SIP D. Wing, Ed.
Internet-Draft Cisco
Intended status: Informational S. Fries
Expires: May 21, 2008 Siemens AG
H. Tschofenig
Nokia Siemens Networks
F. Audet
Nortel
November 18, 2007
Requirements and Analysis of Media Security Management Protocols
draft-ietf-sip-media-security-requirements-01.txt
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Abstract
This documents describes requirements for a protocol to negotiate
security context for SIP-signaled SRTP media. In addition to the
natural security requirements, this negotiation protocol must
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interoperate well with SIP in certain ways. A number of proposals
have been published and a summary of these proposals is in the
appendix of this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Document Organization . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Attack Scenarios . . . . . . . . . . . . . . . . . . . . . . . 5
4. Call Scenarios . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Clipping Media Before Signaling Answer . . . . . . . . . . 7
4.2. Retargeting and Forking . . . . . . . . . . . . . . . . . 8
4.3. Shared Key Conferencing . . . . . . . . . . . . . . . . . 11
4.4. Recording . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5. PSTN gateway . . . . . . . . . . . . . . . . . . . . . . . 13
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Key Management Protocol Requirements . . . . . . . . . . . 14
5.2. Attack Scenario Requirements . . . . . . . . . . . . . . . 16
5.3. Requirements Outside of the Key Management Protocol . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Overview of Keying Mechanisms . . . . . . . . . . . . 22
A.1. Signaling Path Keying Techniques . . . . . . . . . . . . . 23
A.1.1. MIKEY-NULL . . . . . . . . . . . . . . . . . . . . . . 23
A.1.2. MIKEY-PSK . . . . . . . . . . . . . . . . . . . . . . 23
A.1.3. MIKEY-RSA . . . . . . . . . . . . . . . . . . . . . . 24
A.1.4. MIKEY-RSA-R . . . . . . . . . . . . . . . . . . . . . 24
A.1.5. MIKEY-DHSIGN . . . . . . . . . . . . . . . . . . . . . 24
A.1.6. MIKEY-DHHMAC . . . . . . . . . . . . . . . . . . . . . 24
A.1.7. MIKEY-ECIES and MIKEY-ECMQV (MIKEY-ECC) . . . . . . . 25
A.1.8. Security Descriptions with SIPS . . . . . . . . . . . 25
A.1.9. Security Descriptions with S/MIME . . . . . . . . . . 25
A.1.10. SDP-DH (expired) . . . . . . . . . . . . . . . . . . . 25
A.1.11. MIKEYv2 in SDP (expired) . . . . . . . . . . . . . . . 25
A.2. Media Path Keying Technique . . . . . . . . . . . . . . . 26
A.2.1. ZRTP . . . . . . . . . . . . . . . . . . . . . . . . . 26
A.3. Signaling and Media Path Keying Techniques . . . . . . . . 26
A.3.1. EKT . . . . . . . . . . . . . . . . . . . . . . . . . 26
A.3.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . 27
A.3.3. MIKEYv2 Inband (expired) . . . . . . . . . . . . . . . 27
Appendix B. Evaluation Criteria - SIP . . . . . . . . . . . . . . 27
B.1. Secure Retargeting and Secure Forking . . . . . . . . . . 27
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B.2. Clipping Media Before SDP Answer . . . . . . . . . . . . . 30
B.3. Centralized Keying . . . . . . . . . . . . . . . . . . . . 31
B.4. SSRC and ROC . . . . . . . . . . . . . . . . . . . . . . . 33
Appendix C. Evaluation Criteria - Security . . . . . . . . . . . 35
C.1. Public Key Infrastructure . . . . . . . . . . . . . . . . 35
C.2. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 37
C.3. Best Effort Encryption . . . . . . . . . . . . . . . . . . 39
C.4. Upgrading Algorithms . . . . . . . . . . . . . . . . . . . 40
Appendix D. Out-of-Scope . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42
Intellectual Property and Copyright Statements . . . . . . . . . . 44
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1. Introduction
The work on media security started when the Session Initiation
Protocol (SIP) was still in its infancy. With the increased SIP
deployment and the availability of new SIP extensions and related
protocols, the need for end-to-end security was re-evaluated. The
procedure of re-evaluating prior protocol work and design decisions
is not an uncommon strategy and, to some extent, considered necessary
protocol work to ensure that the developed protocols indeed meet the
previously envisioned needs for the users in the Internet.
This document summarizes media security requirements, i.e.,
requirements for mechanisms that negotiate security context such as
cryptographic keys and parameters for SRTP.
1.1. Document Organization
The organization of this document is as follows: Section 2 introduces
terminology, Section 3 describes various attack scenarios against the
signaling path and media path, Section 4 provides an overview about
possible call scenarios, Section 5 lists requirements for media
security. The main part of the document concludes with the security
considerations Section 6, IANA considerations Section 7 and an
acknowledgement section in Section 8. Appendix A lists and compares
available solution proposals. The following Appendix B compares the
different approaches regarding their suitability for the SIP
signaling scenarios described in Appendix A, while Appendix C
provides a comparison regarding security aspects. Appendix D lists
non-goals for this document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119], with the
important qualification that, unless otherwise stated, these terms
apply to the design of the media security key management protocol,
not its implementation or application.
Additionally, the following items are used in this document:
AOR (Address-of-Record): A SIP or SIPS URI that points to a domain
with a location service that can map the URI to another URI where
the user might be available. Typically, the location service is
populated through registrations. An AOR is frequently thought of
as the "public address" of the user.
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SSRC: The 32-bit value that defines the synchronization source, used
in RTP. These are generally unique, but collisions can occur.
two-time pad: The use of the same key and the same key index to
encrypt different data. For SRTP, a two-time pad occurs if two
senders are using the same key and the same RTP SSRC value.
PKI: Public Key Infrastructure (see [RFC3280])
Perfect Forward Secrecy (PFS): The property that disclosure of the
long-term secret keying material that is used to derive an agreed
ephemeral key does not compromise the secrecy of agreed keys from
earlier runs.
active adversary: An active adversary attempts to alter system
resources or affect their operation (see [RFC4949]).
passive adversary: A passive adversary attempts to learn or make use
of information from a system but does not affect resources of that
system (see [RFC4949]).
signaling path: The signaling path is the route taken by SIP
signaling messages transmitted between the calling and called user
agents. This can be either direct signaling between the calling
and called user agents or, more commonly involves the SIP proxy
servers that were involved in the call setup.
media path: The media path is the route taken by media packets
exchanged by the endpoints. In the simplest case, the endpoints
exchange media directly, and the "media path" is defined by a
quartet of IP addresses and TCP/UDP ports, along with an IP route.
In other cases, this path may include RTP relays, mixers,
transcoders, session border controllers, NATs, or media gateways.
3. Attack Scenarios
The discussion in this section refers to requirements R6, R7, R14,
R17, and R27.
This document classifies adversaries according to their access and
their capabilities. An adversary might have access to:
1. only the media path,
2. only the signaling path,
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3. both the media path and the signaling path.
An attacker that can solely be located along the signaling path, and
does not have access to media (item 2), is not considered in this
document.
There are two different types of adversaries, active and passive. An
active adversary may need to be active with regard to the key
exchange relevant information traveling along the media path or
traveling along the signaling path.
Based on their robustness against the adversary capabilities
described above, we can group security mechanisms using the following
labels, ordered from least secure at the top to most secure at the
bottom:
no-signaling-passive-media:
Access to only the media path is sufficient to reveal the content
of the media traffic. This is how unencrypted RTP functions.
passive-signaling-passive-media:
Passive attack on the signaling and passive attack on the media
path is necessary to reveal the content of the media traffic.
active-signaling-passive-media:
Active attack on the signaling path and passive attack on the
media path is necessary to reveal the content of the media
traffic.
active-signaling-active-media:
Active attack on both the signaling path and the media path is
necessary to reveal the content of the media traffic.
detect-attack:
Active attack on both signaling and media path is necessary to
reveal the content of the media traffic (active-signaling-active-
media), but the attack is detectable by the end points when
adversary tampers with the signaling and/or media messages.
For example, Security Descriptions [RFC4568], when protected by TLS
(as it is commonly implemented and deployed), belongs in the passive-
signaling-passive-media category since the adversary needs to learn
the Security Descriptions key by seeing the SIP signaling message at
a SIP proxy (assuming that the adversary is in control of the SIP
proxy). The media traffic can be decrypted using that learned key.
As another example, DTLS-SRTP falls into active-signaling-active-
media category when DTLS-SRTP is used with a public key based
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ciphersuite with self-signed certificates and without SIP-Identity
[RFC4474]. An adversary would have to modify the fingerprint that is
sent along the signaling path and subsequently to modify the
certificates carried in the DTLS handshake that travel along the
media path. If DTLS-SRTP is used with SIP-Identity [RFC4474] and
protects both the offer and the answer, it would belong to the
detect-attack category.
The above discussion of DTLS-SRTP demonstrates how a single security
protocol can be in different classes depending on the mode in which
it is operated. Other protocols can achieve similar effect by adding
functions outside of the on-the-wire key management protocol itself.
Although it may be appropriate to deploy lower-classed mechanisms in
some cases, the ultimate security requirement for a media security
negotiation protocol is that it have a mode of operation available in
which it is detect-attack, which provides protection against the
passive and active attacks and provides detection of such attacks.
That is, there must be a way to use the protocol so that an active
attack is required against both the signaling and media paths, and so
that such attacks are detectable by the endpoints.
4. Call Scenarios
The following subsections describe call scenarios that pose the most
challenge to the key management system for media data in cooperation
with SIP signaling.
4.1. Clipping Media Before Signaling Answer
The discussion in this section refers to requirement R5.
Per the SDP Offer/Answer Model [RFC3264],
"Once the offerer has sent the offer, it MUST be prepared to
receive media for any recvonly streams described by that offer.
It MUST be prepared to send and receive media for any sendrecv
streams in the offer, and send media for any sendonly streams in
the offer (of course, it cannot actually send until the peer
provides an answer with the needed address and port information)."
To meet this requirement with SRTP, the offerer needs to know the
SRTP key for arriving media. If either endpoint receives encrypted
media before it has access to the associated SRTP key, it cannot play
the media -- causing clipping.
For key exchange mechanisms that send the answerer's key in SDP, a
SIP provisional response [RFC3261], such as 183 (session progress),
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is useful. However, the 183 messages are not reliable unless both
the calling and called end point support PRACK [RFC3262], use TCP
across all SIP proxies, implement Security Preconditions [RFC5027],
or the both ends implement ICE [I-D.ietf-mmusic-ice] and the answerer
implements the reliable provisional response mechanism described in
ICE. Unfortunately, there is not wide deployment of any of these
techniques and there is industry reluctance to set requirements
regarding these techniques to avoid the problem described in this
section.
Note that the receipt of an SDP answer is not always sufficient to
allow media to be played to the offerer. Sometimes, the offerer must
send media in order to open up firewall holes or NAT bindings before
media can be received. In this case, even a solution that makes the
key available before the SDP answer arrives will not help.
Fixes to early media might make the requirements to become obsolete,
but at the time of writing no progress has been accomplished.
4.2. Retargeting and Forking
The discussion in this section relates to requirements R1, R2, and
R3.
In SIP, a request sent to a specific AOR but delivered to a different
AOR is called a "retarget". A typical scenario is a "call
forwarding" feature. In Figure 1 Alice sends an Invite in step 1
that is sent to Bob in step 2. Bob responds with a redirect (SIP
response code 3xx) pointing to Carol in step 3. This redirect
typically does not propagate back to Alice but only goes to a proxy
(i.e., the retargeting proxy) that sends the original Invite to Carol
in step 4.
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+-----+
|Alice|
+--+--+
|
| Invite (1)
V
+----+----+
| proxy |
++-+-----++
| ^ |
Invite (2) | | | Invite (4)
& redirect (3) | | |
V | V
++-++ ++----+
|Bob| |Carol|
+---+ +-----+
Figure 1: Retargeting
Using retargeting might lead to situations where the UAC does not
know where its request will be going. This might not immediately
seem like a serious problem; after all, when one places a telephone
call on the PSTN, one never really knows if it will be forwarded to a
different number, who will pick up the line when it rings, and so on.
However, when considering SIP mechanisms for authenticating the
called party, this function can also make it difficult to
differentiate an intermediary that is behaving legitimately from an
attacker. From this perspective, the main problems with retargeting
ares:
Not detectable by the caller: The originating user agent has no
means of anticipating that the condition will arise, nor any means
of determining that it has occurred until the call has already
been set up, i.e. the negative consequences have already been
realized.
Not preventable by the caller: There is no existing security
mechanism that might be employed by the originating user agent in
order to guarantee that the call will not be re-targeted.
The mechanism used by SIP for identifying the calling party is SIP
Identity [RFC3261]. However, due to the nature of retargeting SIP
Identity can only identify the calling party (that is, the party that
initiated the SIP request). Some key exchange mechanisms predate SIP
Identity and include their own identity mechanism. However, those
built-in identity mechanism also suffer from the SIP retargeting
problem. Going forward, Connected Identity [RFC4916] allows
identifying the called party.
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In SIP, 'forking' is the delivery of a request to multiple locations.
This happens when a single AOR is registered more than once. An
example of forking is when a user has a desk phone, PC client, and
mobile handset all registered with the same AOR.
+-----+
|Alice|
+--+--+
|
| Invite
V
+-----+-----+
| proxy |
++---------++
| |
Invite | | Invite
V V
+--+--+ +--+--+
|Bob-1| |Bob-2|
+-----+ +-----+
Figure 2: Forking
With forking, both Bob-1 and Bob-2 might send back SDP answers in SIP
responses. Alice will see those intermediate (18x) and final (200)
responses. It is useful for Alice to be able to associate the SIP
response with the incoming media stream. Although this association
can be done with ICE [I-D.ietf-mmusic-ice], and ICE is useful to make
this association with RTP, it is not desirable to require ICE to
accomplish this association.
Forking and retargeting are often used together. For example, a boss
and secretary might have both phones ring (forking) and rollover to
voice mail if neither phone is answered (retargeting).
To maintain security of the media traffic, only the end point that
answers the call should know the SRTP keys for the session. This is
only an issue when the key management is encrypted with a key
corresponding to the responder. It does not lead to problems with
DH-based approaches. For key exchange mechanisms that do not provide
secure forking or secure retargeting, one workaround is to re-key
immediately after forking or retargeting. However, because the
originator may not be aware that the call forked this mechanism
requires rekeying immediately after every session is established.
This doubles the number of messages processed by the network.
Retargeting securely introduces a more significant problem. With
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retargeting, the actual recipient of the request is not the original
recipient. This means that if the offerer encrypted material (such
as the session key or the SDP) using the original recipient's public
key (or a shared secret established previously), the actual recipient
will not be able to decrypt that material because the recipient won't
have the original recipient's private key. In some cases, this is
the intended behavior, i.e., you wanted to establish a secure
connection with a specific individual. In other cases, it is not
intended behavior (you want all voice media to be encrypted,
regardless of who answers).
Further compounding this problem is a particularity of SIP that when
forking is used, there is always only one final error response
delivered to the sender of the request: the forking proxy is
responsible for choosing which final response to choose in the event
where forking results in multiple final error responses being
received by the forking proxy. This means that if a request is
rejected, say with information that the keying information was
rejected and providing the far end's credentials, it is very possible
that the rejection will never reach the sender. This problem, called
the Heterogeneous Error Response Forking Problem (HERFP)
[I-D.mahy-sipping-herfp-fix], is difficult to solve in SIP. Because
we expect the HERFP to continue to be a problem in SIP for the
foreseeable future, a media security system should function even in
the presence of HERFP behavior.
4.3. Shared Key Conferencing
The consensus on the RTPSEC mailing list was to concentrate on
unicast, point-to-point sessions. Thus, there are no requirements
related to shared key conferencing. This section is retained for
informational purposes.
For efficient scaling, large audio and video conference bridges
operate most efficiently by encrypting the current speaker once and
distributing that stream to the conference attendees. Typically,
inactive participants receive the same streams -- they hear (or see)
the active speaker(s), and the active speakers receive distinct
streams that don't include themselves. In order to maintain
confidentiality of such conferences where listeners share a common
key, all listeners must rekeyed when a listener joins or leaves a
conference.
An important use case for mixers/translators is a conference bridge:
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+----+
A --- 1 --->| |
<-- 2 ----| M |
| I |
B --- 3 --->| X |
<-- 4 ----| E |
| R |
C --- 5 --->| |
<-- 6 ----| |
+----+
Figure 3: Centralized Keying
In the figure above, 1, 3, and 5 are RTP media contributions from
Alice, Bob, and Carol, and 2, 4, and 6 are the RTP flows to those
devices carrying the 'mixed' media.
Several scenarios are possible:
a. Multiple inbound sessions: 1, 3, and 5 are distinct RTP sessions,
b. Multiple outbound sessions: 2, 4, and 6 are distinct RTP
sessions,
c. Single inbound session: 1, 3, and 5 are just different sources
within the same RTP session,
d. Single outbound session: 2, 4, and 6 are different flows of the
same (multi-unicast) RTP session
If there are multiple inbound sessions and multiple outbound sessions
(scenarios a and b), then every keying mechanism behaves as if the
mixer were an end point and can set up a point-to-point secure
session between the participant and the mixer. This is the simplest
situation, but is computationally wasteful, since SRTP processing has
to be done independently for each participant. The use of multiple
inbound sessions (scenario a) doesn't waste computational resources,
though it does consume additional cryptographic context on the mixer
for each participant and has the advantage of non-repudiation of the
originator of the incoming stream.
To support a single outbound session (scenario d), the mixer has to
dictate its encryption key to the participants. Some keying
mechanisms allow the transmitter to determine its own key, and others
allow the offerer to determine the key for the offerer and answerer.
Depending on how the call is established, the offerer might be a
participant (such as a participant dialing into a conference bridge)
or the offerer might be the mixer (such as a conference bridge
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calling a participant). The use of offerless Invites may help some
keying mechanisms reverse the role of offerer/answerer. A
difficulty, however, is knowing a priori if the role should be
reversed for a particular call.
4.4. Recording
The discussion in this section relates to requirement R23.
Some business environments, such as stock brokers, banks, and catalog
call centers, require recording calls with customers. This is the
familiar "this call is being recorded for quality purposes" heard
during calls to these sorts of businesses. In these environments,
media recording is typically performed by an intermediate device
(with RTP, this is typically implemented in a 'sniffer').
When performing such call recording with SRTP, the end-to-end
security is compromised. This is unavoidable, but necessary because
the operation of the business requires such recording. It is
desirable that the media security is not unduly compromised by the
media recording. The endpoint within the organization needs to be
informed that there is an intermediate device and needs to cooperate
with that intermediate device.
This scenario does not place a requirement directly on the key
management protocol. The requirement could be met directly by the
key management protocol (e.g., MIKEY-NULL or [RFC4568]) or through an
external out-of-band-mechanism (e.g., [I-D.wing-sipping-srtp-key]).
4.5. PSTN gateway
The discussion in this section relates to requirement R26.
A typical case of using media security is the one where two entities
are having a VoIP conversation over IP capable networks. However,
there are cases where the other end of the communication is not
connected to an IP capable network. In this kind of setting, there
needs to be some kind of gateway at the edge of the IP network which
converts the VoIP conversation to format understood by the other
network. An example of such gateway is a PSTN gateway sitting at the
edge of IP and PSTN networks.
If media security (e.g., SRTP protection) is employed in this kind of
gateway-setting, then media security and the related key management
needs to be terminated at the gateway. The other network (e.g.,
PSTN) may have its own measures to protect the communication, but
this means that from media security point of view the media security
is not employed end-to-end between the communicating entities.
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5. Requirements
This section is divided into several parts: requirements specific to
the key management protocol (Section 5.1), attack scenarios
(Section 5.2), and requirements which can be met inside the key
management protocol or outside of the key management protocol
(Section 5.3).
5.1. Key Management Protocol Requirements
SIP Forking and Retargeting, from Section 4.2:
R1: The media security key management protocol MUST support forking
and retargeting when all endpoints are willing to use SRTP
without causing the call setup to fail, unless the execution of
the authentication and key exchange protocol leads to a failure
(e.g., an unsuccessful authentication attempt).
R3: The media security key management protocol MUST create
distinct, independent cryptographic contexts for each endpoint
in a forked session.
Security characteristics:
R8: The media security key management protocol MUST be able to
support perfect forward secrecy.
R9: The media security key management protocol MUST support
negotiation of SRTP cipher suites without incurring per-
algorithm computational expense. This allows an offer to be
built without incurring computational expense for each
algorithm.
R12: The media security key management protocol MUST NOT require 3rd
parties to sign certificates.
R13: The media security key management protocol SHOULD use
algorithms that allow FIPS 140-2 [FIPS-140-2] certification.
Note that the United States Government can only purchase and
use crypto implementations that have been validated by the
FIPS-140 [FIPS-140-2] process:
"The FIPS-140 standard is applicable to all Federal agencies
that use cryptographic-based security systems to protect
sensitive information in computer and telecommunication
systems, including voice systems. The adoption and use of this
standard is available to private and commercial
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organizations."[cryptval]
Some commercial organizations, such as banks and defense
contractors, also require or prefer equipment which has
validated by the FIPS-140 process.
R16: The media security key management protocol SHOULD NOT introduce
new denial of service vulnerabilities (e.g., the protocol
should not request the endpoint to perform CPU-intensive
operations without the client being able to validate or
authorize the request).
R18: If two parties share an authentication infrastructure, they
SHOULD be able to make use of it.
R19: The media security key management protocol MUST provide crypto-
agility, i.e., the ability to adapt to evolving cryptography
and security requirements (update of cryptographic algorithms
without substantial disruption to deployed implementations)
R20: The media security key management protocol MUST protect cipher
suite negotiation against downgrading attacks.
R24: <deleted>
Performance considerations:
R4: The media security key management protocol MAY support the re-
use of a previously established security context.
Specialized devices may need to avoid public key operations or
Diffie-Hellman operations as much as possible because of the
computational cost or because of the additional call setup
delay. For example, it can take a second or two to perform a
Diffie-Hellman operation in certain devices. Examples of these
specialized devices would include some handsets, intelligent
SIMs, and PSTN gateways. For the typical case because a phone
call has not yet been established, ancillary processing cycles
can be utilized to perform the PK or DH operation; for example,
in a PSTN gateway the DSP, which is not yet involved with
typical DSP operations, could be used to perform the
calculation, so as to avoid having the central host processor
perform the calculation. Some devices, such as handsets, and
intelligent SIMs do not have such ancillary processing
capability.
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R11: <Requirement R11 has been folded into R4.>
Media considerations:
R5: The media security key management protocol SHOULD avoid
clipping media before SDP answer without requiring PRACK
[RFC3262]. This requirement comes from Section 4.1.
R10: If SRTP key negotiation is performed over the media path (i.e.,
using the same UDP/TCP ports as media packets), the key
negotiation packets MUST NOT pass the RTP validity check
defined in Appendix A.1 of [RFC3550].
R15: The media security key management protocol SHOULD allow
endpoints to start with RTP and then upgrade to SRTP.
Issue: Does this mean
[I-D.ietf-mmusic-sdp-capability-negotiation] (which overlaps
with R1 and R2), allow RTP during early media (and upgrade to
SRTP when the SDP answer arrives), or support an "Encrypt this
call" button on the phone?
R21: The media security key management protocol MUST allow a SIP
User Agent to negotiate media security parameters for each
individual session.
R25: The media security key management protocol MUST work when there
are intermediate nodes (e.g., transcoders), terminating or
processing media, between the end points.
Issue: Unclear how requirement for transcoders is met.
R26: The media security key management protocol MUST support
termination of media security in a PSTN gateway. This
requirement is from Section 4.5.
5.2. Attack Scenario Requirements
This section describes requirements from the attack scenarios
(Section 3).
Required for the passive-signaling-passive-media scenario:
R6: The media security key management protocol MUST have a mode
which prevents a passive adversary with access to the media
path from gaining access to keying material used to protect
SRTP media packets.
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R7: The media security key management protocol MUST have a mode in
which it prevents a passive adversary with access to the
signaling path from gaining access to keying material used to
protect SRTP media packets.
Required for the active-signaling-passive-media scenario:
R14: The media security key management protocol SHOULD include a
mechanism for associating key management messages with both the
signaling traffic that initiated the session and with protected
media traffic. Allowing such an association also allows the
SDP offerer to avoid performing CPU-consuming operations (e.g.,
DH or public key operations) with attackers that have not seen
the signaling messages.
For example, if using a Diffie-Hellman keying technique with
security preconditions that forks to 20 end points, the call
initiator would get 20 provisional responses containing 20
signed Diffie-Hellman key pairs. Calculating 20 DH secrets and
validating signatures can be a difficult task depending on the
device capabilities. Hence, in the case of forking, it is not
desirable to perform a DH or PK operation with every party, but
rather only with the party that answers the call (and incur
some media clipping). To do this, the signaling and media need
to be associated so the calling party knows which key
management needs to be completed. This might be done by using
the transport address indicated in the SDP, although NATs can
complicate this association.
Required for the active-signaling-active-media scenario:
R17: The media security key management protocol SHOULD require the
adversary to have access to the signaling path as well as the
media path for a successful attack to be launched. An
adversary that is located only along the data or only along the
signaling path MUST NOT be able to successfully mount an
attack. A successful attack refers to the ability for the
adversary to obtain keying material to decrypt the SRTP
encrypted media traffic.
Required for the detect-attack scenario::
R27: The media security key management protocol SHOULD include
information in the SDP that can be signed by SIP signaling and
validated by the either party (e.g., SIP-Identity [RFC4474] and
SIP-Connected-Identity [RFC4916]). This allows the both
parties to validate the From: address. The called party can
validate the From: address prior to accepting the call.
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5.3. Requirements Outside of the Key Management Protocol
The requirements in this section can be solved within the key
management protocol itself, or can be solved outside of the key
management protocol itself (e.g., solved in SIP or in SDP).
R2: Even when some end points of a forked or retargeted call are
incapable of using SRTP, the solution MUST be described which
allows the establishment of SRTP associations with SRTP-capable
endpoints and / or RTP associations with non-SRTP-capable
endpoints. This requirement comes from Section 4.2.
R23: The media security key management protocol SHOULD be able to
negotiate keys for SRTP sessions created via different call
signaling protocols (e.g., between Jabber, SIP, H.323, MGCP).
R23: A solution SHOULD be described which supports recording of
decrypted media. This requirement comes from Section 4.4.
6. Security Considerations
This document lists requirements for securing media traffic. As
such, it addresses security throughout the document.
7. IANA Considerations
This document does not require actions by IANA.
8. Acknowledgements
For contributions to the requirements portion of this document, the
authors would like to thank the active participants of the RTPSEC BoF
and on the RTPSEC mailing list. The authors would furthermore like
to thank Wolfgang Buecker, Guenther Horn, Peter Howard, Hans-Heinrich
Grusdt, Srinath Thiruvengadam, Martin Euchner, Eric Rescorla, Matt
Lepinski, Dan York, Werner Dittmann, Richard Barnes, Vesa Lehtovirta,
Colin Perkins, Peter Schneider, and Christer Holmberg for their
feedback to this document.
For contributions to the analysis portion of this document, the
authors would like to thank Special thanks to Steffen Fries and
Dragan Ignjatic for their excellent MIKEY comparison document
[I-D.ietf-msec-mikey-applicability]. The authors would furthermore
like to thank Cullen Jennings, David Oran, David McGrew, Mark
Baugher, Flemming Andreasen, Eric Raymond, Dave Ward, Leo Huang, Eric
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Rescorla, Lakshminath Dondeti, Steffen Fries, Alan Johnston, Dragan
Ignjatic and John Elwell for their feedback to this document.
9. References
9.1. Normative References
[FIPS-140-2]
NIST, "Security Requirements for Cryptographic Modules",
June 2005, <http://csrc.nist.gov/publications/fips/
fips140-2/fips1402.pdf>.
[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.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[cryptval]
NIST, "Cryptographic Module Validation Program",
December 2006,
<http://csrc.nist.gov/cryptval/140-2APP.htm>.
9.2. Informative References
[I-D.baugher-mmusic-sdp-dh]
Baugher, M. and D. McGrew, "Diffie-Hellman Exchanges for
Multimedia Sessions", draft-baugher-mmusic-sdp-dh-00 (work
in progress), February 2006.
[I-D.dondeti-msec-rtpsec-mikeyv2]
Dondeti, L., "MIKEYv2: SRTP Key Management using MIKEY,
revisited", draft-dondeti-msec-rtpsec-mikeyv2-01 (work in
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progress), March 2007.
[I-D.fischl-sipping-media-dtls]
Fischl, J., "Datagram Transport Layer Security (DTLS)
Protocol for Protection of Media Traffic Established with
the Session Initiation Protocol",
draft-fischl-sipping-media-dtls-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-01 (work in progress),
November 2007.
[I-D.ietf-mmusic-ice]
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.
[I-D.ietf-mmusic-sdp-capability-negotiation]
Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-07 (work in
progress), October 2007.
[I-D.ietf-msec-mikey-applicability]
Fries, S. and D. Ignjatic, "On the applicability of
various MIKEY modes and extensions",
draft-ietf-msec-mikey-applicability-06 (work in progress),
July 2007.
[I-D.ietf-msec-mikey-ecc]
Milne, A., "ECC Algorithms for MIKEY",
draft-ietf-msec-mikey-ecc-03 (work in progress),
June 2007.
[I-D.ietf-sip-certs]
Jennings, C., "Certificate Management Service for The
Session Initiation Protocol (SIP)",
draft-ietf-sip-certs-04 (work in progress), July 2007.
[I-D.jennings-sipping-multipart]
Wing, D. and C. Jennings, "Session Initiation Protocol
(SIP) Offer/Answer with Multipart Alternative",
draft-jennings-sipping-multipart-02 (work in progress),
March 2006.
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[I-D.mahy-sipping-herfp-fix]
Mahy, R., "A Solution to the Heterogeneous Error Response
Forking Problem (HERFP) in the Session Initiation
Protocol (SIP)", draft-mahy-sipping-herfp-fix-01 (work in
progress), March 2006.
[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.wing-sipping-srtp-key]
Wing, D., Audet, F., Fries, S., and H. Tschofenig,
"Disclosing Secure RTP (SRTP) Session Keys with a SIP
Event Package", draft-wing-sipping-srtp-key-02 (work in
progress), November 2007.
[I-D.zimmermann-avt-zrtp]
Zimmermann, P., "ZRTP: Media Path Key Agreement for Secure
RTP", draft-zimmermann-avt-zrtp-04 (work in progress),
July 2007.
[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.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 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.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
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[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC4650] Euchner, M., "HMAC-Authenticated Diffie-Hellman for
Multimedia Internet KEYing (MIKEY)", RFC 4650,
September 2006.
[RFC4738] Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY-
RSA-R: An Additional Mode of Key Distribution in
Multimedia Internet KEYing (MIKEY)", RFC 4738,
November 2006.
[RFC4771] Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
Transform Carrying Roll-Over Counter for the Secure Real-
time Transport Protocol (SRTP)", RFC 4771, January 2007.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[RFC5027] Andreasen, F. and D. Wing, "Security Preconditions for
Session Description Protocol (SDP) Media Streams",
RFC 5027, October 2007.
Appendix A. Overview of Keying Mechanisms
Based on how the SRTP keys are exchanged, each SRTP key exchange
mechanism belongs to one general category:
signaling path: All the keying is carried in the call signaling
(SIP or SDP) path.
media path: All the keying is carried in the SRTP/SRTCP media
path, and no signaling whatsoever is carried in the call
signaling path.
signaling and media path: Parts of the keying are carried in the
SRTP/SRTCP media path, and parts are carried in the call
signaling (SIP or SDP) path.
One of the significant benefits of SRTP over other end-to-end
encryption mechanisms, such as for example IPsec, is that SRTP is
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bandwidth efficient and SRTP retains the header of RTP packets.
Bandwidth efficiency is vital for VoIP in many scenarios where access
bandwidth is limited or expensive, and retaining the RTP header is
important for troubleshooting packet loss, delay, and jitter.
Related to SRTP's characteristics is a goal that any SRTP keying
mechanism to also be efficient and not cause additional call setup
delay. Contributors to additional call setup delay include network
or database operations: retrieval of certificates and additional SIP
or media path messages, and computational overhead of establishing
keys or validating certificates.
When examining the choice between keying in the signaling path,
keying in the media path, or keying in both paths, it is important to
realize the media path is generally 'faster' than the SIP signaling
path. The SIP signaling path has computational elements involved
which parse and route SIP messages. The media path, on the other
hand, does not normally have computational elements involved, and
even when computational elements such as firewalls are involved, they
cause very little additional delay. Thus, the media path can be
useful for exchanging several messages to establish SRTP keys. A
disadvantage of keying over the media path is that interworking
different key exchange requires the interworking function be in the
media path, rather than just in the signaling path; in practice this
involvement is probably unavoidable anyway.
A.1. Signaling Path Keying Techniques
A.1.1. MIKEY-NULL
MIKEY-NULL [RFC3830] has the offerer indicate the SRTP keys for both
directions. The key is sent unencrypted in SDP, which means the SDP
must be encrypted hop-by-hop (e.g., by using TLS (SIPS)) or end-to-
end (e.g., by using S/MIME).
MIKEY-NULL requires one message from offerer to answerer (half a
round trip), and does not add additional media path messages.
A.1.2. MIKEY-PSK
MIKEY-PSK (pre-shared key) [RFC3830] requires that all endpoints
share one common key. MIKEY-PSK has the offerer encrypt the SRTP
keys for both directions using this pre-shared key.
MIKEY-PSK requires one message from offerer to answerer (half a round
trip), and does not add additional media path messages.
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A.1.3. MIKEY-RSA
MIKEY-RSA [RFC3830] has the offerer encrypt the keys for both
directions using the intended answerer's public key, which is
obtained from a PKI.
MIKEY-RSA requires one message from offerer to answerer (half a round
trip), and does not add additional media path messages. MIKEY-RSA
requires the offerer to obtain the intended answerer's certificate.
A.1.4. MIKEY-RSA-R
MIKEY-RSA-R [RFC4738] is essentially the same as MIKEY-RSA but
reverses the role of the offerer and the answerer with regards to
providing the keys. That is, the answerer encrypts the keys for both
directions using the offerer's public key. Both the offerer and
answerer validate each other's public keys using a PKI. MIKEY-RSA-R
also enables sending certificates in the MIKEY message.
MIKEY-RSA-R requires one message from offerer to answer, and one
message from answerer to offerer (full round trip), and does not add
additional media path messages. MIKEY-RSA-R requires the offerer
validate the answerer's certificate.
A.1.5. MIKEY-DHSIGN
In MIKEY-DHSIGN [RFC3830] the offerer and answerer derive the key
from a Diffie-Hellman exchange. In order to prevent an active man-
in-the-middle the DH exchange itself is signed using each endpoint's
private key and the associated public keys are validated using a PKI.
MIKEY-DHSIGN requires one message from offerer to answerer, and one
message from answerer to offerer (full round trip), and does not add
additional media path messages. MIKEY-DHSIGN requires the offerer
and answerer to validate each other's certificates. MIKEY-DHSIGN
also enables sending the answerer's certificate in the MIKEY message.
A.1.6. MIKEY-DHHMAC
MIKEY-DHHMAC [RFC4650] uses a pre-shared secret to HMAC the Diffie-
Hellman exchange, essentially combining aspects of MIKEY-PSK with
MIKEY-DHSIGN, but without MIKEY-DHSIGN's need for a PKI to
authenticate the Diffie-Hellman exchange.
MIKEY-DHHMAC requires one message from offerer to answerer, and one
message from answerer to offerer (full round trip), and does not add
additional media path messages.
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A.1.7. MIKEY-ECIES and MIKEY-ECMQV (MIKEY-ECC)
ECC Algorithms For MIKEY [I-D.ietf-msec-mikey-ecc] describes how ECC
can be used with MIKEY-RSA (using ECDSA signature) and with MIKEY-
DHSIGN (using a new DH-Group code), and also defines two new ECC-
based algorithms, Elliptic Curve Integrated Encryption Scheme (ECIES)
and Elliptic Curve Menezes-Qu-Vanstone (ECMQV) .
For the purposes of this paper, the ECDSA signature, MIKEY-ECIES, and
MIKEY-ECMQV function exactly like MIKEY-RSA, and the new DH-Group
code function exactly like MIKEY-DHSIGN. Therefore these ECC
mechanisms aren't discussed separately in this paper.
A.1.8. Security Descriptions with SIPS
Security Descriptions [RFC4568] has each side indicate the key it
will use for transmitting SRTP media, and the keys are sent in the
clear in SDP. Security Descriptions relies on hop-by-hop (TLS via
"SIPS:") encryption to protect the keys exchanged in signaling.
Security Descriptions requires one message from offerer to answerer,
and one message from answerer to offerer (full round trip), and does
not add additional media path messages.
A.1.9. Security Descriptions with S/MIME
This keying mechanism is identical to Appendix A.1.8, except that
rather than protecting the signaling with TLS, the entire SDP is
encrypted with S/MIME.
A.1.10. SDP-DH (expired)
SDP Diffie-Hellman [I-D.baugher-mmusic-sdp-dh] exchanges Diffie-
Hellman messages in the signaling path to establish session keys. To
protect against active man-in-the-middle attacks, the Diffie-Hellman
exchange needs to be protected with S/MIME, SIPS, or SIP-Identity
[RFC4474] and [RFC4474].
SDP-DH requires one message from offerer to answerer, and one message
from answerer to offerer (full round trip), and does not add
additional media path messages.
A.1.11. MIKEYv2 in SDP (expired)
MIKEYv2 [I-D.dondeti-msec-rtpsec-mikeyv2] adds mode negotiation to
MIKEYv1 and removes the time synchronization requirement. It
therefore now takes 2 round-trips to complete. In the first round
trip, the communicating parties learn each other's identities, agree
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on a MIKEY mode, crypto algorithm, SRTP policy, and exchanges nonces
for replay protection. In the second round trip, they negotiate
unicast and/or group SRTP context for SRTP and/or SRTCP.
Furthemore, MIKEYv2 also defines an in-band negotiation mode as an
alternative to SDP (see Appendix A.3.3).
A.2. Media Path Keying Technique
A.2.1. ZRTP
ZRTP [I-D.zimmermann-avt-zrtp] does not exchange information in the
signaling path (although it's possible for endpoints to indicate
support for ZRTP with "a=zrtp" in the initial Offer). In ZRTP the
keys are exchanged entirely in the media path using a Diffie-Hellman
exchange. The advantage to this mechanism is that the signaling
channel is used only for call setup and the media channel is used to
establish an encrypted channel -- much like encryption devices on the
PSTN. ZRTP uses voice authentication of its Diffie-Hellman exchange
by having each person read digits to the other person. Subsequent
sessions with the same ZRTP endpoint can be authenticated using the
stored hash of the previously negotiated key rather than voice
authentication.
ZRTP uses 4 media path messages (Hello, Commit, DHPart1, and DHPart2)
to establish the SRTP key, and 3 media path confirmation messages.
These initial messages are all sent as non-RTP packets.
Note that when ZRTP probing is used, unencrypted RTP is being
exchanged until the SRTP keys are established.
A.3. Signaling and Media Path Keying Techniques
A.3.1. EKT
EKT [I-D.mcgrew-srtp-ekt] relies on another SRTP key exchange
protocol, such as Security Descriptions or MIKEY, for bootstrapping.
In the initial phase, each member of a conference uses an SRTP key
exchange protocol to establish a common key encryption key (KEK).
Each member may use the KEK to securely transport its SRTP master key
and current SRTP rollover counter (ROC), via RTCP, to the other
participants in the session.
EKT requires the offerer to send some parameters (EKT_Cipher, KEK,
and security parameter index (SPI)) via the bootstrapping protocol
such as Security Descriptions or MIKEY. Each answerer sends an SRTCP
message which contains the answerer's SRTP Master Key, rollover
counter, and the SRTP sequence number. Rekeying is done by sending a
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new SRTCP message. For reliable transport, multiple RTCP messages
need to be sent.
A.3.2. DTLS-SRTP
DTLS-SRTP [I-D.ietf-avt-dtls-srtp] exchanges public key fingerprints
in SDP [I-D.fischl-sipping-media-dtls] and then establishes a DTLS
session over the media channel. The endpoints use the DTLS handshake
to agree on crypto suites and establish SRTP session keys. SRTP
packets are then exchanged between the endpoints.
DTLS-SRTP requires one message from offerer to answerer (half round
trip), and, if the offerer wishes to correlate the SDP answer with
the endpoint, requires one message from answer to offerer (full round
trip). DTLS-SRTP uses 4 media path messages to establish the SRTP
key.
This paper assumes DTLS will use TLS_RSA_WITH_3DES_EDE_CBC_SHA as its
cipher suite, which is the mandatory-to-implement cipher suite in TLS
[RFC4346].
A.3.3. MIKEYv2 Inband (expired)
As defined in Appendix A.1.11, MIKEYv2 also defines an in-band
negotiation mode as an alternative to SDP (see Appendix A.3.3). The
details are not sorted out in the draft yet on what in-band actually
means (i.e., UDP, RTP, RTCP, etc.).
Appendix B. Evaluation Criteria - SIP
This section considers how each keying mechanism interacts with SIP
features.
B.1. Secure Retargeting and Secure Forking
Retargeting and forking of signaling requests is described within
Section 4.2. The following builds upon this description.
The following list compares the behavior of secure forking, answering
association, two-time pads, and secure retargeting for each keying
mechanism.
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MIKEY-NULL Secure Forking: No, all AORs see offerer's and
answerer's keys. Answer is associated with media by the SSRC
in MIKEY. Additionally, a two-time pad occurs if two branches
choose the same 32-bit SSRC and transmit SRTP packets.
Secure Retargeting: No, all targets see offerer's and
answerer's keys. Suffers from retargeting identity problem.
MIKEY-PSK
Secure Forking: No, all AORs see offerer's and answerer's keys.
Answer is associated with media by the SSRC in MIKEY. Note
that all AORs must share the same pre-shared key in order for
forking to work at all with MIKEY-PSK. Additionally, a two-
time pad occurs if two branches choose the same 32-bit SSRC and
transmit SRTP packets.
Secure Retargeting: Not secure. For retargeting to work, the
final target must possess the correct PSK. As this is likely
in scenarios were the call is targeted to another device
belonging to the same user (forking), it is very unlikely that
other users will possess that PSK and be able to successfully
answer that call.
MIKEY-RSA
Secure Forking: No, all AORs see offerer's and answerer's keys.
Answer is associated with media by the SSRC in MIKEY. Note
that all AORs must share the same private key in order for
forking to work at all with MIKEY-RSA. Additionally, a two-
time pad occurs if two branches choose the same 32-bit SSRC and
transmit SRTP packets.
Secure Retargeting: No.
MIKEY-RSA-R
Secure Forking: Yes. Answer is associated with media by the
SSRC in MIKEY.
Secure Retargeting: Yes.
MIKEY-DHSIGN
Secure Forking: Yes, each forked endpoint negotiates unique
keys with the offerer for both directions. Answer is
associated with media by the SSRC in MIKEY.
Secure Retargeting: Yes, each target negotiates unique keys
with the offerer for both directions.
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MIKEYv2 in SDP
The behavior will depend on which mode is picked.
MIKEY-DHHMAC
Secure Forking: Yes, each forked endpoint negotiates unique
keys with the offerer for both directions. Answer is
associated with media by the SSRC in MIKEY.
Secure Retargeting: Yes, each target negotiates unique keys
with the offerer for both directions. Note that for the keys
to be meaningful, it would require the PSK to be the same for
all the potential intermediaries, which would only happen
within a single domain.
Security Descriptions with SIPS
Secure Forking: No. Each forked endpoint sees the offerer's
key. Answer is not associated with media.
Secure Retargeting: No. Each target sees the offerer's key.
Security Descriptions with S/MIME
Secure Forking: No. Each forked endpoint sees the offerer's
key. Answer is not associated with media.
Secure Retargeting: No. Each target sees the offerer's key.
Suffers from retargeting identity problem.
SDP-DH
Secure Forking: Yes. Each forked endpoint calculates a unique
SRTP key. Answer is not associated with media.
Secure Retargeting: Yes. The final target calculates a unique
SRTP key.
ZRTP
Secure Forking: Yes. Each forked endpoint calculates a unique
SRTP key. As ZRTP isn't signaled in SDP, there is no
association of the answer with media.
Secure Retargeting: Yes. The final target calculates a unique
SRTP key.
EKT
Secure Forking: Inherited from the bootstrapping mechanism (the
specific MIKEY mode or Security Descriptions). Answer is
associated with media by the SPI in the EKT protocol. Answer
is associated with media by the SPI in the EKT protocol.
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Secure Retargeting: Inherited from the bootstrapping mechanism
(the specific MIKEY mode or Security Descriptions).
DTLS-SRTP
Secure Forking: Yes. Each forked endpoint calculates a unique
SRTP key. Answer is associated with media by the certificate
fingerprint in signaling and certificate in the media path.
Secure Retargeting: Yes. The final target calculates a unique
SRTP key.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
B.2. Clipping Media Before SDP Answer
Clipping media before receiving the signaling answer is described
within Section 4.1. The following builds upon this description.
Furthermore, the problem of clipping gets compounded when forking is
used. For example, if using a Diffie-Hellman keying technique with
security preconditions that forks to 20 endpoints, the call initiator
would get 20 provisional responses containing 20 signed Diffie-
Hellman half keys. Calculating 20 DH secrets and validating
signatures can be a difficult task depending on the device
capabilities.
The following list compares the behavior of clipping before SDP
answer for each keying mechanism.
MIKEY-NULL
Not clipped. The offerer provides the answerer's keys.
MIKEY-PSK
Not clipped. The offerer provides the answerer's keys.
MIKEY-RSA
Not clipped. The offerer provides the answerer's keys.
MIKEY-RSA-R
Clipped. The answer contains the answerer's encryption key.
MIKEY-DHSIGN
Clipped. The answer contains the answerer's Diffie-Hellman
response.
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MIKEY-DHHMAC
Clipped. The answer contains the answerer's Diffie-Hellman
response.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
Clipped. The answer contains the answerer's encryption key.
Security Descriptions with S/MIME
Clipped. The answer contains the answerer's encryption key.
SDP-DH
Clipped. The answer contains the answerer's Diffie-Hellman
response.
ZRTP
Not clipped because the session intially uses RTP. While RTP
is flowing, both ends negotiate SRTP keys in the media path and
then switch to using SRTP.
EKT
Not clipped, as long as the first RTCP packet (containing the
answerer's key) is not lost in transit. The answerer sends its
encryption key in RTCP, which arrives at the same time (or
before) the first SRTP packet encrypted with that key.
Note: RTCP needs to work, in the answerer-to-offerer
direction, before the offerer can decrypt SRTP media.
DTLS-SRTP
Not clipped. Keys are exchanged in the media path without
relying on the signaling path.
MIKEYv2 Inband
Not clipped. Keys are exchanged in the media path without
relying on the signaling path.
B.3. Centralized Keying
Centralized keying is described within Section 4.3. The following
builds upon this description.
The following list describes how each keying mechanism behaves with
centralized keying (scenario d) and rekeying.
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MIKEY-NULL
Keying: Yes, if offerer is the mixer. No, if offerer is the
participant (end user).
Rekeying: Yes, via re-Invite
MIKEY-PSK
Keying: Yes, if offerer is the mixer. No, if offerer is the
participant (end user).
Rekeying: Yes, with a re-Invite
MIKEY-RSA
Keying: Yes, if offerer is the mixer. No, if offerer is the
participant (end user).
Rekeying: Yes, with a re-Invite
MIKEY-RSA-R
Keying: No, if offerer is the mixer. Yes, if offerer is the
participant (end user).
Rekeying: n/a
MIKEY-DHSIGN
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
MIKEY-DHHMAC
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
Keying: Yes, if offerer is the mixer. Yes, if offerer is the
participant.
Rekeying: Yes, with a Re-Invite.
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Security Descriptions with S/MIME
Keying: Yes, if offerer is the mixer. Yes, if offerer is the
participant.
Rekeying: Yes, with a Re-Invite.
SDP-DH
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
ZRTP
Keying: No; a group-key Diffie-Hellman protocol is not
supported.
Rekeying: n/a
EKT
Keying: Yes. After bootstrapping a KEK using Security
Descriptions or MIKEY, each member originating an SRTP stream
can send its SRTP master key, sequence number and ROC via RTCP.
Rekeying: Yes. EKT supports each sender to transmit its SRTP
master key to the group via RTCP packets. Thus, EKT supports
each originator of an SRTP stream to rekey at any time.
DTLS-SRTP
Keying: Yes, because with the assumed cipher suite,
TLS_RSA_WITH_3DES_EDE_CBC_SHA, each end indicates its SRTP key.
Rekeying: via DTLS in the media path.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
B.4. SSRC and ROC
In SRTP, a cryptographic context is defined as the SSRC, destination
network address, and destination transport port number. Whereas RTP,
a flow is defined as the destination network address and destination
transport port number. This results in a problem -- how to
communicate the SSRC so that the SSRC can be used for the
cryptographic context.
Two approaches have emerged for this communication. One, used by all
MIKEY modes, is to communicate the SSRCs to the peer in the MIKEY
exchange. Another, used by Security Descriptions, is to use "late
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bindng" -- that is, any new packet containing a previously-unseen
SSRC (which arrives at the same destination network address and
destination transport port number) will create a new cryptographic
context. Another approach, common amongst techniques with media-path
SRTP key establishment, is to require a handshake over that media
path before SRTP packets are sent. MIKEY's approach changes RTP's
SSRC collision detection behavior by requiring RTP to pre-establish
the SSRC values for each session.
Another related issue is that SRTP introduces a rollover counter
(ROC), which records how many times the SRTP sequence number has
rolled over. As the sequence number is used for SRTP's default
ciphers, it is important that all endpoints know the value of the
ROC. The ROC starts at 0 at the beginning of a session.
Some keying mechanisms cause a two-time pad to occur if two endpoints
of a forked call have an SSRC collision.
Note: A proposal has been made to send the ROC value on every Nth
SRTP packet[RFC4771]. This proposal has not yet been incorporated
into this document.
The following list examines handling of SSRC and ROC:
MIKEY-NULL
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-PSK
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-RSA
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-RSA-R
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEY-DHSIGN
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
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MIKEY-DHHMAC
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
MIKEYv2 in SDP
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
Security Descriptions with SIPS
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
Security Descriptions with S/MIME
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
SDP-DH
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
ZRTP
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
EKT
The SSRC of the SRTCP packet containing an EKT update
corresponds to the SRTP master key and other parameters within
that packet.
DTLS-SRTP
Neither SSRC nor ROC are signaled. SSRC 'late binding' is
used.
MIKEYv2 Inband
Each endpoint indicates a set of SSRCs and the ROC for SRTP
packets it transmits.
Appendix C. Evaluation Criteria - Security
This section evaluates each keying mechanism on the basis of their
security properties.
C.1. Public Key Infrastructure
There are two aspects of PKI requirements -- one aspect is if PKI is
necessary in order for the mechanism to function at all, the other is
if PKI is used to authenticate a certificate. With interactive
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communications it is desirable to avoid fetching certificates that
delay call setup; rather it is preferable to fetch or validate
certificates in such a way that call setup isn't delayed. For
example, a certificate can be validated while the phone is ringing or
can be validated while ring-back tones are being played or even while
the called party is answering the phone and saying "hello".
SRTP key exchange mechanisms that require a particular authentication
infrastructure to operate are gated on the deployment of a such an
infrastructure available to both endpoints. This means that no media
security is achievable until such an infrastructure exists. For SIP,
something like sip-certs [I-D.ietf-sip-certs] might be used to obtain
the certificate of a peer.
Note: Even if sip-certs [I-D.ietf-sip-certs] was deployed, the
retargeting problem (Appendix B.1) would still prevent successful
deployment of keying techniques which require the offerer to
obtain the actual target's public key.
The following list compares the PKI requirements of each keying
mechanism, both if a PKI is required for the key exchange itself, and
if PKI is only used to authenticate the certificate supplied in
signaling.
MIKEY-NULL
PKI not used.
MIKEY-PSK
PKI not used; rather, all endpoints must have some way to
exchange per-endpoint or per-system pre-shared keys.
MIKEY-RSA
The offerer obtains the intended answerer's public key before
initiating the call. This public key is used to encrypt the
SRTP keys. There is no defined mechanism for the offerer to
obtain the answerer's public key, although [I-D.ietf-sip-certs]
might be viable in the future.
MIKEY-RSA-R
The offer contains the offerer's public key. The answerer uses
that public key to encrypt the SRTP keys that will be used by
the offerer and the answerer. A PKI is necessary to validate
the certificates.
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MIKEY-DHSIGN
PKI is used to authenticate the public key that is included in
the MIKEY message, by walking the CA trust chain.
MIKEY-DHHMAC
PKI not used; rather, all endpoints must have some way to
exchange per-endpoint or per-system pre-shared keys.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
PKI not used.
Security Descriptions with S/MIME
PKI is needed for S/MIME. The offerer must obtain the intended
target's public key and encrypt their SDP with that key. The
answerer must obtain the offerer's public key and encrypt their
SDP with that key.
SDP-DH
PKI not used.
ZRTP
PKI not used.
EKT
PKI not used by EKT itself, but might be used by the EKT
bootstrapping keying mechanism (such as certain MIKEY modes).
DTLS-SRTP
Remote party's certificate is sent in media path, and a
fingerprint of the same certificate is sent in the signaling
path.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
C.2. Perfect Forward Secrecy
In the context of SRTP, Perfect Forward Secrecy is the property that
SRTP session keys that protected a previous session are not
compromised if the static keys belonging to the endpoints are
compromised. That is, if someone were to record your encrypted
session content and later acquires either party's private key, that
encrypted session content would be safe from decryption if your key
exchange mechanism had perfect forward secrecy.
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The following list describes how each key exchange mechanism provides
PFS.
MIKEY-NULL
No PFS.
MIKEY-PSK
No PFS.
MIKEY-RSA
No PFS.
MIKEY-RSA-R
No PFS.
MIKEY-DHSIGN
PFS is provided with the Diffie-Hellman exchange.
MIKEY-DHHMAC
PFS is provided with the Diffie-Hellman exchange.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
No PFS.
Security Descriptions with S/MIME
No PFS.
SDP-DH
PFS is provided with the Diffie-Hellman exchange.
ZRTP
PFS is provided with the Diffie-Hellman exchange.
EKT
No PFS.
DTLS-SRTP
PFS is achieved if the negotiated cipher suite includes an
exponential or discrete-logarithmic key exchange (such as
Diffie-Hellman or Elliptic Curve Diffie-Hellman [RFC4492]).
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MIKEYv2 Inband
The behavior will depend on which mode is picked.
C.3. Best Effort Encryption
With best effort encryption, SRTP is used with endpoints that support
SRTP, otherwise RTP is used.
SIP needs a backwards-compatible best effort encryption in order for
SRTP to work successfully with SIP retargeting and forking when there
is a mix of forked or retargeted devices that support SRTP and don't
support SRTP.
Consider the case of Bob, with a phone that only does RTP and a
voice mail system that supports SRTP and RTP. If Alice calls Bob
with an SRTP offer, Bob's RTP-only phone will reject the media
stream (with an empty "m=" line) because Bob's phone doesn't
understand SRTP (RTP/SAVP). Alice's phone will see this rejected
media stream and may terminate the entire call (BYE) and re-
initiate the call as RTP-only, or Alice's phone may decide to
continue with call setup with the SRTP-capable leg (the voice mail
system). If Alice's phone decided to re-initiate the call as RTP-
only, and Bob doesn't answer his phone, Alice will then leave
voice mail using only RTP, rather than SRTP as expected.
Currently, several techniques are commonly considered as candidates
to provide opportunistic encryption:
multipart/alternative
[I-D.jennings-sipping-multipart] describes how to form a
multipart/alternative body part in SIP. The significant issues
with this technique are (1) that multipart MIME is incompatible
with existing SIP proxies, firewalls, Session Border Controllers,
and endpoints and (2) when forking, the Heterogeneous Error
Response Forking Problem (HERFP) [I-D.mahy-sipping-herfp-fix]
causes problems if such non-multipart-capable endpoints were
involved in the forking.
SDP Grouping
A new SDP grouping mechanism (following the idea introduced in
[RFC3388]) has been discussed which would allow a media line to
indicate RTP/AVP and another media line to indicate RTP/SAVP,
allowing non-SRTP-aware endpoints to choose RTP/AVP and SRTP-aware
endpoints to choose RTP/SAVP. As of this writing, this SDP
grouping mechanism has not been published as an Internet Draft.
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session attribute
With this technique, the endpoints signal their desire to do SRTP
by signaling RTP (RTP/AVP), and using an attribute ("a=") in the
SDP. This technique is entirely backwards compatible with non-
SRTP-aware endpoints, but doesn't use the RTP/SAVP protocol
registered by SRTP [RFC3711].
SDP Capability Negotiation
SDP Capability Negotiation
[I-D.ietf-mmusic-sdp-capability-negotiation] provides a backwards-
compatible mechanism to allow offering both SRTP and RTP in a
single offer. This is the preferred technique.
Probing
With this technique, the endpoints first establish an RTP session
using RTP (RTP/AVP). The endpoints send probe messages, over the
media path, to determine if the remote endpoint supports their
keying technique.
The preferred technique, SDP Capability Negotiation
[I-D.ietf-mmusic-sdp-capability-negotiation], can be used with all
key exchange mechanisms. What remains unique is ZRTP, which can also
accomplish its best effort encryption by probing (sending ZRTP
messages over the media path) or by session attribute (see "a=zrtp",
defined in Section 10 of [I-D.zimmermann-avt-zrtp]). Current
implementations of ZRTP use probing.
C.4. Upgrading Algorithms
It is necessary to allow upgrading SRTP encryption and hash
algorithms, as well as upgrading the cryptographic functions used for
the key exchange mechanism. With SIP's offer/answer model, this can
be computionally expensive because the offer needs to contain all
combinations of the key exchange mechanisms (all MIKEY modes,
Security Descriptions) and all SRTP cryptographic suites (AES-128,
AES-256) and all SRTP cryptographic hash functions (SHA-1, SHA-256)
that the offerer supports. In order to do this, the offerer has to
expend CPU resources to build an offer containing all of this
information which becomes computationally prohibitive.
Thus, it is important to keep the offerer's CPU impact fixed so that
offering multiple new SRTP encryption and hash functions incurs no
additional expense.
The following list describes the CPU effort involved in using each
key exchange technique.
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MIKEY-NULL
No significant computaional expense.
MIKEY-PSK
No significant computational expense.
MIKEY-RSA
For each offered SRTP crypto suite, the offerer has to perform
RSA operation to encrypt the TGK
MIKEY-RSA-R
For each offered SRTP crypto suite, the offerer has to perform
public key operation to sign the MIKEY message.
MIKEY-DHSIGN
For each offered SRTP crypto suite, the offerer has to perform
Diffie-Hellman operation, and a public key operation to sign
the Diffie-Hellman output.
MIKEY-DHHMAC
For each offered SRTP crypto suite, the offerer has to perform
Diffie-Hellman operation.
MIKEYv2 in SDP
The behavior will depend on which mode is picked.
Security Descriptions with SIPS
No significant computational expense.
Security Descriptions with S/MIME
S/MIME requires the offerer and the answerer to encrypt the SDP
with the other's public key, and to decrypt the received SDP
with their own private key.
SDP-DH
For each offered SRTP crypto suite, the offerer has to perform
a Diffie-Hellman operation.
ZRTP
The offerer has no additional computational expense at all, as
the offer contains no information about ZRTP or might contain
"a=zrtp".
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EKT
The offerer's Computational expense depends entirely on the EKT
bootstrapping mechanism selected (one or more MIKEY modes or
Security Descriptions).
DTLS-SRTP
The offerer has no additional computational expense at all, as
the offer contains only a fingerprint of the certificate that
will be presented in the DTLS exchange.
MIKEYv2 Inband
The behavior will depend on which mode is picked.
Appendix D. Out-of-Scope
Discussions concluded that key management for shared-key encryption
of conferencing is outside the scope of this document. As the
priority is point-to-point unicast SRTP session keying, resolving
shared-key SRTP session keying is deferred to later and left as an
item for future investigations.
Authors' Addresses
Dan Wing (editor)
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
Steffen Fries
Siemens AG
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: steffen.fries@siemens.com
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Hannes Tschofenig
Nokia Siemens Networks
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@nsn.com
URI: http://www.tschofenig.com
Francois Audet
Nortel
4655 Great America Parkway
Santa Clara, CA 95054
USA
Email: audet@nortel.com
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