What did you say your name was?
Er, am I missing something - the recipient obtains the private key from the repository? How does the repository now that it's the legit recipient asking for it? Because they've pre-registered and been given an authentication credential? And how did they prove that they really were the owner of the identity? Etc, etc.
Are keys static or do they change with every message or are they actually formed from sender/recipient id pairs? If the former, whoppee, I persuade the originator to send me an encrypted message, a simple bit of social engineering, I then get the private key and read all other messages they sent and I've got my hands on. Even better if recipents own computer security is a bit flakey, someone else can get at their collection of private keys. There's a whole new entreprenurial opportunity for the criminally inclined here.