A modern rotor machine

Abstract

The cryptologic literature contains a lot of material on both shift register and rotor machine systems. It is natural to wonder whether these two types of mechanism can be combined in one robust design. During the 1980's, research in clock-controlled shift registers was inspired by rotor machines of the Hagelin type [G1], and a t-shirt appeared at a Crypto conference with a design consisting of a shift register and five rotors [D]. More recently, one writer proposed to filter three linear generators A , B , and C with two permutations ~ and p in order to get a keystream K = A + ~ ( B + pC) IF]; and a rump session paper at Eurocrypt this year showed that if a shift register sequence is filtered through a permutation which acts on m-bit symbols, then a correlation at tack will need m times as many bits as before [B1]. In this article, we propose a different combination, which appears to be the simplest yet; it consists of an Enigma-type rotor machine (without the Umkehrwalze) , in which three wired rotors which each implement a random permutat ion on 256 symbols are turned by a linear feedback shift register. It is straightforward to implement and fairly fast; yet, provided the rotors are kept secret, and the shift register is too long for its state to be guessed, it appears to resist all known attacks.