Slovak University of Technology, Bratislava

Faculty of Electrical Engineering and Information Technology

Degree Course: INFORMATICS

Author: Bc. Andrej Bučka

Diploma thesis: Attacks on Stream Ciphers

Supervisor: Ing. Milan Vojvoda

December 2003

The aim of this thesis is to analyse problems of stream ciphers with intention of their cryptanalysis. Nowadays, their importance becomes greater especially with progress in multimedia where they are often used to encipher multimedia data. Strength of stream cipher is based on its keystream, therefore designers of stream ciphers try to build keystream generators with cryptographic features as best as possible. Each design has also its weaknesses which are used by cryptanalysis in order to find out secret key by using selected attacks. In this work, we have concentrated on the realization of corellation attacks with algorithms A and B by Meier and Staffelbach and P.1, P.2 and P.3 by Mihaljevic and Golic. The suitability of application of these algorithms and their modifications - combo-attacks - was verified by careful testing. Results of the submitted thesis can help to decide which algorithm is more suitable to be used in the divide and conquer strategy of an attack.

Keywords: stream cipher, attack, correlation, Algorithm A, Algorithm B, Algorithm P.1, Algorithm P.2, Algorithm P.3, combo-attack, linear feedback shift register