Mercy (cipher)
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| Designer(s): | Paul Crowley |
|---|---|
| First published: | 2000-04[1] |
| Derived from: | WAKE |
| Key size(s): | 128 bits |
| Block size(s): | 4096 bits |
| Structure: | Feistel network |
| Rounds: | 6 |
| Best public cryptanalysis: | |
| Scott Fluhrer's differential attack is effective up to 7 rounds.[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]
[edit] References
- ^ 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.
- ^ a b Scott Fluhrer (2006-04-02). "Cryptanalysis of the Mercy Block Cipher" (PostScript). Fast Software Encryption Workshop 2001, Royal Park Hotel Nikko, Yokohama, Japan: Cisco Systems, Inc.. Retrieved on 2006-12-15.
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