Modification of the OCFB mode for fast data links

In some communication systems, some of the transmitted symbols are lost or, on the contrary, repeated. Lost or repeated symbols are called slips. In the case of cipher transmission, these slips produce a loss of the synchronization between transmitter and receiver, which is accompanied by the loss of the information being transmitted. Self-synchronizing modes of block ciphers are used for the elimination of this phenomenon. Up to now, the CFB, OCFB and SCFB self-synchronizing modes have been published. In this paper, we describe and analyze a blocked OCFB (so-called BOCFB) mode. This mode is a modification of the self-synchronizing OCFB mode, which consists in the following three adaptations. First, at the beginning of a new keystream generation, the detector of the synchronizing sequence is blocked for the period of transmitting r symbols. Second, if the last r symbols remain in the keystream block, then the generation of a new keystream block is initiated. Third, the synchronization sequence is only one bit long. In the BOCFB mode, there is r-times more time available for the keystream block generation than in the OCFB mode while simultaneously the self-synchronizing ability is preserved. For this reason, the BOCFB mode can be useful in fast data links.