Abstract:
In Europe and North America, the most widely used stream cipher to ensure privacy and confidentiality of conversations in GSM mobile phones is the A5/1. In this thesis, we study the A5/1 and some known attacks on it. We explore the weaknesses of the cipher and suggest certain modifications to the A5/1 encryption scheme with an aim to create a more secure cryptosystem resistant to most of the attacks already known. We have also designed a new attack on the A5/1 stream cipher with a minimum space complexity of around 2^(40) and an average complexity of 2^(48.5), which is much less than the brute-force attack with a complexity of 2^(64). We provide a detailed description of our new attack along with its implementation and results. Various statistical tests for randomness were performed on the suggested variants of the A5/1 which prove that these modified stream ciphers are pseudo random number generators.
