Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.549581
Title: New attacks on FCSR-based stream ciphers
Author: Ali, Arshad
Awarding Body: University of London
Current Institution: University of London
Date of Award: 2011
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Abstract:
This thesis presents a new family of cryptanalytic attacks on a class of binary additive synchronous stream ciphers, the theory of which is based on the properties of 2-adic numbers. We refer to this new family of cryptanalytic attacks as State Transition Attacks (STAs); we identify three variants of this class of attack, namely Conventional State Transition Attacks (CSTAs), Fast State Transition Attacks (FSTAs) and Improved State Transition Attacks (ISTAs). These attack variants give rise to trade-offs between data, time and memory complexities. The thesis describes STAs on a class of binary additive synchronous stream ciphers whose keystream generators use l-sequences, which are generated by binary Feedback with Carry Shift Registers (FCSRs). A new theory of linearisation intervals for FCSR state update functions is also presented, and results on correlations between the feedback bit and the Hamming weights of the main and carry registers of Galois FCSRs are developed. These theoretical findings are used to cryptanalyse an eSTREAM candidate known as F-FCSR-H v2, as well as two variants of this cipher, known as F-FCSR-H and F-FCSR-16. This cryptanalysis yields State Recovery Algorithms (SRAs) for these ciphers. The cryptanalytic attacks on F-FCSR-H v2, F-FCSR-H and F-FCSR-16 presented in this thesis are the most efficient attacks known so far on these ciphers. The thesis also presents a FCSR key recovery algorithm which works in conjunction with the SRAs in order to recover the effective key used in these ciphers. The thesis also presents various techniques, which can be considered as pre-requisite for simulating new attacks on FCSR-based stream ciphers. In order to describe these techniques, the thesis defines a small-scale variant of the F-FCSR-H type keystream generators and names it as T-cipher. The thesis develops a statistical analysis for the T-cipher and uses it to describe various aspects of the sequences generated by such ciphers. These include computing the frequency distribution of linearisation intervals, formulating and solving systems of equations in these intervals. The thesis further presents enumeration and pseudocode algorithms for solving systems of equations in the finite field F2.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.549581  DOI: Not available
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