Encrypted direct messages in Twitter

Just thought about an add-on service for Twitter. Twitter isn´t really an secure communication channel. It would be nice to have an something like an service for public keys. Obviously such an service would need support from the twitter clients. A client would generate a key pair. The private key stays on the system of the user. The public key is transmitted to a server accessible in the internet, the fingerprint and the URL of the public key of the key would be integrated into the twitter profile. When a user wants to transmitt an encrypted direct message to a user, the client checks for the profile, gathers the public key, checks with the fingerprint in the twitter profile and encrypts the direct message with it, prepends the message with a magic (like !C1! to mark it as a crypted message, encrypted with the first public key in the profile) and sends it to the other user. The client of the other user can encrypt it with it´s private key. By using an URL for the key instead of storing the key directly at twitter the public key can reside on any available server. Such an extension of a Twitter client would enable private direct messages trough Twitter.
Just yet another idea while waiting for the water for my tea to start boiling.