Analysis of the Statistical Cipher Feedback Mode of Block Ciphers

In this paper, we examine a recently proposed mode of operation for block ciphers which we refer to as statistical cipher feedback (SCFB) mode. SCFB mode configures the block cipher as a keystream generator for use in a stream cipher such that it has the property of statistical self-synchronization, thereby allowing the stream cipher to recover from bit slips in the communication channel. Statistical self-synchronization involves feeding back ciphertext to the input of the block cipher similar to the conventional cipher feedback (CFB) mode, except that the feedback only occurs when a special synchronization pattern is recognized in the ciphertext. In the paper, we examine the efficiency, resynchronization, and error propagation characteristics of SCFB and compare these to conventional modes such as CFB and output feedback (OFB). In particular, we study these characteristics of SCFB as a function of the synchronization pattern size. As well, we examine implementation issues of SCFB, focusing on the buffer requirements and resulting delay for a practical realization of the cipher. We conclude that SCFB mode can be used to provide practical, efficient, self-synchronizing implementations for stream ciphers. In particular, SCFB mode is best used in circumstances where slips are a concern and where implementation efficiency is a high priority in comparison to encryption latency.