Consider exposing AesGcm::compute_tag

[bryceberger]

(and, to make it usable, AesGcm::init_ctr())

It is useful to be able to generate a hash without caring about the encrypted text, and it would be nice to just use the implementations in the aes_gcm crate directly.

[newpavlov]

Do you mean GMAC? It's better to introduce a separate crate for it in the MACs repository.

[tarcieri]

GMAC is an option, although you can also get similar properties out of AES-GCM itself by using an empty plaintext/ciphertext and passing only the additional associated data to be authenticated

[newpavlov]

Going to close this in favor of RustCrypto/MACs#1