Internet Engineering Task Force Carrara (KTH)
Lehtovirta, Norrman (Ericsson)
INTERNET-DRAFT
EXPIRES: September 2006 March 2006
The Key ID Information Type for the General Extension Payload in MIKEY
<draft-ietf-msec-newtype-keyid-05.txt>
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Abstract
This memo specifies a new Type (the Key ID Information Type) for the
General Extension Payload in the Multimedia Internet KEYing
Protocol. This is used in, e.g., the Multimedia Broadcast/Multicast
Service specified in the 3rd Generation Partnership Project.
INTERNET-DRAFT newtype-keyid March, 2006
TABLE OF CONTENTS
1. Introduction...................................................2
2. Rationale......................................................2
3. Relations to MIKEY and GKMARCH.................................4
4. The Key ID Information Type for the General Extension Payload..4
5. Empty map type definition for the CS ID map type...............5
6. Transport Considerations.......................................6
7. Security Considerations........................................6
8. IANA Considerations............................................7
9. Acknowledgements...............................................8
10. Author's Addresses............................................8
11. References....................................................8
1. Introduction
The 3rd Generation Partnership Project (3GPP) is currently involved
in the development of a multicast and broadcast service, the
Multimedia Broadcast/Multicast Service (MBMS), and its security
architecture [MBMS].
[MBMS] requires the use of the Multimedia Internet KEYing (MIKEY)
Protocol [RFC3830], to convey the keys and related security
parameters needed to secure the multimedia that is multicast or
broadcast.
One of the requirements that MBMS puts on security is the ability to
perform frequent updates of the keys. The rationale behind this is
that it will be costly for subscribers to re-distribute the
decryption keys to non-subscribers. The cost for re-distributing the
keys using the unicast channel should be higher than the cost of
purchasing the keys for this scheme to have an effect. To implement
this, MBMS uses a three level key management, to distribute group
keys to the clients, and be able to re-key by pushing down a new
group key. As illustrated in the section below, MBMS has the need
to identify which types of key are involved in the MIKEY message,
and their identity.
This memo specifies a new Type for the General Extension Payload in
MIKEY, to identify the type and identity of keys involved.
2. Rationale
An application where this extension is used is MBMS key management.
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The key management solution adopted by MBMS uses three level key
management. The keys are used in the way described below.
"Clients" refers to the clients who have subscribed to a given
multicast/broadcast service.
* The MBMS User Key (MUK), point-to-point key between the multicast
server and each client
* The MBMS Service Key (MSK), group key between the multicast server
and all the clients
* The MBMS Traffic Key (MTK), group traffic key between the
multicast server and all clients.
The Traffic Keys are the keys that are regularly updated.
The point-to-point MUK key (first-level key) is shared between the
multicast server and the client via means defined by MBMS [MBMS].
The MUK is used as pre-shared key to run MIKEY with the pre-shared
key method [RFC3830], to deliver (point-to-point) the MSK key. The
same MSK key is pushed to all the clients, to be used as a (second-
level) group key.
Then, the MSK is used to push to all the clients an MTK key (third-
level key), the actual group key that is used for the protection of
the media traffic. For example, the MTK could be the master key for
the Secure Real-time Transport Protocol (SRTP) [RFC3711] in the
streaming case.
A Key Domain identifier defines the domain where the group keys are
valid or applicable. For example it may define a specific service
provider.
To allow the key distribution described above, an indication of the
type and identity of keys being carried in a MIKEY message is
needed. This indication is carried in a new Type of the General
Extension Payload in MIKEY.
It is necessary to specify what Crypto Session ID (CS ID) map type
is associated with a given key. There are cases, for example the
download case in MBMS, where the required parameters are signalled
in-band (each downloaded Digital Rights Management Content Format-
object [DCF] contains the necessary parameters needed by the
receiver to process it). Since the parameters are not transported
by MIKEY, this implies that a CS ID map type needs to be registered
to the "empty map" as defined in Table 3, which is to be used when
the map/policy information is conveyed outside of MIKEY.
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3. Relations to MIKEY and GKMARCH
MIKEY according to [RFC3830] is a registration protocol that
supports re-keying for unicast in the terms of the MSEC Group Key
Management Architecture [RFC4046]. MBMS uses MIKEY both as a
registration protocol and a rekey protocol, and the specified
extension implements the necessary additions to [RFC3830] that
allows MIKEY to function both as a unicast and multicast rekey
protocol in the MBMS setting.
4. The Key ID Information Type for the General Extension Payload
The General Extension payload in MIKEY is defined in Section 6.15 of
[RFC3830]. The General Extension payload type (Key ID Information)
defined in the present document is not restricted to MBMS.
Applications using this General Extension payload type may define
new Key ID types, and these applications MUST define the semantics
and usage of the Key ID Type sub-payloads according to Section 8.
The MBMS use of the Key ID Type sub-payloads defined in Table 2 is
specified in [MBMS].
The Key ID Information Type (Type 3) formats the General Extension
payload as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next payload ! Type ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Key ID Information ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. The Key ID Information General Extension Payload.
Next Payload and Length are defined in Section 6.15 of [RFC3830].
* Type (8 bits): identifies the type of the General Extension
Payload [RFC3830]. This memo adds Type 3 to the ones already
defined in [RFC3830].
Type | Value | Comments
------------------------------------------------------------
Key ID | 3 | information on type and identity of keys
Table 1. Definition of the new General Extension Payload.
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* Key ID Information (variable length): the general payload data
transporting the type and identifier of a key. This field is
formed by Key ID Type sub-payloads as specified below.
The Key ID Type sub-payload is formatted as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Key ID Type ! Key ID Length ! Key ID ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. The Key ID Type sub-payload.
* Key ID Type (8 bits): describes the type of the key ID.
Predefined types are listed in Table 2.
Key ID Type | Value | Comment
--------------------------------------
MBMS Key Domain ID | 0 | ID of the group key domain
MBMS Service Key ID | 1 | ID of the group key
MBMS Transport Key ID | 2 | ID of the group traffic key
Table 2. Type definitions for Key IDs.
* Key ID Length (8 bits): describes the length of the Key ID
field in octets.
* Key ID (variable length): defines the identity of the key.
Note that there may be more than one Key ID Type sub-payload in an
extension, and that the overall length (i.e., the sum of lengths of
all Key ID Type sub-payloads) of the Key ID information field cannot
exceed 2^16 - 1 octets.
5. Empty map type definition for the CS ID map type
When the security policy information is conveyed outside of MIKEY,
the CS ID map type is set to value defined in Table 3 to indicate
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"empty map". In this case, there MUST NOT be any Security Policy
payload present in the message.
CS ID map type | Value | Comments
------------------------------------------------------------
Empty map | 1 | Used when the map/policy information
| | is conveyed outside of MIKEY
Table 3. Definition of the CS ID map type.
6. Transport Considerations
As specified in Section 7 of [RFC3830], the underlying transport of
the MIKEY protocol has to be defined for each type of transport.
When the Key-ID payload is used with MBMS, the transport is UDP, and
the usage of MIKEY over UDP in the MBMS setting is defined in
[MBMS].
7. Security Considerations
The usage of MIKEY for updating the traffic encryption key (MTK) in
the broadcast manner, described in Section 2, deviates from the way
MIKEY [RFC3830] was originally designed. There are mainly two
points that are related to the security of the described usage.
First, the delivery of the MTK is not source origin authenticated,
but rather protected by a group MAC, keyed by the group key (MSK).
The threat this raises is that users that are part of the group are
able to send faked MTK messages to other group members. The origin
of the MTK messages is a node inside the core network, and the trust
model used in MBMS, is that only trusted traffic is allowed to be
sent on the MBMS bearers (from within the operator's network).
However, there is always the risk that traffic is injected on the
air interface between the base stations and the user equipment. It
is possible for members of the group (i.e., with access to the MSK)
to spoof MTK updates to other members of the group. 3GPP decided
that the technical difficulties and costs involved in performing
such an attack are large enough compared to the expected gain for
the attacker, that the risk was deemed acceptable. Note that, since
the delivery of the MTK is not source origin authenticated, there is
nothing gained by adding source origin authentication to the RTP
streams (e.g., using SRTP-TESLA [SRTP_TESLA]). Hence, the current
use of the specified extension is not compatible with SRTP-TESLA,
which requires source origin authentication of the integrity key.
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Note that in MBMS the MSK is protected end-to-end, from the
multicast server to the clients, using a client-unique key MUK, but
the MTK is delivered under protection by the group key MSK, so
source origin authentication is not achieved.
Secondly, the delivery of the MTK is separated from the delivery of
the security policy. The security policy is delivered with the MSK.
The delivery of the MTKs is assumed to be very frequent (some
scenarios require the delivery of MTKs to be as frequent as a few
seconds apart). This would imply that the cost (in terms of
bandwidth) would be very high if the security policy was delivered
together with each MTK. Furthermore, the security policy parameters
of the streaming session are not anticipated to change during the
session, even though there would be an update of the MTK. It was
decided in 3GPP that there was no need for updating the policy
during an ongoing session, and the cost of allowing such a feature,
only to be on the safe side, would be too high. On the other hand,
updating the security policy during an ongoing session would be
possible by updating the MSK.
The Empty map type used when the security policy is delivered in
band relies on the security provided by DCF [DCF], and MIKEY is in
this case only used to provide the master key for the DCF
processing.
8. IANA Considerations
Please add the following to the IANA registry at
http://www.iana.org/assignments/mikey-payloads (To be removed by
after IANA processing).
According to Section 10 of RFC 3830, IETF consensus is required to
register values in the range 0-240 in the Type namespace of the
MIKEY General Extension Payload and the CS ID map type namespace of
the Common Header Payload.
A new value in the MIKEY General Extension Payload Type name space
needs to be registered for this purpose. The registered value for
Key ID is requested to be 3 according to Section 4.
It is also requested to register the value 1 for the Empty map in
the existing CS ID map namespace of the Common Header Payload as
specified in Table 3 in Section 5.
The new name space for the following field in the Key ID information
sub-payload (from Sections 4 and 5) is requested to be managed by
IANA:
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* Key ID Type
It is requested that IANA register the pre-defined types of Table 2
for this namespace. IANA is also requested to manage the definition
of additional values in the future. Values in the range 0-240 for
each name space SHOULD be approved by the process of IETF consensus
and values in the range 241-255 are reserved for Private Use,
according to [RFC2434].
9. Acknowledgements
We would like to thank Fredrik Lindholm.
10. Author's Addresses
Questions and comments should be directed to the authors:
Elisabetta Carrara
Royal Institute of Technology
Stockholm Phone:
Sweden EMail: carrara@kth.se
Vesa Lehtovirta
Ericsson Research
02420 Jorvas Phone: +358 9 2993314
Finland EMail: vesa.lehtovirta@ericsson.com
Karl Norrman
Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@ericsson.com
11. References
Normative
[RFC3830] Arkko et al., "MIKEY: Multimedia Internet KEYing", RFC
3830, August 2004.
[MBMS] 3GPP TS 33.246, "Technical Specification 3rd Generation
Partnership Project; Technical Specification Group Services and
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System Aspects; Security; Security of Multimedia Broadcast/Multicast
Service."
Informative
[RFC3711] Baugher et al., "The Secure Real-time Transport Protocol
(SRTP)", RFC3711, March 2004.
[DCF] OMA, OMA-DRM-DCF-V2_0-20050329-C, "DRM Content Format V2.0",
Candidate Version 2.0 - 29 March 2005
[SRTP_TESLA] Baugher et al., "The Use of TESLA in SRTP", draft-ietf-
msec-srtp-tesla-05.txt. Work in progress.
[RFC4046] Baugher et al., "MSEC Group Key Management Architecture",
RFC 4046, April 2005.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
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