Network Working Group E. Carrara
Request for Comments: 4563 KTH
Category: Standards Track V. LehtovirtaK. Norrman
June 2006 The Key ID Information Type for the General Extension Payload in
Multimedia Internet KEYing (MIKEY)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
This memo specifies a new Type (the Key ID Information Type) for the
General Extension Payload in the Multimedia Internet KEYing (MIKEY)
Protocol. This is used in, for example, the Multimedia
Broadcast/Multicast Service specified in the Third Generation
Table of Contents
1. Introduction ....................................................21.1. Conventions Used in This Document ..........................22. Rationale .......................................................23. Relations to MIKEY and GKMARCH ..................................34. The Key ID Information Type for the General Extension Payload ...45. Empty Map Type Definition for the CS ID Map Type ................56. Transport Considerations ........................................57. Security Considerations .........................................58. IANA Considerations .............................................79. Acknowledgements ................................................710. References .....................................................810.1. Normative References ......................................810.2. Informative References ....................................8
The Third 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
[MBMS] requires the use of the Multimedia Internet KEYing (MIKEY)
Protocol [RFC3830] to convey the keys and related security parameters
needed to secure 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 keys are involved in the MIKEY message and
This memo specifies a new Type for the General Extension Payload in
MIKEY, to identify the type and identity of keys involved.
1.1. Conventions Used in This Document
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 [RFC2119].
An application where this extension is used is MBMS key management.
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
* The MBMS User Key (MUK), a point-to-point key between the multicast
server and each client.
* The MBMS Service Key (MSK), a group key between the multicast
server and all the clients.
* The MBMS Traffic Key (MTK), a 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 (first-level key) is shared between the
multicast server and the client via means defined by MBMS [MBMS].
The MUK is used as a pre-shared key to run MIKEY with the pre-shared
key method [RFC3830], and to deliver (point-to-point) the MSK. The
same MSK is pushed to all the clients, to be used as a (second-level)
Then, the MSK is used to push to all the clients an MTK (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
A Key Domain identifier defines the domain where the group keys are
valid or applicable. For example, it may define a specific service
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.
3. Relations to MIKEY and GKMARCH
According to [RFC3830], MIKEY 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 re-key protocol, and the specified
extension implements the necessary additions to [RFC3830] that allows
MIKEY to function both as a unicast and multicast re-key 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
* 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 Traffic 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 a value defined in Table 3 to indicate
"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 two main 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 fake 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
(from within the operator's network) on the MBMS bearers. 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 [RFC4383]). Hence, the current use
of the specified extension is not compatible with SRTP-TESLA, which
requires source origin authentication of the integrity key.
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 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 that 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
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
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
has been registered for this purpose. The registered value for Key
ID is 3 according to Section 4.
Also, the value 1 has been registered 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) has been established and will be
managed by IANA:
* Key ID Type
The IANA has registered the pre-defined types of Table 2 for this
namespace. IANA will also 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].
We would like to thank Fredrik Lindholm.
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
[MBMS] 3GPP TS 33.246, "Technical Specification 3rd Generation
Partnership Project; Technical Specification Group
Services and System Aspects; Security; Security of
Multimedia Broadcast/Multicast Service".
10.2. Informative References
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[DCF] Open Mobile Alliance, OMA-DRM-DCF-V2_0-20050329-C, "DRM
Content Format V2.0", Candidate Version 2.0 - 29 March
[RFC4383] Baugher, M. and E. Carrara, "The Use of Timed Efficient
Stream Loss-Tolerant Authentication (TESLA) in the Secure
Real-time Transport Protocol (SRTP)", RFC 4383, February
[RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
"Multicast Security (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,
Royal Institute of Technology
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