Longer term, avionic communications systems need secure design
Do they ?
What is the risk of a collision because a valid TCAS warning is ignored because of an expired certificate?
How are you going to keep the keys secure when they have to be handed out to every aircraft operator from North Korea to Narnia?
What is the risk of an attacker using this to cause an accident?
Many people are killed every year in the USA because of false 911 calls. The solution is to persuade Police to think before shooting - not to limit 911 calls to mil-spec secure communication channels.