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In general the AAD itself is not required or won't change the security of the GCM mode of operation itself. It may however directly influence the security of the protocol in which GCM is deployed. For instance, you may have specific configurable parameters outside the ciphertext itself. These parameters may very well include: version number of the ...


AAD has nothing to do with making it "more secure". The aim of AAD is to attach information to the ciphertext that is not encrypted, but is bound to the ciphertext in the sense that it cannot be changed or separated. (Conceptually, the MAC is computed over the AAD and the ciphertext together.)


Any of the ways you listed would work. If you're collecting alternatives, yet another one (which I have seen in practice) is to include the message counter as the AAD (additional authenticated data), which is another input to GCM. When we consider which one would be the best, we note that GCM has absolutely no requirement that the nonce be "random", or for ...

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