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I am evaluating ways to reduce the overhead of (H)MACs on small messages. I was primarily wondering if there is a difference between halving the MAC length or keeping the same MAC length but using it to authenticate a message of twice the size.

My initial thought was that the message/MAC ration would be the same though then it occured to me that this is probably not the complete picture as each individual package would be simpler to attack. Therefore I would probably have to discard both packages if one if them fail an integrity check.

Is my last conclusion correct for them to behave identically or am I missing something?

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  • $\begingroup$ MAC security is independent of message size. $\endgroup$ Commented Mar 24, 2022 at 20:25
  • $\begingroup$ You should be careful about truncating the MAC output, You may give huge advantage to the attacker. Still your question is not clear or wide to answer! $\endgroup$
    – kelalaka
    Commented Mar 24, 2022 at 22:54
  • $\begingroup$ @kelalaka I was more thinking about shrinking the output by using a different Algo (e.g. SHA-512 → SHA-256) though I think SAI Peregrinus already cleared up my misunderstanding $\endgroup$
    – Septatrix
    Commented Mar 25, 2022 at 11:18

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