[Resource Topic] 2002/061: Strengthened Encryption in the CBC Mode

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Strengthened Encryption in the CBC Mode

Authors: Vlastimil Klima, Tomas Rosa


Vaudenay [1] has presented an attack on the CBC mode of block ciphers, which uses padding according to the PKCS#5 standard. One of the countermeasures, which he has assumed, consisted of the encryption of the message M´= M || padding || hash(M || padding) instead of the original M. This can increase the length of the message by several blocks compared with the present padding. Moreover, Wagner [1] showed a security weakness in this proposal. The next correction, which Vaudenay proposed (“A Fix Which May Work”) has a general character and doesn’t solve practical problems with the real cryptographic interfaces used in contemporary applications. In this article we propose three variants of the CBC mode. From the external point of view they behave the same as the present CBC mode with the PKCS#5 padding, but they prevent Vaudenay’s attack.

ePrint: https://eprint.iacr.org/2002/061

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