There's really not much of a difference, except that WhatsApp allows you to verify the fingerprint and will notify you if your chat partner's public key changes

There's really not much of a difference, except that WhatsApp allows you to verify the fingerprint and will notify you if your chat partner's public key changes. WhatsApp doesn't force to you meet your contacts, and there's no "signing each other's public keys" involved at all. There are two levels of authentication or verification - with iMessage, you only get the first one: - The first is WhatsApp telling you "Hey, this is +555 0100. Their public key is 12345. Once upon a time, I sent an SMS to that number, and the device with this key was able to read a code in that message. Looks like the owner to me. Good luck! PS: I might be lying." - (Optional) You compare the fingerprint by meeting in person or communicating through some other secure channel. This will ensure that WhatsApp is not lying to you¹, and that you're actually encrypting messages with a key belonging to the recipient (and vice-versa). ¹ Unless your client has a backdoor, of course.