In Feistel-Network, there would be a wire-crossing at the end (for the next round). Which insufficiency would occur, if the wire crossing would be omitted?


Let's look at a picture of a generic feistel cipher

enter image description here

Notice that no keying material is used during or after that final swap. So, we can conclude that the final swap does not impact security at all.

So, why include it? It is so that all rounds will be identical. This could help with some implementations. That is all.

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It's there to facilitate a simple implementation. As there is no key addition applied afterwards, the final swapping of the halves does not contribute towards security.

The Feistel cipher entry on tutorialspoint explains:

Decryption Process

The process of decryption in Feistel cipher is almost similar. Instead of starting with a block of plaintext, the ciphertext block is fed into the start of the Feistel structure and then the process thereafter is exactly the same as described in the given illustration.

The process is said to be almost similar and not exactly same. In the case of decryption, the only difference is that the subkeys used in encryption are used in the reverse order.

The final swapping of ‘L’ and ‘R’ in last step of the Feistel Cipher is essential. If these are not swapped then the resulting ciphertext could not be decrypted using the same algorithm.

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  • $\begingroup$ Doesn't the image on that page show an extra swap after the last round, so that in effect the halves are not swapped after the last application of the F-function (and the key)? The question mentions a swap "for the next round", which in this case is skipped for the last round. $\endgroup$ – ilkkachu Jun 24 '16 at 19:30
  • $\begingroup$ I'm not really sure, perhaps I misinterpreted @BennoDual's question. I figured they were inquiring about the final swap, as otherwise I would have expected the question to read "why swap the halves at all?" or something similar. Hopefully they'll let me/us know if this answer is not addressing the right question. $\endgroup$ – Ella Rose Jun 24 '16 at 20:04

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