Optimized Self-Synchronizing Mode of Operation

Modes of operation adapt block ciphers to many applications. Among the encryption modes, only CFB (Cipher Feedback) has both of the following properties: Firstly it allows transmission units shorter than the block-cipher length to be encrypted and sent without delay and message expansion. Secondly, it can resynchronize after the loss of such transmission units. However, CFB is ine cient in such applications, since for every transmission unit, regardless how short, a call to the block cipher is needed. We propose a new mode of operation based on CFB which remedies this problem. Our proposal, OCFB, is almost optimally e cient (i.e., almost as many message bits are encrypted as block-cipher output bits produced) and it can self-synchronize after the loss or insertion of transmission units. We prove the security of CFB and OCFB in the sense of modern cryptography.