How does an AMRAAM communicate securely with the aircraft that launched it? From Wikipedia:


  It also incorporates a datalink to guide the missile to a point where
  its active radar turns on and makes terminal intercept of the target.
  An inertial reference unit and micro-computer system makes the missile
  less dependent upon the fire-control system of the aircraft.


What is the nature of this "datalink"? What is it called, what protocol does it use?

How is this datalink secure?
Do you have a need to know?

Fortunately, there is a simpler answer to your question: the original AMRAAM does not communicate with the host aircraft. Only the host aircraft communicates data to the missile. This data is transmitted in the form of fixed-format messages, repeated multiple times with redundancy. 

Security would fall outside the scope here, except in most general terms. In general terms, when secured, unidirectional datalinks use fixed key symmetric encryption. A missile's downlink isn't necessarily secured, as it's only transmitting the enemy's position for guidance correction.

Newer and more complex missiles that do communicate bidirectionally typically use Link 16. That is a secure encrypted protocol. It's well-documented and you can find a fairly good overview of the protocol on the net.