In cryptography, MacGuffin is a block cipher created in 1994 by Bruce Schneier and Matt Blaze at a Fast Software Encryption workshop.

It was intended as a catalyst for analysis of a new cipher structure, known as Generalized Unbalanced Feistel Networks (GUFNs).

Schneier and Blaze based MacGuffin on DES, their main change being that the data block is not split into equal halves in the Feistel network.

Since MacGuffin is a Feistel network, decryption is easy; simply run the encryption algorithm in reverse.

At the same workshop where MacGuffin was introduced, Rijmen and Preneel showed that it was vulnerable to differential cryptanalysis.