Add aead_encrypt and aead_decrypt convenience functions

[thibault-martinez]

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[Wollac]

Two remarks:

• While I am not aware of any particular specification and there are cases with a different order, I would say the "usual" order is nonce || ciphertext || tag. So should we also maybe use that order here?
• I absolutely agree, we should add such a util function. Especially since associated_data and tag is hardly ever needed explicitly. However, I see this a bit differently for the nonce. There are common scenarios where the nonce is not chosen randomly, e.g. all-zero for ephemeral keys or derived from a hash. Good arguments can be made, why the nonce should still be explicit (like e.g. in crypto_secretbox_easy and libsodium is usually a great example for API design).
  However, nonce selection is security critical. So, when this better fits the use-case your are having in mind, let's leave it like this and hide the nonce.

[thibault-martinez]

• While I am not aware of any particular specification and there are cases with a different order, I would say the "usual" order is nonce || ciphertext || tag. So should we also maybe use that order here?

I kind of assumed that was the usual order as it has been chosen this way for the stronghold integration in the wallet. We obviously can't change it now. I would understand if choosing an arbitrary order was a no-go for an inclusion in crypto.rs

However, nonce selection is security critical. So, when this better fits the use-case your are having in mind, let's leave it like this and hide the nonce.

I am not fully certain what your conclusion is. Do you want me to make the nonce a parameter?

[Wollac]

If that order is already in use let's stick with it 👍
Maybe you could add a comment describing the order and that the nonce is initialized randomly.

[thibault-martinez]

Maybe you could add a comment describing the order and that the nonce is initialized randomly.

Done, let me know if this is fine.