I'm implementing IDEA encryption and decryption and I'm trying to implement it in different variants(e.g if we didn't do the output transformation or if we don't do full 8-round encryption).
I got the encryption and decryption right for all variants when there is an output transformation, but I'm having trouble in decryption when there is no output transformation.
I know that decryption in IDEA is basically encrypting with the decryption keys which is computed in this paper like this:
The encryption process is like this:
So I tried decrypting by using the encrypting process with decryption keys without the final output transformation but I got wrong results , I figured maybe that's because the final output transformation uses the decryption keys that's come from inverting the round keys in encryption so I also tried losing the key mixing part of the first round encryption since its keys come from inverting output transfromation keys and keeping the final output transformation but I also got wrong results.
So what's the correct way to do decryption if the encryption was made without the output transformation?
