A Self-Synchronizing Cascaded Cipher System with Dynamic Control of Error Propagation

A cipher system used for secure communication over a noisy channel can automatically synchronize the sender and receiver by computing a stateless function of a key and a limited amount of the recent ciphertext. The more ciphertext feedback is used, the more the errors from the noisy channel are propagated. The less feedback is used, the easier ciphertext-only and chosen-plaintext attacks become. There is a trade-off between security and noise that must be made when a self-synchronizing system is built.This paper presents a self-synchronizing cascaded cipher system that permits most combinations of key and ciphertext feedback lengths and also allows adjustment of the trade-off between security and noise during system operation. At times when maximum security is not needed, the error propagation can be reduced temporarily.As implemented in hardware, the cascaded cipher has a storage register for each stage. The function computed would normally depend on the state of this storage, but different clocks are used at each stage to render the function stateless. The use of a cascade helps to keep the hardware cost down.