Conventional Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 37

y

x u v

2 − PHT

PHT − 1

2 −

u = 2 x + y mod 256 v = x + y mod 256 x = u − v mod 256 y = 2 v − u mod 256

Figure 2.19. Diffusion in SAFER.

2.6 Other Block Cipher Examples

2.6.1 FOX: A Lai-Massey Scheme

FOX is a family of block ciphers which was released in 2003 (see Refs. [96, 97]). It
was designed by Pascal Junod and Serge Vaudenay for the MediaCrypt company. The
family includes block ciphers with 64-bit and 128-bit blocks. Round numbers and key
sizes are flexible. We use an integral number r of rounds between 12 and 255 and a key
of k bits with an integral number of bytes, up to 256 bits. The name FOX64/ k / r refers
to the block cipher of the family characterized by 64-bit blocks, r rounds, and keys of k
bits. Similarly, FOX128/ k / r refers to the block cipher with 128-bit blocks. The nominal
choices denoted by FOX64 and FOX128 refer to FOX64/128/16 and FOX128/256/16
respectively. Namely, we use r

= 16 as a nominal number of rounds and a key length

which corresponds to two blocks.

A key schedule processes the key K and a direction (either “encrypt” or “decrypt”)
and produces a sequence RK 1 ,..., RK r of r round keys in this ordering if the direction

is “encrypt” or the opposite if the direction is “decrypt.” Encryption is performed
through r rounds as depicted in Fig. 2.20. Every round processes a data block and a
round key RK (whose size consists of two blocks) and produces another data block.
The r

− 1 first rounds have identical structure but the last round is a little different.

K Direction

Round

RK 1

Round

RK 2

Key
schedule

.

Round ∗

RK r

Figure 2.20. The FOX skeleton.