Abstract
This paper proposes a blockcipher mode of operation, HCTR, which is a length-preserving encryption mode. HCTR turns an n-bit blockcipher into a tweakable blockcipher that supports arbitrary variable input length which is no less than n bits. The tweak length of HCTR is fixed and can be zero. We prove that HCTR is a strong tweakable pseudorandom permutation ( $\widetilde{sprp}$), when the underlying blockcipher is a strong pseudorandom permutation (sprp). HCTR is shown to be a very efficient mode of operation when some pre-computations are taken into consideration. Arbitrary variable input length brings much flexibility in various application environments. HCTR can be used in disk sector encryption, and other length-preserving encryptions, especially for the message that is not multiple of n bits.
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