Call them and read the number out? I dont think it matters if someone else can see your safety number, you can print it in a newspaper if you really wanted to.
this post was submitted on 17 Feb 2025
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then I do not understand how safety numbers work.
The safety number is not part of the encryption. It just says: this person is who they say they are. So as long as you can trust that the number actually came from that person, it's fine. Afaik, the number is derived from the encryption keys, so it can't be faked, but I would verify that if you're unsure.
Edit: was curious, here's the blog post that introduced them: https://signal.org/blog/safety-number-updates/ Essentially, it's a hash of the public key, so safe to broadcast, similar the HTTPS certificates employed on the web. They even say so: "the share button on the safety number screen and selecting FB, Twitter, email, etc to send the safety number to your contact."
but if somebody else got my safety key for some other person, what could they do with it?
Nothing, it can't be used for anything else. You can't reverse the encryption keys from it. Its like adding all the digits in your phone number and giving that out. People with your phone number can verify it, but to everyone else, its basically useless.