Xor–encrypt–xor

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The xor–encrypt–xor (XEX) is a (tweakable) mode of operation of a block cipher.

Xor Encrypt Xor (XEX) mode encryption.svg

History[edit]

In 1984, to protect DES against exhaustive search attacks, Ron Rivest proposed DESX: XOR a prewhitening key to the plaintext, encrypt the result with DES using a secret key, and then XOR a postwhitening key to the encrypted result to produce the final ciphertext.[3]

In 1991, motivated by Rivest's DESX construction, Even and Mansour proposed a much simpler scheme (the "two-key Even–Mansour scheme"), which they suggested was perhaps the simplest possible block cipher: XOR the plaintext with a prewhitening key, apply a publicly known unkeyed permutation (in practice, a pseudorandom permutation) to the result, and then XOR a postwhitening key to the permuted result to produce the final ciphertext.[3][4]

Studying simple Even–Mansour style block ciphers gives insight into the security of Feistel ciphers (DES-like ciphers) and helps understand block cipher design in general.[5]

Orr Dunkelman, Nathan Keller, and Adi Shamir later proved it was possible to simplify the Even–Mansour scheme even further and still retain the same provable security, producing the "single-key Even–Mansour scheme": XOR the plaintext with the key, apply a publicly known unkeyed permutation to the result, and then XOR the same key to the permuted result to produce the final ciphertext.[3][6]

In 2004, Rogaway presented the XEX scheme.[7]

Rogaway used XEX to allow efficient processing of consecutive blocks (with respect to the cipher used) within one data unit (e.g., a disk sector) for whole-disk encryption.[7]

Many whole-disk encryption systems – BestCrypt, dm-crypt, FreeOTFE, TrueCrypt, DiskCryptor, FreeBSD's geli, OpenBSD softraid disk encryption software, and Mac OS X Lion's FileVault 2 – support XEX-based tweaked-codebook mode with ciphertext stealing (XTS mode).

References[edit]

  1. ^ Barış Ege, Elif Bilge Kavun, and Tolga Yalçın. "Memory Encryption for Smart Cards". 2011.
  2. ^ Emmanuel Prouff. "Smart Card Research and Advanced Applications". 2011, p. 201.
  3. ^ a b c Orr Dunkelman, Nathan Keller, and Adi Shamir. "Minimalism in Cryptography: The Even–Mansour Scheme Revisited".
  4. ^ Joan Daemen, Laboratorium Esat. "Limitations of the Even–Mansour Construction". 1992. doi: 10.1007/3-540-57332-1_46
  5. ^ Craig Gentry and Zulfikar Ramzan. "Eliminating Random Permutation Oracles in the Even–Mansour Cipher". 2004.
  6. ^ Orr Dunkelman; Nathan Keller; and Adi Shamir. "Eurocrypt 2012: Minimalism in Cryptography: The Even-Mansour Scheme Revisited".
  7. ^ a b Rogaway, Phillip (2004-09-24). "Efficient Instantiations of Tweakable Blockciphers and Refinements to Modes OCB and PMAC" (PDF).