The United States Senate has been taking cybersecurity more seriously than ever before, thanks to the DNC leaks and various government cyberattacks. Senate Sergeant at Arms Frank J. Larkin and his team have recently finished encrypting all Senators’ websites, and it turns out he has also approved Signal for official use by Senate staff members. Sen. Ron Wyden, a privacy and encryption advocate, has revealed that Larkin’s office has given one of the most secure messaging apps out there its seal of approval in a letter thanking the Sergeant at Arms for his efforts.
See https://www.engadget.com/2017/05/17/us-senate-approves-signal-for-staff-use/ 
US Senate approves encrypted chat app Signal for staff use
Apparently, Senators and their staff have been officially able to use Signal since March.