Mercy (cipher)

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

↑ 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 City, USA, April 2000. Springer-Verlag.
1 2 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. Retrieved 15 December 2006.

Block ciphers (security summary)

Common algorithms	AES Blowfish DES (Internal Mechanics, Triple DES) Serpent Twofish

Less common algorithms	Camellia CAST-128 IDEA RC2 RC5 RC6 SEED ARIA Skipjack TEA XTEA

Other algorithms	3-Way Akelarre Anubis BaseKing BassOmatic BATON BEAR and LION CAST-256 Chiasmus CIKS-1 CIPHERUNICORN-A CIPHERUNICORN-E CLEFIA CMEA Cobra COCONUT98 Crab Cryptomeria/C2 CRYPTON CS-Cipher DEAL DES-X DFC E2 FEAL FEA-M FROG G-DES GOST Grand Cru Hasty Pudding cipher Hierocrypt ICE IDEA NXT Intel Cascade Cipher Iraqi KASUMI KeeLoq KHAZAD Khufu and Khafre KN-Cipher Kuznyechik Ladder-DES Libelle LOKI (97, 89/91) Lucifer M6 M8 MacGuffin Madryga MAGENTA MARS Mercy MESH MISTY1 MMB MULTI2 MultiSwap New Data Seal NewDES Nimbus NOEKEON NUSH PRESENT Prince Q RC6 REDOC Red Pike S-1 SAFER SAVILLE SC2000 SHACAL SHARK Simon SM4 Speck Spectr-H64 Square SXAL/MBAL Threefish Treyfer UES Xenon xmx XXTEA Zodiac

Design	Feistel network Key schedule Lai-Massey scheme Product cipher S-box P-box SPN Avalanche effect Block size Key size Key whitening (Whitening transformation)

Attack (cryptanalysis)	Brute-force (EFF DES cracker) MITM (Biclique attack, 3-subset MITM attack) Linear (Piling-up lemma) Differential (Impossible Truncated Higher-order) Differential-linear Distinguishing (Known-key) Integral/Square Boomerang Mod n Related-key Slide Rotational Timing XSL Interpolation Partitioning Davies' Rebound Weak key Tau Chi-square Time/memory/data tradeoff

Standardization	AES process CRYPTREC NESSIE

Utilization	Initialization vector Mode of operation Padding

Cryptography

History of cryptography Cryptanalysis Outline of cryptography

Symmetric-key algorithm Block cipher Stream cipher Public-key cryptography Cryptographic hash function Message authentication code Random numbers Steganography

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