And that's why you do not want shadow-IT in your company (not sure if it was in this case, but looks like a pretty good example for me).
Still, trusting Signals encryption itself is a good decision.
@Threadbane
Military comms are designed to only allow personal that are pre vetted. You can't accidentally invite an outsider. The hardware is also secure.
These people have security comms people that travel with them to set up access to secure comms.
The other thing is Signal is not allowed on official phones and can't be downloaded.
This was on personal mobiles, which are unsecure and likely targeted and compromised by foreign intelligence.
It doesn't matter the encryption if Russia has a keylogger and screen capture software installed on the phone.
One of the party had just gone through Russian customs and would have had to hand their phone over and likely had software put on their phone.
China has also been in the US mobile system. So another way to put software on their phones.
Authoritarian countries have used routinely use spyware to surveil journalists, lawyers, political dissidents, and human rights activists
https://en.m.wikipedia.org/wiki/Pegasus_(spyware)
This is not the fault of Signal, but the underlying operating system of the phone. Particularly when up against adversaries with State level resources to target individuals.