What is a safety number?
Each Signal one-to-one chat has a unique safety number that allows you to verify the security of your messages and calls with specific contacts.
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.
Why do I see a safety number change alert?
Signal advises you whenever a safety number has changed. This allows users to check 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 re-installs Signal, but these actions don't always result in a safety number change. However, if a safety number changes frequently or unexpectedly it may be a sign that something is wrong.
Why was my message before the safety number change alert not delivered?
Undelivered messages will not be resent after a safety number has changed. Your messages with Signal are end-to-end encrypted which is like having locks and keys for each message. When the safety number changes it's as if you have changed the locks and keys for your messages. So, messages that were sent before the safety number change are using the old locks and will not be delivered.
Note: This is only created after you've accepted a message request.
- Open a chat with a contact.
- Tap on the chat header or the overflow menu then chat settings.
- Select View Safety Number.
How do I verify the safety number with my contact?
The easiest way to compare safety numbers is to scan your contact's QR code while viewing their safety number. You can also visually or audibly compare the numeric code, or use the share icon to copy it to your clipboard. If the safety number is identical then you can be sure that you are communicating with the right person.
How do I manage the verified status of the safety number?
View the safety number and tap the Mark as Verified or tap Clear Verification button.
How do I know if a safety number has been marked as verified?
A checkmark will appear in the chat header by your contact's name when the safety number is marked as verified. It will remain verified unless the safety number changes or you manually change the verification status.
Why are safety numbers being updated?
The legacy safety number format depended on phone numbers. The new safety number format does not.
In order to support upcoming features like usernames, where you will be able to start a conversation using someone's Signal username without necessarily knowing their phone number, the way that safety numbers are generated needs to be updated too.
On a technical level, the legacy safety number format is a numeric hash of the associated phone numbers and public keys. Instead of using phone numbers, the new safety number format is a numeric hash of the associated account IDs and public keys.
The set of public keys that are verified during the comparison process is exactly the same.
Is there a security difference between the two safety numbers?
No. Both formats are equally secure, and if you have previously marked a safety number as verified you do not need to compare the numbers again.