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RFC 4563 - The Key ID Information Type for the General Extension

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Network Working Group                                         E. Carrara
Request for Comments: 4563                                           KTH
Category: Standards Track                                  V. Lehtovirta
                                                              K. 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 Notice

   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
   Partnership Project.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. Rationale .......................................................2
   3. Relations to MIKEY and GKMARCH ..................................3
   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 ........................................5
   7. Security Considerations .........................................5
   8. IANA Considerations .............................................7
   9. Acknowledgements ................................................7
   10. References .....................................................8
      10.1. Normative References ......................................8
      10.2. Informative References ....................................8

1.  Introduction

   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
   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 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
   their identity.

   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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Rationale

   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
   multicast/broadcast service.

   * 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)
   group key.

   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].

9.  Acknowledgements

   We would like to thank Fredrik Lindholm.

10.  References

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,
              August 2004.

   [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,
              October 1998.

Authors' Addresses

   Elisabetta Carrara
   Royal Institute of Technology

   EMail: carrara@kth.se

   Vesa Lehtovirta
   Ericsson Research
   02420 Jorvas

   Phone: +358 9 2993314
   EMail: vesa.lehtovirta@ericsson.com

   Karl Norrman
   Ericsson Research
   SE-16480 Stockholm

   Phone: +46 8 4044502
   EMail: karl.norrman@ericsson.com

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