Feistel-Network - why wire crossing?

Question

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?

Answer 1

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

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.

Answer 2

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.