Length-Preserving Encryption Based on Single-Key Tweakable Block Cipher

We present a Single-key Length Doubler built on an n-bit Tweakable block cipher (SLDT), which is a length-preserving cipher on the strings with bit length in integer interval \(\left[ n, n+1, \ldots , 2n-1\right] \). SLDT is mainly motivated to reduce the key material size of a length doubler proposed by Chen et al. at FSE2018, since the key management is always challenging in practice. We prove that SLDT is a strong pseudo-random permutation (SPRP) if the underlying tweakable block cipher is SPRP.