In a previous question, it was made clear that in the case of GCM, there is no distinction between "no AAD" and "zero-length AAD". Since I have noticed "zero-length AAD" in several implementations, my question is, does that render such AE schemes less secure?

In the various papers that treat AE security, I think the paper Reconsidering Generic Composition by Namprempre, Rogaway and Shrimpton might have been the most explicit in this regard. According to their nAE scheme enumeration, the first scheme that shows weaker security bounds is A9 where A (i.e. AAD) is dropped from the Tag computation (see Fig. 5). Now of course, they do mention that it's a borderline case (transitional). But they make it clear that its security is not as tight as the A1-A8 schemes (all of which include A in the tag calculation). I have tried to survey other papers on the topic, but I haven't found a paper that gives a similar treatment.

So if AAD is missing from GCM (or if |AAD|=0), is GCM security downgraded in any way or is our security proof a bit weaker? Or, is that not a conclusion we should take from Namprempre's paper above (assuming their results were accepted by the crypto community)?

  • $\begingroup$ As noted: GCM is A5 on the paper. $\endgroup$
    – kelalaka
    Oct 16 at 15:09
  • $\begingroup$ GCM doesn't support noAAD. In GCM, the AAD with zero-length is converted to 128 zeroes. This is clear from the previous answer and NIST's doc. There is always an AAD! in GCM The recipient, therefore, can reject any empty AAD as a forgery. This provides an additional check since AAD is authenticated but not encrypted. If you are asking to modify the CGM to remove AAD, then this is a different scheme. $\endgroup$
    – kelalaka
    Oct 16 at 15:44
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    $\begingroup$ Yes, that is correct, $A || 0^v$ is empty, however, the length is encoded in 64 bits so that one can decode it as empty AAD. So, still, there is always AAD in GCM. $\endgroup$
    – kelalaka
    Oct 16 at 16:47
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    $\begingroup$ No ADD should be something like that; there is no information available about AAD, even length or single bit. However, in this case, the non-existence of AAD may indicate that it is erased or already noAAD. The scheme should distinguish this. GCM simply encodes the length and doesn't allow noAAD ( or we can say converts noADD into zero-length AAD). $\endgroup$
    – kelalaka
    Oct 16 at 17:27
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    $\begingroup$ Got it. So no-AAD here means no information about AAD, not that AAD logic is dropped. I think we're in total agreement now. Thanks for your time! $\endgroup$
    – hlayhel
    Oct 16 at 19:29

AES-GCM falls category A5 in the paper. In AES-GCM doesn't support $\texttt{no-AAD}$, even you don't use AAD during encryption, AES-GCM convert his as a $\texttt{zero-length-AAD}$.

See in the NIST Special Publication 800-38d, page 15;

Algorithm 4: $GCM-AE_K (IV, P, A)$

4. Let $u = 128\cdot\lceil\operatorname{len}(C)/128\rceil - \operatorname{len}(C)$ and let $v = 128\cdot\lceil\operatorname{len}(A)/128\rceil - \operatorname{len}(A)$.
5. Define a block, $S$, as follows: $$S = \operatorname{GHASH}_H(\mathbf{A \mathbin\| \mathtt 0^v} \mathbin\| C \mathbin\| \mathtt 0^u \mathbin\| \mathbf{[\operatorname{len}(A)]_{64}} \mathbin\| [\operatorname{len}(C)]_{64}).$$
In Steps 4 and 5, the AAD and the ciphertext are each appended with the minimum number of ‘$\mathtt 0$’ bits, possibly none, so that the bit lengths of the resulting strings are multiples of the block size (The bolds, are mine).

$A$ is associated data, $len(A) = 0$ therefore, $\mathtt{v} = 0$. Even in this case, we have $[\operatorname{len}(A)]_{64}$, this 64-bit encoding of the length of th associated data and this will always indicate the exsitance of AAD, zero-length or not!.

If an attacker, removes the associated data, to make it seem like zero-length, during the decryption, one must halt with the tag mismatch ( Always halt and stop the decryption at once).

A scheme that supports no-AAD, must not carry information about the missing AAD, otherwise, it falls into zero-length AAD.

In the end, we have extra protection against forgeries with zero-length AAD.


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