One step encryption/decryption? I'd like to put them in a separate PR. Ideally they're as simple as:

```python
    def encrypt(self, data):
        encryptor = self.encryptor()
        return encryptor.update(data) + encryptor.finalize()

    def decrypt(self, data):
        decryptor = self.decryptor()
        return decryptor.update(data) + decryptor.finalize()
```

but that may cause us problems with AEAD, so it may take a bit of discussion.