International Association
for Cryptologic Research

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.