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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.

1 SAFER K and SAFER SK

The first SAFER cipher was SAFER K-64, published by Massey in 1993, with a 64-bit block size. The "K-64" denotes a key size of 64 bits. There was some demand for a version with a larger 128-bit key, and the following year Massey published such a variant incorporating new key schedule designed by the Singapore Ministry for Home affairs: SAFER K-128. 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 SK-64 and SAFER SK-128 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 SK-40, to comply with 40-bit40-bit encryption is a key size for symmetric encryption representing a low-level of security where the key is forty bits in length (five bytes). Forty bits can represent a total of 240 possible keys. Although this is a large number (about a trillion, and export restrictions.

All of these ciphers use the same round function consisting of four stages, as shown in the diagram: a key-mixing stage, a substitution layer, another key-mixing stage, and finally a diffusion layer. In the first key-mixing stage, the plaintext block is divided into eight 8-bit 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 S-boxes, each the inverse of each other, derived from discrete exponentiationIn mathematics, exponentiation is a process generalized from repeated multiplication, in much the same way that multiplication is a process generalized from repeated addition. The next operation after exponentiation is sometimes called tetration; repeatin (45^x) and logarithmIn mathematics, the logarithm functions are the inverses of the exponential functions. Logarithms are numbers that are substituted in computation for other numbers, to which they bear such a relation that the operations to be performed on the latter are r (log₄₅x) functions. After a second key-mixing stage there is the diffusion layer: a novel cryptographic component termed a Pseudo-Hadamard Transform (PHT). The PHT was later used in the TwofishIn cryptography, Twofish is a symmetric key block cipher with a block size of 128 bits and key sizes up to 256 bits. It was one of the five finalists of the AES contest, but was not selected for standardisation. Twofish is related to the earlier block cip cipher.)