All of the best site to site encryption is based on Signal.
Most of the supposed “secure” sites are NOT based on it.
Signal has an excellent reputation
Hate to break it to you but compared to other countries in NATO, America is WAY behind.
Other alliance countries are well down the road of using very modern multifactor authentication approaches (there’s even talk of Free Republic adopting it) and Europe stopped using SMS messaging ages ago. Telegram , Signal, and WhatsApp, are totally prohibited for live discussion of ongoing military operations.
Has the USA done all of the same? Not really. Which is even more concerning when you have a senior politician sat IN THE KREMLIN using a Russian owned app over a Russian provided internet connection.
For secure messaging, in NATO , there’s XO Mail and NI2CE Messenger, with its security reinforced through Armour Mobile and MFA.
All of this information by the way is UNCLASSIFIED.