Mercy (cipher)

{{Short description|Block cipher}}

{{Infobox block cipher

| name = Mercy

| image =

| caption =

| designers = Paul Crowley

| publish date = April 2000

| derived from = WAKE

| derived to =

| key size = 128 bits

| block size = 4096 bits

| structure = Feistel network

| rounds = 6

| cryptanalysis = Scott Fluhrer's differential attack breaks the cipher.

}}

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.