What is a safety number?
Each Signal conversation has a unique safety number that allows you to verify the security of your messages and calls with specific contacts.
Why do I see a safety number change alert?
Signal advises you whenever a safety number has changed. This allows users to verify the privacy of their communication with a contact and helps protect against any attempted man-in-the-middle attacks.
The most common scenarios where a safety number advisory is displayed are when a contact switches to a new phone or reinstalls Signal. However, if a safety number changes frequently or unexpectedly it may be a sign that something is wrong.
Verification of safety numbers is a good security practice for sensitive communication. If a safety number has been marked as verified, any change must be manually approved before sending a new message.