I was wondering how decryption would work for FPE in the case of cycled Feistel ciphers. I understand that simply reiterating through the networks with reversed key order will generate the ciphertext inputted at the beginning of the network.
The ciphertext is a bit representation of some number, so I'm just expanding the number to search for correct length before stopping.
However, the question that I have is that I am using AES as my round function. However, I need significant padding (0s), as the AES block size is 128 and the block size (of each half) is significantly smaller at 10-30 bits. I am simply appending these 0s to the end of my string, then using AES to get an encrypted 128 bit output.
At this point, I am not too sure what to do. I truncate by removing the last ~100 bits and only taking the beginning of the string. However, this seems to me as if I am losing data, and I cannot understand how I would be able to decrypt using this same method.
Is this simple method of truncating actually the correct way of implementing AES for smaller blocks, or should I be padding/truncating in a different way? Also, if it is correct, could someone explain why it is or link me to some relevant literature?
I have seen the related link Recover plaintext from truncated ciphertext using AES for FPE , but I either don't understand the comments or a solution was never posted.