# RotWord in Rijndael's Algorithm

I am currently learning Rijndael's Algorithm through this site where there is a slide show animation step-by-step.
I understand nearly all of the encryption process except part of Add Round Key.
Looking at slides 16 & 17, it says:

Applying the RotWord ... to the previous word Wi-1

What is the RotWord and how do we apply it? Is it the same transformation in all rounds?

I can deduct (from the result on slide 17) that the RotWord transformation has done a shift upwards by 1 on the word:

+----+                 +----+
| 09 |                 | CF |
+----+                 +----+
| CF |                 | 4F |
+----+ --- RotWord --> +----+
| 4F |                 | 3C |
+----+                 +----+
| 3C |                 | 09 |
+----+                 +----+


Is my assumption of this transformation correct?
If so, will the amount shifted by change every round or does it remain constant?

• Yes. See also en.wikipedia.org/wiki/Advanced_Encryption_Standard. There is a pseudo-code in NIST's AES standard document. I have a Python code that very closely follows that pseudo-code in s13.zetaboards.com/Crypto/topic/7385224/1/ Aug 3, 2016 at 8:28
• @Mok-KongShen: if this is an answer, please submit it as an answer (so that it can be accepted) Aug 3, 2016 at 21:14