| Abstract: | Metaheuristic algorithms are general algorithms allowing to solve various types of computational
problems, usually optimization ones. In the dissertation, new versions of selected
metaheuristic algorithms were developed: Tabu Search and Ant Colony Optimization algorithms.
They have been adapted to solve the problem of cryptanalysis of stream ciphers, which
are an important element of data protection processed and stored in information systems.
Attempts to hide information from unauthorized persons have a long history. As early as
the 5th century BC there was a simple Atbash substitution cipher among the Hebrew scholars.
Although a lot has changed since then, and the art of encrypting information has undergone
a significant transformation, the issue of confidentiality of communication is still important.
Encryption is used wherever protection of transmitted or stored data, especially in information
systems, is of key importance. Encryption is used when talking on the phone or logging in via
the Internet to a bank account. It is also of great importance in the military. Encryption is an
issue with a long history, still important and topical.
The proposed Tabu Search and Ant Colony Optimization algorithms adapted to cryptanalysis
were tested using three stream ciphers: RC4, VMPC and RC4+. This enabled the development
of an attack independent of the design of the cipher itself, assuming that the internal state
of the cipher can be represented as a permutation of numbers from a given range.
For all proposed metaheuristic algorithms, four types of fitness functions have been tested,
three of which are original ones. The original fitness functions enabled achieving better results
for all three analysed metaheuristic algorithms compared to a function known from the
literature.
Each of the proposed algorithms were tested in terms of the impact of parameters values on
the results they achieved. Also the results achieved by all three metaheuristic algorithms were
compared to one another. The results obtained during cryptanalysis of smaller and full versions
of the analysed ciphers with the use of Tabu Search were compared with the results obtained
by other metaheuristic algorithms, showing that Tabu Search leads to better results than other
metaheuristics. The results obtained using the Tabu Search algorithm were also compared to
attacks known from the literature on selected stream ciphers. The results of the experiments
indicate that for the VMPC and RC4+ ciphers, the proposed cryptanalysis algorithm using Tabu
Search may be better than the cryptanalysis algorithms known so far. The results achieved by
other metaheuristic algorithms considered were not as good as for Tabu Search, although it
cannot be ruled out that further enhancement of these algorithms could improve the results. |