Remote Audio Output Protocol (RAOP), is an RTSP/RTP based protocol made by Apple Inc. It powers the AirPlay technology built into AirPort Express wireless (802.11b/g/n) access point as well as the Apple TV. The major difference from the RTSP protocol is an initial Asymmetric key verification made by iTunes to verify it is communicating with an AirPort Express (as opposed to a simulation), and vice versa. The data channel is also encrypted by AES, with a random key protected by the asymmetric key mentioned above.
The RSA public key stored in iTunes has been extracted by Jon Lech Johansen, enabling 3rd party software to stream music to an Airport Express [1].
The RSA private key stored in the AirPort Express has been extracted by James Laird, enabling simulation of an Airport Express. [2]. The Shairport application is available[3]