# New Approaches to the Design of Self-Synchronizing Stream Ciphers

## Ueli Maurer

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```Self-synchronizing stream ciphers (SSSC) are a commonly used encryption
technique for channels with low bit error rate but for which bit
synchronization can present a problem. Most presently used such ciphers
are based on a block cipher (e.g. DES) in 1-bit cipher feedback mode.
In this paper, several alternative design approaches for SSSCs are
proposed that are superior to the design based on a block cipher with
respect to encryption speed and potentially also with respect to
security. A method for combining several SSSCs is presented that
allows to prove that the combined SSSC is at least as secure as any of
the component ciphers. The problem of designing SSSCs is contrasted
with the problem of designing conventional synchronous additive stream
ciphers and it is shown that different security criteria must be
applied.

Furthermore, an efficient algorithm is presented for finding a function
of low degree that approximates a given Boolean function, if such an
approximation exists. Its significance for the cryptographic security
of SSSCs and its applications in coding theory are discussed.