A Demonstrable Break of PCBC Mode

Modes have long been used to help ciphers remain secure under cryptanalysis. The goal of modes is to randomize the cipher so that for any two identical input blocks the cipher text output is likely to be different. This apparent morphing of the mapping between input and output is generally thought to make decryption more difficult. However, this may not be true. Side channel attacks against some modes have proven that they carry sufficient information to be able to return the original message without having to also break the cipher. Of the identified and approved modes, one quarter have been broken (return the key for the cipher) and this paper presents a verifiable break for another mode. Further, a concept for breaking a fourth mode is also presented. If more modes are broken, then the concept of modes may be proved ineffective, and abandoning modes may be indicated.