Introduction to the

Block cipher is a very good encryption structure, many commonly used encryption algorithms are used to add the grouping algorithm, such as DES. Safer is also a block cipher algorithm. Let’s take a look.

Introduction of SAFER

Safer means Secure And Fast Encryption. In cryptography, Safer is mainly a set of block passwords designed by James Massey (one of the designers of IDEA) on behalf of Cylink.

There are four main types of Safer: Safer K, Safer SK, Safer + and Safer +.

Safer K and Safer SK are relatively early designs, sharing the same encryption function, but the rounds and key scheduling are different.

Safer + and Safer ++ are proposed to Nessie as candidates for the AES algorithm. None of the algorithms in the Safer series are patented and are free to use.

Safer K and Safer SK

The first Safer password was the Safer K-64, which was released by Massey in 1993 and has a 64-bit block size. “K-64” represents the 64-bit key size.

Because 64-bit blocks are too small to encrypt large data, the next year, Massey released a 128-bit variant called the Safer K-128.

However, Lars Knudsen and Sean Murphy found that there were some problems with this version, so they redesigned the key scheduling according to Knudsen’s suggestion. These variants are named Safer SK-64 and Safer SK-128.

The word “SK” stands for “Strengthened Key Schedule “.

In addition, there’s a 40-bit block-size variant, the Safer SK-40.

Let’s use a diagram to look at the basic flow of the Safer K algorithm:

Starting at the top is the input phase of the plaintext, each of which is divided into eight pieces.

Then comes the key mixing stage, in which the plaintext is additionally modulo 256 or XOR with the subkeys, respectively.

Then comes the replacement stage, in which the results generated in the previous stage are mapped to the new data through two opposite S-boxes.

The two S-boxes are derived from the 45x and log45 functions, respectively. If you want to know about the S box, you can refer to the article “Cryptography Series: Blowfish Symmetrical Key Group Algorithm”.

Then comes the second key mixing phase.

After the second key mixing stage, the diffusion is performed using the Pseudo-Hadamard Transform (PHT).

These four stages make up a round of encryption.


Safer + and Safer ++ are improvements on the original Safer encryption algorithm. They were designed by Armenian cryptographers Gurgen Khachatrian (University of Armenia) and Melsik Kuregian in collaboration with Massey.

Safer + was proposed in 1998, but was submitted as a candidate algorithm for AES, with a 128-bit block size. Bluetooth keys are implemented based on the Safer + custom algorithm (called E21 and E22),

The Safer ++ was submitted to the Nessie Project in 2000 in two versions, one 64-bit and the other 128-bit.

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