ABC: A New Fast Flexible Stream Cipher

ABC is a synchronous stream cipher optimized for software applications. Its key length is 128 bits. It accommodates a 128-bit initial vector. Here a version of ABC with a 128-bit key and 32-bit internal variables is presented. A new approach to the design of stream ciphers has been used which results in a cipher based upon keyand clock-dependent state transition and filter functions. More precisely, ABC combines two building blocks: a wreath product of a LFSR and a non-linear single-cycle T-function, as well as a derivation of the knapsack function. Our techniques guarantee the period of 2 ∙ (2 − 1) words, uniform distribution, and high linear complexity of the keystream of ABC. The allowed length of a single stream (for a given key/IV pair) for ABC is 2 32-bit words. During Phase 1 of eSTREAM the previous versions of ABC ([10] and [11]) were cryptanalyzed. [13], [18] and [26] discovered successful attacks. [19] contains erroneous results that are not applicable [9] to ABC. The current version of ABC includes tweaks against all of the mounted attacks (a longer LFSR, another T-function, modified key key setup procedure). It offers a security level of 2. No hidden weaknesses have been incorporated in the design of ABC. ABC can be efficiently implemented in software. Our C implementation encryption performance is about 4 clocks per byte on a standard 1.73 GHz Pentium M processor. The flexibility property results in the possibility of its efficient application on a variety of computer platforms by choosing proper implementation parameters.