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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?

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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.

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  • $\begingroup$ I'm obviously only asking for public information. I find this interesting because there seems to be quite a big attack surface here from an information security perspective. AMRAAM is quite old, if it uses old ciphers, they may have vulnerabilities, etc. $\endgroup$
    – AlphaCentauri
    Aug 10, 2019 at 14:52
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    $\begingroup$ There's actually an information security SE! Specific information regarding the ciphers is classified due to DOD policy, so no one who knows can tell you whether AMRAAM uses a cipher at all. What they can tell you is that encrypted protocols of that era and that application use Triple-DES with a fixed key. Triple-DES is not especially vulnerable and it's not broken. Key compromise is a different concern. But the midcourse update feature is a very minor factor in the AIM-120's performance. $\endgroup$
    – Therac - Peace for Palestine
    Aug 10, 2019 at 15:03
  • $\begingroup$ You would think that, given plane and missile initially being physically connected, they could exchange a one time pad for use that the planes computer generates. Key exchange seems trivial, given that initially there wouldn’t be any trust issues. But I think this is a fascinating question in general, obviously not many people need to know, but that doesn’t make it any less interesting. Lots of interesting questions here, I’d love to read a paper from a pentester that looked at it (obviously I know that such a document will never make it before my eyes. But it’s interesting regardless) $\endgroup$
    – JustSid
    Aug 10, 2019 at 21:44
  • $\begingroup$ @JustSid I was thinking the same thing. Not just the missile and the aircraft, but all aircraft in the package that will operate together should ideally exchange one time pads before the mission starts --- especially since the missile can receive target updates from AWACS or other friendly aircraft. This is actually a quite complex problem. $\endgroup$
    – AlphaCentauri
    Aug 12, 2019 at 3:54
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    $\begingroup$ I imagine, in the case of a missle and its launching aircraft, you can get away with pretty weak encryption. Since the missle has limited sending power, and it is active only for a short time, the window of opportunity (both temporally and spatially) is quite narrow. Furthermore, the aircraft that's being attacked by the missle needs to have special hacking equippment on board to successfully exploit any weakness. Comms between aircraft is a different kettle of fish, since they are active for years, and probably transmit much further than a simple air-air missle. $\endgroup$
    – Dohn Joe
    May 6, 2020 at 7:17

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