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. To view a safety number, open a conversation, tap on the contact's name to access the conversation settings, and choose "View safety number".
You can then compare the numbers or scan a single QR code to make sure the same number is displayed on both your phone and your contact's phone and optionally mark the safety number as verified to store the result of the comparison.
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.