And I don't mean "I learned how to do textbook RSA in college", I mean more of, "Can tease a previously undiscovered cache-timing side-channel out of a crypto library". (For example, the recent libgcrypt advisories.)

If the words "padding oracle" or acronyms like AEAD sound strange and foreign, that person is not qualified to fill that role.

I would wager most developers lack the background to make a messaging service that is actually secure. (To anyone reading this: Please don't let this fact magnify any sense of impostor syndrome you may have. You're far from alone. Even the experts won't embark on this endeavor without peer review.)