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Can someone explain to me why does using a key schedule make's AES more secure if instead of calculating and adding your round key's, you just keep on adding the cipher key. ?

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Not using separate keys for each AES round would make your cipher vulnerable to slide attacks. Using two plaintexts M and M' with M' being the result of an AES round after calculating M as input, you can differentiate between those plaintexts by calculating the output of an AES round which takes the ciphertext C as input.

Due to the birthday problem, this attack schould reduce the complexity of breaking the AES to $\mathcal{O}(2^{n/2})$.

Source: Silde attack

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Many block ciphers, including AES, encrypt using multiple/iterated rounds (10, 12 or 14 for AES depending on key size). Each round requires a key, called sub-key or round key (always 128-bit for AES), and it would be a weakness¹ if the same sub-key was used at each round.

The key schedule's job is to expand the original key (128, 192 or 256-bit for AES) into one sub-key per round. Without somewhat executing the key schedule, we would not get the right sub-keys, and the result of AES encryption or decryption would be wrong.

The key schedule does not really need to be performed before executing AES, at least for encryption: it can be performed while executing AES, and that's pretty common in hardware, and even in software when speed is less important than key ability and memory size.


¹ This other answer gives an attack, I would not bet there are not others even for AES-128, and for larger key sizes much of the key bits would go unused.

PS: much of the question is about the reasons for multiple subkeys, thus my answer is lacking; upvote another!

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    $\begingroup$ " it would be a weakness if the same sub-key was used at each round." <- I think the OP is interested in said weakness $\endgroup$
    – SEJPM
    Apr 9 '20 at 11:53
  • $\begingroup$ @SEJPM: yes, but something comprehensive on this would make the answer may times bigger, even if I stick to AES; and I'm not sure how I would start. $\endgroup$
    – fgrieu
    Apr 9 '20 at 11:59
  • $\begingroup$ +1 for mentioning the subkey derivation during each round rather than all at once. $\endgroup$
    – tum_
    Apr 9 '20 at 12:02

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