I have read that the only difference between enc and dec of a Feistel Cipher is the order of the subkeys. But on the following figure, the harves block are also inverted, no? enter image description here

I don't understand why it doesn't count as a difference... To me, take the R block or the L block into the XOR has not the same effect.


2 Answers 2


You need to have another look at the picture. The output of the encryption process actually denotes $R_{n+1}$ on the left side and $L_{n+1}$ on the right side. And the input to the decryption function has the exact same order, so you don't have to swap anything manually, they are just 'labeled wrong'.

The reason here is that the last round is actually not exactly like the ones before, because it does not contain swapping the two halves. A more obvious way to express this would have been to do one final step like $L_{out} = R_{n+1}$, etc. and then start on the decryption with Left/Right labeling.

Also, I don't understand what they want to indicate by the green and red arrows. The coloring in both sides is either inconsistent or simply wrong. It also indicates that $n$ is an even number.


Where did you read that the only difference is inverting the subkeys?

You also need to use the reverse order of the corresponding blocks to decrypt the corresponding blocks.

It doesn't make sense to use the same blocks as when you encrypted (You need to reverse the process you did)


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