Data Encryption Standard (DES)

• The Data encryption standard (DES) is a symmetric-key block cipher published by National Institute of Standards and Technology (NIST).

• DES is an implementation of a Fiestel cipher. It uses 16 bit Fiestel structure, ie it contains 16 rounds of encryption. 

• The plaintext and ciphertext is of 64 bit.

• Though the key length is of length 64 bit, only 56 bits of it is used, the remaining 8 of the 64 bits of the key are used as check  bits.

                                    

• initial and Final Permutations: Straight  P-boxes that are inverses of each other. They have no cryptographic significance in DES. They are used to provide shifting to the plaintext and the ciphertext.

• Round Function: It is the heart of this cipher. The DES function applies a 48 bit key to the right most 32 bits to produce a 32-bit a 32 bit output.

• Expansion Permutation Box: Used to expand the right side 32 bit input in to 48 bits.

• XOR: The  key is then  XOR with the right side expanded 48 bit code.

• S-box: it is used to take 6 bit input and produce 4 bit output. There are a total of eight S-boxes in DES. it provides substitution to the taken input.

• Straight Permutation: the 32 bit data from the S-box is subjected to straight permutations.
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• Key generation: The round  key generator is used to derive 16 48bit subkeys from the 64 bit cipher key.