International Association for Cryptologic Research (IACR)
Abstract
Tweakable blockcipher (TBC) is an extension of standard blockcipher introduced by Liskov, Rivest and Wagner in 2002. TBC is a versatile building block for efficient symmetric-key cryptographic functions, such as authenticated encryption.
In this paper we study the problem of extending tweak of a given TBC of fixed-length tweak,
which is a variant of popular problem of converting a blockcipher into a TBC, i.e., blockcipher mode of operation.
The problem is particularly important for known dedicated TBCs since they have relatively short tweak.
We propose a simple and efficient solution, called XTX, for this problem.
XTX converts a TBC of fixed-length tweak into another TBC of arbitrarily long tweak, by extending the scheme of Liskov, Rivest and Wagner that converts a blockcipher into a TBC.
Given a TBC of n-bit block and m-bit tweak, XTX provides (n+m)/2-bit security while conventional methods provide n/2 or m/2-bit security.
We also show that XTX is even useful when combined with some blockcipher modes for building TBC having security beyond the birthday bound