Secure message transmission is of extreme importance in today's information-based society. Stream encryption is a practical means to this end. This monograph is devoted to a new aspect of stream ciphers, namely the stability theory of stream ciphers, with the purpose of developing bounds on complexity which can form part of the basis for a general theory of data security and of stabilizing stream-cipher systems. The topic is treated by introducing measure indexes on the security of stream ciphers, developing lower bounds on these indexes, and establishing connections among them. The treatment...