Data encryption standard is one of the widely used algorithm to encrypt and decrypt the messages.

  • DES is a secret key encryption algorithm that was designed by IBM and adopted by the U.S government
  • DES encrypt-bit block of plain-text using-bit key
  • The key is actually contains  56 bits as the last bit of the 8 bytes in the key is a parity bit for those bytes i.e 8 out of 64 are parity bits.
  • DES can also encrypt the message larger than 64 bits
  • DES has three distinct phases :
  1. The 64-bits in the block are permuted and shuffled.
  2. Sixteen rounds of an identical operations are applied to the resulting data and the key.
  3. The inverse of the original permutation of step 1 is applied to the resulting data to get the ultimate cipher-text.
  •  In the initial permutation the various bits are shuffled with each other and is not dependent on key.
  • During each round the 64-bit block is broken into two halves the left half and the right half and each is of 32-bits.
  • The key used in each round is of 48 bits and is derived from 56-bits key by rotating the bits.
  • If we denote the left and right half of the block of the block at round i as Li and Ri respectively, and key at round i as Ki then these pieces are combined using round i according to the following rules
    Li = Ri-1
    Ri = Li-1 ⊕ F(Ri-1, Ki)
    Where F is a combine function and ⊕ is he exclusive – OR(XOR) operation.
  • The function F contains the following 4 steps :
    1. A 48- bit number, E, is constructed by expanding the 32-bit Ri-1, according to fixed permutation and duplication rule.
    2. E and Ki are XORed together.
    3. The output is then partitioned into eight groups of 6 bits each, each is of fed into a different S-box. S-box stands for substitution box which reduces each 6 bit chunk into bits. There are eight different which reduces S-boxed one for each of the 6 bits chunks. S-box perform many to one mapping.
    4. Finally 8×4 bits are passed through P-box. P-box stands for permutation box used to effect a permutation on 8-bit input.
  • Each round takes two 32 bits inputs and produces two 32 bit outputs. The left output is simply a copy of the right input.
  • The right output is the bit-wise XOR of the left input and a function of the right and the key for this stage.
  • The last stage is exact inverse of initial permutation. For the message having length more than 64 bits are made and a technique called cipher block chaining (CBC) is used.
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