draft-ietf-avtcore-rtp-security-options-10.txt   rfc7201.txt 
Network Working Group M. Westerlund Internet Engineering Task Force (IETF) M. Westerlund
Internet-Draft Ericsson Request for Comments: 7201 Ericsson
Intended status: Informational C. Perkins Category: Informational C. Perkins
Expires: July 19, 2014 University of Glasgow ISSN: 2070-1721 University of Glasgow
January 15, 2014 April 2014
Options for Securing RTP Sessions Options for Securing RTP Sessions
draft-ietf-avtcore-rtp-security-options-10
Abstract Abstract
The Real-time Transport Protocol (RTP) is used in a large number of The Real-time Transport Protocol (RTP) is used in a large number of
different application domains and environments. This heterogeneity different application domains and environments. This heterogeneity
implies that different security mechanisms are needed to provide implies that different security mechanisms are needed to provide
services such as confidentiality, integrity and source authentication services such as confidentiality, integrity, and source
of RTP/RTCP packets suitable for the various environments. The range authentication of RTP and RTP Control Protocol (RTCP) packets
of solutions makes it difficult for RTP-based application developers suitable for the various environments. The range of solutions makes
to pick the most suitable mechanism. This document provides an it difficult for RTP-based application developers to pick the most
overview of a number of security solutions for RTP, and gives suitable mechanism. This document provides an overview of a number
guidance for developers on how to choose the appropriate security of security solutions for RTP and gives guidance for developers on
mechanism. how to choose the appropriate security mechanism.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
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approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
This Internet-Draft will expire on July 19, 2014. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7201.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Point-to-Point Sessions . . . . . . . . . . . . . . . . . 4 2.1. Point-to-Point Sessions . . . . . . . . . . . . . . . . . 5
2.2. Sessions Using an RTP Mixer . . . . . . . . . . . . . . . 4 2.2. Sessions Using an RTP Mixer . . . . . . . . . . . . . . . 5
2.3. Sessions Using an RTP Translator . . . . . . . . . . . . 5 2.3. Sessions Using an RTP Translator . . . . . . . . . . . . 6
2.3.1. Transport Translator (Relay) . . . . . . . . . . . . 5 2.3.1. Transport Translator (Relay) . . . . . . . . . . . . 6
2.3.2. Gateway . . . . . . . . . . . . . . . . . . . . . . . 6 2.3.2. Gateway . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.3. Media Transcoder . . . . . . . . . . . . . . . . . . 7 2.3.3. Media Transcoder . . . . . . . . . . . . . . . . . . 8
2.4. Any Source Multicast . . . . . . . . . . . . . . . . . . 7 2.4. Any Source Multicast . . . . . . . . . . . . . . . . . . 8
2.5. Source-Specific Multicast . . . . . . . . . . . . . . . . 7 2.5. Source-Specific Multicast . . . . . . . . . . . . . . . . 8
3. Security Options . . . . . . . . . . . . . . . . . . . . . . 9 3. Security Options . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Secure RTP . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Secure RTP . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1. Key Management for SRTP: DTLS-SRTP . . . . . . . . . 11 3.1.1. Key Management for SRTP: DTLS-SRTP . . . . . . . . . 12
3.1.2. Key Management for SRTP: MIKEY . . . . . . . . . . . 13 3.1.2. Key Management for SRTP: MIKEY . . . . . . . . . . . 14
3.1.3. Key Management for SRTP: Security Descriptions . . . 14 3.1.3. Key Management for SRTP: Security Descriptions . . . 15
3.1.4. Key Management for SRTP: Encrypted Key Transport . . 15 3.1.4. Key Management for SRTP: Encrypted Key Transport . . 16
3.1.5. Key Management for SRTP: ZRTP and Other Solutions . . 15 3.1.5. Key Management for SRTP: ZRTP and Other Solutions . . 17
3.2. RTP Legacy Confidentiality . . . . . . . . . . . . . . . 16 3.2. RTP Legacy Confidentiality . . . . . . . . . . . . . . . 17
3.3. IPsec . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.3. IPsec . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.4. RTP over TLS over TCP . . . . . . . . . . . . . . . . . . 16 3.4. RTP over TLS over TCP . . . . . . . . . . . . . . . . . . 18
3.5. RTP over Datagram TLS (DTLS) . . . . . . . . . . . . . . 17 3.5. RTP over Datagram TLS (DTLS) . . . . . . . . . . . . . . 18
3.6. Media Content Security/Digital Rights Management . . . . 17 3.6. Media Content Security/Digital Rights Management . . . . 19
3.6.1. ISMA Encryption and Authentication . . . . . . . . . 18 3.6.1. ISMA Encryption and Authentication . . . . . . . . . 19
4. Securing RTP Applications . . . . . . . . . . . . . . . . . . 18 4. Securing RTP Applications . . . . . . . . . . . . . . . . . . 20
4.1. Application Requirements . . . . . . . . . . . . . . . . 19 4.1. Application Requirements . . . . . . . . . . . . . . . . 20
4.1.1. Confidentiality . . . . . . . . . . . . . . . . . . . 19 4.1.1. Confidentiality . . . . . . . . . . . . . . . . . . . 20
4.1.2. Integrity . . . . . . . . . . . . . . . . . . . . . . 20 4.1.2. Integrity . . . . . . . . . . . . . . . . . . . . . . 21
4.1.3. Source Authentication . . . . . . . . . . . . . . . . 20 4.1.3. Source Authentication . . . . . . . . . . . . . . . . 22
4.1.4. Identifiers and Identity . . . . . . . . . . . . . . 22 4.1.4. Identifiers and Identity . . . . . . . . . . . . . . 23
4.1.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . 23 4.1.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . 24
4.2. Application Structure . . . . . . . . . . . . . . . . . . 23 4.2. Application Structure . . . . . . . . . . . . . . . . . . 25
4.3. Automatic Key Management . . . . . . . . . . . . . . . . 24 4.3. Automatic Key Management . . . . . . . . . . . . . . . . 25
4.4. End-to-End Security vs Tunnels . . . . . . . . . . . . . 24 4.4. End-to-End Security vs. Tunnels . . . . . . . . . . . . . 25
4.5. Plain Text Keys . . . . . . . . . . . . . . . . . . . . . 24 4.5. Plaintext Keys . . . . . . . . . . . . . . . . . . . . . 26
4.6. Interoperability . . . . . . . . . . . . . . . . . . . . 25 4.6. Interoperability . . . . . . . . . . . . . . . . . . . . 26
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1. Media Security for SIP-established Sessions using DTLS- 5.1. Media Security for SIP-Established Sessions Using
SRTP . . . . . . . . . . . . . . . . . . . . . . . . . . 25 DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . . 27
5.2. Media Security for WebRTC Sessions . . . . . . . . . . . 26 5.2. Media Security for WebRTC Sessions . . . . . . . . . . . 27
5.3. IP Multimedia Subsystem (IMS) Media Security . . . . . . 27 5.3. IP Multimedia Subsystem (IMS) Media Security . . . . . . 28
5.4. 3GPP Packet Based Streaming Service (PSS) . . . . . . . . 28 5.4. 3GPP Packet-Switched Streaming Service (PSS) . . . . . . 29
5.5. RTSP 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 29 5.5. RTSP 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 30
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 6. Security Considerations . . . . . . . . . . . . . . . . . . . 31
7. Security Considerations . . . . . . . . . . . . . . . . . . . 30 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 8. Informative References . . . . . . . . . . . . . . . . . . . 31
9. Informative References . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction 1. Introduction
Real-time Transport Protocol (RTP) [RFC3550] is widely used in a The Real-time Transport Protocol (RTP) [RFC3550] is widely used in a
large variety of multimedia applications, including Voice over IP large variety of multimedia applications, including Voice over IP
(VoIP), centralized multimedia conferencing, sensor data transport, (VoIP), centralized multimedia conferencing, sensor data transport,
and Internet television (IPTV) services. These applications can and Internet television (IPTV) services. These applications can
range from point-to-point phone calls, through centralised group range from point-to-point phone calls, through centralized group
teleconferences, to large-scale television distribution services. teleconferences, to large-scale television distribution services.
The types of media can vary significantly, as can the signalling The types of media can vary significantly, as can the signaling
methods used to establish the RTP sessions. methods used to establish the RTP sessions.
This multi-dimensional heterogeneity has so far prevented development So far, this multidimensional heterogeneity has prevented development
of a single security solution that meets the needs of the different of a single security solution that meets the needs of the different
applications. Instead significant number of different solutions have applications. Instead, a significant number of different solutions
been developed to meet different sets of security goals. This makes have been developed to meet different sets of security goals. This
it difficult for application developers to know what solutions exist, makes it difficult for application developers to know what solutions
and whether their properties are appropriate. This memo gives an exist and whether their properties are appropriate. This memo gives
overview of the available RTP solutions, and provides guidance on an overview of the available RTP solutions and provides guidance on
their applicability for different application domains. It also their applicability for different application domains. It also
attempts to provide indication of actual and intended usage at time attempts to provide an indication of actual and intended usage at the
of writing as additional input to help with considerations such as time of writing as additional input to help with considerations such
interoperability, availability of implementations etc. The guidance as interoperability, availability of implementations, etc. The
provided is not exhaustive, and this memo does not provide normative guidance provided is not exhaustive, and this memo does not provide
recommendations. normative recommendations.
It is important that application developers consider the security It is important that application developers consider the security
goals and requirements for their application. The IETF considers it goals and requirements for their application. The IETF considers it
important that protocols implement secure modes of operation and important that protocols implement secure modes of operation and
makes them available to users [RFC3365]. Because of the makes them available to users [RFC3365]. Because of the
heterogeneity of RTP applications and use cases, however, a single heterogeneity of RTP applications and use cases, however, a single
security solution cannot be mandated security solution cannot be mandated [RFC7202]. Instead, application
[I-D.ietf-avt-srtp-not-mandatory]. Instead, application developers developers need to select mechanisms that provide appropriate
need to select mechanisms that provide appropriate security for their security for their environment. It is strongly encouraged that
environment. It is strongly encouraged that common mechanisms are common mechanisms be used by related applications in common
used by related applications in common environments. The IETF environments. The IETF publishes guidelines for specific classes of
publishes guidelines for specific classes of applications, so it is applications, so it is worth searching for such guidelines.
worth searching for such guidelines.
The remainder of this document is structured as follows. Section 2 The remainder of this document is structured as follows. Section 2
provides additional background. Section 3 outlines the available provides additional background. Section 3 outlines the available
security mechanisms at the time of this writing, and lists their key security mechanisms at the time of this writing and lists their key
security properties and constraints. That is followed by guidelines security properties and constraints. Section 4 provides guidelines
and important aspects to consider when securing an RTP application in and important aspects to consider when securing an RTP application.
Section 4. Finally, we give some examples of application domains Finally, in Section 5, we give some examples of application domains
where guidelines for security exist in Section 5. where guidelines for security exist.
2. Background 2. Background
RTP can be used in a wide variety of topologies due to its support RTP can be used in a wide variety of topologies due to its support
for point-to-point sessions, multicast groups, and other topologies for point-to-point sessions, multicast groups, and other topologies
built around different types of RTP middleboxes. In the following we built around different types of RTP middleboxes. In the following,
review the different topologies supported by RTP to understand their we review the different topologies supported by RTP to understand
implications for the security properties and trust relations that can their implications for the security properties and trust relations
exist in RTP sessions. that can exist in RTP sessions.
2.1. Point-to-Point Sessions 2.1. Point-to-Point Sessions
The most basic use case is two directly connected end-points, shown The most basic use case is two directly connected endpoints, shown in
in Figure 1, where A has established an RTP session with B. In this Figure 1, where A has established an RTP session with B. In this
case the RTP security is primarily about ensuring that any third case, the RTP security is primarily about ensuring that any third
party can't compromise the confidentiality and integrity of the media party be unable to compromise the confidentiality and integrity of
communication. This requires confidentiality protection of the RTP the media communication. This requires confidentiality protection of
session, integrity protection of the RTP/RTCP packets, and source the RTP session, integrity protection of the RTP/RTCP packets, and
authentication of all the packets to ensure no man-in-the-middle source authentication of all the packets to ensure no man-in-the-
attack is taking place. middle (MITM) attack is taking place.
The source authentication can also be tied to a user or an end- The source authentication can also be tied to a user or an endpoint's
point's verifiable identity to ensure that the peer knows who they verifiable identity to ensure that the peer knows with whom they are
are communicating with. Here the combination of the security communicating. Here, the combination of the security protocol
protocol protecting the RTP session (and hence the RTP and RTCP protecting the RTP session (and, hence, the RTP and RTCP traffic) and
traffic) and the key-management protocol becomes important to the key management protocol becomes important to determine what
determine what security claims can be made. security claims can be made.
+---+ +---+ +---+ +---+
| A |<------->| B | | A |<------->| B |
+---+ +---+ +---+ +---+
Figure 1: Point-to-point topology Figure 1: Point-to-Point Topology
2.2. Sessions Using an RTP Mixer 2.2. Sessions Using an RTP Mixer
An RTP mixer is an RTP session-level middlebox that one can build a An RTP mixer is an RTP session-level middlebox around which one can
multi-party RTP based conference around. The RTP mixer might build a multiparty RTP-based conference. The RTP mixer might
actually perform media mixing, like mixing audio or compositing video actually perform media mixing, like mixing audio or compositing video
images into a new media stream being sent from the mixer to a given images into a new media stream being sent from the mixer to a given
participant; or it might provide a conceptual stream, for example the participant, or it might provide a conceptual stream; for example,
video of the current active speaker. From a security point of view, the video of the current active speaker. From a security point of
the important features of an RTP mixer is that it generates a new view, the important features of an RTP mixer are that it generates a
media stream, and has its own source identifier, and does not simply new media stream, has its own source identifier, and does not simply
forward the original media. forward the original media.
An RTP session using a mixer might have a topology like that in An RTP session using a mixer might have a topology like that in
Figure 2. In this example, participants A through D each send Figure 2. In this example, participants A through D each send
unicast RTP traffic to the RTP mixer, and receive an RTP stream from unicast RTP traffic to the RTP mixer, and receive an RTP stream from
the mixer, comprising a mixture of the streams from the other the mixer, comprising a mixture of the streams from the other
participants. participants.
+---+ +------------+ +---+ +---+ +------------+ +---+
| A |<---->| |<---->| B | | A |<---->| |<---->| B |
+---+ | | +---+ +---+ | | +---+
| Mixer | | Mixer |
+---+ | | +---+ +---+ | | +---+
| C |<---->| |<---->| D | | C |<---->| |<---->| D |
+---+ +------------+ +---+ +---+ +------------+ +---+
Figure 2: Example RTP mixer Topology Figure 2: Example RTP Mixer Topology
A consequence of an RTP mixer having its own source identifier, and A consequence of an RTP mixer having its own source identifier and
acting as an active participant towards the other end-points is that acting as an active participant towards the other endpoints is that
the RTP mixer needs to be a trusted device that has access to the the RTP mixer needs to be a trusted device that has access to the
security context(s) established. The RTP mixer can also become a security context(s) established. The RTP mixer can also become a
security enforcing entity. For example, a common approach to secure security-enforcing entity. For example, a common approach to secure
the topology in Figure 2 is to establish a security context between the topology in Figure 2 is to establish a security context between
the mixer and each participant independently, and have the mixer the mixer and each participant independently and have the mixer
source authenticate each peer. The mixer then ensures that one source authenticate each peer. The mixer then ensures that one
participant cannot impersonate another. participant cannot impersonate another.
2.3. Sessions Using an RTP Translator 2.3. Sessions Using an RTP Translator
RTP translators are middleboxes that provide various levels of in- RTP translators are middleboxes that provide various levels of
network media translation and transcoding. Their security properties in-network media translation and transcoding. Their security
vary widely, depending on which type of operations they attempt to properties vary widely, depending on which type of operations they
perform. We identify three different categories of RTP translator: attempt to perform. We identify and discuss three different
transport translators, gateways, and media transcoders. We discuss categories of RTP translators: transport translators, gateways, and
each in turn. media transcoders.
2.3.1. Transport Translator (Relay) 2.3.1. Transport Translator (Relay)
A transport translator [RFC5117] operates on a level below RTP and A transport translator [RFC5117] operates on a level below RTP and
RTCP. It relays the RTP/RTCP traffic from one end-point to one or RTCP. It relays the RTP/RTCP traffic from one endpoint to one or
more other addresses. This can be done based only on IP addresses more other addresses. This can be done based only on IP addresses
and transport protocol ports, with each receive port on the and transport protocol ports, and each receive port on the translator
translator can have a very basic list of where to forward traffic. can have a very basic list of where to forward traffic. Transport
Transport translators also need to implement ingress filtering to translators also need to implement ingress filtering to prevent
prevent random traffic from being forwarded that isn't coming from a random traffic from being forwarded that isn't coming from a
participant in the conference. participant in the conference.
Figure 3 shows an example transport translator, where traffic from Figure 3 shows an example transport translator, where traffic from
any one of the four participants will be forwarded to the other three any one of the four participants will be forwarded to the other three
participants unchanged. The resulting topology is very similar to participants unchanged. The resulting topology is very similar to an
Any Source Multicast (ASM) session (as discussed in Section 2.4), but Any Source Multicast (ASM) session (as discussed in Section 2.4) but
implemented at the application layer. is implemented at the application layer.
+---+ +------------+ +---+ +---+ +------------+ +---+
| A |<---->| |<---->| B | | A |<---->| |<---->| B |
+---+ | Relay | +---+ +---+ | Relay | +---+
| Translator | | Translator |
+---+ | | +---+ +---+ | | +---+
| C |<---->| |<---->| D | | C |<---->| |<---->| D |
+---+ +------------+ +---+ +---+ +------------+ +---+
Figure 3: RTP relay translator topology Figure 3: RTP Relay Translator Topology
A transport translator can often operate without needing access to A transport translator can often operate without needing access to
the security context, as long as the security mechanism does not the security context, as long as the security mechanism does not
provide protection over the transport-layer information. A transport provide protection over the transport-layer information. A transport
translator does, however, make the group communication visible, and translator does, however, make the group communication visible and,
so can complicate keying and source authentication mechanisms. This thus, can complicate keying and source authentication mechanisms.
is further discussed in Section 2.4. This is further discussed in Section 2.4.
2.3.2. Gateway 2.3.2. Gateway
Gateways are deployed when the endpoints are not fully compatible. Gateways are deployed when the endpoints are not fully compatible.
Figure 4 shows an example topology. The functions a gateway provides Figure 4 shows an example topology. The functions a gateway provides
can be diverse, and range from transport layer relaying between two can be diverse and range from transport-layer relaying between two
domains not allowing direct communication, via transport or media domains not allowing direct communication, via transport or media
protocol function initiation or termination, to protocol or media protocol function initiation or termination, to protocol- or media-
encoding translation. The supported security protocol might even be encoding translation. The supported security protocol might even be
one of the reasons a gateway is needed. one of the reasons a gateway is needed.
+---+ +-----------+ +---+ +---+ +-----------+ +---+
| A |<---->| Gateway |<---->| B | | A |<---->| Gateway |<---->| B |
+---+ +-----------+ +---+ +---+ +-----------+ +---+
Figure 4: RTP gateway topology Figure 4: RTP Gateway Topology
The choice of security protocol, and the details of the gateway The choice of security protocol, and the details of the gateway
function, will determine if the gateway needs to be trusted with function, will determine if the gateway needs to be trusted with
access to the application security context. Many gateways need to be access to the application security context. Many gateways need to be
trusted by all peers to perform the translation; in other cases some trusted by all peers to perform the translation; in other cases, some
or all peers might not be aware of the presence of the gateway. The or all peers might not be aware of the presence of the gateway. The
security protocols have different properties depending on the degree security protocols have different properties depending on the degree
of trust and visibility needed. Ensuring communication is possible of trust and visibility needed. Ensuring communication is possible
without trusting the gateway can be strong incentive for accepting without trusting the gateway can be a strong incentive for accepting
different security properties. Some security solutions will be able different security properties. Some security solutions will be able
to detect the gateways as manipulating the media stream, unless the to detect the gateways as manipulating the media stream, unless the
gateway is a trusted device. gateway is a trusted device.
2.3.3. Media Transcoder 2.3.3. Media Transcoder
A Media transcoder is a special type of gateway device that changes A media transcoder is a special type of gateway device that changes
the encoding of the media being transported by RTP. The discussion the encoding of the media being transported by RTP. The discussion
in Section 2.3.2 applies. A media transcoder alters the media data, in Section 2.3.2 applies. A media transcoder alters the media data
and thus needs to be trusted with access to the security context. and, thus, needs to be trusted with access to the security context.
2.4. Any Source Multicast 2.4. Any Source Multicast
Any Source Multicast [RFC1112] is the original multicast model where Any Source Multicast [RFC1112] is the original multicast model where
any multicast group participant can send to the multicast group, and any multicast group participant can send to the multicast group and
get their packets delivered to all group members (see Figure 5). get their packets delivered to all group members (see Figure 5).
This form of communication has interesting security properties, due This form of communication has interesting security properties due to
to the many-to-many nature of the group. Source authentication is the many-to-many nature of the group. Source authentication is
important, but all participants with access to group security context important, but all participants with access to the group security
will have the necessary secrets to decrypt and verify integrity of context will have the necessary secrets to decrypt and verify the
the traffic. Thus use of any group security context fails if the integrity of the traffic. Thus, use of any group security context
goal is to separate individual sources; alternate solutions are fails if the goal is to separate individual sources; alternate
needed. solutions are needed.
+-----+ +-----+
+---+ / \ +---+ +---+ / \ +---+
| A |----/ \---| B | | A |----/ \---| B |
+---+ / Multi- \ +---+ +---+ / \ +---+
+ Cast + + Multicast +
+---+ \ Network / +---+ +---+ \ Network / +---+
| C |----\ /---| D | | C |----\ /---| D |
+---+ \ / +---+ +---+ \ / +---+
+-----+ +-----+
Figure 5: Any source multicast (ASM) group Figure 5: Any Source Multicast (ASM) Group
In addition the potential large size of multicast groups creates some In addition, the potential large size of multicast groups creates
considerations for the scalability of the solution and how the key- some considerations for the scalability of the solution and how the
management is handled. key management is handled.
2.5. Source-Specific Multicast 2.5. Source-Specific Multicast
Source-Specific Multicast [RFC4607] allows only a specific end-point Source-Specific Multicast (SSM) [RFC4607] allows only a specific
to send traffic to the multicast group, irrespective of the number of endpoint to send traffic to the multicast group, irrespective of the
RTP media sources. The end-point is known as the media Distribution number of RTP media sources. The endpoint is known as the media
Source. For RTP session to function correctly with RTCP over an SSM distribution source. For the RTP session to function correctly with
session extensions have been defined in [RFC5760]. Figure 6 shows a RTCP over an SSM session, extensions have been defined in [RFC5760].
sample SSM-based RTP session where several media sources, MS1...MSm, Figure 6 shows a sample SSM-based RTP session where several media
all send media to a Distribution Source, which then forwards the sources, MS1...MSm, all send media to a distribution source, which
media data to the SSM group for delivery to the receivers, R1...Rn, then forwards the media data to the SSM group for delivery to the
and the Feedback Targets, FT1...FTn. RTCP reception quality feedback receivers, R1...Rn, and the feedback targets, FT1...FTn. RTCP
is sent unicast from each receiver to one of the Feedback Targets. reception quality feedback is sent unicast from each receiver to one
The feedback targets aggregate reception quality feedback and forward of the feedback targets. The feedback targets aggregate reception
it upstream towards the distribution source. The distribution source quality feedback and forward it upstream towards the distribution
forwards (possibly aggregated and summarised) reception feedback to source. The distribution source forwards (possibly aggregated and
the SSM group, and back to the original media sources. The feedback summarized) reception feedback to the SSM group and back to the
targets are also members of the SSM group and receive the media data, original media sources. The feedback targets are also members of the
so they can send unicast repair data to the receivers in response to SSM group and receive the media data, so they can send unicast repair
feedback if appropriate. data to the receivers in response to feedback if appropriate.
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
| MS1 | | MS2 | .... | MSm | | MS1 | | MS2 | .... | MSm |
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
^ ^ ^ ^ ^ ^
| | | | | |
V V V V V V
+---------------------------------+ +---------------------------------+
| Distribution Source | | Distribution Source |
+--------+ | +--------+ |
skipping to change at page 8, line 48 skipping to change at page 9, line 42
: : / \ : : : : / \ : :
: : / \ : : : : / \ : :
: : / \ : : : : / \ : :
: ./\ /\. : : ./\ /\. :
: /. \ / .\ : : /. \ / .\ :
: V . V V . V : : V . V V . V :
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
| R1 | | R2 | ... |Rn-1| | Rn | | R1 | | R2 | ... |Rn-1| | Rn |
+----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
Figure 6: Example SSM-based RTP session with two feedback targets Figure 6: Example SSM-Based RTP Session with Two Feedback Targets
The use of SSM makes it more difficult to inject traffic into the The use of SSM makes it more difficult to inject traffic into the
multicast group, but not impossible. Source authentication multicast group, but not impossible. Source authentication
requirements apply for SSM sessions too, and an individual requirements apply for SSM sessions, too; an individual verification
verification of who sent the RTP and RTCP packets is needed. An RTP of who sent the RTP and RTCP packets is needed. An RTP session using
session using SSM will have a group security context that includes SSM will have a group security context that includes the media
the media sources, distribution source, feedback targets, and the sources, distribution source, feedback targets, and the receivers.
receivers. Each has a different role and will be trusted to perform Each has a different role and will be trusted to perform different
different actions. For example, the distribution source will need to actions. For example, the distribution source will need to
authenticate the media sources to prevent unwanted traffic being authenticate the media sources to prevent unwanted traffic from being
distributed via the SSM group. Similarly, the receivers need to distributed via the SSM group. Similarly, the receivers need to
authenticate both the distribution source and their feedback target, authenticate both the distribution source and their feedback target
to prevent injection attacks from malicious devices claiming to be to prevent injection attacks from malicious devices claiming to be
feedback targets. An understanding of the trust relationships and feedback targets. An understanding of the trust relationships and
group security context is needed between all components of the group security context is needed between all components of the
system. system.
3. Security Options 3. Security Options
This section provides an overview of security requirements, and the This section provides an overview of security requirements and the
current RTP security mechanisms that implement those requirements. current RTP security mechanisms that implement those requirements.
This cannot be a complete survey, since new security mechanisms are This cannot be a complete survey, since new security mechanisms are
defined regularly. The goal is to help applications designer by defined regularly. The goal is to help applications designers by
reviewing the types of solution that are available. This section reviewing the types of solutions that are available. This section
will use a number of different security related terms, described in will use a number of different security-related terms, as described
the Internet Security Glossary, Version 2 [RFC4949]. in the Internet Security Glossary, Version 2 [RFC4949].
3.1. Secure RTP 3.1. Secure RTP
The Secure RTP (SRTP) protocol [RFC3711] is one of the most commonly The Secure Real-time Transport Protocol (SRTP) [RFC3711] is one of
used mechanisms to provide confidentiality, integrity protection, the most commonly used mechanisms to provide confidentiality,
source authentication and replay protection for RTP. SRTP was integrity protection, source authentication, and replay protection
developed with RTP header compression and third party monitors in for RTP. SRTP was developed with RTP header compression and third-
mind. Thus the RTP header is not encrypted in RTP data packets, and party monitors in mind. Thus, the RTP header is not encrypted in RTP
the first 8 bytes of the first RTCP packet header in each compound data packets, and the first 8 bytes of the first RTCP packet header
RTCP packet are not encrypted. The entirety of RTP packets and in each compound RTCP packet are not encrypted. The entirety of RTP
compound RTCP packets are integrity protected. This allows RTP packets and compound RTCP packets are integrity protected. This
header compression to work, and lets third party monitors determine allows RTP header compression to work and lets third-party monitors
what RTP traffic flows exist based on the SSRC fields, but protects determine what RTP traffic flows exist based on the synchronization
the sensitive content. source (SSRC) fields, but it protects the sensitive content.
SRTP works with transforms where different combinations of encryption SRTP works with transforms where different combinations of encryption
algorithm, authentication algorithm, and pseudo-random function can algorithm, authentication algorithm, and pseudorandom function can be
be used, and the authentication tag length can be set to any value. used, and the authentication tag length can be set to any value.
SRTP can also be easily extended with additional cryptographic SRTP can also be easily extended with additional cryptographic
transforms. This gives flexibility, but requires more security transforms. This gives flexibility but requires more security
knowledge by the application developer. To simplify things, SDP knowledge by the application developer. To simplify things, Session
Security Descriptions (see Section 3.1.3) and DTLS-SRTP (see Description Protocol (SDP) security descriptions (see Section 3.1.3)
Section 3.1.1) use pre-defined combinations of transforms, known as and Datagram Transport Layer Security Extension for SRTP (DTLS-SRTP)
SRTP crypto suites and SRTP protection profiles, that bundle together (see Section 3.1.1) use predefined combinations of transforms, known
transforms and other parameters, making them easier to use but as SRTP crypto suites and SRTP protection profiles, that bundle
reducing flexibility. The MIKEY protocol (see Section 3.1.2) together transforms and other parameters, making them easier to use
provides flexibility to negotiate the full selection of transforms. but reducing flexibility. The Multimedia Internet Keying (MIKEY)
At the time of this writing, the following transforms, SRTP crypto protocol (see Section 3.1.2) provides flexibility to negotiate the
suites, and SRTP protection profiles are defined or under definition: full selection of transforms. At the time of this writing, the
following transforms, SRTP crypto suites, and SRTP protection
profiles are defined or under definition:
AES-CM and HMAC-SHA-1: AES Counter Mode encryption with 128-bit keys AES-CM and HMAC-SHA-1: AES Counter Mode encryption with 128-bit keys
combined with 160-bit keyed HMAC-SHA-1 with 80-bit authentication combined with 160-bit keyed HMAC-SHA-1 with an 80-bit
tag. This is the default cryptographic transform that needs to be authentication tag. This is the default cryptographic transform
supported. The transforms are defined in SRTP [RFC3711], with the that needs to be supported. The transforms are defined in SRTP
corresponding SRTP crypto suite in [RFC4568] and SRTP protection [RFC3711], with the corresponding SRTP crypto suite defined in
profile in [RFC5764]. [RFC4568] and SRTP protection profile defined in [RFC5764].
AES-f8 and HMAC-SHA-1: AES f8 mode encryption using 128-bit keys AES-f8 and HMAC-SHA-1: AES f8-mode encryption using 128-bit keys
combined with keyed HMAC-SHA-1 using 80-bit authentication. The combined with keyed HMAC-SHA-1 using 80-bit authentication. The
transforms are defined in [RFC3711], with the corresponding SRTP transforms are defined in [RFC3711], with the corresponding SRTP
crypto suite in [RFC4568]. The corresponding SRTP protection crypto suite defined in [RFC4568]. The corresponding SRTP
profile is not defined. protection profile is not defined.
SEED: A Korean national standard cryptographic transform that is SEED: A Korean national standard cryptographic transform that is
defined to be used with SRTP in [RFC5669]. Three options are defined to be used with SRTP in [RFC5669]. Three options are
defined, one using SHA-1 authentication, one using Counter mode defined: one using SHA-1 authentication, one using Counter Mode
with CBC-MAC, and finally one using Galois Counter mode. with Cipher Block Chaining Message Authentication Code (CBC-MAC),
and one using Galois Counter Mode.
ARIA: A Korean block cipher [I-D.ietf-avtcore-aria-srtp], that ARIA: A Korean block cipher [ARIA-SRTP] that supports 128-, 192-,
supports 128-, 192- and 256- bit keys. It also defines three and 256-bit keys. It also defines three options: Counter Mode
options, Counter mode where combined with HMAC-SHA-1 with 80 or 32 where combined with HMAC-SHA-1 with 80- or 32-bit authentication
bits authentication tags, Counter mode with CBC-MAC and Galois tags, Counter Mode with CBC-MAC, and Galois Counter Mode. It also
Counter mode. It also defines a different key derivation function defines a different key derivation function than the AES-based
than the AES based systems. systems.
AES-192-CM and AES-256-CM: Cryptographic transforms for SRTP based AES-192-CM and AES-256-CM: Cryptographic transforms for SRTP based
on AES-192 and AES-256 counter mode encryption and 160-bit keyed on AES-192 and AES-256 Counter Mode encryption and 160-bit keyed
HMAC-SHA-1 with 80- and 32-bit authentication tags. These provide HMAC-SHA-1 with 80- and 32-bit authentication tags. These provide
192- and 256-bit encryption keys, but otherwise match the default 192- and 256-bit encryption keys, but otherwise match the default
128-bit AES-CM transform. The transforms are defined in [RFC3711] 128-bit AES-CM transform. The transforms are defined in [RFC3711]
and [RFC6188], with the SRTP crypto suites in [RFC6188]. and [RFC6188], and the SRTP crypto suites are defined in
[RFC6188].
AES-GCM and AES-CCM: AES Galois Counter Mode and AES Counter with AES-GCM and AES-CCM: AES Galois Counter Mode and AES Counter Mode
CBC MAC for AES-128 and AES-256. This authentication is included with CBC-MAC for AES-128 and AES-256. This authentication is
in the cipher text which becomes expanded with the length of the included in the cipher text, which becomes expanded with the
authentication tag instead of using the SRTP authentication tag. length of the authentication tag instead of using the SRTP
This is defined in [I-D.ietf-avtcore-srtp-aes-gcm]. authentication tag. This is defined in [AES-GCM].
NULL: SRTP [RFC3711] also provides a NULL cipher that can be used NULL: SRTP [RFC3711] also provides a NULL cipher that can be used
when no confidentiality for RTP/RTCP is requested. The when no confidentiality for RTP/RTCP is requested. The
corresponding SRTP protection profile is defined in [RFC5764]. corresponding SRTP protection profile is defined in [RFC5764].
The source authentication guarantees provided by SRTP depend on the The source authentication guarantees provided by SRTP depend on the
cryptographic transform and key-management used. Some transforms cryptographic transform and key management used. Some transforms
give strong source authentication even in multiparty sessions; others give strong source authentication even in multiparty sessions; others
give weaker guarantees and can authenticate group membership but not give weaker guarantees and can authenticate group membership but not
sources. TESLA [RFC4383] offers a complement to the regular sources. Timed Efficient Stream Loss-Tolerant Authentication (TESLA)
symmetric keyed authentication transforms, like HMAC-SHA-1, and can [RFC4383] offers a complement to the regular symmetric keyed
provide per-source authentication in some group communication authentication transforms, like HMAC-SHA-1, and can provide
scenarios. The downside is need for buffering the packets for a per-source authentication in some group communication scenarios. The
while before authenticity can be verified. downside is the need for buffering the packets for a while before
authenticity can be verified.
[RFC4771] defines a variant of the authentication tag that enables a [RFC4771] defines a variant of the authentication tag that enables a
receiver to obtain the Roll over Counter for the RTP sequence number receiver to obtain the Roll over Counter for the RTP sequence number
that is part of the Initialization vector (IV) for many cryptographic that is part of the Initialization Vector (IV) for many cryptographic
transforms. This enables quicker and easier options for joining a transforms. This enables quicker and easier options for joining a
long lived secure RTP group, for example a broadcast session. long-lived RTP group; for example, a broadcast session.
RTP header extensions are normally carried in the clear and only RTP header extensions are normally carried in the clear and are only
integrity protected in SRTP. This can be problematic in some cases, integrity protected in SRTP. This can be problematic in some cases,
so [RFC6904] defines an extension to also encrypt selected header so [RFC6904] defines an extension to also encrypt selected header
extensions. extensions.
SRTP is specified and deployed in a number of RTP usage contexts; SRTP is specified and deployed in a number of RTP usage contexts;
Significant support in SIP-established VoIP clients including IMS; significant support is provided in SIP-established VoIP clients,
RTSP [I-D.ietf-mmusic-rfc2326bis] and RTP based media streaming. including IP Multimedia Subsystems (IMS), and in the Real Time
Thus SRTP in general is widely deployed. When it comes to Streaming Protocol (RTSP) [RTSP] and RTP-based media streaming.
cryptographic transforms the default (AES-CM and HMAC-SHA-1) is the Thus, SRTP in general is widely deployed. When it comes to
cryptographic transforms, the default (AES-CM and HMAC-SHA-1) is the
most commonly used, but it might be expected that AES-GCM, most commonly used, but it might be expected that AES-GCM,
AES-192-CM, and AES-256-CM will gain usage in future, especially due AES-192-CM, and AES-256-CM will gain usage in future, especially due
to the AES- and GCM-specific instructions in new CPUs. to the AES- and GCM-specific instructions in new CPUs.
SRTP does not contain an integrated key-management solution, and SRTP does not contain an integrated key management solution; instead,
instead relies on an external key management protocol. There are it relies on an external key management protocol. There are several
several protocols that can be used. The following sections outline protocols that can be used. The following sections outline some
some popular schemes. popular schemes.
3.1.1. Key Management for SRTP: DTLS-SRTP 3.1.1. Key Management for SRTP: DTLS-SRTP
A Datagram Transport Layer Security extension exists for establishing A Datagram Transport Layer Security (DTLS) extension exists for
SRTP keys [RFC5763][RFC5764]. This extension provides secure key- establishing SRTP keys [RFC5763][RFC5764]. This extension provides
exchange between two peers, enabling Perfect Forward Secrecy (PFS) secure key exchange between two peers, enabling Perfect Forward
and binding strong identity verification to an end-point. Perfect Secrecy (PFS) and binding strong identity verification to an
Forward Secrecy is a property of the key-agreement protocol that endpoint. PFS is a property of the key agreement protocol that
ensures that a session key derived from a set of long-term keys will ensures that a session key derived from a set of long-term keys will
not be compromised if one of the long-term keys is compromised in the not be compromised if one of the long-term keys is compromised in the
future. The default key generation will generate a key that contains future. The default key generation will generate a key that contains
material contributed by both peers. The key-exchange happens in the material contributed by both peers. The key exchange happens in the
media plane directly between the peers. The common key-exchange media plane directly between the peers. The common key exchange
procedures will take two round trips assuming no losses. TLS procedures will take two round trips assuming no losses. Transport
resumption can be used when establishing additional media streams Layer Security (TLS) resumption can be used when establishing
with the same peer, and reduces the set-up time to one RTT for these additional media streams with the same peer, and it reduces the setup
streams (see [RFC5764] for a discussion of TLS resumption in this time to one RTT for these streams (see [RFC5764] for a discussion of
context). TLS resumption in this context).
The actual security properties of an established SRTP session using The actual security properties of an established SRTP session using
DTLS will depend on the cipher suites offered and used, as well as DTLS will depend on the cipher suites offered and used, as well as
the mechanism for identifying the end-points of the hand-shake. For the mechanism for identifying the endpoints of the handshake. For
example some cipher suits provide PFS , while other do not. When example, some cipher suites provide PFS, while others do not. When
using DTLS, the application designer needs to select which cipher using DTLS, the application designer needs to select which cipher
suites DTLS-SRTP can offer and accept so that the desired security suites DTLS-SRTP can offer and accept so that the desired security
properties are achieved. The next choice is how to verify the properties are achieved. The next choice is how to verify the
identity of the peer end-point. One choice can be to rely on the identity of the peer endpoint. One choice can be to rely on the
certificates and use a PKI to verify them to make an identity certificates and use a PKI to verify them to make an identity
assertion. However, this is not the most common way, instead self- assertion. However, this is not the most common way; instead, self-
signed certificate are common to use, and instead establish trust signed certificates are common to use to establish trust through
through signalling or other third party solutions. signaling or other third-party solutions.
DTLS-SRTP key management can use the signalling protocol in four DTLS-SRTP key management can use the signaling protocol in four ways:
ways. First, to agree on using DTLS-SRTP for media security. First, to agree on using DTLS-SRTP for media security. Second, to
Secondly, to determine the network location (address and port) where determine the network location (address and port) where each side is
each side is running a DTLS listener to let the parts perform the running a DTLS listener to let the parts perform the key management
key-management handshakes that generate the keys used by SRTP. handshakes that generate the keys used by SRTP. Third, to exchange
Thirdly, to exchange hashes of each side's certificates to bind these hashes of each side's certificates to bind these to the signaling and
to the signalling, and ensure there is no man-in-the-middle attack. ensure there is no MITM attack. This assumes that one can trust the
This assumes that one can trust the signalling solution to be signaling solution to be resistant to modification and not be in
resistant to modification, and not be in collaboration with an collaboration with an attacker. Finally, to provide an asserted
attacker. Finally to provide an assertable identity, e.g. [RFC4474] identity, e.g., [RFC4474], that can be used to prevent modification
that can be used to prevent modification of the signalling and the of the signaling and the exchange of certificate hashes. That way,
exchange of certificate hashes. That way enabling binding between it enables binding between the key exchange and the signaling.
the key-exchange and the signalling.
This usage is well defined for SIP/SDP in [RFC5763], and in most This usage is well defined for SIP/SDP in [RFC5763] and, in most
cases can be adopted for use with other bi-directional signalling cases, can be adopted for use with other bidirectional signaling
solutions. It is to be noted that there is work underway to revisit solutions. It is to be noted that there is work underway to revisit
the SIP Identity mechanism [RFC4474] in the IETF STIR working group. the SIP Identity mechanism [RFC4474] in the IETF STIR working group.
The main question regarding DTLS-SRTP's security properties is how The main question regarding DTLS-SRTP's security properties is how
one verifies any peer identity or at least prevents man-in-the-middle one verifies any peer identity or at least prevents MITM attacks.
attacks. This do requires trust in some DTLS-SRTP external party, This does require trust in some DTLS-SRTP external parties: either a
either a PKI, a signalling system or some identity provider. PKI, a signaling system, or some identity provider.
DTLS-SRTP usage is clearly on the rise. It is mandatory to support DTLS-SRTP usage is clearly on the rise. It is mandatory to support
in WebRTC. It has growing support among SIP end-points. DTLS-SRTP in Web Real-Time Communication (WebRTC). It has growing support
was developed in IETF primarily to meet security requirements for RTP among SIP endpoints. DTLS-SRTP was developed in IETF primarily to
based media established using SIP. The requirements considered can meet security requirements for RTP-based media established using SIP.
be reviewed in "Requirements and Analysis of Media Security The requirements considered can be reviewed in "Requirements and
Management Protocols." [RFC5479]. Analysis of Media Security Management Protocols" [RFC5479].
3.1.2. Key Management for SRTP: MIKEY 3.1.2. Key Management for SRTP: MIKEY
Multimedia Internet Keying (MIKEY) [RFC3830] is a keying protocol Multimedia Internet Keying (MIKEY) [RFC3830] is a keying protocol
that has several modes with different properties. MIKEY can be used that has several modes with different properties. MIKEY can be used
in point-to-point applications using SIP and RTSP (e.g., VoIP calls), in point-to-point applications using SIP and RTSP (e.g., VoIP calls)
but is also suitable for use in broadcast and multicast applications, but is also suitable for use in broadcast and multicast applications
and centralized group communications. and centralized group communications.
MIKEY can establish multiple security contexts or cryptographic MIKEY can establish multiple security contexts or cryptographic
sessions with a single message. It is useable in scenarios where one sessions with a single message. It is usable in scenarios where one
entity generates the key and needs to distribute the key to a number entity generates the key and needs to distribute the key to a number
of participants. The different modes and the resulting properties of participants. The different modes and the resulting properties
are highly dependent on the cryptographic method used to establish are highly dependent on the cryptographic method used to establish
the session keys actually used by the security protocol, like SRTP. the session keys actually used by the security protocol, like SRTP.
MIKEY has the following modes of operation: MIKEY has the following modes of operation:
Pre-Shared Key: Uses a pre-shared secret for symmetric key crypto Pre-Shared Key: Uses a pre-shared secret for symmetric key crypto
used to secure a keying message carrying the already generated used to secure a keying message carrying the already-generated
session key. This system is the most efficient from the session key. This system is the most efficient from the
perspective of having small messages and processing demands. The perspective of having small messages and processing demands. The
downside is scalability, where usually the effort for the downside is scalability, where usually the effort for the
provisioning of pre-shared keys is only manageable if the number provisioning of pre-shared keys is only manageable if the number
of endpoints is small. of endpoints is small.
Public Key encryption: Uses a public key crypto to secure a keying Public Key Encryption: Uses a public key crypto to secure a keying
message carrying the already-generated session key. This is more message carrying the already-generated session key. This is more
resource intensive but enables scalable systems. It does require resource intensive but enables scalable systems. It does require
a public key infrastructure to enable verification. a public key infrastructure to enable verification.
Diffie-Hellman: Uses Diffie-Hellman key-agreement to generate the Diffie-Hellman: Uses Diffie-Hellman key agreement to generate the
session key, thus providing perfect forward secrecy. The downside session key, thus providing perfect forward secrecy. The downside
is high resource consumption in bandwidth and processing during is high resource consumption in bandwidth and processing during
the MIKEY exchange. This method can't be used to establish group the MIKEY exchange. This method can't be used to establish group
keys as each pair of peers performing the MIKEY exchange will keys as each pair of peers performing the MIKEY exchange will
establish different keys. establish different keys.
HMAC-Authenticated Diffie-Hellman: [RFC4650] defines a variant of HMAC-Authenticated Diffie-Hellman: [RFC4650] defines a variant of
the Diffie-Hellman exchange that uses a pre-shared key in a keyed the Diffie-Hellman exchange that uses a pre-shared key in a keyed
HMAC to verify authenticity of the keying material instead of a Hashed Message Authentication Code (HMAC) to verify authenticity
digital signature as in the previous method. This method is still of the keying material instead of a digital signature as in the
restricted to point-to-point usage. previous method. This method is still restricted to
point-to-point usage.
RSA-R: MIKEY-RSA in Reverse mode [RFC4738] is a variant of the RSA-R: MIKEY-RSA in Reverse mode [RFC4738] is a variant of the
public key method which doesn't rely on the initiator of the key- public key method, which doesn't rely on the initiator of the key
exchange knowing the responder's certificate. This method lets exchange knowing the responder's certificate. This method lets
both the initiator and the responder to specify the session keying both the initiator and the responder specify the session keying
material depending on use case. Usage of this mode requires one material depending on the use case. Usage of this mode requires
round-trip time. one round-trip time.
TICKET: [RFC6043] is a MIKEY extension using a trusted centralized TICKET: Ticket Payload (TICKET) [RFC6043] is a MIKEY extension using
key management service (KMS). The Initiator and Responder do not a trusted centralized key management service (KMS). The initiator
share any credentials; instead, they trust a third party, the KMS, and responder do not share any credentials; instead, they trust a
with which they both have or can establish shared credentials. third party, the KMS, with which they both have or can establish
shared credentials.
IBAKE: [RFC6267] uses a key management services (KMS) infrastructure IBAKE: Identity-Based Authenticated Key Exchange (IBAKE) [RFC6267]
but with lower demand on the KMS. Claims to provides both perfect uses a KMS infrastructure but with lower demand on the KMS. It
forward and backwards secrecy. claims to provide both perfect forward and backwards secrecy.
SAKKE: [RFC6509] provides Sakai-Kasahara Key Encryption in MIKEY. SAKKE: [RFC6509] provides Sakai-Kasahara Key Encryption (SAKKE) in
Based on Identity based Public Key Cryptography and a KMS MIKEY. It is based on Identity-based Public Key Cryptography and
infrastructure to establish a shared secret value and certificate a KMS infrastructure to establish a shared secret value and
less signatures to provide source authentication. Its features certificateless signatures to provide source authentication. Its
include simplex transmission, scalability, low-latency call set- features include simplex transmission, scalability, low-latency
up, and support for secure deferred delivery. call setup, and support for secure deferred delivery.
MIKEY messages have several different transports. [RFC4567] defines MIKEY messages have several different transports. [RFC4567] defines
how MIKEY messages can be embedded in general SDP for usage with the how MIKEY messages can be embedded in general SDP for usage with the
signalling protocols SIP, SAP and RTSP. There also exist a 3GPP signaling protocols SIP, Session Announcement Protocol (SAP), and
defined usage of MIKEY that sends MIKEY messages directly over UDP RTSP. There also exists a usage of MIKEY defined by the Third
[T3GPP.33.246] to key the receivers of Multimedia Broadcast and Generation Partnership Project (3GPP) that sends MIKEY messages
Multicast Service (MBMS) [T3GPP.26.346]. [RFC3830] defines the directly over UDP [T3GPP.33.246] to key the receivers of Multimedia
application/mikey media type allowing MIKEY to be used in, e.g., Broadcast and Multicast Service (MBMS) [T3GPP.26.346]. [RFC3830]
email and HTTP. defines the application/mikey media type, allowing MIKEY to be used
in, e.g., email and HTTP.
Based on the many choices it is important to consider the properties Based on the many choices, it is important to consider the properties
needed in ones solution and based on that evaluate which modes that needed in one's solution and based on that evaluate which modes are
are candidates for ones usage. More information on the applicability candidates for use. More information on the applicability of the
of the different MIKEY modes can be found in [RFC5197]. different MIKEY modes can be found in [RFC5197].
MIKEY with pre-shared keys are used by 3GPP MBMS [T3GPP.33.246] and MIKEY with pre-shared keys is used by 3GPP MBMS [T3GPP.33.246], and
IMS media security [T3GPP.33.328] specifies the use of the TICKET IMS media security [T3GPP.33.328] specifies the use of the TICKET
mode transported over SIP and HTTP. RTSP 2.0 mode transported over SIP and HTTP. RTSP 2.0 [RTSP] specifies use of
[I-D.ietf-mmusic-rfc2326bis] specifies use of the RSA-R mode. There the RSA-R mode. There are some SIP endpoints that support MIKEY.
are some SIP end-points that support MIKEY. The modes they use are The modes they use are unknown to the authors.
unknown to the authors.
3.1.3. Key Management for SRTP: Security Descriptions 3.1.3. Key Management for SRTP: Security Descriptions
[RFC4568] provides a keying solution based on sending plain text keys [RFC4568] provides a keying solution based on sending plaintext keys
in SDP [RFC4566]. It is primarily used with SIP and the SDP Offer/ in SDP [RFC4566]. It is primarily used with SIP and the SDP Offer/
Answer model, and is well-defined in point-to-point sessions where Answer model and is well defined in point-to-point sessions where
each side declares its own unique key. Using Security Descriptions each side declares its own unique key. Using security descriptions
to establish group keys is less well defined, and can have security to establish group keys is less well defined and can have security
issues since it's difficult to guarantee unique SSRCs (as needed to issues since it's difficult to guarantee unique SSRCs (as needed to
avoid a "two-time pad" attack - see Section 9 of [RFC3711]). avoid a "two-time pad" attack -- see Section 9 of [RFC3711]).
Since keys are transported in plain text in SDP, they can easily be Since keys are transported in plaintext in SDP, they can easily be
intercepted unless the SDP carrying protocol provides strong end-to- intercepted unless the SDP carrying protocol provides strong
end confidentiality and authentication guarantees. This is not end-to-end confidentiality and authentication guarantees. This is
normally the case, where instead hop-by-hop security is provided not normally the case; instead, hop-by-hop security is provided
between signalling nodes using TLS. This leaves the keying material between signaling nodes using TLS. This leaves the keying material
sensitive to capture by the traversed signalling nodes. Thus, in sensitive to capture by the traversed signaling nodes. Thus, in most
most cases, the security properties of security descriptions are cases, the security properties of security descriptions are weak.
weak. The usage of security descriptions usually requires additional The usage of security descriptions usually requires additional
security measures, e.g. the signalling nodes be trusted and protected security measures; for example, the signaling nodes are trusted and
by strict access control. Usage of security descriptions requires protected by strict access control. Usage of security descriptions
careful design in order to ensure that the security goals can be met. requires careful design in order to ensure that the security goals
can be met.
Security Descriptions is the most commonly deployed keying solution Security descriptions are the most commonly deployed keying solution
for SIP-based end-points, where almost all end-points that support for SIP-based endpoints, where almost all endpoints that support SRTP
SRTP also support Security Descriptions. It is also used for access also support security descriptions. It is also used for access
protection in IMS Media Security [T3GPP.33.328]. protection in IMS Media Security [T3GPP.33.328].
3.1.4. Key Management for SRTP: Encrypted Key Transport 3.1.4. Key Management for SRTP: Encrypted Key Transport
Encrypted Key Transport (EKT) [I-D.ietf-avtcore-srtp-ekt] is an SRTP Encrypted Key Transport (EKT) [EKT] is an SRTP extension that enables
extension that enables group keying despite using a keying mechanism group keying despite using a keying mechanism like DTLS-SRTP that
like DTLS-SRTP that doesn't support group keys. It is designed for doesn't support group keys. It is designed for centralized
centralized conferencing, but can also be used in sessions where end- conferencing, but it can also be used in sessions where endpoints
points connect to a conference bridge or a gateway, and need to be connect to a conference bridge or a gateway and need to be
provisioned with the keys each participant on the bridge or gateway provisioned with the keys each participant on the bridge or gateway
uses to avoid decryption and encryption cycles on the bridge or uses to avoid decryption and encryption cycles. This can enable
gateway. This can enable interworking between DTLS-SRTP and other interworking between DTLS-SRTP and other keying systems where either
keying systems where either party can set the key (e.g., interworking party can set the key (e.g., interworking with security
with security descriptions). descriptions).
The mechanism is based on establishing an additional EKT key which The mechanism is based on establishing an additional EKT key, which
everyone uses to protect their actual session key. The actual everyone uses to protect their actual session key. The actual
session key is sent in a expanded authentication tag to the other session key is sent in an expanded authentication tag to the other
session participants. This key is only sent occasionally or session participants. This key is only sent occasionally or
periodically depending on use cases and depending on what periodically depending on use cases and depending on what
requirements exist for timely delivery or notification. requirements exist for timely delivery or notification.
The only known deployment of EKT so far are in some Cisco video The only known deployment of EKT so far is in some Cisco video
conferencing products. conferencing products.
3.1.5. Key Management for SRTP: ZRTP and Other Solutions 3.1.5. Key Management for SRTP: ZRTP and Other Solutions
The ZRTP [RFC6189] key-management system for SRTP was proposed as an The ZRTP [RFC6189] key management system for SRTP was proposed as an
alternative to DTLS-SRTP. ZRTP provides best effort encryption alternative to DTLS-SRTP. ZRTP provides best effort encryption
independent of the signalling protocol and utilizes key continuity, independent of the signaling protocol and utilizes key continuity,
Short Authentication Strings, or a PKI for authentication. ZRTP Short Authentication Strings, or a PKI for authentication. ZRTP
wasn't adopted as an IETF standards track protocol, but was instead wasn't adopted as an IETF Standards Track protocol, but was instead
published as an informational RFC. Commercial implementations exist. published as an Informational RFC in the IETF stream. Commercial
implementations exist.
Additional proprietary solutions are also known to exist. Additional proprietary solutions are also known to exist.
3.2. RTP Legacy Confidentiality 3.2. RTP Legacy Confidentiality
Section 9 of the RTP standard [RFC3550] defines a DES or 3DES based Section 9 of the RTP standard [RFC3550] defines a Data Encryption
encryption of RTP and RTCP packets. This mechanism is keyed using Standard (DES) or 3DES-based encryption of RTP and RTCP packets.
plain text keys in SDP [RFC4566] using the "k=" SDP field. This This mechanism is keyed using plaintext keys in SDP [RFC4566] using
method can provide confidentiality but, as discussed in Section 9 of the "k=" SDP field. This method can provide confidentiality but, as
[RFC3550], it has extremely weak security properties and is not to be discussed in Section 9 of [RFC3550], it has extremely weak security
used. properties and is not to be used.
3.3. IPsec 3.3. IPsec
IPsec [RFC4301] can be used in either tunnel or transport mode to IPsec [RFC4301] can be used in either tunnel or transport mode to
protect RTP and RTCP packets in transit from one network interface to protect RTP and RTCP packets in transit from one network interface to
another. This can be sufficient when the network interfaces have a another. This can be sufficient when the network interfaces have a
direct relation, or in a secured environment where it can be direct relation or in a secured environment where it can be
controlled who can read the packets from those interfaces. controlled who can read the packets from those interfaces.
The main concern with using IPsec to protect RTP traffic is that in The main concern with using IPsec to protect RTP traffic is that in
most cases using a VPN approach that terminates the security most cases, using a VPN approach that terminates the security
association at some node prior to the RTP end-point leaves the association at some node prior to the RTP endpoint leaves the traffic
traffic vulnerable to attack between the VPN termination node and the vulnerable to attack between the VPN termination node and the
end-point. Thus usage of IPsec requires careful thought and design endpoint. Thus, usage of IPsec requires careful thought and design
of its usage so that it meets the security goals. A important of its usage so that it meets the security goals. An important
question is how one ensures the IPsec terminating peer and the question is how one ensures the IPsec terminating peer and the
ultimate destination are the same. Applications can have issues ultimate destination are the same. Applications can have issues
using existing APIs with determining if IPsec is being used or not, using existing APIs when determining if IPsec is being used or not
and when used who the authenticated peer entity is. and when determining who the authenticated peer entity is when IPsec
is used.
IPsec with RTP is more commonly used as a security solution between IPsec with RTP is more commonly used as a security solution between
infrastructure nodes that exchange many RTP sessions and media infrastructure nodes that exchange many RTP sessions and media
streams. The establishment of a secure tunnel between such nodes streams. The establishment of a secure tunnel between such nodes
minimizes the key-management overhead. minimizes the key management overhead.
3.4. RTP over TLS over TCP 3.4. RTP over TLS over TCP
Just as RTP can be sent over TCP [RFC4571], it can also be sent over Just as RTP can be sent over TCP [RFC4571], it can also be sent over
TLS over TCP [RFC4572], using TLS to provide point-to-point security TLS over TCP [RFC4572], using TLS to provide point-to-point security
services. The security properties TLS provides are confidentiality, services. The security properties TLS provides are confidentiality,
integrity protection and possible source authentication if the client integrity protection, and possible source authentication if the
or server certificates are verified and provide a usable identity. client or server certificates are verified and provide a usable
When used in multi-party scenarios using a central node for media identity. When used in multiparty scenarios using a central node for
distribution, the security provide is only between the central node media distribution, the security provided is only between the central
and the peers, so the security properties for the whole session are node and the peers, so the security properties for the whole session
dependent on what trust one can place in the central node. are dependent on what trust one can place in the central node.
RTSP 1.0 [RFC2326] and 2.0 [I-D.ietf-mmusic-rfc2326bis] specifies the RTSP 1.0 [RFC2326] and 2.0 [RTSP] specify the usage of RTP over the
usage of RTP over the same TLS/TCP connection that the RTSP messages same TLS/TCP connection that the RTSP messages are sent over. It
are sent over. It appears that RTP over TLS/TCP is also used in some appears that RTP over TLS/TCP is also used in some proprietary
proprietary solutions that uses TLS to bypass firewalls. solutions that use TLS to bypass firewalls.
3.5. RTP over Datagram TLS (DTLS) 3.5. RTP over Datagram TLS (DTLS)
Datagram Transport Layer Security (DTLS) [RFC6347] is a based on TLS DTLS [RFC6347] is based on TLS [RFC5246] but designed to work over an
[RFC5246], but designed to work over a unreliable datagram oriented unreliable datagram-oriented transport rather than requiring reliable
transport rather than requiring reliable byte stream semantics from byte stream semantics from the transport protocol. Accordingly, DTLS
the transport protocol. Accordingly, DTLS can provide point-to-point can provide point-to-point security for RTP flows analogous to that
security for RTP flows analogous to that provided by TLS, but over an provided by TLS but over a datagram transport such as UDP. The two
datagram transport such as UDP. The two peers establish an DTLS peers establish a DTLS association between each other, including the
association between each other, including the possibility to do possibility to do certificate-based source authentication when
certificate-based source authentication when establishing the establishing the association. All RTP and RTCP packets flowing will
association. All RTP and RTCP packets flowing will be protected by be protected by this DTLS association.
this DTLS association.
Note that using DTLS for RTP flows is different to using DTLS-SRTP Note that using DTLS for RTP flows is different from using DTLS-SRTP
key management. DTLS-SRTP uses the same key-management steps as key management. DTLS-SRTP uses the same key management steps as
DTLS, but uses SRTP for the per packet security operations. Using DTLS, but uses SRTP for the per-packet security operations. Using
DTLS for RTP flows uses the normal datagram TLS data protection, DTLS for RTP flows uses the normal datagram TLS data protection,
wrapping complete RTP packets. When using DTLS for RTP flows, the wrapping complete RTP packets. When using DTLS for RTP flows, the
RTP and RTCP packets are completely encrypted with no headers in the RTP and RTCP packets are completely encrypted with no headers in the
clear; when using DTLS-SRTP, the RTP headers are in the clear and clear; when using DTLS-SRTP, the RTP headers are in the clear and
only the payload data is encrypted. only the payload data is encrypted.
DTLS can use similar techniques to those available for DTLS-SRTP to DTLS can use similar techniques to those available for DTLS-SRTP to
bind a signalling-side agreement to communicate to the certificates bind a signaling-side agreement to communicate to the certificates
used by the end-point when doing the DTLS handshake. This enables used by the endpoint when doing the DTLS handshake. This enables use
use without having a certificate-based trust chain to a trusted without having a certificate-based trust chain to a trusted
certificate root. certificate root.
There does not appear to be significant usage of DTLS for RTP. There does not appear to be significant usage of DTLS for RTP.
3.6. Media Content Security/Digital Rights Management 3.6. Media Content Security/Digital Rights Management
Mechanisms have been defined that encrypt only the media content, Mechanisms have been defined that encrypt only the media content
operating within the RTP payload data and leaving the RTP headers and operating within the RTP payload data and leaving the RTP headers and
RTCP unaffected. There are several reasons why this might be RTCP unaffected. There are several reasons why this might be
appropriate, but a common rationale is to ensure that the content appropriate, but a common rationale is to ensure that the content
stored by RTSP streaming servers has the media content in a protected stored by RTSP streaming servers has the media content in a protected
format that cannot be read by the streaming server (this is mostly format that cannot be read by the streaming server (this is mostly
done in the context of Digital Rights Management). These approaches done in the context of Digital Rights Management). These approaches
then use a key-management solution between the rights provider and then use a key management solution between the rights provider and
the consuming client to deliver the key used to protect the content the consuming client to deliver the key used to protect the content
and do not give the media server access to the security context. and do not give the media server access to the security context.
Such methods have several security weaknesses such as the fact that Such methods have several security weaknesses such as the fact that
the same key is handed out to a potentially large group of receiving the same key is handed out to a potentially large group of receiving
clients, increasing the risk of a leak. clients, increasing the risk of a leak.
Use of this type of solution can be of interest in environments that Use of this type of solution can be of interest in environments that
allow middleboxes to rewrite the RTP headers and select which streams allow middleboxes to rewrite the RTP headers and select which streams
are delivered to an end-point (e.g., some types of centralised video are delivered to an endpoint (e.g., some types of centralized video
conference systems). The advantage of encrypting and possibly conference systems). The advantage of encrypting and possibly
integrity protecting the payload but not the headers is that the integrity protecting the payload but not the headers is that the
middlebox can't eavesdrop on the media content, but can still provide middlebox can't eavesdrop on the media content, but it can still
stream switching functionality. The downside of such a system is provide stream switching functionality. The downside of such a
that it likely needs two levels of security: the payload level system is that it likely needs two levels of security: the payload-
solution to provide confidentiality and source authentication, and a level solution, to provide confidentiality and source authentication,
second layer with additional transport security ensuring source and a second layer with additional transport security ensuring source
authentication and integrity of the RTP headers associated with the authentication and integrity of the RTP headers associated with the
encrypted payloads. This can also results in the need to have two encrypted payloads. This can also result in the need to have two
different key-management systems as the entity protecting the packets different key management systems as the entity protecting the packets
and payloads are different with different set of keys. and payloads are different with a different set of keys.
The aspect of two tiers of security are present in ISMACryp (see The aspect of two tiers of security are present in ISMACryp (see
Section 3.6.1) and the deprecated 3GPP Packet Based Streaming Service Section 3.6.1) and the deprecated 3GPP Packet-switched Streaming
Annex.K [T3GPP.26.234R8] solution. Service solution; see Annex K of [T3GPP.26.234R8].
3.6.1. ISMA Encryption and Authentication 3.6.1. ISMA Encryption and Authentication
The Internet Streaming Media Alliance (ISMA) has defined ISMA The Internet Streaming Media Alliance (ISMA) has defined ISMA
Encryption and Authentication 2.0 [ISMACryp2]. This specification Encryption and Authentication 2.0 [ISMACryp2]. This specification
defines how one encrypts and packetizes the encrypted application defines how one encrypts and packetizes the encrypted application
data units (ADUs) in an RTP payload using the MPEG-4 Generic payload data units (ADUs) in an RTP payload using the MPEG-4 generic payload
format [RFC3640]. The ADU types that are allowed are those that can format [RFC3640]. The ADU types that are allowed are those that can
be stored as elementary streams in an ISO Media File format based be stored as elementary streams in an ISO Media File format-based
file. ISMACryp uses SRTP for packet level integrity and source file. ISMACryp uses SRTP for packet-level integrity and source
authentication from a streaming server to the receiver. authentication from a streaming server to the receiver.
Key-management for a ISMACryp based system can be achieved through Key management for an ISMACryp-based system can be achieved through
Open Mobile Alliance (OMA) Digital Rights Management 2.0 [OMADRMv2], Open Mobile Alliance (OMA) Digital Rights Management 2.0 [OMADRMv2],
for example. for example.
4. Securing RTP Applications 4. Securing RTP Applications
In the following we provide guidelines for how to choose appropriate In the following, we provide guidelines for how to choose appropriate
security mechanisms for RTP applications. security mechanisms for RTP applications.
4.1. Application Requirements 4.1. Application Requirements
This section discusses a number of application requirements that need This section discusses a number of application requirements that need
be considered. An application designer choosing security solutions to be considered. An application designer choosing security
requires a good understanding of what level of security is needed and solutions requires a good understanding of what level of security is
what behaviour they strive to achieve. needed and what behavior they strive to achieve.
4.1.1. Confidentiality 4.1.1. Confidentiality
When it comes to confidentiality of an RTP session there are several When it comes to confidentiality of an RTP session, there are several
aspects to consider: aspects to consider:
Probability of compromise: When using encryption to provide media Probability of compromise: When using encryption to provide media
confidentiality, it is necessary to have some rough understanding confidentiality, it is necessary to have some rough understanding
of the security goal and how long one expect the protected content of the security goal and how long one can expect the protected
to remain confidential. National or other regulations might content to remain confidential. National or other regulations
provide additional requirements on a particular usage of an RTP. might provide additional requirements on a particular usage of an
From that, one can determine which encryption algorithms are to be RTP. From that, one can determine which encryption algorithms are
used from the set of available transforms. to be used from the set of available transforms.
Potential for other leakage: RTP based security in most of its forms Potential for other leakage: RTP-based security in most of its forms
simply wraps RTP and RTCP packets into cryptographic containers. simply wraps RTP and RTCP packets into cryptographic containers.
This commonly means that the size of the original RTP payload is This commonly means that the size of the original RTP payload is
visible to observers of the protected packet flow. This can visible to observers of the protected packet flow. This can
provide information to those observers. A well-documented case is provide information to those observers. A well-documented case is
the risk with variable bit-rate speech codecs that produce the risk with variable bitrate speech codecs that produce
different sized packets based on the speech input [RFC6562]. different sized packets based on the speech input [RFC6562].
Potential threats such as these need to be considered and, if they Potential threats such as these need to be considered and, if they
are significant, then restrictions will be needed on mode choices are significant, then restrictions will be needed on mode choices
in the codec, or additional padding will need to be added to make in the codec, or additional padding will need to be added to make
all packets equal size and remove the informational leakage. all packets equal size and remove the informational leakage.
Another case is RTP header extensions. If SRTP is used, header Another case is RTP header extensions. If SRTP is used, header
extensions are normally not protected by the security mechanism extensions are normally not protected by the security mechanism
protecting the RTP payload. If the header extension carries protecting the RTP payload. If the header extension carries
information that is considered sensitive, then the application information that is considered sensitive, then the application
needs to be modified to ensure that mechanisms used to protect needs to be modified to ensure that mechanisms used to protect
against such information leakage are employed. against such information leakage are employed.
Who has access: When considering the confidentiality properties of a Who has access: When considering the confidentiality properties of a
system, it is important to consider where the media handled in the system, it is important to consider where the media handled in the
clear. For example, if the system is based on an RTP mixer that clear. For example, if the system is based on an RTP mixer that
needs the keys to decrypt the media, process, and repacketize it, needs the keys to decrypt the media, process it, and repacketize
then is the mixer providing the security guarantees expected by it, then is the mixer providing the security guarantees expected
the other parts of the system? Furthermore, it is important to by the other parts of the system? Furthermore, it is important to
consider who has access to the keys. The policies for the consider who has access to the keys. The policies for the
handling of the keys, and who can access the keys, need to be handling of the keys, and who can access the keys, need to be
considered along with the confidentiality goals. considered along with the confidentiality goals.
As can be seen the actual confidentiality level has likely more to do As can be seen, the actual confidentiality level has likely more to
with the application's usage of centralized nodes, and the details of do with the application's usage of centralized nodes, and the details
the key-management solution chosen, than with the actual choice of of the key management solution chosen, than with the actual choice of
encryption algorithm (although, of course, the encryption algorithm encryption algorithm (although, of course, the encryption algorithm
needs to be chosen appropriately for the desired security level). needs to be chosen appropriately for the desired security level).
4.1.2. Integrity 4.1.2. Integrity
Protection against modification of content by a third party, or due Protection against modification of content by a third party, or due
to errors in the network, is another factor to consider. The first to errors in the network, is another factor to consider. The first
aspect that one considers is what resilience one has against aspect that one assesses is what resilience one has against
modifications to the content. Some media types are extremely modifications to the content. Some media types are extremely
sensitive to network bit errors, whereas others might be able to sensitive to network bit errors, whereas others might be able to
tolerate some degree of data corruption. Equally important is to tolerate some degree of data corruption. Equally important is to
consider the sensitivity of the content, who is providing the consider the sensitivity of the content, who is providing the
integrity assertion, what is the source of the integrity tag, and integrity assertion, what is the source of the integrity tag, and
what are the risks of modifications happening prior to that point what are the risks of modifications happening prior to that point
where protection is applied? These issues affect what cryptographic where protection is applied. These issues affect what cryptographic
algorithm is used, and the length of the integrity tags, and whether algorithm is used, the length of the integrity tags, and whether the
the entire payload is protected. entire payload is protected.
RTP applications that rely on central nodes need to consider if hop- RTP applications that rely on central nodes need to consider if
by-hop integrity is acceptable, or if true end-to-end integrity hop-by-hop integrity is acceptable or if true end-to-end integrity
protection is needed? Is it important to be able to tell if a protection is needed. Is it important to be able to tell if a
middlebox has modified the data? There are some uses of RTP that middlebox has modified the data? There are some uses of RTP that
require trusted middleboxes that can modify the data in a way that require trusted middleboxes that can modify the data in a way that
doesn't break integrity protection as seen by the receiver, for doesn't break integrity protection as seen by the receiver, for
example local advertisement insertion in IPTV systems; there are also example, local advertisement insertion in IPTV systems. There are
uses where it is essential that such in-network modification be also uses where it is essential that such in-network modification be
detectable. RTP can support both, with appropriate choices of detectable. RTP can support both with appropriate choices of
security mechanisms. security mechanisms.
Integrity of the data is commonly closely tied to the question of Integrity of the data is commonly closely tied to the question of
source authentication. That is, it becomes important to know who source authentication. That is, it becomes important to know who
makes an integrity assertion for the data. makes an integrity assertion for the data.
4.1.3. Source Authentication 4.1.3. Source Authentication
Source authentication is about determining who sent a particular RTP Source authentication is about determining who sent a particular RTP
or RTCP packet. It is normally closely tied with integrity, since a or RTCP packet. It is normally closely tied with integrity, since a
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what the source really sent, so source authentication without what the source really sent, so source authentication without
integrity is not particularly useful. Similarly, integrity integrity is not particularly useful. Similarly, integrity
protection without source authentication is also not particularly protection without source authentication is also not particularly
useful; a claim that a packet is unchanged that cannot itself be useful; a claim that a packet is unchanged that cannot itself be
validated as from the source (or some from other known and trusted validated as from the source (or some from other known and trusted
party) is meaningless. party) is meaningless.
Source authentication can be asserted in several different ways: Source authentication can be asserted in several different ways:
Base level: Using cryptographic mechanisms that give authentication Base level: Using cryptographic mechanisms that give authentication
with some type of key-management provide an implicit method for with some type of key management provide an implicit method for
source authentication. Assuming that the mechanism has sufficient source authentication. Assuming that the mechanism has sufficient
strength to not be circumvented in the time frame when you would strength not to be circumvented in the time frame when you would
accept the packet as valid, it is possible to assert a source- accept the packet as valid, it is possible to assert a source-
authenticated statement; this message is likely from a source that authenticated statement; this message is likely from a source that
has the cryptographic key(s) to this communication. has the cryptographic key(s) to this communication.
What that assertion actually means is highly dependent on the What that assertion actually means is highly dependent on the
application and how it handles the keys. If only the two peers application and how it handles the keys. If only the two peers
have access to the keys, this can form a basis for a strong trust have access to the keys, this can form a basis for a strong trust
relationship that traffic is authenticated coming from one of the relationship that traffic is authenticated coming from one of the
peers. However, in a multi-party scenario where security contexts peers. However, in a multiparty scenario where security contexts
are shared among participants, most base-level authentication are shared among participants, most base-level authentication
solutions can't even assert that this packet is from the same solutions can't even assert that this packet is from the same
source as the previous packet. source as the previous packet.
Binding the source and the signalling: A step up in the assertion Binding the source and the signaling: A step up in the assertion
that can be done in base-level systems is to tie the signalling to that can be done in base-level systems is to tie the signaling to
the key-exchange. Here, the goal is to at least be able to assert the key exchange. Here, the goal is to at least be able to assert
that the source of the packets is the same entity that the that the source of the packets is the same entity with which the
receiver established the session with. How feasible this is receiver established the session. How feasible this is depends on
depends on the properties of the key-management system, the the properties of the key management system, the ability to tie
ability to tie the signalling to a particular source, and the the signaling to a particular source, and the degree of trust the
degree of trust the receiver places on the different nodes receiver places on the different nodes involved.
involved.
For example, systems where the key-exchange is done using the For example, systems where the key exchange is done using the
signalling systems, such as Security Descriptions [RFC4568], signaling systems, such as security descriptions [RFC4568] enable
enable a direct binding between signalling and key-exchange. In a direct binding between signaling and key exchange. In such
such systems, the actual security depends on the trust one can systems, the actual security depends on the trust one can place in
place in the signalling system to correctly associate the peer's the signaling system to correctly associate the peer's identifier
identifier with the key-exchange. with the key exchange.
Using Identifiers: If the applications have access to a system that Using identifiers: If the applications have access to a system that
can provide verifiable identifiers, then the source authentication can provide verifiable identifiers, then the source authentication
can be bound to that identifier. For example, in a point-to-point can be bound to that identifier. For example, in a point-to-point
communication even symmetric key crypto, where the key-management communication, even symmetric key crypto, where the key management
can assert that the key has only been exchanged with a particular can assert that the key has only been exchanged with a particular
identifier, can provide a strong assertion about the source of the identifier, can provide a strong assertion about the source of the
traffic. SIP identity [RFC4474] provides one example of how this traffic. SIP Identity [RFC4474] provides one example of how this
can be done, and could be used to bind DTLS-SRTP certificates used can be done and could be used to bind DTLS-SRTP certificates used
by an end-point to the identity provider's public key to by an endpoint to the identity provider's public key to
authenticate the source of a DTLS-SRTP flow. authenticate the source of a DTLS-SRTP flow.
Note that all levels of the system need to have matching Note that all levels of the system need to have matching
capability to assert identifiers. If the signalling can assert capability to assert identifiers. If the signaling can assert
that only a given entity in a multiparty session has a key, then that only a given entity in a multiparty session has a key, then
the media layer might be able to provide guarantees about the the media layer might be able to provide guarantees about the
identifier used by the media sender. However, using an signalling identifier used by the media sender. However, using a signaling
authentication mechanism built on a group key can limit the media authentication mechanism built on a group key can limit the media
layer to asserting only group membership. layer to asserting only group membership.
4.1.4. Identifiers and Identity 4.1.4. Identifiers and Identity
There exist many different types of systems providing identifiers There exist many different types of systems providing identifiers
with different properties (e.g., SIP identity [RFC4474]). In the with different properties (e.g., SIP Identity [RFC4474]). In the
context of RTP applications, the most important property is the context of RTP applications, the most important property is the
possibility to perform source authentication and verify such possibility to perform source authentication and verify such
assertions in relation to any claimed identifiers. What an assertions in relation to any claimed identifiers. What an
identifier really represent can also vary but, in the context of identifier really represents can also vary but, in the context of
communication, one of the most obvious is the identifiers communication, one of the most obvious is the identifiers
representing the identity of the human user one communicates with. representing the identity of the human user with which one
However, the human user can also have additional identifiers in a communicates. However, the human user can also have additional
particular role. For example, the human Alice, can also be a police identifiers in a particular role. For example, the human (Alice) can
officer and in some cases a identifier for her role as police officer also be a police officer, and in some cases, an identifier for her
will be more relevant than one that assert that she is Alice. This role as police officer will be more relevant than one that asserts
is common in contact with organizations, where it is important to that she is Alice. This is common in contact with organizations,
prove the persons right to represent the organization. Some examples where it is important to prove the person's right to represent the
of identifier/Identity mechanisms that can be used: organization. Some examples of identifier/identity mechanisms that
can be used:
Certificate based: A certificate is used to assert the identifiers Certificate based: A certificate is used to assert the identifiers
used to claim an identity, by having access to the private part of used to claim an identity; by having access to the private part of
the certificate one can perform signing to assert ones identity. the certificate, one can perform signing to assert one's identity.
Any entity interested in verifying the assertion then needs the Any entity interested in verifying the assertion then needs the
public part of the certificate. By having the certificate, one public part of the certificate. By having the certificate, one
can verify the signature against the certificate. The next step can verify the signature against the certificate. The next step
is to determine if one trusts the certificate's trust chain. is to determine if one trusts the certificate's trust chain.
Commonly by provisioning the verifier with the public part of a Commonly, by provisioning the verifier with the public part of a
root certificate, this enables the verifier to verify a trust root certificate, this enables the verifier to verify a trust
chain from the root certificate down to the identifier in the chain from the root certificate down to the identifier in the
certificate. However, the trust is based on all steps in the certificate. However, the trust is based on all steps in the
certificate chain being verifiable and trusted. Thus provisioning certificate chain being verifiable and trusted. Thus, the
of root certificates and the ability to revoke compromised provisioning of root certificates and the ability to revoke
certificates are aspects that will require infrastructure. compromised certificates are aspects that will require
infrastructure.
Online Identity Providers: An online identity provider (IdP) can Online identity providers: An online identity provider (IdP) can
authenticate a user's right to use an identifier, then perform authenticate a user's right to use an identifier and then perform
assertions on their behalf or provision the requester with short- assertions on their behalf or provision the requester with short-
term credentials to assert the identifiers. The verifier can then term credentials to assert the identifiers. The verifier can then
contact the IdP to request verification of a particular contact the IdP to request verification of a particular
identifier. Here the trust is highly dependent on how much one identifier. Here, the trust is highly dependent on how much one
trusts the IdP. The system also becomes dependent on having trusts the IdP. The system also becomes dependent on having
access to the relevant IdP. access to the relevant IdP.
In all of the above examples, an important part of the security In all of the above examples, an important part of the security
properties are related to the method for authenticating the access to properties is related to the method for authenticating the access to
the identity. the identity.
4.1.5. Privacy 4.1.5. Privacy
RTP applications need to consider what privacy goals they have. As RTP applications need to consider what privacy goals they have. As
RTP applications communicate directly between peers in many cases, RTP applications communicate directly between peers in many cases,
the IP addresses of any communication peer will be available. The the IP addresses of any communication peer will be available. The
main privacy concern with IP addresses is related to geographical main privacy concern with IP addresses is related to geographical
location and the possibility to track a user of an end-point. The location and the possibility to track a user of an endpoint. The
main way of avoid such concerns is the introduction of relay (e.g., a main way to avoid such concerns is the introduction of relay (e.g., a
TURN server [RFC5766]) or centralized media mixers or forwarders that Traversal Using Relay NAT (TURN) server [RFC5766]) or centralized
hides the address of a peer from any other peer. The security and media mixers or forwarders that hide the address of a peer from any
trust placed in these relays obviously needs to be carefully other peer. The security and trust placed in these relays obviously
considered. needs to be carefully considered.
RTP itself can contribute to enabling a particular user to be tracked RTP itself can contribute to enabling a particular user to be tracked
between communication sessions if the CNAME is generated according to between communication sessions if the Canonical Name (CNAME) is
the RTP specification in the form of user@host. Such RTCP CNAMEs are generated according to the RTP specification in the form of
likely long term stable over multiple sessions, allowing tracking of user@host. Such RTCP CNAMEs are likely long-term stable over
users. This can be desirable for long-term fault tracking and multiple sessions, allowing tracking of users. This can be desirable
diagnosis, but clearly has privacy implications. Instead for long-term fault tracking and diagnosis, but it clearly has
cryptographically random ones could be used as defined by Guidelines privacy implications. Instead, cryptographically random ones could
for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs) be used as defined by "Guidelines for Choosing RTP Control Protocol
[RFC7022]. (RTCP) CNAMEs" [RFC7022].
If there exist privacy goals, these need to be considered, and the If privacy goals exist, they need to be considered and the system
system designed with them in mind. In addition certain RTP features designed with them in mind. In addition, certain RTP features might
might have to be configured to safeguard privacy, or have have to be configured to safeguard privacy or have requirements on
requirements on how the implementation is done. how the implementation is done.
4.2. Application Structure 4.2. Application Structure
When it comes to RTP security, the most appropriate solution is often When it comes to RTP security, the most appropriate solution is often
highly dependent on the topology of the communication session. The highly dependent on the topology of the communication session. The
signalling also impacts what information can be provided, and if this signaling also impacts what information can be provided and if this
can be instance specific, or common for a group. In the end the key- can be instance specific or common for a group. In the end, the key
management system will highly affect the security properties achieved management system will highly affect the security properties achieved
by the application. At the same time, the communication structure of by the application. At the same time, the communication structure of
the application limits what key management methods are applicable. the application limits what key management methods are applicable.
As different key-management have different requirements on underlying As different key management methods have different requirements on
infrastructure it is important to take that aspect into consideration underlying infrastructure, it is important to take that aspect into
early in the design. consideration early in the design.
4.3. Automatic Key Management 4.3. Automatic Key Management
The Guidelines for Cryptographic Key Management [RFC4107] provide an The guidelines for Cryptographic Key Management [RFC4107] provide an
overview of why automatic key management is important. They also overview of why automatic key management is important. They also
provide a strong recommendation on using automatic key management. provide a strong recommendation on using automatic key management.
Most of the security solutions reviewed in this document provide or Most of the security solutions reviewed in this document provide or
support automatic key management, at least to establish session keys. support automatic key management, at least to establish session keys.
In some more long term use cases, credentials might in certain cases In some more long-term use cases, credentials might need to be
need to be be manually deployed. manually deployed in certain cases.
For SRTP an important aspect of automatic key management is to ensure For SRTP, an important aspect of automatic key management is to
that two time pads do not occur, in particular by preventing multiple ensure that two-time pads do not occur, in particular by preventing
end points using the same session key and SSRC. In these cases multiple endpoints using the same session key and SSRC. In these
automatic key management methods can have strong dependencies on cases, automatic key management methods can have strong dependencies
signalling features to function correctly. If those dependencies on signaling features to function correctly. If those dependencies
can't be fulfilled, additional constrains on usage, e.g., per-end can't be fulfilled, additional constrains on usage, e.g., per-
point session keys, might be needed to avoid the issue. endpoint session keys, might be needed to avoid the issue.
When selecting security mechanisms for an RTP application it is When selecting security mechanisms for an RTP application, it is
important to consider the properties of the key management. Using important to consider the properties of the key management. Using
key management that is both automatic and integrated will provide key management that is both automatic and integrated will provide
minimal interruption for the user, and is important to ensure that minimal interruption for the user and is important to ensure that
security can, and will remain, to be on by default. security can, and will remain, to be on by default.
4.4. End-to-End Security vs Tunnels 4.4. End-to-End Security vs. Tunnels
If the security mechanism only provides a secured tunnel, for example If the security mechanism only provides a secured tunnel, for
like some common uses of IPsec (Section 3.3), it is important to example, like some common uses of IPsec (Section 3.3), it is
consider the full end-to-end properties of the system. How does one important to consider the full end-to-end properties of the system.
ensure that the path from the endpoint to the local tunnel ingress/ How does one ensure that the path from the endpoint to the local
egress is secure and can be trusted (and similarly for the other end tunnel ingress/egress is secure and can be trusted (and similarly for
of the tunnel)? How does one handle the source authentication of the the other end of the tunnel)? How does one handle the source
peer, as the security protocol identifies the other end of the authentication of the peer, as the security protocol identifies the
tunnel. These are some of the issues that arise when one considers a other end of the tunnel? These are some of the issues that arise
tunnel based security protocol rather than an end-to-end. Even with when one considers a tunnel-based security protocol rather than an
clear requirements and knowledge that one still can achieve the end-to-end one. Even with clear requirements and knowledge that one
security properties using a tunnel based solution, one ought to still can achieve the security properties using a tunnel-based
prefer to use end-to-end mechanisms, as they are much less likely to solution, one ought to prefer to use end-to-end mechanisms, as they
violate any assumptions made about deployment. These assumptions can are much less likely to violate any assumptions made about
also be difficult to automatically verify. deployment. These assumptions can also be difficult to automatically
verify.
4.5. Plain Text Keys 4.5. Plaintext Keys
Key management solutions that use plain text keys, like SDP Security Key management solutions that use plaintext keys, like SDP security
Descriptions (Section 3.1.3), require care to ensure a secure descriptions (Section 3.1.3), require care to ensure a secure
transport of the signalling messages that contain the plain text transport of the signaling messages that contain the plaintext keys.
keys. For plain text keys the security properties of the system For plaintext keys, the security properties of the system depend on
depend on how securely the plain text keys are protected end-to-end how securely the plaintext keys are protected end-to-end between the
between the sender and receiver(s). Not only does one need to sender and receiver(s). Not only does one need to consider what
consider what transport protection is provided for the signalling transport protection is provided for the signaling message, including
message including the keys, but also the degree to which any the keys, but also the degree to which any intermediaries in the
intermediaries in the signalling are trusted. Untrusted signaling are trusted. Untrusted intermediaries can perform MITM
intermediaries can perform man in the middle attacks on the attacks on the communication or can log the keys, resulting in the
communication, or can log the keys with the result in encryption encryption being compromised significantly after the actual
being compromised significantly after the actual communication communication occurred.
occurred.
4.6. Interoperability 4.6. Interoperability
Few RTP applications exist as independent applications that never Few RTP applications exist as independent applications that never
interoperate with anything else. Rather, they enable communication interoperate with anything else. Rather, they enable communication
with a potentially large number of other systems. To minimize the with a potentially large number of other systems. To minimize the
number of security mechanisms that need to be implemented, it is number of security mechanisms that need to be implemented, it is
important to consider if one can use the same security mechanisms as important to consider if one can use the same security mechanisms as
other applications. This can also reduce problems of determining other applications. This can also reduce problems with determining
what security level is actually negotiated in a particular session. what security level is actually negotiated in a particular session.
The desire to be interoperable can, in some cases, be in conflict The desire to be interoperable can, in some cases, be in conflict
with the security requirements of an application. To meet the with the security requirements of an application. To meet the
security goals, it might be necessary to sacrifice interoperability. security goals, it might be necessary to sacrifice interoperability.
Alternatively, one can implement multiple security mechanisms, this Alternatively, one can implement multiple security mechanisms; this,
however introduces the complication of ensuring that the user however, introduces the complication of ensuring that the user
understands what it means to use a particular security system. In understands what it means to use a particular security system. In
addition, the application can then become vulnerable to bid-down addition, the application can then become vulnerable to bid-down
attack. attacks.
5. Examples 5. Examples
In the following we describe a number of example security solutions In the following, we describe a number of example security solutions
for applications using RTP services or frameworks. These examples for applications using RTP services or frameworks. These examples
are provided to illustrate the choices available. They are not are provided to illustrate the choices available. They are not
normative recommendations for security. normative recommendations for security.
5.1. Media Security for SIP-established Sessions using DTLS-SRTP 5.1. Media Security for SIP-Established Sessions Using DTLS-SRTP
The IETF evaluated media security for RTP sessions established using In 2009, the IETF evaluated media security for RTP sessions
point-to-point SIP sessions in 2009. A number of requirements were established using point-to-point SIP sessions. A number of
determined, and based on those, the existing solutions for media requirements were determined, and based on those, the existing
security and especially the keying methods were analysed. The solutions for media security and especially the keying methods were
resulting requirements and analysis were published in [RFC5479]. analyzed. The resulting requirements and analysis were published in
Based on this analysis and working group discussion, DTLS-SRTP was [RFC5479]. Based on this analysis and working group discussion,
determined to be the best solution. DTLS-SRTP was determined to be the best solution.
The security solution for SIP using DTLS-SRTP is defined in the The security solution for SIP using DTLS-SRTP is defined in
Framework for Establishing a Secure Real-time Transport Protocol "Framework for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer Security (SRTP) Security Context Using Datagram Transport Layer Security
(DTLS) [RFC5763]. On a high level the framework uses SIP with SDP (DTLS)" [RFC5763]. On a high level, the framework uses SIP with SDP
offer/answer procedures to exchange the network addresses where the offer/answer procedures to exchange the network addresses where the
server end-point will have a DTLS-SRTP enable server running. The server endpoint will have a DTLS-SRTP-enabled server running. The
SIP signalling is also used to exchange the fingerprints of the SIP signaling is also used to exchange the fingerprints of the
certificate each end-point will use in the DTLS establishment certificate each endpoint will use in the DTLS establishment process.
process. When the signalling is sufficiently completed, the DTLS- When the signaling is sufficiently completed, the DTLS-SRTP client
SRTP client performs DTLS handshakes and establishes SRTP session performs DTLS handshakes and establishes SRTP session keys. The
keys. The clients also verify the fingerprints of the certificates clients also verify the fingerprints of the certificates to verify
to verify that no man in the middle has inserted themselves into the that no man in the middle has inserted themselves into the exchange.
exchange.
DTLS has a number of good security properties. For example, to DTLS has a number of good security properties. For example, to
enable a man in the middle someone in the signalling path needs to enable a MITM, someone in the signaling path needs to perform an
perform an active action and modify both the signalling message and active action and modify both the signaling message and the DTLS
the DTLS handshake. There also exists solutions that enables the handshake. Solutions also exist that enable the fingerprints to be
fingerprints to be bound to identities. SIP Identity provides an bound to identities. SIP Identity provides an identity established
identity established by the first proxy for each user [RFC4474]. by the first proxy for each user [RFC4474]. This reduces the number
This reduces the number of nodes the connecting user User Agent has of nodes the connecting User Agent has to trust to include just the
to trust to include just the first hop proxy, rather than the full first-hop proxy rather than the full signaling path. The biggest
signalling path. The biggest security weakness of this system is its security weakness of this system is its dependency on the signaling.
dependency on the signalling. SIP signalling passes multiple nodes SIP signaling passes multiple nodes and there is usually no message
and there is usually no message security deployed, only hop-by-hop security deployed, only hop-by-hop transport security, if any,
transport security, if any, between the nodes. between the nodes.
5.2. Media Security for WebRTC Sessions 5.2. Media Security for WebRTC Sessions
Web Real-Time Communication (WebRTC) [I-D.ietf-rtcweb-overview] is a Web Real-Time Communication (WebRTC) [WebRTC] is a solution providing
solution providing JavaScript web applications with real-time media JavaScript web applications with real-time media directly between
directly between browsers. Media is transported using RTP protected browsers. Media is transported using RTP and protected using a
using a mandatory application of SRTP [RFC3711], with keying done mandatory application of SRTP [RFC3711], with keying done using DTLS-
using DTLS-SRTP [RFC5764]. The security configuration is further SRTP [RFC5764]. The security configuration is further defined in
defined in the WebRTC Security Architecture "WebRTC Security Architecture" [WebRTC-SEC].
[I-D.ietf-rtcweb-security-arch].
A hash of the peer's certificate is provided to the JavaScript web A hash of the peer's certificate is provided to the JavaScript web
application, allowing that web application to verify identity of the application, allowing that web application to verify identity of the
peer. There are several ways in which the certificate hashes can be peer. There are several ways in which the certificate hashes can be
verified. An approach identified in the WebRTC security architecture verified. An approach identified in the WebRTC security architecture
[I-D.ietf-rtcweb-security-arch] is to use an identity provider. In [WebRTC-SEC] is to use an identity provider. In this solution, the
this solution the Identity Provider, which is a third party to the identity provider, which is a third party to the web application,
web application, signs the DTLS-SRTP hash combined with a statement signs the DTLS-SRTP hash combined with a statement on the validity of
on the validity of the user identity that has been used to sign the the user identity that has been used to sign the hash. The receiver
hash. The receiver of such an identity assertion can then of such an identity assertion can then independently verify the user
independently verify the user identity to ensure that it is the identity to ensure that it is the identity that the receiver intended
identity that the receiver intended to communicate with, and that the to communicate with, and that the cryptographic assertion holds; this
cryptographic assertion holds; this way a user can be certain that way, a user can be certain that the application also can't perform a
the application also can't perform a MITM and acquire the keys to the MITM and acquire the keys to the media communication. Other ways of
media communication. Other ways of verifying the certificate hashes verifying the certificate hashes exist; for example, they could be
exist, for example they could be verified against a hash carried in verified against a hash carried in some out-of-band channel (e.g.,
some out of band channel (e.g., compare with a hash printed on a compare with a hash printed on a business card) or using a verbal
business card), or using a verbal short authentication string (e.g., short authentication string (e.g., as in ZRTP [RFC6189]) or using
as in ZRTP [RFC6189]), or using hash continuity. hash continuity.
In the development of WebRTC there has also been attention given to In the development of WebRTC, there has also been attention given to
privacy considerations. The main RTP-related concerns that have been privacy considerations. The main RTP-related concerns that have been
raised are: raised are:
Location Disclosure: As ICE negotiation [RFC5245] provides IP Location disclosure: As Interactive Connectivity Establishment (ICE)
addresses and ports for the browser, this leaks location negotiation [RFC5245] provides IP addresses and ports for the
information in the signalling to the peer. To prevent this one browser, this leaks location information in the signaling to the
can block the usage of any ICE candidate that isn't a relay peer. To prevent this, one can block the usage of any ICE
candidate, i.e. where the IP and port provided belong to the candidate that isn't a relay candidate, i.e., where the IP and
service providers media traffic relay. port provided belong to the service providers media traffic relay.
Prevent tracking between sessions: static RTP CNAMEs and DTLS-SRTP Prevent tracking between sessions: Static RTP CNAMEs and DTLS-SRTP
certificates provide information that is re-used between session certificates provide information that is reused between session
instances. Thus to prevent tracking, such information is ought instances. Thus, to prevent tracking, such information ought not
not be re-used between sessions, or the information ought not sent be reused between sessions, or the information ought not be sent
in the clear. Note, that generating new certificates each time in the clear. Note that generating new certificates each time
prevents continuity in authentication, however, as WebRTC users prevents continuity in authentication, however, as WebRTC users
are expected to use multiple devices to access the same are expected to use multiple devices to access the same
communication service, such continuity can't be expected anyway, communication service, such continuity can't be expected anyway;
instead the above described identity mechanism has to be relied instead, the above-described identity mechanism has to be relied
on. on.
Note: The above cases are focused on providing privacy from other Note: The above cases are focused on providing privacy from other
parties, not on providing privacy from the web server that provides parties, not on providing privacy from the web server that provides
the WebRTC Javascript application. the WebRTC JavaScript application.
5.3. IP Multimedia Subsystem (IMS) Media Security 5.3. IP Multimedia Subsystem (IMS) Media Security
In IMS, the core network is controlled by a single operator, or by In IMS, the core network is controlled by a single operator or by
several operators with high trust in each other. Except for some several operators with high trust in each other. Except for some
types of accesses, the operator is in full control, and no packages types of accesses, the operator is in full control, and no packages
are routed over the Internet. Nodes in the core network offer are routed over the Internet. Nodes in the core network offer
services such as voice mail, interworking with legacy systems (PSTN, services such as voice mail, interworking with legacy systems (Public
GSM, and 3G), and transcoding. End-points are authenticated during Switched Telephone Network (PSTN), Global System for Mobile
the SIP registration using either IMS-AKA (using SIM credentials) or Communications (GSM), and 3G), and transcoding. Endpoints are
SIP Digest (using password). authenticated during the SIP registration using either IMS and
Authentication and Key Agreement (AKA) (using Subscriber Identity
Module (SIM) credentials) or SIP Digest (using a password).
In IMS media security [T3GPP.33.328], end-to-end encryption is In IMS media security [T3GPP.33.328], end-to-end encryption is,
therefore not seen as needed or desired as it would hinder for therefore, not seen as needed or desired as it would hinder, for
example interworking and transcoding, making calls between example, interworking and transcoding, making calls between
incompatible terminals impossible. Because of this IMS media incompatible terminals impossible. Because of this, IMS media
security mostly uses end-to-access-edge security where SRTP is security mostly uses end-to-access-edge security where SRTP is
terminated in the first node in the core network. As the SIP terminated in the first node in the core network. As the SIP
signaling is trusted and encrypted (with TLS or IPsec), security signaling is trusted and encrypted (with TLS or IPsec), security
descriptions [RFC4568] is considered to give good protection against descriptions [RFC4568] is considered to give good protection against
eavesdropping over the accesses that are not already encrypted (GSM, eavesdropping over the accesses that are not already encrypted (GSM,
3G, LTE). Media source authentication is based on knowledge of the 3G, and Long Term Evolution (LTE)). Media source authentication is
SRTP session key and trust in that the IMS network will only forward based on knowledge of the SRTP session key and trust in that the IMS
media from the correct end-point. network will only forward media from the correct endpoint.
For enterprises and government agencies, which might have weaker For enterprises and government agencies, which might have weaker
trust in the IMS core network and can be assumed to have compatible trust in the IMS core network and can be assumed to have compatible
terminals, end-to-end security can be achieved by deploying their own terminals, end-to-end security can be achieved by deploying their own
key management server. key management server.
Work on Interworking with WebRTC is currently ongoing; the security Work on interworking with WebRTC is currently ongoing; the security
will still be end-to-access-edge, but using DTLS-SRTP [RFC5763] will still be end-to-access-edge but using DTLS-SRTP [RFC5763]
instead of security descriptions. instead of security descriptions.
5.4. 3GPP Packet Based Streaming Service (PSS) 5.4. 3GPP Packet-Switched Streaming Service (PSS)
The 3GPP Release 11 PSS specification of the Packet Based Streaming The 3GPP Release 11 PSS specification of the Packet-switched
Service (PSS) [T3GPP.26.234R11] defines, in Annex R, a set of Streaming Service (PSS) [T3GPP.26.234R11] defines, in Annex R, a set
security mechanisms. These security mechanisms are concerned with of security mechanisms. These security mechanisms are concerned with
protecting the content from being copied, i.e. Digital Rights protecting the content from being copied, i.e., Digital Rights
Management. To meet these goals with the specified solution, the Management (DRM). To meet these goals with the specified solution,
client implementation and the application platform are trusted to the client implementation and the application platform are trusted to
protect against access and modification by an attacker. protect against access and modification by an attacker.
PSS is RTSP 1.0 [RFC2326] controlled media streaming over RTP. Thus PSS is media controlled by RTSP 1.0 [RFC2326] streaming over RTP.
an RTSP client whose user wants to access a protected content will Thus, an RTSP client whose user wants to access a protected content
request a session description (SDP [RFC4566]) for the protected will request a session description (SDP [RFC4566]) for the protected
content. This SDP will indicate that the media is ISMACryp 2.0 content. This SDP will indicate that the media is protected by
[ISMACryp2] protected media encoding application units (AUs). The ISMACryp 2.0 [ISMACryp2] encoding application units (AUs). The
key(s) used to protect the media are provided in either of two ways. key(s) used to protect the media is provided in one of two ways. If
If a single key is used then the client uses some DRM system to a single key is used, then the client uses some DRM system to
retrieve the key as indicated in the SDP. Commonly OMA DRM v2 retrieve the key as indicated in the SDP. Commonly, OMA DRM v2
[OMADRMv2] will be used to retrieve the key. If multiple keys are to [OMADRMv2] will be used to retrieve the key. If multiple keys are to
be used, then an additional RTSP stream for key-updates in parallel be used, then an additional RTSP stream for key updates in parallel
with the media streams is established, where key updates are sent to with the media streams is established, where key updates are sent to
the client using Short Term Key Messages defined in the "Service and the client using Short Term Key Messages defined in the "Service and
Content Protection for Mobile Broadcast Services" section of the OMA Content Protection for Mobile Broadcast Services" part [OMASCP] of
Mobile Broadcast Services [OMABCAST]. the OMA Mobile Broadcast Services [OMABCAST].
Worth noting is that this solution doesn't provide any integrity Worth noting is that this solution doesn't provide any integrity
verification method for the RTP header and payload header verification method for the RTP header and payload header
information, only the encoded media AU is protected. 3GPP has not information; only the encoded media AU is protected. 3GPP has not
defined any requirement for supporting any solution that could defined any requirement for supporting any solution that could
provide that service. Thus, replay or insertion attacks are provide that service. Thus, replay or insertion attacks are
possible. Another property is that the media content can be possible. Another property is that the media content can be
protected by the ones providing the media, so that the operators of protected by the ones providing the media, so that the operators of
the RTSP server has no access to unprotected content. Instead all the RTSP server have no access to unprotected content. Instead, all
that want to access the media is supposed to contact the DRM keying that want to access the media are supposed to contact the DRM keying
server and if the device is acceptable they will be given the key to server, and if the device is acceptable, they will be given the key
decrypt the media. to decrypt the media.
To protect the signalling, RTSP 1.0 supports the usage of TLS. This To protect the signaling, RTSP 1.0 supports the usage of TLS. This
is, however, not explicitly discussed in the PSS specification. is, however, not explicitly discussed in the PSS specification.
Usage of TLS can prevent both modification of the session description Usage of TLS can prevent both modification of the session description
information and help maintain some privacy of what content the user information and help maintain some privacy of what content the user
is watching as all URLs would then be confidentiality protected. is watching as all URLs would then be confidentiality protected.
5.5. RTSP 2.0 5.5. RTSP 2.0
Real-time Streaming Protocol 2.0 [I-D.ietf-mmusic-rfc2326bis] offers The Real-time Streaming Protocol 2.0 [RTSP] offers an interesting
an interesting comparison to the PSS service (Section 5.4) that is comparison to the PSS service (Section 5.4) that is based on RTSP 1.0
based on RTSP 1.0 and service requirements perceived by mobile and service requirements perceived by mobile operators. A major
operators. A major difference between RTSP 1.0 and RTSP 2.0 is that difference between RTSP 1.0 and RTSP 2.0 is that 2.0 is fully defined
2.0 is fully defined under the requirement to have mandatory to under the requirement to have a mandatory-to-implement security
implement security mechanism. As it specifies how one transport mechanism. As it specifies one transport media over RTP, it is also
media over RTP it is also defining security mechanisms for the RTP defining security mechanisms for the RTP-transported media streams.
transported media streams.
The security goals for RTP in RTSP 2.0 is to ensure that there is The security goal for RTP in RTSP 2.0 is to ensure that there is
confidentiality, integrity and source authentication between the RTSP confidentiality, integrity, and source authentication between the
server and the client. This to prevent eavesdropping on what the RTSP server and the client. This to prevent eavesdropping on what
user is watching for privacy reasons and to prevent replay or the user is watching for privacy reasons and to prevent replay or
injection attacks on the media stream. To reach these goals, the injection attacks on the media stream. To reach these goals, the
signalling also has to be protected, requiring the use of TLS between signaling also has to be protected, requiring the use of TLS between
the client and server. the client and server.
Using TLS-protected signalling the client and server agree on the Using TLS-protected signaling, the client and server agree on the
media transport method when doing the SETUP request and response. media transport method when doing the SETUP request and response.
The secured media transport is SRTP (SAVP/RTP) normally over UDP. The secured media transport is SRTP (SAVP/RTP) normally over UDP.
The key management for SRTP is MIKEY using RSA-R mode. The RSA-R The key management for SRTP is MIKEY using RSA-R mode. The RSA-R
mode is selected as it allows the RTSP Server to select the key mode is selected as it allows the RTSP server to select the key
despite having the RTSP Client initiate the MIKEY exchange. It also despite having the RTSP client initiate the MIKEY exchange. It also
enables the reuse of the RTSP servers TLS certificate when creating enables the reuse of the RTSP server's TLS certificate when creating
the MIKEY messages thus ensuring a binding between the RTSP server the MIKEY messages, thus ensuring a binding between the RTSP server
and the key exchange. Assuming the SETUP process works, this will and the key exchange. Assuming the SETUP process works, this will
establish a SRTP crypto context to be used between the RTSP Server establish a SRTP crypto context to be used between the RTSP server
and the Client for the RTP transported media streams. and the client for the RTP-transported media streams.
6. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section can be removed on publication as an
RFC.
7. Security Considerations 6. Security Considerations
This entire document is about security. Please read it. This entire document is about security. Please read it.
8. Acknowledgements 7. Acknowledgements
We thank the IESG for their careful review of We thank the IESG for their careful review of [RFC7202], which led to
[I-D.ietf-avt-srtp-not-mandatory] which led to the writing of this the writing of this memo. John Mattsson has contributed the IMS
memo. John Mattsson has contributed the IMS Media Security example Media Security example (Section 5.3).
(Section 5.3).
The authors wished to thank Christian Correll, Dan Wing, Kevin Gross, The authors wish to thank Christian Correll, Dan Wing, Kevin Gross,
Alan Johnston, Michael Peck, Ole Jacobsen, Spencer Dawkins, Stephen Alan Johnston, Michael Peck, Ole Jacobsen, Spencer Dawkins, Stephen
Farrell, John Mattsson, and Suresh Krishnan for review and proposals Farrell, John Mattsson, and Suresh Krishnan for their reviews and
for improvements of the text. proposals for improvements to the text.
9. Informative References
[I-D.ietf-avt-srtp-not-mandatory] 8. Informative References
Perkins, C. and M. Westerlund, "Securing the RTP Protocol
Framework: Why RTP Does Not Mandate a Single Media
Security Solution", draft-ietf-avt-srtp-not-mandatory-14
(work in progress), October 2013.
[I-D.ietf-avtcore-aria-srtp] [AES-GCM] McGrew, D. and K. Igoe, "AES-GCM and AES-CCM
Kim, W., Lee, J., Kim, D., Park, J., and D. Kwon, "The Authenticated Encryption in Secure RTP (SRTP)", Work in
ARIA Algorithm and Its Use with the Secure Real-time Progress, September 2013.
Transport Protocol(SRTP)", draft-ietf-avtcore-aria-srtp-06
(work in progress), November 2013.
[I-D.ietf-avtcore-srtp-aes-gcm] [ARIA-SRTP] Kim, W., Lee, J., Kim, D., Park, J., and D. Kwon, "The
McGrew, D. and K. Igoe, "AES-GCM and AES-CCM Authenticated ARIA Algorithm and Its Use with the Secure Real-time
Encryption in Secure RTP (SRTP)", draft-ietf-avtcore-srtp- Transport Protocol(SRTP)", Work in Progress, November
aes-gcm-10 (work in progress), September 2013. 2013.
[I-D.ietf-avtcore-srtp-ekt] [EKT] McGrew, D. and D. Wing, "Encrypted Key Transport for
McGrew, D. and D. Wing, "Encrypted Key Transport for Secure RTP", Work in Progress, February 2014.
Secure RTP", draft-ietf-avtcore-srtp-ekt-01 (work in
progress), October 2013.
[I-D.ietf-mmusic-rfc2326bis] [ISMACryp2] Internet Streaming Media Alliance (ISMA), "ISMA
Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., Encryption and Authentication Version 2.0", November
and M. Stiemerling, "Real Time Streaming Protocol 2.0 2007, <http://www.oipf.tv/images/site/DOCS/mpegif/ISMA/
(RTSP)", draft-ietf-mmusic-rfc2326bis-38 (work in isma_easpec2.0.pdf>.
progress), October 2013.
[I-D.ietf-rtcweb-overview] [OMABCAST] Open Mobile Alliance, "Mobile Broadcast Services Version
Alvestrand, H., "Overview: Real Time Protocols for Brower- 1.0", February 2009,
based Applications", draft-ietf-rtcweb-overview-08 (work <http://technical.openmobilealliance.org/Technical/
in progress), September 2013. release_program/bcast_v1_0.aspx>.
[I-D.ietf-rtcweb-security-arch] [OMADRMv2] Open Mobile Alliance, "OMA Digital Rights Management
Rescorla, E., "WebRTC Security Architecture", draft-ietf- V2.0", July 2008,
rtcweb-security-arch-07 (work in progress), July 2013. <http://technical.openmobilealliance.org/
Technical/release_program/drm_v2_0.aspx>.
[ISMACryp2] [OMASCP] Open Mobile Alliance, "Service and Content Protection for
Internet Streaming Media Alliance (ISMA), "ISMA Encryption Mobile Broadcast Services", January 2013,
and Authentication, Version 2.0 release version", November <http://technical.openmobilealliance.org/Technical/
2007. release_program/docs/BCAST/V1_0_1-20130109-A/
OMA-TS-BCAST_SvcCntProtection-V1_0_1-20130109-A.pdf>.
[OMABCAST] [RFC1112] Deering, S., "Host extensions for IP multicasting", STD
Open Mobile Alliance, "OMA Mobile Broadcast Services 5, RFC 1112, August 1989.
V1.0", February 2009.
[OMADRMv2] [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Open Mobile Alliance, "OMA Digital Rights Management Streaming Protocol (RTSP)", RFC 2326, April 1998.
V2.0", July 2008.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5, [RFC3365] Schiller, J., "Strong Security Requirements for Internet
RFC 1112, August 1989. Engineering Task Force Standard Protocols", BCP 61, RFC
3365, August 2002.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Streaming Protocol (RTSP)", RFC 2326, April 1998. Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3365] Schiller, J., "Strong Security Requirements for Internet [RFC3640] van der Meer, J., Mackie, D., Swaminathan, V., Singer,
Engineering Task Force Standard Protocols", BCP 61, RFC D., and P. Gentric, "RTP Payload Format for Transport of
3365, August 2002. MPEG-4 Elementary Streams", RFC 3640, November 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Jacobson, "RTP: A Transport Protocol for Real-Time Norrman, "The Secure Real-time Transport Protocol
Applications", STD 64, RFC 3550, July 2003. (SRTP)", RFC 3711, March 2004.
[RFC3640] van der Meer, J., Mackie, D., Swaminathan, V., Singer, D., [RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
and P. Gentric, "RTP Payload Format for Transport of Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
MPEG-4 Elementary Streams", RFC 3640, November 2003. August 2004.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [RFC4107] Bellovin, S. and R. Housley, "Guidelines for
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Cryptographic Key Management", BCP 107, RFC 4107, June
RFC 3711, March 2004. 2005.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, Internet Protocol", RFC 4301, December 2005.
August 2004.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic [RFC4383] Baugher, M. and E. Carrara, "The Use of Timed Efficient
Key Management", BCP 107, RFC 4107, June 2005. Stream Loss-Tolerant Authentication (TESLA) in the Secure
Real-time Transport Protocol (SRTP)", RFC 4383, February
2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the [RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Internet Protocol", RFC 4301, December 2005. Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4383] Baugher, M. and E. Carrara, "The Use of Timed Efficient [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Stream Loss-Tolerant Authentication (TESLA) in the Secure Description Protocol", RFC 4566, July 2006.
Real-time Transport Protocol (SRTP)", RFC 4383, February
2006.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for [RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Authenticated Identity Management in the Session Carrara, "Key Management Extensions for Session
Initiation Protocol (SIP)", RFC 4474, August 2006. Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", RFC 4567, July 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol", RFC 4566, July 2006. Description Protocol (SDP) Security Descriptions for
Media Streams", RFC 4568, July 2006.
[RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. [RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
Carrara, "Key Management Extensions for Session and RTP Control Protocol (RTCP) Packets over Connection-
Description Protocol (SDP) and Real Time Streaming Oriented Transport", RFC 4571, July 2006.
Protocol (RTSP)", RFC 4567, July 2006.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session [RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Description Protocol (SDP) Security Descriptions for Media Transport Layer Security (TLS) Protocol in the Session
Streams", RFC 4568, July 2006. Description Protocol (SDP)", RFC 4572, July 2006.
[RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
and RTP Control Protocol (RTCP) Packets over Connection- IP", RFC 4607, August 2006.
Oriented Transport", RFC 4571, July 2006.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the [RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for
Transport Layer Security (TLS) Protocol in the Session Multimedia Internet KEYing (MIKEY)", RFC 4650, September
Description Protocol (SDP)", RFC 4572, July 2006. 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for [RFC4738] Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY-
IP", RFC 4607, August 2006. RSA-R: An Additional Mode of Key Distribution in
Multimedia Internet KEYing (MIKEY)", RFC 4738, November
2006.
[RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for [RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Multimedia Internet KEYing (MIKEY)", RFC 4650, September Transform Carrying Roll-Over Counter for the Secure Real-
2006. time Transport Protocol (SRTP)", RFC 4771, January 2007.
[RFC4738] Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY- [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
RSA-R: An Additional Mode of Key Distribution in 4949, August 2007.
Multimedia Internet KEYing (MIKEY)", RFC 4738, November
2006.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
Transform Carrying Roll-Over Counter for the Secure Real- January 2008.
time Transport Protocol (SRTP)", RFC 4771, January 2007.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC [RFC5197] Fries, S. and D. Ignjatic, "On the Applicability of
4949, August 2007. Various Multimedia Internet KEYing (MIKEY) Modes and
Extensions", RFC 5197, June 2008.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
January 2008. (ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5197] Fries, S. and D. Ignjatic, "On the Applicability of [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
Various Multimedia Internet KEYing (MIKEY) Modes and (TLS) Protocol Version 1.2", RFC 5246, August 2008.
Extensions", RFC 5197, June 2008.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment [RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet,
(ICE): A Protocol for Network Address Translator (NAT) "Requirements and Analysis of Media Security Management
Traversal for Offer/Answer Protocols", RFC 5245, April Protocols", RFC 5479, April 2009.
2010.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5669] Yoon, S., Kim, J., Park, H., Jeong, H., and Y. Won, "The
(TLS) Protocol Version 1.2", RFC 5246, August 2008. SEED Cipher Algorithm and Its Use with the Secure Real-
Time Transport Protocol (SRTP)", RFC 5669, August 2010.
[RFC5479] Wing, D., Fries, S., Tschofenig, H., and F. Audet, [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
"Requirements and Analysis of Media Security Management Protocol (RTCP) Extensions for Single-Source Multicast
Protocols", RFC 5479, April 2009. Sessions with Unicast Feedback", RFC 5760, February 2010.
[RFC5669] Yoon, S., Kim, J., Park, H., Jeong, H., and Y. Won, "The [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
SEED Cipher Algorithm and Its Use with the Secure Real- for Establishing a Secure Real-time Transport Protocol
Time Transport Protocol (SRTP)", RFC 5669, August 2010. (SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, May 2010.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Protocol (RTCP) Extensions for Single-Source Multicast Security (DTLS) Extension to Establish Keys for the
Sessions with Unicast Feedback", RFC 5760, February 2010. Secure Real-time Transport Protocol (SRTP)", RFC 5764,
May 2010.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal
for Establishing a Secure Real-time Transport Protocol Using Relays around NAT (TURN): Relay Extensions to
(SRTP) Security Context Using Datagram Transport Layer Session Traversal Utilities for NAT (STUN)", RFC 5766,
Security (DTLS)", RFC 5763, May 2010. April 2010.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer [RFC6043] Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based
Security (DTLS) Extension to Establish Keys for the Secure Modes of Key Distribution in Multimedia Internet KEYing
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. (MIKEY)", RFC 6043, March 2011.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using [RFC6188] McGrew, D., "The Use of AES-192 and AES-256 in Secure
Relays around NAT (TURN): Relay Extensions to Session RTP", RFC 6188, March 2011.
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6043] Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based [RFC6189] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
Modes of Key Distribution in Multimedia Internet KEYing Path Key Agreement for Unicast Secure RTP", RFC 6189,
(MIKEY)", RFC 6043, March 2011. April 2011.
[RFC6188] McGrew, D., "The Use of AES-192 and AES-256 in Secure [RFC6267] Cakulev, V. and G. Sundaram, "MIKEY-IBAKE: Identity-Based
RTP", RFC 6188, March 2011. Authenticated Key Exchange (IBAKE) Mode of Key
Distribution in Multimedia Internet KEYing (MIKEY)", RFC
6267, June 2011.
[RFC6189] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Path Key Agreement for Unicast Secure RTP", RFC 6189, Security Version 1.2", RFC 6347, January 2012.
April 2011.
[RFC6267] Cakulev, V. and G. Sundaram, "MIKEY-IBAKE: Identity-Based [RFC6509] Groves, M., "MIKEY-SAKKE: Sakai-Kasahara Key Encryption
Authenticated Key Exchange (IBAKE) Mode of Key in Multimedia Internet KEYing (MIKEY)", RFC 6509,
Distribution in Multimedia Internet KEYing (MIKEY)", RFC February 2012.
6267, June 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC6562] Perkins, C. and JM. Valin, "Guidelines for the Use of
Security Version 1.2", RFC 6347, January 2012. Variable Bit Rate Audio with Secure RTP", RFC 6562, March
2012.
[RFC6509] Groves, M., "MIKEY-SAKKE: Sakai-Kasahara Key Encryption in [RFC6904] Lennox, J., "Encryption of Header Extensions in the
Multimedia Internet KEYing (MIKEY)", RFC 6509, February Secure Real-time Transport Protocol (SRTP)", RFC 6904,
2012. April 2013.
[RFC6562] Perkins, C. and JM. Valin, "Guidelines for the Use of [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
Variable Bit Rate Audio with Secure RTP", RFC 6562, March "Guidelines for Choosing RTP Control Protocol (RTCP)
2012. Canonical Names (CNAMEs)", RFC 7022, September 2013.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure [RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP Protocol
Real-time Transport Protocol (SRTP)", RFC 6904, April Framework: Why RTP Does Not Mandate a Single Media
2013. Security Solution", RFC 7202, April 2014.
[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, [RTSP] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
"Guidelines for Choosing RTP Control Protocol (RTCP) and M. Stiemerling, "Real Time Streaming Protocol 2.0
Canonical Names (CNAMEs)", RFC 7022, September 2013. (RTSP)", Work in Progress, February 2014.
[T3GPP.26.234R11] [T3GPP.26.234R11]
3GPP, "Technical Specification Group Services and System 3GPP, "Technical Specification Group Services and System
Aspects; Transparent end-to-end Packet-switched Streaming Aspects; Transparent end-to-end Packet-switched Streaming
Service (PSS); Protocols and codecs", 3GPP TS 26.234 Service (PSS); Protocols and codecs", 3GPP TS 26.234
11.1.0, September 2012. 11.1.0, September 2012,
<http://www.3gpp.org/DynaReport/26234.htm>.
[T3GPP.26.234R8] [T3GPP.26.234R8]
3GPP, "Technical Specification Group Services and System 3GPP, "Technical Specification Group Services and System
Aspects; Transparent end-to-end Packet-switched Streaming Aspects; Transparent end-to-end Packet-switched Streaming
Service (PSS); Protocols and codecs", 3GPP TS 26.234 Service (PSS); Protocols and codecs", 3GPP TS 26.234
8.4.0, September 2009. 8.4.0, September 2009,
<http://www.3gpp.org/DynaReport/26234.htm>.
[T3GPP.26.346] [T3GPP.26.346]
3GPP, "Multimedia Broadcast/Multicast Service (MBMS); 3GPP, "Multimedia Broadcast/Multicast Service (MBMS);
Protocols and codecs", 3GPP TS 26.346 10.7.0, March 2013. Protocols and codecs", 3GPP TS 26.346 10.7.0, March 2013,
<http://www.3gpp.org/DynaReport/26346.htm>.
[T3GPP.33.246] [T3GPP.33.246]
3GPP, "3G Security; Security of Multimedia Broadcast/ 3GPP, "3G Security; Security of Multimedia Broadcast/
Multicast Service (MBMS)", 3GPP TS 33.246 12.1.0, December Multicast Service (MBMS)", 3GPP TS 33.246 11.1.0,
2012. December 2012,
<http://www.3gpp.org/DynaReport/33246.htm>.
[T3GPP.33.328] [T3GPP.33.328]
3GPP, "IP Multimedia Subsystem (IMS) media plane 3GPP, "IP Multimedia Subsystem (IMS) media plane
security", 3GPP TS 33.328 12.1.0, December 2012. security", 3GPP TS 33.328 12.1.0, December 2012,
<http://www.3gpp.org/DynaReport/33328.htm>.
[WebRTC-SEC]
Rescorla, E., "WebRTC Security Architecture", Work in
Progress, February 2014.
[WebRTC] Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", Work in Progress, February
2014.
Authors' Addresses Authors' Addresses
Magnus Westerlund Magnus Westerlund
Ericsson Ericsson
Farogatan 6 Farogatan 6
SE-164 80 Kista SE-164 80 Kista
Sweden Sweden
Phone: +46 10 714 82 87 Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com EMail: magnus.westerlund@ericsson.com
Colin Perkins Colin Perkins
University of Glasgow University of Glasgow
School of Computing Science School of Computing Science
Glasgow G12 8QQ Glasgow G12 8QQ
United Kingdom United Kingdom
Email: csp@csperkins.org EMail: csp@csperkins.org
URI: http://csperkins.org/ URI: http://csperkins.org/
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