How can one share information using the 'host-proof' paradigm?
Getting data from user A to user B securely generally works via asymmetric encryption. Encrypt the data with B's public key and store the result on the server. B then retrieves the encrypted data and decrypts it using his private key.

This assumes the server does not store the private key, or at least stores it in a form that can only be decrypted by the owner.