Cryptanalysis of 2-cascade finite automata generator with functional key | Prikladnaya Diskretnaya Matematika - Applied Discrete Mathematics. 2018. № 42. DOI: 10.17223/20710410/42/3

A cryptographic generator under consideration is a serial connection G = A? · A₂ of two finite state machines (finite automata) A? = (F^,F₂,g₍,f₍) (it is autonomous) and A₂ = (F₂, Fn, F₂,g₂, f₂). The key of the generator is the function f₍ and possibly the initial states x(1),y(1) of the automata A₍, A₂. The cryptanalysis problem for G is the following: given an output sequence 7 = z(1)z(2)... z(l), find the generator's key. Two algorithms for analysis of A₂ are presented, they allow to find a preimage u(1).. . u(l) of 7 in general case and in the case when A₂ is the Moore automaton with the transition function g₂(u, y) = -ug^ (y) + ug^T(y) for some g : F™ ^ F™ and 5, т e N. This preimage is an input to A₂ and an output from A₍. The values u(t) equal the values f?(x(t)) where x(t) is the state of A? at a time t, t = 1,2,..., l. If the initial state x(1) and a function class C containing f₁ are known, then f₁ can be determined by its specifying in the class C₍.