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Encryption in RTP

protocol transport time real

Definition: Encryption in Real-time Transport Protocol is used to ensure the confidentiality of the media content.

To transport multimedia over the Internet, appropriate protocol is needed. For instance, RTP, Real-time Transport Protocol , are created as a standard protocol for the end-to-end network transport of real-time data, including audio and video. Today, RTP is often used through Internet that can not be considered secure. To ensure the confidentiality of the media content, encryption has to be used. To support on-demand and live streaming media services, a streaming media system needs to support pre-encryption and live-encryption respectively. Further, the system must be capable of delivering the decryption key to the authorized clients securely.

In RTP, when encryption is desired, all the octets that will be encapsulated for transmission in a single lower-layer packet are encrypted as a unit. The presence of encryption and the use of the correct key are confirmed by the receiver through header or payload validity checks. The default encryption algorithm in RTP is specified to be Data Encryption Standard (DES) algorithm in cipher block chaining (CBC) mode.

Strong encryption algorithms, such as Triple-DES, can be used in place of the default algorithm for better security. In addition, profiles may define additional payload types for encrypted encodings. RTP consists of a data and a control part. The latter is called RTCP. A framework for encryption of RTP and RTCP streams is provided in SRTP, the Secure Real-time Transport Protocol. SRTP is a profile of RTP that defines a set of default cryptographic transforms. It allows new transforms to be introduced in the future. With appropriate key management, SRTP is secure for unicast and multicast RTP applications.

In RTP, mixer is used to perform remixing of RTP streams, including encrypted streams. They are able to decrypt and re-encrypt streams. Translators are another type of application level devices in RTP. They perform payload format conversions, tunnel the packets through firewalls, and add or remove encryption and enable the coexistence of the different networking technologies. Since in many applications, not all information should be confidential, the use of mixer and translator can help to optimize network usage. As RTP is often used for transferring huge amounts of time critical data, for example video, it is essential that all security features are implemented with minimal delay and jitter. It should be evident that with huge transmission rates even a small timing overhead easily amounts to huge loss of bandwidth. RTP, implemented on an application bases, provides the flexibility to allow splitting of packets into encrypted and unencrypted parts.

Interested readers are directed to IETF’s Real-time Transport Protocol and The Secure Real-time Transport Protocol Internet draft documents for more details about the protocols.

Enders, John Franklin [next] [back] Emotion and Multimedia Content

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