The client fetches the pub key off the server which is decentralized? There's no part in the protocol that authenticates whether or not a pub key is legit. If its replaced by an attacker and someone subsequently goes to fetch a key they can read those messages. I mean, pub key infrastructure is meant to solve that. With SSL and such... that's why you its a federated chain of certificates with providers vouching that names = pub keys.

This is a very common problem. There is potential to possibly make this more decentralized with smart card technology. Like imagine a smart phone with access to pub keys in the hardware tied to an account cryptographically. Then you can say something like phone number = subscriber = pub key. Encrypted messaging apps seem to bootstrap off of ownership for numbers in the mobile system (mobile system security is very bad so there are dragons here.) The other apps like pidgin with OTR plugins they have unique phrases that help with the issue.

When you start looking at decentralized pub key infrastructure tied to human-meaningful names you start to run into zookos triangle:

https://en.wikipedia.org/wiki/Zooko%27s_triangle

human-meaningful, decentralized, secure -- pick two