Internet Engineering Task Force                       Carrara (KTH)
                                        Lehtovirta, Norrman (Ericsson)
   EXPIRES: August 2006                                  February 2006

 The Key ID Information Type for the General Extension Payload in MIKEY

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   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.

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    1. Introduction...................................................2
    2. Rationale......................................................2
    3. Relations to MIKEY and GMARCH..................................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. Security Considerations........................................6
    7. IANA Considerations............................................7
    8. Acknowledgements...............................................8
    9. Author's Addresses.............................................8
    10. 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

   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

   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 GMARCH

   MIKEY is according to [RFC3830] not a rekey protocol, but a
   registration protocol in the terms of the MSEC Group Key Management
   Architecture [RFC4046].  However, 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 as a 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

   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.

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   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.  Applications using this general extension
   payload have to define the semantics and usage of the Key ID Type

   The MBMS use of the Key ID Type sub-payloads is as follows.  Since
   each MTK is protected by a particular MSK, there is a need to
   indicate which MSK is used.  This is done by including the MSK
   identity as well as the MTK identity in the Extension payload with
   MTK delivery messages.  The MSK itself is of course not included in
   the message, only its identity.

5. Empty map type definition for the CS ID map type

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   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
   "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. 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.
   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.

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   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

7. IANA Considerations

   Please add the following to the IANA registry at (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

   * 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

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   and values in the range 241-255 are reserved for Private Use,
   according to [RFC2434].

8. Acknowledgements

   We would like to thank Fredrik Lindholm.

9. Author's Addresses

   Questions and comments should be directed to the authors:

      Elisabetta Carrara
      Royal Institute of Technology
      Stockholm              Phone:
      Sweden                 EMail:

      Vesa Lehtovirta
      Ericsson Research
      02420 Jorvas           Phone:  +358 9 2993314
      Finland                EMail:

      Karl Norrman
      Ericsson Research
      SE-16480 Stockholm     Phone:  +46 8 4044502
      Sweden                 EMail:

10. References


   [RFC3830] Arkko et al., "MIKEY: Multimedia Internet KEYing", RFC
   3830, August 2004.


   [RFC3711] Baugher et al., "The Secure Real-time Transport Protocol
   (SRTP)", RFC3711, March 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

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   [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

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