All the real documentation for this is likely to be classified, but here's

a backdoor: https://www.fsf.org/blogs/community/replicant-developers-fin...

a patent for doing it in "civilian" applications: https://patents.google.com/patent/WO2013114317A1/en

sure, the NSA will respect that

Well damn. I had a vague feeling that something like that was possible, but this is eye opening, thank you.