http://www.rfc-editor.org/rfc/rfc4568.txt
RFC 4568: Session Description Protocol (SDP) Security Descriptions for Media Streams, F. Andreasen, M. Baugher, D. Wing2006-07-01T23:00:00-00:00This document defines a Session Description Protocol (SDP) cryptographic attribute for unicast media streams. The attribute describes a cryptographic key and other parameters that serve to configure security for a unicast media stream in either a single message or a roundtrip exchange. The attribute can be used with a variety of SDP media transports, and this document defines how to use it for the Secure Real-time Transport Protocol (SRTP) unicast media streams. The SDP crypto attribute requires the services of a data security protocol to secure the SDP message. [STANDARDS-TRACK]