Cryptanalysis of the EPBC Authenticated Encryption Mode

A large variety of methods for using block ciphers, so called ‘modes of operation’, have been proposed, including some designed to provide both confidentiality and integrity protection. Such modes, usually known as ‘authenticated encryption’ modes, are increasingly important given the variety of issues now known with the use of unauthenticated encryption. In this paper we show that a mode known as EPBC (Efficient error-Propagating Block Chaining), proposed in 1997 by Zúquete and Guedes, is insecure. Specifically we show that given a modest amount of known plaintext for a single enciphered message, new enciphered messages can be constructed which will pass tests for authenticity. That is, we demonstrate a message forgery attack.