c43f563dc7
Factoring this out, as computing these keys is something that we need to do both when generating the ArchiveEncryptionState, as well as when performing a decryption. This also renames "authKey" and "encKey" in ArchiveEncryptionState to use "backupAuthKey" and "backupEncKey", as these are more in-line with what the encryption design document uses (and because there are "authKeys" and "encKeys" that will be used by the encryption mechanism that are distinct from the backupAuthKey and backupEncKey). Differential Revision: https://phabricator.services.mozilla.com/D212858