In cryptography, SAFER (Secure And Fast Encryption Routine) is the name of a family of block ciphers designed primarily by James Massey (one of the designers of IDEA) on behalf of Cylink Corporation. The early SAFER K and SAFER SK designs share the same encryption function, but differ in the number of rounds and the key schedule. More recent versions — SAFER+ and SAFER++ — were submitted as candidates to the AES process and the NESSIE project respectively. All of the algorithms in the SAFER family are unpatented and available for unrestricted use.
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The first SAFER cipher was SAFER K64, published by Massey in 1993, with a 64bit block size. The "K64" denotes a key size of 64 bits. There was some demand for a version with a larger 128bit key, and the following year Massey published such a variant incorporating new key schedule designed by the Singapore Ministry for Home affairs: SAFER K128. However, both Lars Knudsen and Sean Murphy found minor weaknesses in this version, prompting a redesign of the key schedule to one suggested by Knudsen; these variants were named SAFER SK64 and SAFER SK128 respectively — the "SK" standing for "Strengthened Key schedule", though the RSA FAQ reports that, "one joke has it that SK really stands for 'Stop Knudsen', a wise precaution in the design of any block cipher". Another variant with a reduced key size was published, SAFER SK40, to comply with 40bit export restrictions.
All of these ciphers use the same round function consisting of four stages, as shown in the diagram: a keymixing stage, a substitution layer, another keymixing stage, and finally a diffusion layer. In the first keymixing stage, the plaintext block is divided into eight 8bit segments, and subkeys are added using either addition modulo 256 (denoted by a "+" in a square) or XOR (denoted by a "+" in a circle). The substitution layer consists of two Sboxes, each the inverse of each other, derived from discrete exponentiation (45^{x}) and logarithm (log_{45}x) functions. After a second keymixing stage there is the diffusion layer: a novel cryptographic component termed a pseudoHadamard transform (PHT). (The PHT was also later used in the Twofish cipher.)
There are two morerecent members of the SAFER family that have made changes to the main encryption routine, designed by the Armenian cryptographers Gurgen Khachatrian and Melsik Kuregian in conjunction with Massey.
