A5/1/2 stream ciphers family is one of the attractive algorithms due to their key role in GSM Crypto-Core protocol. Among them, the original encryption algorithm for GSM was A5/1. However, A5/1 was export restricted, and as the network grew beyond Europe there was a need for an encryption algorithm without export restrictions. As a result, a new encryption algorithm A5/2 was developed. The design of both algorithms was kept secret until the internal design of both A5/1 and A5/2 was reverse engineered from an actual GSM phone by Briceno in 1999. After the reverse engineering of A5/1 and A5/2, it was demonstrated that A5/1 and A5/2 do not provide an adequate level of security for GSM. So far several effective attacks have been introduced on these algorithms, among them, the Correlation and the Algebraic attacks have obtained considerable results on A5/1 and A5/2 algorithms, respectively. For instance, A5/2 was initially cryptanalyzed by Goldberg, Wagner and Green immediately after the reverse engineering. This attack on A5/2 works in a negligible time complexity and it requires only two known-plaintext data frames which are exactly 1326 data frames apart. This reveals the general weaknesses of these algorithms. Nevertheless one of the important issues related to these algorithms is the possibility of cryptanalysis in the presence of reception error while the above attacks assume that there are no reception errors. Accordingly, the objective is to present a method to analyze these stream ciphers even in the presence of error in the received ciphertext. Among the cryptanalysis methods against the stream ciphers, some of them such as the Correlation attacks due to their statistical nature are somewhat resistant against reception error while some others such as the Algebric attacks are vulnerable to this error. As mentioned before, the A5/2 cryptanalysis methods are algebraic and hence the attacks against it are still ineffective to this kind of error. This issue has been studied before, only for erasure bit errors which is a special case of reception error. In this thesis we introduce a new algebraic attack on A5/2 algorithm which is unconditionally applicable to ciphertexts with reception error. Key Words: A5/2 Stream Cipher, Cryptanalysis, Reception error
