I wanted to know how does the increase the block size or rounds in AES is not a good idea

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    $\begingroup$ What's unadvisable is trying to increase the number of rounds or using a larger block size to a published (standardized) block cipher without a deep of knowledge of cryptanalysis, or keeping using the standard's name. For AES (which by definition is a 128-bit block cipher), the reasonable approach if needing a wider block is sticking to what's proposed by the authors of Rijndael, which defined 192-bit and 256-bit block modes. $\endgroup$
    – fgrieu
    Feb 12 '18 at 6:57
  • $\begingroup$ an AES derivative would benefit more from a change in key schedule than it would from additional rounds $\endgroup$ Feb 13 '18 at 1:43

Just in general messing with well studied cryptographic standards is inadvisable. Especially if there is no clear benefit.

Adding rounds to AES is fairly safe, before it became AES the number of rounds in Rijndael was discussed. You need to make sure you continue to use a reasonable key schedule. This may have even some value, as we have seen attacks on reduced round AES making the safety margin smaller than originally believed.

Changing block size is far from trivial. It is not all obvious how to change AES block size while preserving it's desired properties. This is far more likely to end up in a weak slow cipher.

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    $\begingroup$ Safely adding rounds to AES is fairly easy only if one does not need to resit related-key attacks. $\endgroup$
    – fgrieu
    Feb 12 '18 at 7:09
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    $\begingroup$ You can add rounds as much as you want as long as you generate the round keys using an independent key. You could formally prove that this is no worse than AES in such a case. $\endgroup$ Feb 12 '18 at 8:59
  • $\begingroup$ so does adding rounds reduce performance? $\endgroup$ Feb 14 '18 at 4:17
  • $\begingroup$ Obviously, you can't add rounds in parallel. $\endgroup$
    – Meir Maor
    Feb 14 '18 at 4:19
  • $\begingroup$ can you please elaborate what you mean by that? $\endgroup$ Feb 14 '18 at 4:27

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