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Fiestel Cipher Model
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Fiestel Cipher

  • It is a design model from which many different block ciphers are divided.
  • DES is an example.
  • Cryptographic system based on Fiestel Cipherstructure uses the same algorithm for both encryption and decryption.
  • Consists of multiple rounds of processing of the plaintext, each round consisting of a substitution step followed by a permutation step.

[m2-4]

Encryption steps using a Fiestal Cipher

  1. Split the 64 bits of plaintext into left 32 bits $L_0$ and right 32 bits $R_0$
  2. Apply carefully designed mathematical function of that takes as i/p the key K & $R_0$ and computes o/p f($R_0$,K)
  3. XOR the o/p of the mathematical function with $L_0$, to compute a new 32 bit sequence X=$L_0\oplus f(R_0,K)$
  4. Let the new right 32 bits be X
  5. let the new left 32 bits $L_1$ be the previous right 32 bits $R_0$
  6. Repeat the process from step 2 to step 5 except using $R_1$ instead of $R_0$ & $L_1$ instead of $L_0$. This sequence of steps (steps 2 to 5) is known as a round of the block cipher. The function used is often referred to as the round function.
  7. Repeat step 6 for as many rounds as specified by the design algorithm. Once the last round (round m) is completed then the last left 32 bits $L_m$ are joined with the last right 32 bits $R_m$ to form the 64 bits of the Ciphertext, which is formed by concatenating $R_m$ & $L_m$ (in that order).
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