DES -- The Algorithm

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DES Operates on blocks of 64 bits of plaintext, encrypted with a block of 56 key bits. It is constructed as successive rounds comprised of the most popular crypto primitives: transposition, substitution, XORing, Concatenation.

Three successive elements:

  • Key Independent transposition
  • Key bit-wise operation
  • Key Independent transposition

    The 56 bits key is expressed as 8 7bits strings. Usually each string is trailed with a parity bit, combining into a 64 bits block.

    Deciphering DES works the same as encrypting with the one difference that the 16 48bits extracted keys are used in reverse: the 16th key is used first, and the first key is used last.

    DES comes with slight modifications for international use, sometimes known as DEA: data encryption algorithm (the final permutation in the f function is omitted).

    The First Key Independent Transposition:

    The 64 plaintext block is transposed by a speicifc Transposition operator:

    T=58, 50, 42, 34, 26, 18, ..., 23, 15, 7,

    namely: in the transposed list the original 58th bit becomes the first, the 50s bit becomes the second,...

    The Second Key Independent Transposition:

    Reverse transposition of the starting action, T-1

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