1
$\begingroup$

24 bytes generate methodIs it possible to generate a 24 bytes CMAC using AES-192 or AES-256 and if so, how?

According to AN10922AN10922.0df,looks like we can generate 24 bytes and 32 bytes key using div constant ?

$\endgroup$
1
  • $\begingroup$ I didn't recall seeing that PDF in your initial revision of the question. Now that we know the semiconductor manufacturer NXP provide this sort of guidance, could you share with us, what environment are you developing on/for? $\endgroup$
    – DannyNiu
    Commented May 23 at 7:04

1 Answer 1

3
$\begingroup$

You can't. AES only has 16-byte (i.e. 128-bit) block size. The only alternatives would be to

  • use Rijndael variants with wider blocks (the derivation of subkeys $K_1$ and $K_2$ will have to change to use other polynomials),
  • iterate the final block through something like OFB (output feedback) mode.
$\endgroup$
1
  • 1
    $\begingroup$ Just iterating the final block won't make it more secure than an 128-bit MAC, though. An option might be to use some modified construction which uses a wider state, but that won't be CMAC. $\endgroup$ Commented May 22 at 21:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.