SIP J. Fischl
Internet-Draft CounterPath Corporation
Intended status: Standards Track H. Tschofenig
Expires: April 4, 2009 Nokia Siemens Networks
E. Rescorla
RTFM, Inc.
October 1, 2008
Framework for Establishing an SRTP Security Context using DTLS
draft-ietf-sip-dtls-srtp-framework-04.txt
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Abstract
This document specifies how to use the Session Initiation Protocol
(SIP) to establish an Secure Real-time Transport Protocol (SRTP)
security context using the Datagram Transport Layer Security (DTLS)
protocol. It describes a mechanism of transporting a fingerprint
attribute in the Session Description Protocol (SDP) that identifies
the key that will be presented during the DTLS handshake. The key
exchange travels along the media path as opposed to the signaling
path. The SIP Identity mechanism can be used to protect the
integrity of the fingerprint attribute from modification by
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intermediate proxies.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Establishing a Secure Channel . . . . . . . . . . . . . . . . 8
6. Miscellaneous Considerations . . . . . . . . . . . . . . . . . 10
6.1. Anonymous Calls . . . . . . . . . . . . . . . . . . . . . 10
6.2. Early Media . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. Forking . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.4. Delayed Offer Calls . . . . . . . . . . . . . . . . . . . 11
6.5. Multiple Associations . . . . . . . . . . . . . . . . . . 11
6.6. Session Modification . . . . . . . . . . . . . . . . . . . 11
6.7. Middlebox Interaction . . . . . . . . . . . . . . . . . . 12
6.7.1. ICE Interaction . . . . . . . . . . . . . . . . . . . 12
6.7.2. Latching Control Without ICE . . . . . . . . . . . . . 12
6.8. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.9. Conference Servers and Shared Encryptions Contexts . . . . 13
6.10. Media over SRTP . . . . . . . . . . . . . . . . . . . . . 13
6.11. Best Effort Encryption . . . . . . . . . . . . . . . . . . 14
7. Example Message Flow . . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8.1. Responder Identity . . . . . . . . . . . . . . . . . . . . 21
8.2. SIPS . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.3. S/MIME . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.4. Continuity of Authentication . . . . . . . . . . . . . . . 22
8.5. Short Authentication String . . . . . . . . . . . . . . . 23
8.6. Limits of Identity Assertions . . . . . . . . . . . . . . 23
8.7. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.1. Normative References . . . . . . . . . . . . . . . . . . . 25
11.2. Informational References . . . . . . . . . . . . . . . . . 26
Appendix A. Requirements Analysis . . . . . . . . . . . . . . . . 28
A.1. Forking and retargeting (R-FORK-RETARGET,
R-BEST-SECURE, R-DISTINCT) . . . . . . . . . . . . . . . . 29
A.2. Distinct Cryptographic Contexts (R-DISTINCT) . . . . . . . 29
A.3. Reusage of a Security Context (R-REUSE) . . . . . . . . . 29
A.4. Clipping (R-AVOID-CLIPPING) . . . . . . . . . . . . . . . 29
A.5. Passive Attacks on the Media Path (R-PASS-MEDIA) . . . . . 29
A.6. Passive Attacks on the Signaling Path (R-PASS-SIG) . . . . 30
A.7. (R-SIG-MEDIA, R-ACT-ACT) . . . . . . . . . . . . . . . . . 30
A.8. Binding to Identifiers (R-ID-BINDING) . . . . . . . . . . 30
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A.9. Perfect Forward Secrecy (R-PFS) . . . . . . . . . . . . . 30
A.10. Algorithm Negotiation (R-COMPUTE) . . . . . . . . . . . . 30
A.11. RTP Validity Check (R-RTP-VALID) . . . . . . . . . . . . . 31
A.12. 3rd Party Certificates (R-CERTS, R-EXISTING) . . . . . . . 31
A.13. FIPS 140-2 (R-FIPS) . . . . . . . . . . . . . . . . . . . 31
A.14. Linkage between Keying Exchange and SIP Signaling
(R-ASSOC) . . . . . . . . . . . . . . . . . . . . . . . . 31
A.15. Denial of Service Vulnerability (R-DOS) . . . . . . . . . 31
A.16. Crypto-Agility (R-AGILITY) . . . . . . . . . . . . . . . . 31
A.17. Downgrading Protection (R-DOWNGRADE) . . . . . . . . . . . 31
A.18. Media Security Negotation (R-NEGOTIATE) . . . . . . . . . 32
A.19. Signaling Protocol Independence (R-OTHER-SIGNALING) . . . 32
A.20. Media Recording (R-RECORDING) . . . . . . . . . . . . . . 32
A.21. Interworking with Intermediaries (R-TRANSCODER) . . . . . 32
A.22. PSTN Gateway Termination (R-PSTN) . . . . . . . . . . . . 32
A.23. R-ALLOW-RTP . . . . . . . . . . . . . . . . . . . . . . . 32
A.24. R-HERFP . . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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1. Introduction
The Session Initiation Protocol (SIP) [RFC3261] and the Session
Description Protocol (SDP) [RFC4566] are used to set up multimedia
sessions or calls. SDP is also used to set up TCP [RFC4145] and
additionally TCP/TLS connections for usage with media sessions
[RFC4572]. The Real-time Transport Protocol (RTP) [RFC3550] is used
to transmit real time media on top of UDP and TCP [RFC4571].
Datagram TLS [RFC4347] was introduced to allow TLS functionality to
be applied to datagram transport protocols, such as UDP and DCCP.
This draft provides guidelines on how to establish SRTP [RFC3711]
security over UDP using an extension to DTLS (see
[I-D.ietf-avt-dtls-srtp]).
The goal of this work is to provide a key negotiation technique that
allows encrypted communication between devices with no prior
relationships. It also does not require the devices to trust every
call signaling element that was involved in routing or session setup.
This approach does not require any extra effort by end users and does
not require deployment of certificates that are signed by a well-
known certificate authority to all devices.
The media is transported over a mutually authenticated DTLS session
where both sides have certificates. It is very important to note
that certificates are being used purely as a carrier for the public
keys of the peers. This is required because DTLS does not have a
mode for carrying bare keys, but it is purely an issue of formatting.
The certificates can be self-signed and completely self-generated.
All major TLS stacks have the capability to generate such
certificates on demand. However, third party certificates MAY also
be used for extra security. The certificate fingerprints are sent in
SDP over SIP as part of the offer/answer exchange.
The fingerprint mechanism allows one side of the connection to verify
that the certificate presented in the DTLS handshake matches the
certificate used by the party in the signalling. However, this
requires some form of integrity protection on the signalling. S/MIME
signatures, as described in RFC 3261, or SIP Identity, as described
in [RFC4474] provides the highest level of security because they are
not susceptible to modification by malicious intermediaries.
However, even hop-by-hop security such as provided by SIPS provides
some protection against modification by attackers who are not in
control of on-path sigaling elements.
This approach differs from previous attempts to secure media traffic
where the authentication and key exchange protocol (e.g., MIKEY
[RFC3830]) is piggybacked in the signaling message exchange. With
DTLS-SRTP, establishing the protection of the media traffic between
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the endpoints is done by the media endpoints without involving the
SIP/SDP communication. It allows RTP and SIP to be used in the usual
manner when there is no encrypted media.
In SIP, typically the caller sends an offer and the callee may
subsequently send one-way media back to the caller before a SIP
answer is received by the caller. The approach in this
specification, where the media key negotiation is decoupled from the
SIP signaling, allows the early media to be set up before the SIP
answer is received while preserving the important security property
of allowing the media sender to choose some of the keying material
for the media. This also allows the media sessions to be changed,
re-keyed, and otherwise modified after the initial SIP signaling
without any additional SIP signaling.
Design decisions that influence the applicability of this
specification are discussed in Section 3.
2. Overview
Endpoints wishing to set up an RTP media session do so by exchanging
offers and answers in SDP messages over SIP. In a typical use case,
two endpoints would negotiate to transmit audio data over RTP using
the UDP protocol.
Figure 1 shows a typical message exchange in the SIP Trapezoid.
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+-----------+ +-----------+
|SIP | SIP/SDP |SIP |
+------>|Proxy |----------->|Proxy |-------+
| |Server X | (+finger- |Server Y | |
| +-----------+ print, +-----------+ |
| +auth.id.) |
| SIP/SDP SIP/SDP |
| (+fingerprint) (+fingerprint,|
| +auth.id.) |
| |
| v
+-----------+ Datagram TLS +-----------+
|SIP | <-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-> |SIP |
|User Agent | Media |User Agent |
|Alice@X | <=================================> |Bob@Y |
+-----------+ +-----------+
Legend:
------>: Signaling Traffic
<-+-+->: Key Management Traffic
<=====>: Data Traffic
Figure 1: DTLS Usage in the SIP Trapezoid
Consider Alice wanting to set up an encrypted audio session with Bob.
Both Bob and Alice could use public-key based authentication in order
to establish a confidentiality protected channel using DTLS.
Since providing mutual authentication between two arbitrary end
points on the Internet using public key based cryptography tends to
be problematic, we consider more deployment-friendly alternatives.
This document uses one approach and several others are discussed in
Section 8.
Alice sends an SDP offer to Bob over SIP. If Alice uses only self-
signed certificates for the communication with Bob, a fingerprint is
included in the SDP offer/answer exchange. This fingerprint binds
the DTLS key exchange in the media plane to the signaling plane.
The fingerprint alone protects against active attacks on the media
but not active attacks on the signalling. In order to prevent active
attacks on the signalling, Enhancements for Authenticated Identity
Management in SIP [RFC4474] may be is used. When Bob receives the
offer, the peers establish some number of DTLS connections (depending
on the number of media sessions) with mutual DTLS authentication
(i.e., both sides provide certificates) At this point, Bob can verify
that Alice's credentials offered in TLS match the fingerprint in the
SDP offer, and Bob can begin sending media to Alice. Once Bob
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accepts Alice's offer and sends an SDP answer to Alice, Alice can
begin sending confidential media to Bob over the appropriate streams.
Alice and Bob will verify the fingerprints from the certificates
received over the DTLS handshakes match with the fingerprints
received in the SDP of the SIP signaling. This provides the security
property that Alice knows that the media traffic is going to Bob and
vice-versa without necessarily requiring global PKI certificates for
Alice and Bob. (see Section 8 for detailed security analysis.)
3. Motivation
Although there is already prior work in this area (e.g., Security
Descriptions for SDP [RFC4568], Key Management Extensions [RFC4567]
combined with MIKEY [RFC3830] for authentication and key exchange),
this specification is motivated as follows:
o TLS will be used to offer security for connection-oriented media.
The design of TLS is well-known and implementations are widely
available.
o This approach deals with forking and early media without requiring
support for PRACK [RFC3262] while preserving the important
security property of allowing the offerer to choose keying
material for encrypting the media.
o The establishment of security protection for the media path is
also provided along the media path and not over the signaling
path. In many deployment scenarios, the signaling and media
traffic travel along a different path through the network.
o When RFC 4474 Identity is used, this solution works even when the
SIP proxies downstream of the authentication service are not
trusted. There is no need to reveal keys in the SIP signaling or
in the SDP message exchange. Retargeting of a dialog-forming
request (changing the value of the Request-URI), the UA that
receives it (the User Agent Server, UAS) can have a different
identity from that in the To header field. When RFC 4916 is used
then it is possible to supply its identity to the peer UA by means
of a request in the reverse direction, and for that identity to be
signed by an Authentication Service.
o In this method, SSRC collisions do not result in any extra SIP
signaling.
o Many SIP endpoints already implement TLS. The changes to existing
SIP and RTP usage are minimal even when DTLS-SRTP
[I-D.ietf-avt-dtls-srtp] is used.
4. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
DTLS/TLS uses the term "session" to refer to a long-lived set of
keying material that spans associations. In this document,
consistent with SIP/SDP usage, we use it to refer to a multimedia
session and use the term "TLS session" to refer to the TLS construct.
We use the term "association" to refer to a particular DTLS
ciphersuite and keying material set which is associated with a single
host/port quartet. The same DTLS/TLS session can be used to
establish the keying material for multiple associations. For
consistency with other SIP/SDP usage, we use the term "connection"
when what's being referred to is a multimedia stream that is not
specifically DTLS/TLS.
In this document, the term "Mutual DTLS" indicates that both the DTLS
client and server present certificates even if one or both
certificates are self-signed.
5. Establishing a Secure Channel
The two endpoints in the exchange present their identities as part of
the DTLS handshake procedure using certificates. This document uses
certificates in the same style as described in Comedia over TLS in
SDP [RFC4572].
If self-signed certificates are used, the content of the
subjectAltName attribute inside the certificate MAY use the uniform
resource identifier (URI) of the user. This is useful for debugging
purposes only and is not required to bind the certificate to one of
the communication endpoints. The integrity of the certificate is
ensured through the fingerprint attribute in the SDP. The
subjectAltName is not an important component of the certificate
verification.
The generation of public/private key pairs is relatively expensive.
Endpoints are not required to generate certificates for each session.
The offer/answer model, defined in [RFC3264], is used by protocols
like the Session Initiation Protocol (SIP) [RFC3261] to set up
multimedia sessions. In addition to the usual contents of an SDP
[RFC4566] message, each media description ('m' line and associated
parameters) will also contain several attributes as specified in
[I-D.ietf-avt-dtls-srtp], [RFC4145] and [RFC4572].
When an endpoint wishes to set up a secure media session with another
endpoint it sends an offer in a SIP message to the other endpoint.
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This offer includes, as part of the SDP payload, the fingerprint of
the certificate that the endpoint wants to use. The endpoint SHOULD
send the SIP message containing the offer to the offerer's sip proxy
over an integrity protected channel. The proxy SHOULD add an
Identity header field according to the procedures outlined in
[RFC4474]. The SIP message containing the offer SHOULD be sent to
the offerer's sip proxy over an integrity protected channel. When
the far endpoint receives the SIP message it can verify the identity
of the sender using the Identity header field. Since the Identity
header field is a digital signature across several SIP header fields,
in addition to the body of the SIP message, the receiver can also be
certain that the message has not been tampered with after the digital
signature was applied and added to the SIP message.
The far endpoint (answerer) may now establish a DTLS association with
DTLS to the offerer. Alternately, it can indicate in its answer that
the offerer is to initiate the TLS association. In either case,
mutual DTLS certificate-based authentication will be used. After
completing the DTLS handshake, information about the authenticated
identities, including the certificates, are made available to the
endpoint application. The answerer is then able to verify that the
offerer's certificate used for authentication in the DTLS handshake
can be associated to the certificate fingerprint contained in the
offer in the SDP. At this point the answerer may indicate to the end
user that the media is secured. The offerer may only tentatively
accept the answerer's certificate since it may not yet have the
answerer's certificate fingerprint.
When the answerer accepts the offer, it provides an answer back to
the offerer containing the answerer's certificate fingerprint. At
this point the offerer can accept or reject the peer's certificate
and the offerer can indicate to the end user that the media is
secured.
Note that the entire authentication and key exchange for securing the
media traffic is handled in the media path through DTLS. The
signaling path is only used to verify the peers' certificate
fingerprints.
The offer and answer MUST be conform to the following requirements.
o The endpoint MUST use the setup attribute defined in [RFC4145].
The endpoint which is the offerer MUST use the setup attribute
value of setup:actpass and be prepared to receive a client_hello
before it receives the answer. The answerer MUST use either a
setup attribute value of setup:active or setup:passive. Note that
if the answerer uses setup:passive, then the DTLS handshake will
not begin until the answerer is received, which adds additional
latency. setup:active allows the answer and the DTLS handshake to
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occur in parallel. Thus, setup:active is RECOMMENDED. Whichever
party is active MUST initiate a DTLS handshake by sending a
ClientHello over each flow (host/port quartet).
o The endpoint MUST NOT use the connection attribute defined in
[RFC4145].
o The endpoint MUST use the certificate fingerprint attribute as
specified in [RFC4572].
o The certificate presented during the DTLS handshake MUST match the
fingerprint exchanged via the signaling path in the SDP. The
security properties of this mechanism are described in Section 8.
o If the fingerprint does not match the hashed certificate then the
endpoint MUST tear down the media session immediately. Note that
it is permissible to wait until the other side's fingerprint has
been received before establishing the connection, however this may
have undesirable latency effects.
6. Miscellaneous Considerations
6.1. Anonymous Calls
The use of DTLS-SRTP does not provide anonymous calling, however it
also does not prevent it. However, if care is not taken when
anonymous calling features such as those described in [RFC3325] or
[I-D.ietf-sip-ua-privacy] are used DTLS-SRTP may allow deanonymizing
an otherwise anonymous call. When anonymous calls are being made,
the following procedures SHOULD be used to prevent deanonymization.
When making anonymous calls, a new self-signed certificate SHOULD be
used for each call so that the calls can not be correlated as to
being from the same caller. In situations where some degree of
correlation is acceptable, the same certificate SHOULD be used for a
number of calls in order to enable continuity of authentication, see
Section 8.4.
Additionally note that in networks that deploy [RFC3325], RFC 3325
requires that the Privacy header field value defined in [RFC3323]
needs to be set to 'id'. This is used in conjunction with the SIP
identity mechanism to ensure that the identity of the user is not
asserted when enabling anonymous calls. Furthermore, the content of
the subjectAltName attribute inside the certificate MUST NOT contain
information that either allows correlation or identification of the
user that wishes to place an anonymous call. Note that following
this recommendation is not sufficient to provide anonymization.
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6.2. Early Media
If an offer is received by an endpoint that wishes to provide early
media, it MUST take the setup:active role and can immediately
establish a DTLS association with the other endpoint and begin
sending media. The setup:passive endpoint may not yet have validated
the fingerprint of the active endpoint's certificate. The security
aspects of media handling in this situation are discussed in
Section 8.
6.3. Forking
In SIP, it is possible for a request to fork to multiple endpoints.
Each forked request can result in a different answer. Assuming that
the requester provided an offer, each of the answerers' will provide
a unique answer. Each answerer will form a DTLS association with the
offerer. The offerer can then securely correlate the SDP answer
received in the SIP message by comparing the fingerprint in the
answer to the hashed certificate for each DTLS association.
6.4. Delayed Offer Calls
An endpoint may send a SIP INVITE request with no offer in it. When
this occurs, the receiver(s) of the INVITE will provide the offer in
the response and the originator will provide the answer in the
subsequent ACK request or in the PRACK request [RFC3262] if both
endpoints support reliable provisional responses. In any event, the
active endpoint still establishes the DTLS association with the
passive endpoint as negotiated in the offer/answer exchange.
6.5. Multiple Associations
When there are multiple flows (e.g., multiple media streams, non-
multiplexed RTP and RTCP, etc.) the active side MAY perform the DTLS
handshakes in any order. Appendix B of [I-D.ietf-avt-dtls-srtp]
provides some guidance on the performance of parallel DTLS
handshakes. Note that if the answerer ends up being active, it may
only initiate handshakes on some subset of the potential streams
(e.g., if audio and video are offered but it only wishes to do
audio.) If the offerer ands up being active, the complete answer
will be received before the offerer begins initiating handshakes.
6.6. Session Modification
Once an answer is provided to the offerer, either endpoint MAY
request a session modification which MAY include an updated offer.
This session modification can be carried in either an INVITE or
UPDATE request. The peers can reuse the the existing associations if
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they are compatible (i.e., they have the same key fingerprints and
transport parameters), or establish a new one following the same
rules are for initial exchanges, tearing down the existing
association as soon as the offer/answer exchange is completed. Note
that if the active/passive status of the endpoints changes, a new
connection MUST be established.
6.7. Middlebox Interaction
There are a number of potentially bad interactions between DTLS-SRTP
and middleboxes, as documented in
[I-D.ietf-mmusic-media-path-middleboxes], which also provides
recommendations for avoiding such problems.
6.7.1. ICE Interaction
Interactive Connectivity Establishment (ICE), as specified in
[I-D.ietf-mmusic-ice], provides a methodology of allowing
participants in multi-media sessions to verify mutual connectivity.
When ICE is being used the ICE connectivity checks are performed
before the DTLS handshake begins. Note that if aggressive nomination
mode is used, multiple candidate pairs may be marked valid before ICE
finally converges on a single candidate pair. Implementations MUST
treat all ICE candidate pairs associated with a single component as
part of the same DTLS association. Thus, there will be only one DTLS
handshake even if there are multiple valid candidate pairs. Note
that this may mean adjusting the endpoint IP addresses if the
selected candidate pair shifts, just as if the DTLS packets were an
ordinary media stream.
Note that STUN packets are sent directly over UDP, not over DTLS.
[I-D.ietf-avt-dtls-srtp] describes how to demultiplex STUN packets
from DTLS packets and SRTP packets.
6.7.2. Latching Control Without ICE
If ICE is not being used, then there is potential for a bad
interaction with SBCs via "latching", as described in
[I-D.ietf-mmusic-media-path-middleboxes]. In order to avoid this
issue, if ICE is not being used and the DTLS handshake has not
completed, upon receiving the other side's SDP then the passive side
MUST do a single unauthenticated STUN [I-D.ietf-behave-rfc3489bis]
connectivity check in order to open up the appropriate pinhole. All
implementations MUST be prepared to answer this request during the
handshake period even if they do not otherwise do ICE. However, the
active side MUST proceed with the DTLS handshake as appopriate even
if no such STUN check is received and the passive MUST NOT wait for a
STUN answer before sending its ServerHello.
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6.8. Rekeying
As with TLS, DTLS endpoints can rekey at any time by redoing the DTLS
handshake. While the rekey is under way, the endpoints continue to
use the previously established keying material for usage with DTLS.
Once the new session keys are established the session can switch to
using these and abandon the old keys. This ensures that latency is
not introduced during the rekeying process.
Further considerations regarding rekeying in case the SRTP security
context is established with DTLS can be found in Section 3.7 of
[I-D.ietf-avt-dtls-srtp].
6.9. Conference Servers and Shared Encryptions Contexts
It has been proposed that conference servers might use the same
encryption context for all of the participants in a conference. The
advantage of this approach is that the conference server only needs
to encrypt the output for all speakers instead of once per
participant.
This shared encryption context approach is not possible under this
specification because each DTLS handshake establishes fresh keys
which are not completely under the control of either side. However,
it is argued that the effort to encrypt each RTP packet is small
compared to the other tasks performed by the conference server such
as the codec processing.
Future extensions such as [I-D.mcgrew-srtp-ekt] or
[I-D.wing-avt-dtls-srtp-key-transport] could be used to provide this
functionality in concert with the mechanisms described in this
specification.
6.10. Media over SRTP
Because DTLS's data transfer protocol is generic, it is less highly
optimized for use with RTP than is SRTP [RFC3711], which has been
specifically tuned for that purpose. DTLS-SRTP
[I-D.ietf-avt-dtls-srtp], has been defined to provide for the
negotiation of SRTP transport using a DTLS connection, thus allowing
the performance benefits of SRTP with the easy key management of
DTLS. The ability to reuse existing SRTP software and hardware
implementations may in some environments provide another important
motivation for using DTLS-SRTP instead of RTP over DTLS.
Implementations of this specification MUST support DTLS-SRTP
[I-D.ietf-avt-dtls-srtp].
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6.11. Best Effort Encryption
[I-D.ietf-sip-media-security-requirements] describes a requirement
for best effort encryption where SRTP is used where both endpoints
support it and key negotiation succeeds, otherwise RTP is used.
[I-D.ietf-mmusic-sdp-capability-negotiation] describes a mechanism
which can signal both RTP and SRTP as an alternative. This allows an
offerer to express a preference for SRTP, but RTP is the default and
will be understood by endpoints that do not understand SRTP or this
key exchange mechanism. Implementations of this document MUST
support [I-D.ietf-mmusic-sdp-capability-negotiation].
7. Example Message Flow
Prior to establishing the session, both Alice and Bob generate self-
signed certificates which are used for a single session or, more
likely, reused for multiple sessions. In this example, Alice calls
Bob. In this example we assume that Alice and Bob share the same
proxy.
The example shows the SIP message flows where Alice acts as the
passive endpoint and Bob acts as the active endpoint meaning that as
soon as Bob receives the INVITE from Alice, with DTLS specified in
the 'm' line of the offer, Bob will begin to negotiate a DTLS
association with Alice for both RTP and RTCP streams. Early media
(RTP and RTCP) starts to flow from Bob to Alice as soon as Bob sends
the DTLS finished message to Alice. Bi-directional media (RTP and
RTCP) can flow after Alice receives the SIP 200 response and once
Alice has sent the DTLS finished message.
The SIP signaling from Alice to her proxy is transported over TLS to
ensure an integrity protected channel between Alice and her identity
service. Transport between proxies should also be protected somehow,
especialy if Identity is not in use. Note that all other signaling
is transported over TCP in this example although it could be done
over any supported transport.
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Alice Proxies Bob
|(1) INVITE | |
|---------------->| |
| |(2) INVITE |
| |----------------->|
| |(3) hello |
|<-----------------------------------|
|(4) hello | |
|----------------------------------->|
| |(5) finished |
|<-----------------------------------|
| |(6) media |
|<-----------------------------------|
|(7) finished | |
|----------------------------------->|
| |(8) 200 OK |
|<-----------------------------------|
| |(9) media |
|----------------------------------->|
|(10) ACK | |
|----------------------------------->|
Message (1): INVITE Alice -> Proxy
This shows the initial INVITE from Alice to Bob carried over the
TLS transport protocol to ensure an integrity protected channel
between Alice and her proxy which acts as Alice's identity
service. Note that Alice has requested to be either the active or
passive endpoint by specifying a=setup:actpass. Bob chooses to
act as the DTLS client and will initiate the session. Also note
that there is a fingerprint attribute in the SDP. This is
computed from Alice's self-signed certificate.
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INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/TLS 192.168.1.101:5060;branch=z9hG4bK-0e53sadfkasldkfj
Max-Forwards: 70
Contact: <sip:alice@192.168.1.103:6937;transport=TLS>
To: <sip:bob@example.com>
From: "Alice"<sip:alice@example.com>;tag=843c7b0b
Call-ID: 6076913b1c39c212@REVMTEpG
CSeq: 1 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE
Content-Type: application/sdp
Content-Length: xxxx
v=0
o=- 1181923068 1181923196 IN IP4 192.168.1.103
s=example1
c=IN IP4 192.168.1.103
a=setup:actpass
a=fingerprint: \
SHA-1 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
t=0 0
m=audio 6056 RTP/AVP 0
a=sendrecv
a=tcap:1 UDP/TLS/RTP/SAVP RTP/AVP
a=pcfg:1 t=1
Message (2): INVITE Proxy -> Bob
This shows the INVITE being relayed to Bob from Alice (and Bob's)
proxy. Note that Alice's proxy has inserted an Identity and
Identity-Info header. This example only shows one element for
both proxies for the purposes of simplification. Bob verifies the
identity provided with the INVITE. Note that this offer includes
a default m-line offering RTP in case the answerer does not
support SRTP. However, the potential configuration utilizing a
transport of SRTP is preferred. See
[I-D.ietf-mmusic-sdp-capability-negotiation] for more details on
the details of SDP capability negotiation.
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INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/TLS 192.168.1.101:5060;branch=z9hG4bK-0e53sadfkasldkfj
Via: SIP/2.0/TCP 192.168.1.100:5060;branch=z9hG4bK-0e53244234324234
Via: SIP/2.0/TCP 192.168.1.103:6937;branch=z9hG4bK-0e5b7d3edb2add32
Max-Forwards: 70
Contact: <sip:alice@192.168.1.103:6937;transport=TLS>
To: <sip:bob@example.com>
From: "Alice"<sip:alice@example.com>;tag=843c7b0b
Call-ID: 6076913b1c39c212@REVMTEpG
CSeq: 1 INVITE
Identity: CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFC
HYWGCl0nB2sNsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=
Identity-Info: https://example.com/cert
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE
Content-Type: application/sdp
Content-Length: xxxx
v=0
o=- 1181923068 1181923196 IN IP4 192.168.1.103
s=example1
c=IN IP4 192.168.1.103
a=setup:actpass
a=fingerprint: \
SHA-1 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
t=0 0
m=audio 6056 RTP/AVP 0
a=sendrecv
a=tcap:1 UDP/TLS/RTP/SAVP RTP/AVP
a=pcfg:1 t=1
Message (3): ClientHello Bob -> Alice
Assuming that Alice's identity is valid, Line 3 shows Bob sending
a DTLS ClientHello(s) directly to Alice. In this case two DTLS
ClientHello messages would be sent to Alice: one to
192.168.1.103:6056 for RTP and another to port 6057 for RTCP, but
only one arrow is drawn for compactness of the figure.
Message (4): ServerHello+Certificate Alice -> Bob
Alice sends back a ServerHello, Certificate, ServerHelloDone for
both RTP and RTCP associations. Note that the same certificate is
used for both the RTP and RTCP associations. If RTP/RTCP
multiplexing [I-D.ietf-avt-rtp-and-rtcp-mux] were being used only
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a single association would be required.
Message (5): Certificate Bob -> Alice
Bob sends a Certificate, ClientKeyExchange, CertificateVerify,
change_cipher_spec and Finished for both RTP and RTCP
associations. Again note that Bob uses the same server
certificate for both associations.
Message (6): Early Media Bob -> Alice
At this point, Bob can begin sending early media (RTP and RTCP) to
Alice. Note that Alice can't yet trust the media since the
fingerprint has not yet been received. This lack of trusted,
secure media is indicated to Alice via the UA user interface.
Message (7): Finished Alice -> Bob
After Message 7 is received by Bob, Alice sends change_cipher_spec
and Finished.
Message (8): 200 OK Bob -> Alice
When Bob answers the call, Bob sends a 200 OK SIP message which
contains the fingerprint for Bob's certificate. When Alice
receives the message and validates the certificate presented in
Message 7. The endpoint now shows Alice that the call as secured.
Note that in this case, Bob signals the actual transport protocol
configuration of SRTP over DTLS in the acfg parameter.
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SIP/2.0 200 OK
To: <sip:bob@example.com>;tag=6418913922105372816
From: "Alice" <sip:alice@example.com>;tag=843c7b0b
Via: SIP/2.0/TCP 192.168.1.103:6937;branch=z9hG4bK-0e5b7d3edb2add32
Call-ID: 6076913b1c39c212@REVMTEpG
CSeq: 1 INVITE
Contact: <sip:192.168.1.104:5060;transport=TCP>
Content-Type: application/sdp
Content-Length: xxxx
v=0
o=- 6418913922105372816 2105372818 IN IP4 192.168.1.104
s=example2
c=IN IP4 192.168.1.104
a=setup:active
a=fingerprint:\
SHA-1 FF:FF:FF:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
t=0 0
m=audio 12000 UDP/TLS/RTP/SAVP 0
a=acfg:1 t=1
Message (9): RTP+RTCP Alice -> Bob
At this point, Alice can also start sending RTP and RTCP to Bob.
Message (10): ACK Alice -> Bob
Finally, Alice sends the SIP ACK to Bob.
In this example, the DTLS handshake has already completed by the time
Alice receives Bob's 200 OK (8). Therefore, no STUN check is sent.
However, if Alice had a NAT, then Bob's ClientHello might get blocked
by that NAT, in which case Alice would send the the STUN check
described in Section 6.7.1 upon receiving the 200 OK, as shown below:
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Alice Proxies Bob
|(1) INVITE | |
|---------------->| |
| |(2) INVITE |
| |----------------->|
| |(3) hello |
| X<-----------------|
| |(4) 200 OK |
|<-----------------------------------|
| (5) conn-check | |
|----------------------------------->|
| |(6) conn-response |
|<-----------------------------------|
| |(7) hello |
|<-----------------------------------|
|(8) hello (rtx) | |
|----------------------------------->|
| |(9) finished |
|<-----------------------------------|
| |(10) media |
|<-----------------------------------|
|(11) finished | |
|----------------------------------->|
| |(11) media |
|----------------------------------->|
|(12) ACK | |
|----------------------------------->|
The messages here are the same as in the previous example, with the
following three new messages:
Message (5): STUN connectivity-check Alice -> Bob
Section 6.7.1 describes an approach to avoid an SBC interaction
issue where the endpoints do not support ICE. Alice (the passive
endpoint) sends a STUN connectivity check to Bob. This opens a
pinhole in Alice's NAT/firewall.
Message (6): STUN connectivity-check response Bob -> Alice
Bob (the active endpoint) sends a response to the STUN
connectivity check (Message 3) to Alice. This tells Alice that
her connectivity check has succeeded and she can stop the
retransmit state machine.
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Message (7): Hello (retransmit) Bob -> Alice
Bob retransmits his DTLS ClientHello which now passes through the
pinhole created in Alice's firewall. At this point, the DTLS
handshake proceeds as before.
8. Security Considerations
DTLS or TLS media signalled with SIP requires a way to ensure that
the communicating peers' certificates are correct.
The standard TLS/DTLS strategy for authenticating the communicating
parties is to give the server (and optionally the client) a PKIX
[RFC3280] certificate. The client then verifies the certificate and
checks that the name in the certificate matches the server's domain
name. This works because there are a relatively small number of
servers with well-defined names; a situation which does not usually
occur in the VoIP context.
The design described in this document is intended to leverage the
authenticity of the signaling channel (while not requiring
confidentiality). As long each side of the connection can verify the
integrity of the SDP received from the other side, then the DTLS
handshake cannot be hijacked via a man-in-the-middle attack. This
integrity protection is easily provided by the caller to the callee
(see Alice to Bob in Section 7) via the SIP Identity [RFC4474]
mechanism. Other mechanisms, such as the S/MIME mechanism described
in RFC 3261, or perhaps future mechanisms yet to be defined could
also serve this purpose.
While this mechanism can still be used without such integrity
mechanisms, the security provided is limited to defense against
passive attack by intermediaries. An active attack on the signaling
plus an active attack on the media plane can allow an attacker to
attack the connection (R-SIG-MEDIA in the notation of
[I-D.ietf-sip-media-security-requirements]).
8.1. Responder Identity
SIP Identity does not support signatures in responses. Ideally Alice
would want to know that Bob's SDP had not been tampered with and who
it was from so that Alice's User Agent could indicate to Alice that
there was a secure phone call to Bob. [RFC4916] defines an approach
for a UA to supply its identity to its peer UA and for this identity
to be signed by an authentication service. For example, using this
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approach, Bob sends an answer, then immediately follows up with an
UPDATE that includes the fingerprint and uses the SIP Identity
mechanism to assert that the message is from Bob@example.com. The
downside of this approach is that it requires the extra round trip of
the UPDATE. However, it is simple and secure even when not all of
the proxies are trusted. In this example, Bob only needs to trust
his proxy. Answerers SHOULD use this UPDATE mechanism.
In some cases, answerers will not send an UPDATE and in many calls,
some media will be sent before the UPDATE is received. In these
cases, no integrity is provided for the fingerprint from Bob to
Alice. In this approach, an attacker that was on the signaling path
could tamper with the fingerprint and insert themselves as a man-in-
the-middle on the media. Alice would know that she had a secure call
with someone but would not know if it was with Bob or a man-in-the-
middle. Bob would know that an attack was happening. The fact that
one side can detect this attack means that in most cases where Alice
and Bob both wish the communications to be encrypted there is not a
problem. Keep in mind that in any of the possible approaches Bob
could always reveal the media that was received to anyone. We are
making the assumption that Bob also wants secure communications. In
this do nothing case, Bob knows the media has not been tampered with
or intercepted by a third party and that it is from
Alice@example.com. Alice knows that she is talking to someone and
that whoever that is has probably checked that the media is not being
intercepted or tampered with. This approach is certainly less than
ideal but very usable for many situations.
8.2. SIPS
If SIP Identity is not used, but the signaling is protected by SIPS,
the security guarantees are weaker. Some security is still provided
as long as all proxies are trusted. This provides integrity for the
fingerprint in a chain-of-trust security model. Note, however, that
if the proxies are not trusted, then the level of security provided
is limited.
8.3. S/MIME
RFC 3261 [RFC3261] defines a S/MIME security mechanism for SIP that
could be used to sign that the fingerprint was from Bob. This would
be secure.
8.4. Continuity of Authentication
One desirable property of a secure media system is to provide
continuity of authentication: being able to ensure cryptographically
that you are talking to the same person as before. With DTLS,
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continuity of authentication is achieved by having each side use the
same public key/self-signed certificate for each connection (at least
with a given peer entity). It then becomes possible to cache the
credential (or its hash) and verify that it is unchanged. Thus, once
a single secure connection has been established, an implementation
can establish a future secure channel even in the face of future
insecure signalling.
In order to enable continuity of authentication, implementations
SHOULD attempt to keep a constant long-term key. Verifying
implementations SHOULD maintain a cache of the key used for each peer
identity and alert the user if that key changes.
8.5. Short Authentication String
An alternative available to Alice and Bob is to use human speech to
verify each others' identity and then to verify each others'
fingerprints also using human speech. Assuming that it is difficult
to impersonate another's speech and seamlessly modify the audio
contents of a call, this approach is relatively safe. It would not
be effective if other forms of communication were being used such as
video or instant messaging. DTLS supports this mode of operation.
The minimal secure fingerprint length is around 64 bits.
ZRTP [I-D.zimmermann-avt-zrtp] includes Short Authentication String
mode in which a unique per-connection bitstring is generated as part
of the cryptographic handshake. The SAS can be as short as 25 bits
and so is somewhat easier to read. DTLS does not natively support
this mode. Based on the level of deployment interest a TLS extension
[RFC3546] could provide support for it. Note that SAS schemes only
work well when the endpoints recognize each other's voices, which is
not true in many settings (e.g., call centers).
8.6. Limits of Identity Assertions
When RFC 4474 is used to bind the media keying material to the SIP
signalling, the assurances about the provenance and security of the
media are only as good as those for the signalling. There are two
important cases to note here:
o RFC 4474 assumes that the proxy with the certificate "example.com"
controls the namespace "example.com". Therefore the RFC 4474
authentication service which is authoritative for a given
namespace can control which user is assigned each name. Thus, the
authentication service can take an address formerly assigned to
Alice and transfer it to Bob. This is an intentional design
feature of RFC 4474 and a direct consequence of the SIP namespace
architecture.
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o When phone number URIs (e.g.,
'sip:+17005551008@chicago.example.com' or
'sip:+17005551008@chicago.example.com;user=phone') are used, there
is no structural reason to trust that the domain name is
authoritative for a given phone number, although individual
proxies and UAs may have private arrangements that allow them to
trust other domains. This is a structural issue in that PSTN
elements are trusted to assert their phone number correctly and
that there is no real concept of a given entity being
authoritative for some number space.
In both of these cases, the assurances taht DTLS-SRTP provides in
terms of data origin integrity and confidentiality are necessarily no
better than SIP provides for signalling integrity when RFC 4474 is
used. Implementors should therefore take care not to indicate
misleading peer identity information in the user interface. e.g. If
the peer's identity is sip:+17005551008@chicago.example.com, it is
not sufficient to display that the identity of the peer as
+17005551008, unless there is some policy that states that the domain
"chicago.example.com" is trusted to assert the E.164 numbers it is
asserting. In cases where the UA can determine that the peer
identity is clearly an E.164 number, it may be less confusing to
simply identify the call as encrypted but to an unknown peer.
In addition, some middleboxes (B2BUAs and Session Border Controllers)
are known to modify portions of the SIP message which are included in
the RFC 4474 signature computation, thus breaking the signature.
This sort of man-in-the-middle operation is precisely the sort of
message modification that 4474 is intended to detect. In cases where
the middlebox is itself permitted to generate valid RFC 4474
signatures (e.g., it is within the same administrative domain as the
RFC 4474 authentication service), then it may generate a new
signature on the modified message. Alternately, the middlebox may be
able to sign with some other identity that it is permitted to assert.
Otherwise, the recipient cannot rely on the RFC 4474 Identity
assertion and the UA MUST NOT indicate to the user that a secure call
has been established to the claimed identity. Implementations which
are configured to only establish secure calls SHOULD terminate the
call in this case.
If SIP Identity or an equivalent mechanism is not used, then only
protection against attackers who cannot actively change the signaling
is provided. while this is still superior to previous mechanisms, the
security provided is inferior to that provided if integrity is
provided for the signaling.
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8.7. Perfect Forward Secrecy
One concern about the use of a long-term key is that compromise of
that key may lead to compromise of past communications. In order to
prevent this attack, DTLS supports modes with Perfect Forward Secrecy
using Diffie-Hellman and Elliptic-Curve Diffie-Hellman cipher suites.
When these modes are in use, the system is secure against such
attacks. Note that compromise of a long-term key may still lead to
future active attacks. If this is a concern, a backup authentication
channel such as manual fingerprint establishment or a short
authentication string should be used.
9. IANA Considerations
This specification does not require any IANA actions.
10. Acknowledgments
Cullen Jennings contributed substantial text and comments to this
document. This document benefited from discussions with Francois
Audet, Nagendra Modadugu, and Dan Wing. Thanks also for useful
comments by Flemming Andreasen, Jonathan Rosenberg, Rohan Mahy, David
McGrew, Miguel Garcia, Steffen Fries, Brian Stucker, Robert Gilman,
David Oran, and Peter Schneider.
We would like to thank Thomas Belling, Guenther Horn, Steffen Fries,
Brian Stucker, Francois Audet, Dan Wing, Jari Arkko, and Vesa
Lehtovirta for their input regarding traversal of SBCs.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
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[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-18 (work in progress),
July 2008.
11.2. Informational References
[RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
and RTP Control Protocol (RTCP) Packets over Connection-
Oriented Transport", RFC 4571, July 2006.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[I-D.ietf-mmusic-ice]
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Internet-Draft DTLS-SRTP Framework October 2008
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session
Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", RFC 4567, July 2006.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[I-D.zimmermann-avt-zrtp]
Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
Path Key Agreement for Secure RTP",
draft-zimmermann-avt-zrtp-09 (work in progress),
September 2008.
[I-D.mcgrew-srtp-ekt]
McGrew, D., "Encrypted Key Transport for Secure RTP",
draft-mcgrew-srtp-ekt-03 (work in progress), July 2007.
[I-D.ietf-avt-dtls-srtp]
McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for Secure
Real-time Transport Protocol (SRTP)",
draft-ietf-avt-dtls-srtp-05 (work in progress),
September 2008.
[I-D.ietf-sip-media-security-requirements]
Wing, D., Fries, S., Tschofenig, H., and F. Audet,
"Requirements and Analysis of Media Security Management
Protocols", draft-ietf-sip-media-security-requirements-07
(work in progress), June 2008.
[I-D.ietf-mmusic-sdp-capability-negotiation]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-09 (work in
progress), July 2008.
[I-D.ietf-avt-rtp-and-rtcp-mux]
Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port",
draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress),
August 2007.
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[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[I-D.wing-sipping-srtp-key]
Wing, D., Audet, F., Fries, S., Tschofenig, H., and A.
Johnston, "Secure Media Recording and Transcoding with the
Session Initiation Protocol",
draft-wing-sipping-srtp-key-03 (work in progress),
February 2008.
[I-D.wing-avt-dtls-srtp-key-transport]
Wing, D., "DTLS-SRTP Key Transport",
draft-wing-avt-dtls-srtp-key-transport-02 (work in
progress), July 2008.
[I-D.ietf-mmusic-media-path-middleboxes]
Stucker, B. and H. Tschofenig, "Analysis of Middlebox
Interactions for Signaling Protocol Communication along
the Media Path",
draft-ietf-mmusic-media-path-middleboxes-01 (work in
progress), July 2008.
[I-D.ietf-sip-ua-privacy]
Munakata, M., Schubert, S., and T. Ohba, "UA-Driven
Privacy Mechanism for SIP", draft-ietf-sip-ua-privacy-02
(work in progress), July 2008.
Appendix A. Requirements Analysis
[I-D.ietf-sip-media-security-requirements] describes security
requirements for media keying. This section evaluates this proposal
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with respect to each requirement.
A.1. Forking and retargeting (R-FORK-RETARGET, R-BEST-SECURE,
R-DISTINCT)
In this draft, the SDP offer (in the INVITE) is simply an
advertisement of the capability to do security. This advertisement
does not depend on the identity of the communicating peer, so forking
and retargeting work work when all the endpoints will do SRTP. When
a mix of SRTP and non-SRTP endpoints are present, we use the SDP
capabilities mechanism currently being defined
[I-D.ietf-mmusic-sdp-capability-negotiation] to transparently
negotiate security where possible. Because DTLS establishes a new
key for each session, only the entity with which the call is finally
established gets the media encryption keys (R3).
A.2. Distinct Cryptographic Contexts (R-DISTINCT)
DTLS performs a new DTLS handshake with each endpoint, which
establishes distinct keys and cryptographic contexts for each
endpoint.
A.3. Reusage of a Security Context (R-REUSE)
DTLS allows sessions to be resumed with the 'TLS session resumption'
functionality. This feature can be used to lower the amount of
cryptographic computation that needs to be done when two peers re-
initiates the communication. See [I-D.ietf-avt-dtls-srtp] for more
on session resumption in this context.
A.4. Clipping (R-AVOID-CLIPPING)
Because the key establishment occurs in the media plane, media need
not be clipped before the receipt of the SDP answer. Note, however,
that only confidentiality is provided until the offerer receives the
answer: the answerer knows that they are not sending data to an
attacker but the offerer cannot know that they are receiving data
from the answerer.
A.5. Passive Attacks on the Media Path (R-PASS-MEDIA)
The public key algorithms used by DTLS ciphersuites, such as RSA,
Diffie-Hellman, and Elliptic Curve Diffie-Hellman, are secure against
passive attacks.
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A.6. Passive Attacks on the Signaling Path (R-PASS-SIG)
DTLS provides protection against passive attacks by adversaries on
the signaling path since only a fingerprint is exchanged using SIP
signaling.
A.7. (R-SIG-MEDIA, R-ACT-ACT)
An attacker who controls the media channel but not the signalling
channel can perform a MITM attack on the DTLS handshake but this will
change the certificates which will cause the fingerprint check to
fail. Thus, any successful attack requires that the attacker modify
the signalling messages to replace the fingerprints.
If RFC 4474 Identity or an equivalent mechanism is used, a attacker
who controls the signalling channel at any point between the proxies
performing the Identity signatures cannot modify the fingerprints
without invalidating the signature. Thus, even an attacker who
controls both signalling and media paths cannot successfully attack
the media traffic. Note that the channel between the UA and the
authentication service MUST be secured and the authentication service
MUST verify the UA's identity in order for this mechanism to be
secure.
Note that an attacker who controls the authentication service can
impersonate the UA using that authentication service. This is an
intended feature of SIP Identity--the authentication service owns the
namespace and therefore defines which user has which identity.
A.8. Binding to Identifiers (R-ID-BINDING)
When an end-to-end mechanism such as SIP-Identity [RFC4474] and SIP-
Connected-Identity [RFC4916] or S/MIME are used, they bind the
endpoint's certificate fingerprints to the From: address in the
signalling. The fingerprint is covered by the Identity signature.
When other mechanisms (e.g., SIPS) are used, then the binding is
correspondingly weaker.
A.9. Perfect Forward Secrecy (R-PFS)
DTLS supports Diffie-Hellman and Elliptic Curve Diffie-Hellman cipher
suites which provide PFS.
A.10. Algorithm Negotiation (R-COMPUTE)
DTLS negotiates cipher suites before performing significant
cryptographic computation and therefore supports algorithm
negotiation and multiple cipher suites without additional
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computational expense.
A.11. RTP Validity Check (R-RTP-VALID)
DTLS packets do not pass the RTP validity check. The first byte of a
DTLS packet is the content type and All current DTLS content types
have the first two bits set to zero, resulting in a version of 0,
thus failing the first validity check. DTLS packets can also be
distinguished from STUN packets. See [I-D.ietf-avt-dtls-srtp] for
details on demultiplexing.
A.12. 3rd Party Certificates (R-CERTS, R-EXISTING)
Third party certificates are not required because signalling (e.g.,
[RFC4474]) is used to authenticate the certificates used by DTLS.
However, if the parties share an authentication infrastructure that
is compatible with TLS (3rd party certificates or shared keys) it can
be used.
A.13. FIPS 140-2 (R-FIPS)
TLS implementations already may be FIPS 140-2 approved and the
algorithms used here are consistent with the approval of DTLS and
DTLS-SRTP.
A.14. Linkage between Keying Exchange and SIP Signaling (R-ASSOC)
The signaling exchange is linked to the key management exchange using
the fingerprints carried in SIP and the certificates are exchanged in
DTLS.
A.15. Denial of Service Vulnerability (R-DOS)
DTLS offers some degree of DoS protection as a built-in feature (see
Section 4.2.1 or RFC 4347).
A.16. Crypto-Agility (R-AGILITY)
DTLS allows ciphersuites to be negotiated and hence new algorithms
can be incrementally deployed. Work on replacing the fixed MD5/SHA-1
key derivation function is ongoing.
A.17. Downgrading Protection (R-DOWNGRADE)
DTLS provides protection against downgrading attacks since the
selection of the offered ciphersuites is confirmed in a later stage
of the handshake. This protection is efficient unless an adversary
is able to break a ciphersuite in real-time. RFC 4474 is able to
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prevent an active attacker on the signalling path from downgrading
the call from SRTP to RTP.
A.18. Media Security Negotation (R-NEGOTIATE)
DTLS allows a User Agent to negotiate media security parameters for
each individual session.
A.19. Signaling Protocol Independence (R-OTHER-SIGNALING)
The DTLS-SRTP framework does not rely on SIP; every protocol that is
capable of exchanging a fingerprint and the media description can be
secured.
A.20. Media Recording (R-RECORDING)
An extension, see [I-D.wing-sipping-srtp-key], has been specified to
support media recording that does not require intermediaries to act
as a MITM.
When media recording is done by intermediaries then they need to act
as a MITM.
A.21. Interworking with Intermediaries (R-TRANSCODER)
In order to interface with any intermediary that transcodes the
media, the transcoder must have access to the keying material and be
treated as an endpoint for the purposes of this document.
A.22. PSTN Gateway Termination (R-PSTN)
The DTLS-SRTP framework allows the media security to terminate at a
PSTN gateway. This does not provide end-to-end security, but is
consistent with the security goals of this framework because the
gateway is authorized to speak for the PSTN namespace.
A.23. R-ALLOW-RTP
DTLS-SRTP allows RTP media to be received by the calling party until
SRTP has been negotiated with the answerer, after which SRTP is
preferred over RTP.
A.24. R-HERFP
The Heterogeneous Error Response Forking Problem (HERFP) is not
applicable to DTLS-SRTP since the key exchange protocol will be
executed along the media path and hence error messages are
communicated along this path and proxies do not need to progress
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them.
Authors' Addresses
Jason Fischl
CounterPath Corporation
Suite 300, One Bentall Centre, 505 Burrard Street
Vancouver, BC V7X 1M3
Canada
Phone: +1 604 320-3340
Email: jason@counterpath.com
Hannes Tschofenig
Nokia Siemens Networks
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@nsn.com
URI: http://www.tschofenig.com
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@rtfm.com
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Fischl, et al. Expires April 4, 2009 [Page 34]
