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2. Goals and Features

2. Goals and Features​

The security goals for SRTP are to ensure:

  • the confidentiality of the RTP and RTCP payloads, and

  • the integrity of the entire RTP and RTCP packets, together with protection against replayed packets.

These security services are optional and independent from each other, except that SRTCP integrity protection is mandatory (malicious or erroneous alteration of RTCP messages could otherwise disrupt the processing of the RTP stream).

Other, functional, goals for the protocol are:

  • a framework that permits upgrading with new cryptographic transforms,

  • low bandwidth cost, i.e., a framework preserving RTP header compression efficiency,

and, asserted by the pre-defined transforms:

  • a low computational cost,

  • a small footprint (i.e., small code size and data memory for keying information and replay lists),

  • limited packet expansion to support the bandwidth economy goal,

  • independence from the underlying transport, network, and physical layers used by RTP, in particular high tolerance to packet loss and re-ordering.

These properties ensure that SRTP is a suitable protection scheme for RTP/RTCP in both wired and wireless scenarios.

2.1. Features​

Besides the above mentioned direct goals, SRTP provides for some additional features. They have been introduced to lighten the burden on key management and to further increase security. They include:

  • A single "master key" can provide keying material for confidentiality and integrity protection, both for the SRTP stream and the corresponding SRTCP stream. This is achieved with a key derivation function (see Section 4.3), providing "session keys" for the respective security primitive, securely derived from the master key.

  • In addition, the key derivation can be configured to periodically refresh the session keys, which limits the amount of ciphertext produced by a fixed key, available for an adversary to cryptanalyze.

  • "Salting keys" are used to protect against pre-computation and time-memory tradeoff attacks [MF00] [BS00].

Detailed rationale for these features can be found in Section 7.

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