Signal app, widely considered the most secure of all other encrypted messaging apps, stores minimum information about its users. Signal app was put to the test earlier this year when an FBI subpoena and gag order demanded a wide range of information on two Signal users. But unfortunately for the government, Signal keeps as little data as possible on its users, and therefore Open Whisper Systems was unable to hand over anything useful to the FBI agents. Signal is also the force behind the Signal Protocol that powers the encryption built into WhatsApp, Facebook Messenger and Google Allo’s Incognito mode.
Source: https://thehackernews.com/2016/10/signal-messenger-fbi-subpoena.html