Data Encryption Standard
SDES encrypts 64-bit blocks using a 56-bit key and produces a 64-bit ciphertext.
Same steps, with the same key, used to reverse the encryption with the order of the keys reversed.
The DES widely used.
DES Encryption: Data Encryption Standard
- The DES encryption is shown in the figure below:
Encryption function has two inputs: the plaintext to be encrypted and the key.
- The processing of the plaintext proceeds in three phases.
- The permuted output then passed through sixteen rounds of the same function, which involves both permutation and substitution functions. The left and right halves from the last round swapped to produce pre output.
- The pre-output passed through a permutation that is the inverse of the initial permutation function, to produce the 64-bit ciphertext.
- Moreover, The 64-bit plaintext passes through an initial permutation (IP) that rearranges the bits to produce the permuted input.
- The right-hand portion of the figure shows the way in which the 56-bit key used.
- Initially, the key passed through a permutation function.
- Then, a subkey (ki) produced for each of the sixteen rounds by the combination of a left circular shift and a permutation.
- The permutation functions the same for each round, but a different subkey produced because of the repeated shifts of the key bits.
Initial Permutation and Inverse Initial Permutatioin: Data Encryption Standard
- Moreover, The initial permutation and its inverse defined by tables.
- The tables are to interpreted as follows.
- The input to a table consists of 64 bits numbered from 1 to 64. o The 64 entries in the permutation table contain a permutation of the numbers from 1 to 64.
- Each entry in the permutation table indicates the position of an input bit in the output.
- Inverse permutation table nullifies the effect of initial permutation.