It is now a decade since the appearance of W. Diffie and M. E.
Hellmann's startling paper, "New Directions in Cryptography". This paper
not only established the new field of public-key cryptography but also
awakened scientific interest in secret-key cryptography, a field that
had been the almost exclusive domain of secret agencies and mathematical
hobbyist. A number of ex- cellent books on the science of cryptography
have appeared since 1976. In the main, these books thoroughly treat both
public-key systems and block ciphers (i. e. secret-key ciphers with no
memo- ry in the enciphering transformation) but give short shrift to
stream ciphers (i. e., secret-key ciphers wi th memory in the
enciphering transformation). Yet, stream ciphers, such as those .
implemented by rotor machines, have played a dominant role in past
cryptographic practice, and, as far as I can determine, re- main still
the workhorses of commercial, military and diplomatic secrecy systems.
My own research interest in stream ciphers found a natural re- sonance
in one of my doctoral students at the Swiss Federal Institute of
Technology in Zurich, Rainer A. Rueppe1. As Rainer was completing his
dissertation in late 1984, the question arose as to where he should
publish the many new results on stream ciphers that had sprung from his
research.