CITS3002 Computer Networks  
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The Steps of the DES Algorithm

The algorithm has 19 steps for encryption; decryption simply performs the steps in the reverse order.

  • Step 1. Transposition of plaintext, independent of key.
  • Step 19. Inverse of Step 1.
  • Step 18. Exchange left 32 bits with right 32 bits.
  • Steps 2-17. Use a function of the key for each stage, which we shall call Ki. 
     Left out := Right in
Right out := XOR(left in, f(Right in, Ki))
                      where f is a 4 step function.

What are the steps of this magic function f?
  1. E := R1 (which is 32 bits) expanded to 48 bits.
  2. D := XOR(E, Ki).
  3. Divide D into 8x6 bits;
    feed each of these 6 bits into a different S box each producing 4 bits.
  4. Feed these 8x4 bits (= 32 bits) through a P box.

How is the DES key used?
  • K0 := 56 transposition cipher of K.
  • Divide K0 into 2x28 bits.
  • ROLeft each part.
  • Ki := 56 bit transition of the number formed.


CITS3002 Computer Networks, Lecture 12, Cryptography's role in networking, p9, 22nd May 2024.