The input block is split in the middle to give to equal sized sub-blocks.
Let us call the initial left-hand side sub-block
and the initial right-hand side sub-block
Let the function
Fki be the round function which uses
ith round key,
These round keys are generated by the key schedule which is unspecified.
If we have
r rounds, then for
Li = Ri-1 Ri = Li-1⊕Fki(Ri-1)
Lr = Rr-1⊕Fkr(Lr-1) Rr = Lr-1
The process of decryption is identical to the one for encryption as long as the round keys are used in reverse order. The figure below shows decryption.
When compared to a substitution/permutation network we see that the Feistel network has an advantage in that it does not require that the round function have an inverse.
L. R. Knudsen, M. J. B. Robshaw, The Block Cipher Companion. Springer, 2011.
Copyright © 2014 Barry Watson. All rights reserved.