Reference: RSA Data Security Conference, Cryptographer's Track (RSA CT '02) Lecture Notes in Computer Science, vol. 1872, Springer, February 2002.
Abstract: We explore the problem of enciphering members of a finite set M where k=|M| is arbitrary (in particular, it need not be a power of two). We want to achieve this goal starting from a block cipher (which requires a message space of size N=2n, for some n). We look at a few solutions to this problem, focusing on the case when M=[0, k-1]. We see ciphers with arbitrary domains as a worthwhile primitive in its own right, and as a potentially useful one for making higher-level protocols.
Rogaway's home page.