International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: The Security of the Extended Codebook (XCB) Mode of Operation

David A. McGrew
Scott R. Fluhrer
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Abstract: The XCB mode of operation was outlined in 2004 as a contribution to the IEEE Security in Storage effort, but no security analysis was provided. In this paper, we provide a proof of security for XCB, and show that it is a secure tweakable (super) pseudorandom permutation. Our analysis makes several new contributions: it uses an algebraic property of XCB's internal universal hash function to simplify the proof, and it defines a nonce mode in which XCB can be securely used even when the plaintext is shorter than twice the width of the underlying block cipher. We also show minor modifications that improve the performance of XCB and make it easier to analyze. XCB is interesting because it is highly efficient in both hardware and software, it has no alignment restrictions on input lengths, it can be used in nonce mode, and it uses the internal functions of the Galois/Counter Mode (GCM) of operation, which facilitates design re-use and admits multi-purpose implementations.
  title={The Security of the Extended Codebook (XCB) Mode of Operation},
  booktitle={IACR Eprint archive},
  keywords={secret-key cryptography / tweakable pseudorandom permutation},
  note={14th Workshop on Selected Areas in Cryptography (SAC 2007) 13726 received 1 Aug 2007},
  author={David A. McGrew and Scott R. Fluhrer},