4737

4456

كارشناسي ارشد

مخابرات

اصفهان: دانشگاه صنعتي اصفهان، دانشكده برق و كامپيوتر

1388

نه،86،[II]ص.:نمودار،جدول

ص.ع.به فارسي و انگليسي

محمد دخيل عليان

محمود مدرس هاشمي

88/8/16

مهندسي برق و كامپيوتر

ID4456

به فارسي و انگليسي: قابل رويت در نسخه ديجيتالي

A5 2 Algorithm Cryptanalysis in the presence of reception error Mohsen Shakiba m shakiba@ec iut ac ir Date of Submission 2009 04 18 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor M Dakhil Alian mdalian@cc iut ac irAbstract A5 1 2 stream ciphers family is one of the attractive algorithms due to their key role in GSMCrypto 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 foran encryption algorithm without export restrictions As a result a new encryption algorithm A5 2was developed The design of both algorithms was kept secret until the internal design of bothA5 1 and A5 2 was reverse engineered from an actual GSM phone by Briceno in 1999 After thereverse engineering of A5 1 and A5 2 it was demonstrated that A5 1 and A5 2 do not provide anadequate level of security for GSM So far several effective attacks have been introduced on thesealgorithms among them the Correlation and the Algebraic attacks have obtained considerableresults on A5 1 and A5 2 algorithms respectively For instance A5 2 was initially cryptanalyzedby Goldberg Wagner and Green immediately after the reverse engineering This attack on A5 2works in a negligible time complexity and it requires only two known plaintext data frames whichare 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 ofcryptanalysis in the presence of reception error while the above attacks assume that there are noreception errors Accordingly the objective is to present a method to analyze these stream cipherseven in the presence of error in the received ciphertext Among the cryptanalysis methods againstthe stream ciphers some of them such as the Correlation attacks due to their statistical nature aresomewhat resistant against reception error while some others such as the Algebric attacks arevulnerable to this error As mentioned before the A5 2 cryptanalysis methods are algebraic andhence the attacks against it are still ineffective to this kind of error This issue has been studiedbefore only for erasure bit errors which is a special case of reception error In this thesis weintroduce a new algebraic attack on A5 2 algorithm which is unconditionally applicable tociphertexts with reception error Key Words A5 2 Stream Cipher Cryptanalysis Reception error

محمد دخيل عليان

محمود مدرس هاشمي