AES key and block size

I am trying to understand $AES$ and I wondered what happens in the case of a $128$ bit block size and a $256$ bit key. All documents I read about $AES$ told me the key expansions algorithm for $128$ bit keys and $128$ bit blocks. But I have a $256$ bit key ($8$ words) and $128$ bit block ($4$ words) and I am not sure how the key expansion works now. Should I expand it how usual but instead beginning in the second round beginning in the third round? Like this graphic shows: • Sorry it was a typo... – Michael Jan 27 '16 at 16:33
• The answer to your question should be in FIPS 197 – SEJPM Jan 27 '16 at 16:51
• Then that's well specified in the standard document FIPS-197 of NIST, where the processing of the AES scheme is nicely described with pseudo-codes. For a concrete implementation in Python that closely follows that, see: s13.zetaboards.com/Crypto/topic/7385224/1 – Mok-Kong Shen Jan 27 '16 at 16:52
• @Mok-KongShen Could you turn that into an answer? "Read the standard" is not much of an answer in itself, but maybe you can quote the pseudo code? Otherwise I think the question has to be closed. – Maarten Bodewes Jan 27 '16 at 23:39
• @Maarten Bodewes: P.20 of FIPS-197 has "Figure 11 Pseudo Code for Key Expansion" and also notes that its "Appendix A presents examples of Key Expansion". – Mok-Kong Shen Jan 28 '16 at 15:05

Well, as SEJPM and Mok-Kong Shen pointed out, it's described in FIPS 197, but let's explain it in details:

Here's the Key Expansion algorithm pseudo-code from FIPS 197: Where

• $Nk$ is the number of $32$-bit words that composed the key
• $Nr$ is the number of rounds
• $Nb$ is the number of columns in the state block (which is always $4$).
• $word$ is a $32$-bit word (can be seen as an array of $4$ bytes).
• $w$ denotes a $4$-byte word array where the number of elements is equal to $Nr + 1$ (because AES uses a different key at each round)

For a $256$-bit key, we have $Nk = 8$ and $Nr = 14$ according to FIPS 197.

When you are asking

Should I expand it how usual but instead beginning in the second round beginning in the third round?

Don't you mean the fourth round? Let me explain.

As you can see with the above pseudo-code, the first loop just copy the secret key at the beginning of the array $w$. Then, we get into the second loop to compute rounds key which are derived from the given one.

So for a $128$-bit key, you start to derive round keys as soon as $i=4$ but only when $i=8$ for a $256$-bit key (because the key is longer).

Also, as mentionned in FIPS 197, note that the routine isn't the same regarding to the key length (which is $Nk$) because there are arithmetic conditions between $i$ and $Nk$ to get into different branches of the algorithm. For example, only $256$-bit keys will satisfy the condition else if (Nk >6 and i mod Nk = 4).

Hope it's more clear now.