Mercy (cipher)

This article is about the block cipher. For other uses, see Mercy (disambiguation).

Mercy

General
Designers	Paul Crowley
First published	April 2000[1]
Derived from	WAKE
Cipher detail
Key sizes	128 bits
Block sizes	4096 bits
Structure	Feistel network
Rounds	6
Best public cryptanalysis
Scott Fluhrer's differential attack breaks the cipher.[2]

In cryptography, Mercy is a tweakable block cipher designed by Paul Crowley for disk encryption.

The block size is 4096 bits—unusually large for a block cipher, but a standard disk sector size. Mercy uses a 128-bit secret key, along with a 128-bit non-secret tweak for each block. In disk encryption, the sector number would be used as a tweak. Mercy uses a 6-round Feistel network structure with partial key whitening. The round function uses a key-dependent state machine which borrows some structure from the stream cipher WAKE, with key-dependent S-boxes based on the Nyberg S-boxes also used in AES.

Scott Fluhrer has discovered a differential attack that works against the full 6 rounds of Mercy. This attack can even be extended to a seven-round variant.^[2]

References

1. ^ Paul Crowley, Mercy: A fast large block cipher for disk sector encryption. In Bruce Schneier, editor, Fast Software Encryption: 7th International Workshop, volume 1978 of Lecture Notes in Computer Science, pages 49-63, New York, USA, April 2000. Springer-Verlag.
2. ^ ^{a b} Scott Fluhrer (2 April 2006). "Cryptanalysis of the Mercy Block Cipher" (PostScript). Fast Software Encryption Workshop 2001. Royal Park Hotel Nikko, Yokohama, Japan: Cisco Systems, Inc.. http://www.ciphergoth.org/crypto/mercy/fluhrer-dc.html. Retrieved 15 December 2006.