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I have read that there is no corelation between the key length and the block length in AES. But it doesn't make sense to me, because the key will XOR the block, how they can be in different lengths?

Eran Nahshon
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2 Answers2

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There is no relationship between key size and block size in AES, because it it not a simple XOR. Block size is always 128 bits, but key size can be 128, 192, and 256 bits based on algorithm rounds.

AES is a complex algorithm with a lot of steps. key is used in key scheduler and then its result will be used in encryption process. You can check details in here.

Afshin
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  • So a longer key will derive more fixed length key? The length of the key affects the number of subkeys and rounds? – Eran Nahshon Oct 21 '18 at 06:52
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    @EranNahshon yes. 128, 192, and 256 bit key is used for a 10, 12, and 14 rounds algorithm. If you want to understand it, please read that wikipedia page at least. – Afshin Oct 21 '18 at 06:54
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AES has 10, 12 or 14 rounds. Having that many separate keys is clumsy to handle and keep secure. Instead a single key is used to generate the 11, 13 or 15 round keys that are used, one for each round plus one extra. The process of deriving the round keys from the initial single key is called the Round Schedule. The round keys do not have to be the same size as the initial single key.

rossum
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