Input Output Chaining (IOC) AE Mode Revisited

Input Output Chaining (IOC) is an authenticated encryption (AE) mode that can be used with any block cipher. IOC main interest is that each message block is ciphered just once, as when only implementing confidentiality, while the added complexity by the accompanying integrity service is negligible. The core integrity concept in IOC is based on a novel, minimal and appealing chaining mechanism already presented by the first published IOC mode proposal [6], which so far has resisted public scrutiny. This paper upgrades some details of the former IOC specification and its security demonstration concluding that whatever resources could be spent to forge IOC integrity mechanism, assuming an ideal block cipher, the success probability of such attack will not be higher than 2-(n-1.25), where n is the cipher block size.