How does the Key Expansion Step Work for AES-192

Epicko Corporation

8/13/24, 7:11 AM

I now understand the initial key from all the round keys will be the original 4x4 block for 128-bit keys, but I do not know how it would work for something else, like AES-256 or 192. Would it be that you copy the rest of the key and put it into the second block, but what would you do with the third and fourth columns? Thanks in advance!

1 + 1

aes

ecb

DannyNiu

8/15/24, 9:09 AM

For starters, AES rounds don't work over a single 128-bit key block. The key (regardless of length) is first "expanded" to a multiply of 128 bits (11x128 for AES-128, 13x128 for AES-192, and 15x128 for AES-256), afterwards, each round take one block of the expanded key schedule successively. If this clarifies your confusion I'll post it as an answer. If there's something unclear, [edit](https://crypto.stackexchange.com/posts/107604/edit) the question or post a comment down here.

Score:0

Crypto

Mustafa Hakan Solmaz

8/13/24, 12:56 PM

For a graphical description of the algorithm Click here. Key expansion is described for 128 bits in this video. The algorithm's block size is same, 128-bits, for all key-lengths 128, 192, and 256. So for each round a round key of 128 bits is produced.

Key expansion is different for 128, 192 and 256 bits. It is described in section 5.2 Figure 11 of fips-197. Follow Appendix A.1, A.2 and A.3 of fips-197 document for key expansion vectors. The crucial part is that you need longer keys for key expansion of 192 and 256-bit versions of the algorithm.

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Epicko Corporation

8/14/24, 4:04 AM

Yeah, I've read that document, and I actually found the flash animation for that video, but I still don't understand how they handle the extra bits

DannyNiu

8/15/24, 8:56 AM

@EpickoCorporation What are the "extra" bits that you're confused with? The 64 bits beyond the initial 128? Or the key schedule derived from the inital 192?

Epicko Corporation

8/19/24, 7:17 AM

The 64 bits beyond the initial 128!

Elon Musk

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