The following is an excerpt from the book Securing VoIP Networks: Threats, Vulnerabilities and Countermeasures. In this section of Chapter 6: Media Protection Mechanisms (.pdf), authors Peter Thermos and Ari Takanen discuss the strengths and weaknesses of SRTP.
MEDIA PROTECTION MECHANISMS
Any multimedia application—such as video, voice, or gaming—uses a distinct
set of protocols to set up sessions between end points (for example,
SIP, H.323) and a distinct protocol to transmit the media streams. The
standard protocol used to exchange media streams is RTP1 (Real Time
Protocol), which is defined in RFC 3550. As discussed in Chapter 3,
"Threats and Attacks," RTP streams can be intercepted and manipulated
in order to perform various attacks. Although IPSec can be used to protect
RTP, its limitations require a more scalable and versatile solution that alleviates
the NAT traversal issue, dynamic allocation of sessions,2 and the
need for a PKI. This has led to the development of SRTP3 (Secure Real
Time Protocol). The use of SRTP requires a mechanism to exchange cryptographic
keys before sending any media. Therefore, key management
protocols such as MIKEY and SDescriptions4 have been proposed to provide
the necessary keying material and management mechanisms to maintain
the security of multimedia sessions. Currently, there is not a single
key-exchange mechanism considered to be the industry standard because
each has strengths and weaknesses. The most logical approach: to combine
SRTP with the appropriate key-exchange mechanism is to identify the
requirements that need to be supported by the environment and evaluate
the applicability of each of the existing key management mechanisms.
Alternatives to using SRTP include DTLS (Datagram Transport Layer
Security) and IPSec, which were discussed in Chapter 5, "Signaling
Protection Mechanisms." The following sections describe SRTP and discuss
its strengths and limitations.
SRTP
The Secure Real Time Protocol (SRTP) is a profile for the Real Time
Protocol (RTP, IETF RFC 3550) to provide confidentiality, integrity, and
authentication to media streams and is defined in the IETF RFC 3711.
Although there are several signaling protocols (for example, SIP, H.323,
Skinny) and several key-exchange mechanisms (for example, MIKEY, SDESCRIPTIONS,
ZRTP), SRTP is considered one of the standard mechanism
for protecting real-time media (voice and video) in multimedia applications.
In addition to protecting the RTP packets, it provides protection for the
RTCP (Real-time Transport Control Protocol) messages. RTCP is used primarily
to provide QoS feedback (for example, round-trip delay, jitter, bytes
and packets sent) to the participating end points of a session. The RTCP
messages are transmitted separately from the RTP messages, and separate
ports are used for each of the protocols. Therefore, both RTP and RTCP
need to be protected during a multimedia session. If RTCP is left unprotected,
an attacker can manipulate the RTCP messages between participants
and cause service disruption or perform traffic analysis.
The designers of SRTP focused on developing a protocol that can
provide adequate protection for media streams but also maintain key properties
to support wired and wireless networks in which bandwidth or
underlying transport limitations may exist. Some of the highlighted properties
are as follows:
- The ability to incorporate new cryptographic transforms.
- Maintain low bandwidth and computational cost.
- Conservative in the size of implementation code. This is useful for
devices with limited memory (for example, cell phones).
- Underlying transport independence, including network and physical
layers that may be used, and perhaps prone to reordering and
packet loss.
These properties make the implementation of SRTP feasible even for
mobile devices that have limited memory and processing capabilities.
Similar design properties are found in MIKEY (Multimedia Internet
KEYing). Therefore, the use of MIKEY for key exchange and SRTP for
media protection is one combination of mechanisms to provide adequate
security for Internet multimedia applications, including VoIP, video, and
conferencing.
The application that implements SRTP has to convert RTP packets to
SRTP packets before sending them across the network. The same process
is used in reverse to decrypt SRTP packets and convert them to RTP packets.
To see how the conversion process works and to learn more about key management defense measures, download the rest of Chapter 6: Media Protection Mechanisms (.pdf).
1 H. Schulzrinne, et al. "RTP: A Transport Protocol for Real-Time Applications," IETF RFC 3550,
July 2003.
2 P. Thermos, T. Bowen, J. Haluska, and Steve Ungar. Using IPSec and Intrusion Detection to protect
SIP implanted IP telephony. IEEE GlobeCom, 2004.
3 M. Baugher, D. McGrew, M. Naslund, E. Carrara, and K. Norrman. "The Secure Real-time
Transport Protocol (SRTP)," IETF RFC 3711, March 2004.
4 F. Andreasen, M. Baugher, and D. Wing. Session Description Protocol Security Descriptions for
Media Streams, IETF draft draft-ietf-mmusic-sdescriptions-12.txt, 2005.
Reproduced from the book Securing VoIP Networks: Threats, Vulnerabilities and Countermeasures Copyright [2007], Addison Wesley Professional. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other users.
