Construction Details
Let be the round function and let be the sub-keys for the rounds respectively.
Then the basic operation is as follows:
Split the plaintext block into two equal pieces, (, )
For each round, compute
- .
Then the ciphertext is .
Decryption of a ciphertext is accomplished by computing for
- .
Then is the plaintext again.
One advantage of the Feistel model compared to a substitution-permutation network is that the round function does not have to be invertible.
The diagram illustrates both encryption and decryption. Note the reversal of the subkey order for decryption; this is the only difference between encryption and decryption.
Read more about this topic: Feistel Cipher
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