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 mak

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. [0]: https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm