http://y7yea4pmqqtznb33qiugvysyn2bob5v62e4pvoadoibrwkq3tsddjeyd.onion/ps/module/key-verification
For instance, if you ask to verify Signal fingerprints using a FaceTime video chat, the fake person would have to be able to both run a fake Signal account and a fake FaceTime account, and disguise themselves as your friend on video.