The aim of the IACR Ph.D. database is twofold. On the first hand, we want to offer an overview of Ph.D. already completed
in the domain of cryptology. Where possible, this should also include a subject classification, an abstract, and
access to the full text.
On the second hand, it deals with Ph.D. subjects
currently under investigation. This way, we provide a timely
map of contemporary research in cryptology.
All entries or changes need to be approved by an editor. You can contact them via phds (at) iacr.org.
Arshad Ali (#741)
Topic of his/her doctorate.
New Attacks on FCSR-based Stream Ciphers
Year of completion
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 eective 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.
Arshad.Ali.2008 (at) live.rhul.ac.uk