Enc/Dec both need update and finalize (which you'd get by providing `CipherContext`), they also both need `authenticate_additional_data` (which can be on an `AEADCipherContext`), and then only encryption needs to provide tag (so that can go on `AEADEncryptionContext`)

`_AEADEncryptionContext` can just wrap an `_AEADCipherContext` and the backend implementation of `_CipherContext` could still reasonably provide all the interfaces.

Just a thought.