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Mathematical Methods of Cryptography
Cryptautomata with functional keys
G. P. Agibalov National Research Tomsk State University, Tomsk, Russia
Abstract:
In this paper, we describe the cryptautomata and some cryptanalysis techniques for them. In cryptographic systems, the cryptautomata are widely used as its primitives including key-stream generators, s-boxes, cryptofilters, cryptocombiners, key hash functions as well as symmetric and public-key ciphers, digital signature schemes. Here, a cryptautomaton is defined as a class
$C$ of automata networks of a fixed structure
$N$ constructed by means of the series, parallel, and feedback connection operations over initial finite automata (finite state machines) with transition and output functions taken from some predetermined functional classes. A cryptautomaton key can include initial states, transition and output functions of some components in
$N$. The choosing a certain key
$k$ produces a certain network
$N_k$ from
$C$ to be a cryptographic algorithm. In case of invertibility of
$N_k$, this algorithm can be used for encryption. The operation (functioning) of any network
$N_k$ in the discrete time is described by the canonical system of equations of its automaton. The structure of
$N_k$ is described by the union of canonical systems of equations of its components. The cryptanalysis problems for a cryptautomaton are considered as the problems of solving the operational or structural system of equations of
$N_k$ with the corresponding unknowns that are key
$k$ variables and (or) plaintexts (input sequences). For solving such a system
$E$, the method DSS is used. It is the iteration of the following three actions: 1)
$E$ is Divided into subsystems
$E'$ and
$E''$, where
$E'$ is easy solvable; 2)
$E'$ is Solved; 3) the solutions of
$E'$ are Substituted into
$E''$ by turns. The definition and cryptanalysis of a cryptautomaton are illustrated by giving the example of the autonomous cryptautomaton with the alternative control. It is a generalization of the LFSR-based cryptographic alternating step generator. We present a number of attacks on this cryptautomaton with the states or output functions of its components as a key.
Keywords:
finite automaton, automata network, cryptautomaton, cryptautomaton generator with alternative control, cryptanalysis, linearization attack, “devide-and-solve-and-substitute”, partially defined function completion.
UDC:
519.7
DOI:
10.17223/20710410/36/5