Data Encryption Standard DES principle

The Data Encryption Standard (DES) was developed by IBM in the United States in the mid 1970s and was published by the U.S. National Standards Office as a data encryption standard for a packet encryption algorithm.

The DES algorithm has a packet size of 64 bits, and the encryption or decryption key used is also 64 bits. Because there are 8 bits that are used to make parity, only 56 bits of the 64-bit real key function. Des encryption and decryption algorithms used in addition to the sub-key in a different order, the other parts are exactly the same.

1) Feistel Network

The DES cipher structure is based on a structure called a Feistel network. Feistel network was invented by Horst Feistel in the design of Lucifer Block cipher, and was used by Des, FEAL, Twofish, RC5 and other algorithms.

Feistel defines an iterative block cipher algorithm, which essentially uses the product cipher to execute multiple basic cipher systems sequentially, making the password strength of the final result higher than the result of each basic cipher system.

For an R-round Feistel cipher with a packet length of 2n bits, each iteration performs the same operation, and the input per round depends on the output of the previous round. Take the first round as an example, describing the operation process for each round of iterations as follows:

① the output m of the i-1 wheel as input to this wheel, and divides the input into two parts with equal left and right lengths, recorded as M=l R, and L and R are the left and right n bits of M respectively.

The ② will swap the left and right parts of the position, and r directly as the left half of the output m of the current wheel L. At the same time, the right half R and the wheel key K as the input of the function f, the output of the function f and the left half of the L is a bitwise XOR operation, the results obtained as the right half of the output M R.

The decryption process of the Feistel network is exactly the same as the encryption process, with the use of each round of sub-keys in the opposite number of times, which makes the block cipher based on the Feistel network design easy to implement with hardware.

To be continued 、、、、、

Data Encryption Standard DES principle