Bruce Schneier and I have been designing a block cipher, called "MacGuffin", based on a new variant of the standard Feistel (S-P) network structure. We are presenting a paper describing the cipher, entitled "The MacGuffin Block Cipher Algorithm", at the Leuven crypto algorithms workshop later this month. Here's the abstract: \begin{abstract} This paper introduces MacGuffin, a 64 bit ``codebook'' block cipher. Many of its characteristics (block size, application domain, performance and implementation structure) are similar to those of the U.S. Data Encryption Standard (DES). It is based on a Feistel network, in which the cleartext is split into two sides with one side repeatedly modified according to a keyed function of the other. Previous block ciphers of this design, such as DES, operate on equal length sides. MacGuffin is unusual in that it is based on a {\em generalized unbalanced Feistel network (GUFN)} in which each round of the cipher modifies only 16 bits according to a function of the other 48. We describe the general characteristics of MacGuffin architecture and implementation and give a complete specification for the 32-round, 128-bit key version of the cipher. \end{abstract} A PostScript preprint of the paper is available via anonymous FTP from: ftp://research.att.com/dist/mab/mcg.ps A forthcoming paper will discuss the characteristics GUFN structure in more detail. Comments and analysis greatly appreciated. -matt