But you can? Open a conversation -> Menu -> Conversation settings -> View safety number.
Note that the safety number is basically a combination of your and your contact's (DHKE-negotiated) keys and is thus going to be different for every conversation. The reason both keys are not shown separately is that it apparently confused users.
Interesting, and thanks! Normally DHKE is authenticated though with public key cryptography? The way I understand is DHKE establishes a secret... with someone but in order to ensure it was your intended party, not a hostile government, usually a public key signature exchange takes place.
That's correct, it's an authenticated Diffie-Hellman key exchange. I'm not sure why they don't show the actual public keys anywhere but I suppose it doesn't make a lot of sense for the general public and it'd only be confusing.
In fact, the cryptography is even more complicated: The Signal protocol uses a so-called double ratchet algorithm[0] which derives from the session/conversation keys a new ephemeral message key whenever possible (again using a DHKE). This has the added advantage of providing forward secrecy.
Note that the safety number is basically a combination of your and your contact's (DHKE-negotiated) keys and is thus going to be different for every conversation. The reason both keys are not shown separately is that it apparently confused users.