Internet Engineering Task Force J. Arkko
MMUSIC Working Group E. Carrara
INTERNET-DRAFT F. Lindholm
Expires: August 2003 M. Naslund
K. Norrman
Ericsson
February, 2003
Key Management Extensions for SDP and RTSP
<draft-ietf-mmusic-kmgmt-ext-06.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document defines general extensions for SDP and RTSP to carry
the security information needed by a key management protocol, in
order to secure the media. These extensions are presented as a
framework, to be used by one or more key management protocols. As
such, its use is meaningful only when it is completed by the key
management protocol in use.
General guidelines are also given on how the framework should be used
together with SIP and RTSP.
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TABLE OF CONTENTS
1. Introduction.....................................................2
1.1. Notational Conventions.........................................3
2. Extensions to SDP and RTSP.......................................3
2.1. SDP Extensions.................................................4
2.2. RTSP Extensions................................................4
3. Usage with SIP and RTSP..........................................5
3.1. General SDP processing.........................................5
3.2. SIP usage......................................................6
3.3. RTSP usage.....................................................7
3.4. Example scenarios..............................................7
4. Adding further Key management protocols.........................10
5. Security Considerations.........................................10
6. IANA Considerations.............................................11
7. Conclusions.....................................................11
8. Acknowledgments.................................................11
9. Author's Addresses..............................................11
10. References.....................................................12
10.1. Normative References.........................................12
10.2. Informative References.......................................12
1. Introduction
There has recently been work to define a security framework for the
protection of real-time applications running over RTP, [SRTP].
However, a security protocol needs a key management infrastructure to
exchange keys and security parameters, managing and refreshing keys,
etc.
A key management protocol is executed prior to the security protocol
execution. The key management protocol's main goal is to, in a secure
and reliable way, establish a so-called security association for the
security protocol. This includes one or several cryptographic keys
and a set of necessary parameters for the security protocol, e.g.,
cipher and authentication algorithm to be used. The key management
protocol has similarities with, e.g., SIP [SIP] and RTSP [RTSP] in
the sense that it negotiates necessary information in order to be
able to setup the session.
The focus in the following sections is to describe SDP attribute
extensions and RTSP header extensions to support key management, and
a possible integration within SIP and RTSP. A framework is therefore
described in the following. Such a framework will need to be
completed by one or more key management protocols, to describe how
the framework is used, e.g. which is the data to be carried in the
extensions.
Some of the motivations to create a framework with the possibility to
include the key management in the session establishment are:
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* Just as the codec information is a description of how to encode and
decode the audio (or video) stream, the key management data is a
description of how to encrypt and decrypt the data.
* The possibility to negotiate the security for the entire multimedia
session at the same time.
* The knowledge of the media at the session establishment makes it
easy to tie the key management to the multimedia sessions.
* This approach may be more efficient than setting up the security
later, as that approach might force extra roundtrips, possibly
also a separate set-up for each stream, hence implying more delay
to the actual setup of the media session.
Currently in SDP [SDPnew], one field exists to transport keys, i.e.
the "key=" field. However, this is not enough for a key management
protocol as there are many more parameters that need to be
transported. The approach here is to use and extend the SDP
description to transport the key management offer/answer and also to
associate it with the media sessions. SIP uses the offer/answer model
[OAM] whereby extensions to SDP will be enough. However, RTSP [RTSP]
does not use the offer/answer model. This makes it impossible to send
back an answer to the server. To solve this, a new header is
introduced in which the key management data can be included.
1.1. Notational Conventions
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 RFC-2119.
2. Extensions to SDP and RTSP
This section describes common attributes that are to be included in
an SDP description or in an RTSP header when an integrated key
management protocol is used. The attribute values MUST follow the
general SDP or RTSP guidelines.
For the SDP description, the key management attributes MAY be defined
at session level (i.e. before the media descriptor lines) and/or at
media level. If the key management attributes are defined at media
level, they will only apply to that specific media. If the key
management attributes are defined at both session and media level,
the media level definition overrides the session level definition for
that specific media.
The following SDP attribute is defined:
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key-mgmt:<identifier> <opaque-data>
<identifier> is the name of the key management protocol and the
opaque-data is a field to transport the key management protocol data.
The key management protocol data contains the necessary information
to establish the security protocol, e.g., keys and cryptographic
parameters. All parameters and keys are protected by the key
management. Note that if the key management protocol fails, e.g., the
receiver does not accept any of the proposed security parameters, or
simply does not understand the key management protocol, the security
setup will fail. Consequently, it is impossible to establish a secure
session. So, if the key management fails, the offer must be rejected.
2.1. SDP Extensions
This section provides an Augmented Backus-Naur Form (ABNF) grammar
(as used in [SDPnew]) for the key management extensions to SDP.
Note that the new definitions are compliant with the definition of an
attribute field, i.e.
attribute = (att-field ":" att-value) | att-field
One new attribute for SDP is defined:
key-mgmt = "key-mgmt: " prtcl-id keymgmt-data
prtcl-id = non-ws-string
; e.g. "mikey"
keymgmt-data = text
where non-ws-string and text are as defined in SDP [SDPnew]. The
attribute may be used at session level, media level or at both
levels. An attribute defined at media level overrides an attribute
defined at session level.
2.2. RTSP Extensions
To support the needed attribute described, the following RTSP header
is defined:
KeyMgmt = "keymgmt" ":" 1#key-mgmt-spec
key-mgmt-spec = "prot" "=" token ";" "data" "=" quoted-string
token and quoted-string are as defined in the RTSP specification
[RTSP].
The KeyMgmt header should be possible to use in the messages
described in the table below.
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Method Direction Requirement
DESCRIBE C->S required
SETUP C->S required
ANNOUNCE C->S, S->C optional (required: if re-key should
be supported)
3. Usage with SIP and RTSP
This section gives recommendations of how/when to include the defined
key management attribute when SIP and/or RTSP are used together with
SDP.
When a key management protocol is integrated with SIP/SDP and RTSP,
the following requirements are put on the key management:
* It MUST be possible to execute the key management protocol in at
most one roundtrip in case the answerer accepts the offer.
* It MUST be possible from the SIP/SDP and RTSP application, using
the key management API, to receive key management data, and
information of whether a message is accepted or not.
Today, the MIKEY protocol [MIKEY] has adopted the key management
extensions to work together with SIP and RTSP. Other protocols MAY
use the described attribute and header, e.g. Kerberos [KERB].
3.1. General SDP processing
When an SDP message is created, the following procedure should be
applied:
* The identifier of the key management protocol used (e.g. MIKEY or
Kerberos) MUST be put in the prtcl-id field.
* The keymgmt-data field MUST be created with the data received from
the key management protocol (this data MUST be base64 encoded).
The data may e.g. be a MIKEY message or Kerberos ticket.
A received SDP message that contains the key management attributes
SHOULD process these attributes in the following manner:
* The key management protocol used MUST be identified by checking the
prtcl-id field in the key management attribute.
* The key management data from the keymgmt-data field MUST be
extracted and given to the key management protocol. Note that
depending on key management protocol, some extra parameters might
of course be requested, such as the source/destination network
address/port(s) for the specified media.
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* Depending on the outcome of the key management processing (i.e.
whether it was accepted or not), the processing can proceed
according to normal processing (e.g. according to the offer/answer
model, see also Section 3.2).
Note that the attribute MAY be repeated more than once (e.g., one at
session level and one at media level). Consequently, the process is
repeated for each key management attribute detected.
If more than one key management protocol is supported, multiple
instances of the key management attribute MAY be included in the
initial offer, each transporting a different key management data,
thus indicating alternatives supported.
If the sender includes more than one key management protocol
attributes at session level (analogous for the media level), these
SHOULD be listed in order of preference (with the first being the
preferred). The receiver chooses the key management protocol it
supports. When answering, only the accepted key management protocol
attribute MUST be included. If the receiver does not support any of
the sender's suggested key management protocols, the receiver answers
with an error message (see SIP and RTSP), whereby the sender MUST put
down the current setup procedure.
Note that by placing multiple key management offers in a single
message has the disadvantage that the message expands and the
computational workload for the offerer will increase drastically. The
possibility to support multiple key management protocols may
introduce bidding down attacks. To avoid this, the list of
identifiers of the proposed key management protocols MUST be
authenticated, which MUST be done by each key management. This puts
the requirement that it MUST be specified (in the key management
protocol itself or in a companion document) how the protocol
identifiers could be authenticated from the offerer to the responder
by the use of the specific key management protocol. Note that even if
only one key management protocol is used, that still must
authenticate its own protocol identifier.
If more than one protocol is supported by the offerer, it is
RECOMMENDED that he offers all to him acceptable protocols in the
first offer, rather than making single, subsequent alternative offers
in response to error messages, see "Security Considerations".
3.2. SIP usage
The offerer SHOULD include the key management data within an offer
that contains the media description it should apply to. The answerer
MUST check with the key management protocol if the attribute values
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are valid, and then obtain from the key management the data to
include in the answer.
If the offer is not accepted, the answerer SHOULD return a "606 Not
Acceptable" message, including one or more Warning headers (at least
a 306). The offer MUST then abort the security setup.
Re-keying can be handled as a new offer, i.e. a re-INVITE should be
sent with the new proposed parameters. The answerer treats this as a
new offer where the key management is the issue of change. In
general, the re-INVITE (and the key exchange) must be finalized
before the security protocol can change the keys. The synchronization
method used when changing keys are dependent on the security and key
management protocol used.
3.3. RTSP usage
RTSP does not use the offer/answer model, as SIP does. This causes
some problems, as it is not possible (without abusing RTSP) to send
back an answer to the server (as the server will in most cases be the
one initiating the security parameter exchange). To solve this, a new
header has been introduced (Section 2.2). This also assumes that the
key management also has some kind of binding to the media, so that
the response to the server will be processed as required.
The processing of a key management header in RTSP should be done
analogous of the SDP message processing. The initial key management
message from a server should be sent to the client using SDP. When
responding to this, the client uses the new RTSP header to send back
an answer (included in the SETUP message). If a server receives a
SETUP message in which it expects a key management message, but none
is included, a 403 Forbidden SHOULD be returned to the client,
whereby the current setup MUST be aborted.
The server MAY provide re-keying/updating facilities by sending a new
key management message in an ANNOUNCE messages. The ANNOUNCE message
contains an SDP message including the key management parameters. The
response message is put in the new RTSP header in the response from
the client to the server. Note that the ANNOUNCE messages MUST be
supported if this feature is to be used.
3.4. Example scenarios
Example 1 (SIP)
A SIP call is taking place between Alice and Bob. Alice sends an
Invite message consisting of the following offer:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.somewhere.com
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s=Cool stuff
e=alice@w-land.org
t=0 0
c=IN IP4 lost.somewhere.com
a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 49000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52230 RTP/SAVP 31
a=rtpmap:31 H261/90000
i.e. Alice proposes to set up one audio stream and one video stream
that run over SRTP. To set up the security parameters for SRTP, she
uses MIKEY. Note that MIKEY is negotiating the crypto suite for both
streams (as it is placed at the session level).
Bob accepts the offer and sends an answer back to Alice:
v=0
o=bob 2891092897 2891092897 IN IP4 found.somewhere.com
s=Cool stuff
e=bob@null.org
t=0 0
c=IN IP4 found.somewhere.com
a=key-mgmt:mikey skaoqDeMkdwRW278HjKVB...
m=audio 49030 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52230 RTP/SAVP 31
a=rtpmap:31 H261/90000
Example 2 (SDP)
This example shows how Alice would have done in the previous example
if she wished to protect only the audio stream.
v=0
o=alice 2891092738 2891092738 IN IP4 lost.somewhere.com
s=Cool stuff
e=alice@w-land.org
t=0 0
c=IN IP4 lost.somewhere.com
m=audio 49000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
m=video 52230 RTP/AVP 31
a=rtpmap:31 H261/90000
Note that even if the key management attribute is specified at
session level, the video part will not be affected by this (as a
security profile is not used).
Example 3 (RTSP)
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A client wants to set up a streaming session and requests a media
description from the streaming server.
DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/1.0
CSeq: 312
Accept: application/sdp
From: user@client.com
The server sends back an OK message including an SDP description.
RTSP/1.0 200 OK
CSeq: 312
Date: 23 Jan 1997 15:35:06 GMT
Content-Type: application/sdp
v=0
o=actionmovie 2891092738 2891092738 IN IP4 movie.somewhere.com
s=Action Movie
e=action@movie.somewhere.com
t=0 0
c=IN IP4 movie.somewhere.com
a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 0 RTP/SAVP 98
a=rtpmap:98 AMR/8000
control:rtsp://movie.somewhere.com/action/audio
m=video 0 RTP/SAVP 31
a=rtpmap:31 H261/90000
control:rtsp://movie.somewhere.com/action/video
The client is now ready to setup the sessions. It includes the key
management data in the first message going back to the server (i.e.
the SETUP message).
SETUP rtsp://movie.somewhere.com/action/audio RTSP/1.0
CSeq: 313
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057
keymgmt: prot=mikey; data="skaoqDeMkdwRW278HjKVB..."
The server processes the request including checking the validity of
the key management header.
RTSP/1.0 200 OK
CSeq: 313
Session: 12345678
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057;
server_port=5000-5001
The RTSP then proceeds as usual (with e.g. a SETUP message for the
video followed by a PLAY message).
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4. Adding further Key management protocols
This framework cannot be used with all key management protocols. The
key management protocol needs to comply with the requirements
described in Section 3. To be able to use a key management protocol
with this framework, the following MUST be specified:
* the key management protocol identifier that should be used in the
protocol identifier fields in both SDP and RTSP (e.g. "mikey" for
MIKEY).
* the information the key management needs from SDP and RTSP (Section
3 gives a guideline of what SDP and RTSP needs from the key
management). The exact API is implementation specific, but it
SHOULD at least support to exchange the specified information.
Note that in particular, the key management MUST always be given
the protocol identifier(s) of the key management protocol(s)
included in the offer in the correct order as they appear.
The key management data MUST be base64 encoded in the SDP and RTSP
fields. Therefore, considerations of possible conversion from the
normal key management representation to base64 SHOULD be taken into
account.
5. Security Considerations
The nature of this document is to allow SDP and RTSP to support
security of the media sessions. It is therefore not a primary
intention of this document to describe possible security solutions or
to define possible security problems. The defined SDP and RTSP
extensions are not believed to introduce any new security risks to
SDP and RTSP, if used as specified.
Note that the purpose of the key management fields is to provide
information to secure the media streams. Under the assumption that
the key management schemes are secure, the SDP can be passed along
unprotected without affecting the key management, and the media
streams will still be secure even if some attackers gained knowledge
of the SDP contents.
However, if the SDP messages are not sent authenticated between the
parties, it is possible for an active attacker to change attributes
without being detected. As the key management protocol may
(indirectly) rely on some of the session information from SDP (e.g.,
address information), an attack on SDP may have indirect consequences
on the key management. In general, it is therefore a good thing, not
only to try to secure the session, but also to secure the session
setup.
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Note that it is impossible to assure the authenticity of a declined
offer, since even if it comes from the true respondent, the fact that
he/she declines the offer usually means that he/she does not support
the protocol(s) offered, and consequently cannot be expected to
authenticate the response either. This means that if the initiator is
unsure of which protocol(s) the responder supports, we RECOMMEND that
the initiator offers all acceptable protocols in a single offer. If
not, this opens up the possibility for a "man-in-the-middle" (MITM)
to affect the outcome of the eventually agreed upon protocol, by
faking unauthenticated error messages until the initiator eventually
offers a protocol "to the liking" of the MITM. This is not really a
security problem, but rather a mild form of denial of service that
can be avoided by following the above recommendation.
6. IANA Considerations
New attribute fields for SDP (see Section 2.1) and RTSP header are
registered (see Section 2.2).
7. Conclusions
A security solution for real-time applications needs a key management
infrastructure. Integrating the key management scheme with the
session establishment protocol could be done efficiently in most of
the scenarios. This draft proposes a framework that integrates a key
management protocol (e.g., MIKEY) into SIP and RTSP, and which can be
accompanied by different key management protocols. A set of new
attributes and headers has been defined in SDP and RTSP to support
this.
8. Acknowledgments
Thanks to: Rolf Blom, Magnus Westerlund, and the rest involved in the
MMUSIC WG and the MSEC WG.
A special thanks to Joerg Ott and Colin Perkins.
9. Author's Addresses
Jari Arkko
Ericsson
02420 Jorvas Phone: +358 40 5079256
Finland Email: jari.arkko@ericsson.com
Elisabetta Carrara
Ericsson Research
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SE-16480 Stockholm Phone: +46 8 50877040
Sweden EMail: elisabetta.carrara@era.ericsson.se
Fredrik Lindholm
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58531705
Sweden EMail: fredrik.lindholm@era.ericsson.se
Mats Naslund
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58533739
Sweden EMail: mats.naslund@era.ericsson.se
Karl Norrman
Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@era.ericsson.se
10. References
10.1. Normative References
[OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with
the Session Description Protocol (SDP)", IETF, 3264.
[RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time
Streaming Protocol (RTSP)", IETF, RFC 2326.
[SDPnew] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
Description Protocol", Internet Draft, IETF, Work in progress
(MMUSIC).
[SIP] Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J.,
"SIP: Session Initiation Protocol", IETF, RFC 2543.
10.2. Informative References
[KERB] Kohl, J., Neuman, C., "The Kerberos Network Authentication
Service (V5)", IETF, RFC 1510.
[MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
Norrman, K., "MIKEY: Multimedia Internet KEYing", Internet Draft,
IETF, Work in progress (MSEC).
[SRTP] Baugher, M., Blom, R., Carrara, E., McGrew, D., Naslund, M,
Norrman, K., and Oran, D., "The Secure Real Time Transport Protocol",
Internet Draft, IETF, Work in Progress (AVT).
This Internet-Draft expires in August 2003.
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