirement. Experimental results using an implementation of Groebner basis
algorithm are presented. It has been found that state bits of the cipher can be recovered in fewer number of data frames than
required in a previous efficient attack against GSM communication. Number of data frames required for both known-plaintext
and ciphertext-only attacks can be reduced if linearization is replaced by Groebner basis technique without changing the time
complexity of attack.
[Show abstract][Hide abstract] ABSTRACT: Cryptanalysis is also a tool to measure the strength of a cipher in terms of its resistance against different types of attacks. Every new proposed design ensures at least that it is resistant to the existing attacks. Algebraic attack is now quite a familiar threat for stream ciphers. Moreover, to make out the design components that can strengthen a cipher against algebraic cryptanalysis must also be of interest to stream cipher designers. Algebraic cryptanalysis, in its general form, aims to recover the internal secret state bits of the registers of the cipher by solving non-linear algebraic equations. That is why it is considered to be not applicable on stream ciphers where registers are updated non-linearly. Since in this case degree of algebraic equations, which relate internal states with key-stream bits, increase with each clock. However different designs with nonlinear update may offer disparate levels of resistance. In this article we compare some recently proposed structures of stream ciphers and identify the level of resistance their design shows against gaining the secret internal states. We analyze eSTREAM proposed stream ciphers Grain, Trivium and Mickey and also compare their key generating structures. Mickey and Grain-128 are found to be more resistant than Grain-v1 and Trivium.
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