_MACParams represents the parameters necessary to compute SSH MAC (Message Authenticate Codes).
_MACParams is a tuple subclass to maintain compatibility with older versions of the code. The elements of a _MACParams are:
0. The digest object used for the MAC
1. The inner pad ("ipad") string
2. The outer pad ("opad") string
3. The size of the digest produced by the digest object
_MACParams is also an object lesson in why tuples are a bad type for public APIs.
| Instance Variable | key |
The HMAC key which will be used. |