As I understand it, Signal takes a "trust on first appearance" approach and only really starts bugging you about verifying keys if they suddenly change.
Only the latter really, but yes. It doesn't need to be required -- see the footnote text where I elaborate: Threema also doesn't require it, but it's very visible, so people do it and thereby avoid having to trust the server.
> only really starts bugging you about verifying keys if they suddenly change
Does it? I haven't seen that, though I think the only key changes so far have been in group chats, perhaps that's different. This would be quite an improvement over WhatsApp that I wasn't aware of (if it really does this).
> 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.
https://support.signal.org/hc/en-us/articles/360007060632-Wh...