cient Stream Cipher with Variable Internal

This paper presents an eecient stream cipher using an internal state with variable structure and evolution. Arbitrarily large internal states can be used in order to defeat brute-force guessing attacks without compromising the performance of cipher, and possibly improving it. Attacking is made even more complicated by dynamically chosing diierent topologies and evo-lutions for the cipher's internal state. The cipher controls the evolution of its internal state by using both an external keyed pseudo-random generator (EKPRG), either cryptographically strong or weak, and plaintext feedback. The plaintext feedback reduces the probability of producing cyclic keystreams without compromising the security of the cipher. The parameters controlling the structure and evolution of the cipher's internal state can be chosen in order to achieve diierent levels of security , memory consumption and performance. In terms of security, we evaluate the impact of this parameters in the strength of the cipher against brute-force guessing attacks. Concerning performance , we evaluate the encryption speed of the cipher using two diierent EKPRGs { 8-bit ARC4 and DES working in 8-bit OFB { with several topologies and evolutions of the internal state assuring very high security levels. When comparing against the ciphers used as external EKPRGs, we obtain a minimum encryption speedup of 7 % and 428 %, respectively, and a maximum speedup of 63 % and 2556 %.