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.
Key generation: The round key generator is used to derive 16 48bit subkeys from the 64 bit cipher key.
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