I worked at a videoconferencing hardware/software company. We provided systems to USA government offices like the NSA and State Department and provided an input for which they gave us hardware specs but told us nothing else, the customer did the final testing on it to make sure it worked as specified. We assumed it was for some sort of encryption method of which they revealed to us as little as possible, the hardware engineers who saw it tested only saw a large, portable black box. Otherwise our system used the standard encoding/decoding methods of the day in the 1990s.

The most secure method of communication is a one-time pad, a pre-shared private key.

"A one-time pad (OTP) is considered theoretically the most secure method of communication — when it’s implemented correctly. That means: 1. The key (pad) is truly random. 2. The key is at least as long as the message. 3. The key is used only once. 4. The key is securely shared in advance and kept completely secret.

When all these conditions are met, a one-time pad provides perfect secrecy — an eavesdropper cannot learn anything about the message, even with infinite computing power."

And you need to do that on paper, not on consumer device.

There are significantly fewer concerns about symmetric encryption, and while it doesn't scale to the size or budget of a service like Signal, it's exactly the type of thing the military is good at:

Distribute a bunch of physical artifacts (smartcards) across the globe; guard a central facility (a symmetric key exchange center) extremely well etc.

The military can also afford to run its (encrypted or plaintext) communications over infrastructure it fully controls. The same isn't true for a service provided out of public clouds, on the public Internet.