Hello there,
I'm going to make a long story short. It's been a while that I want to implement my own AES encryption/decryption program. The encryption program went well and encrypting without any error or strange output (Since I have compared my program's output with a working commercial one and result was the same).
Wikipedia was (is) my guide in this implementation within which I read "A set of reverse rounds are applied to transform ciphertext back into the original plaintext using the same encryption key."
There are couple of modules that I implemented:
- Add round key
- Shift rows
- Sub bytes
- Mix Column
I also implemented couple of reverse implementation of the above modules:
- Reverse shift rows
- Reverse Sub Byte
- Reverse Mix Column
NOTE: I didn't implement reverse round key since, It's XOR ing the plaintext with the encryption key, and reverse of XOR is XOR itself (correct me if I am wrong)
So I putted this modules in the reverse order that I did encryption, but never I got my plain-text back:
expandkey128(key);
rev_subbytes(data);
rev_shiftrows(data);
addroundkey(data,key,10);
for(int i = 9; i>= 1; i--) {
rev_subbytes(data);
rev_shiftrows(data);
rev_mixColum(data);
addroundkey(data,key,i);
}
addroundkey(data,key,0);
// Please note that I also did from 0 to 10 ,
// instead of 10 to 0 and didn't workout
And also I thought , maybe I should not implement reverse model of the modules, maybe I have to use those modules that I did encryption with, only in reverse order; well guess what? didn't work! :
expandkey128(key);
addroundkey(data,key,0);
for(int i = 1; i<= 9; i++) {
subbytes(data);
shiftrows(data);
mixColum(data);
addroundkey(data,key,i);
}
subbytes(data);
shiftrows(data);
addroundkey(data,key,10);
So here is the question: what is wrong? || what is the correct sequence of applying these so called modules or functions if you will?