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Data encryption Standard (DES) is a packet encryption algorithm developed by IBM in the mid-1970s and published as a data encryption standard by the National Bureau of Standards of the United States.

The packet size of the DES algorithm is 64 bits, and the encryption or decryption keys used are also 64 bits. Because 8 of these bits are used for parity, only 56 of the 64 bits really act as keys. The algorithms used for DES encryption and decryption are exactly the same except for the order of the subkeys.

1) Feistel network

The DES cryptographic structure is based on a structure called Feistel network. Feistel network is invented by Horst Feistel when designing Lucifer block ciphers and is used by DES, FEAL, Twofish, RC5 and other algorithms.

Feistel defines an iterative block cipher algorithm, whose essence is to use product ciphers to execute several basic ciphers sequentially, so that the cipher strength of the final result is higher than that of each basic cryptosystem.

For an r-round Feistel cipher with a packet length of 2n bits, each iteration performs the same operation, and the input of each round depends on the output of the previous round. Taking the first round as an example, the operation process of each iteration is described as follows:

① takes the output m of the I-1 round as the input of this round, and divides the input into two parts of equal length, which are recorded as the left and right bits of m L R, L and R respectively.

② exchanges positions between the left and right parts, and takes the right half R directly as the left half L of the current round output m. At the same time, the right part R and the wheel key K are taken as the input of the theory function F, and the output of the theory function F is calculated bit by bit with the left part L, and the result is taken as the right part R of the output m.

The decryption process of Feistel network is completely consistent with the encryption process, but the sub-key of each round is used in the opposite number of times, which makes the block cipher designed based on Feistel network easy to be implemented in hardware.

To be continued,

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