The following is a quote from Wikipedia:

One key insight is that the same encryption key can be used for both, provided that the counter values used in the encryption do not collide with the (pre-)initialization vector used in the authentication.

The entire article can be found at: https://en.wikipedia.org/wiki/CCM_mode

I do not understand what it is saying. I am hoping somebody can explain it to me.

Thanks Bob


1 Answer 1


You want both confidentiality and authenticity. You use e.g. encryption for confidentiality and a MAC for authenticity.

There are a host of problem that you can get if you use the same key for both. (This is imho the canonical example.) So that unfortunately in general means you should use two keys or in other words a combined key of twice the size.

The sentence you quote advertises that given certain other assumptions this is unnecessary for CCM.

For reference: RFC and Security proof

  • $\begingroup$ Can you comment about how the counter values might collide with the (pre-)initialization vector. I do not see how they are related. Thanks. Bob. $\endgroup$
    – Bob
    Mar 24, 2017 at 14:20
  • 1
    $\begingroup$ The security proof requires that the inputs to the block cipher from the counter are different from the first block being MACed. The RFC achieves this by appropriate formatting of those blocks. $\endgroup$
    – Elias
    Mar 24, 2017 at 17:26

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