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Network Working Group E. CarraraRequest for Comments: 4563 KTHCategory: Standards Track V. Lehtovirta K. Norrman Ericsson June 2006The Key ID Information Type for the General Extension Payload inMultimedia 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).Abstract 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 Contents1. 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 ....................................8Carrara, et al. Standards Track [Page 1]

RFC 4563 Key ID for General Extension Payload June 20061. 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described inRFC 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.Carrara, et al. Standards Track [Page 2]

RFC 4563 Key ID for General Extension Payload June 2006 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 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.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.Carrara, et al. Standards Track [Page 3]

RFC 4563 Key ID for General Extension Payload June 20064. The Key ID Information Type for the General Extension Payload The General Extension payload in MIKEY is defined inSection 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 toSection 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 inSection 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-payloadCarrara, et al. Standards Track [Page 4]

RFC 4563 Key ID for General Extension Payload June 2006 * 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 inSection 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 inSection 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.Carrara, et al. Standards Track [Page 5]

RFC 4563 Key ID for General Extension Payload June 2006 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 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.Carrara, et al. Standards Track [Page 6]

RFC 4563          Key ID for General Extension Payload         June 20068.  IANA Considerations   According toSection 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 toSection 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, inSection 5.   The new name space for the following field in the Key ID information   sub-payload (from Sections4 and5) 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.Carrara, et al.             Standards Track                     [Page 7]

RFC 4563          Key ID for General Extension Payload         June 200610.  References10.1.  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              2005.   [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              2006.   [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.Carrara, et al.             Standards Track                     [Page 8]

RFC 4563          Key ID for General Extension Payload         June 2006Authors' Addresses   Elisabetta Carrara   Royal Institute of Technology   Stockholm   Sweden   EMail: carrara@kth.se   Vesa Lehtovirta   Ericsson Research   02420 Jorvas   Finland   Phone: +358 9 2993314   EMail: vesa.lehtovirta@ericsson.com   Karl Norrman   Ericsson Research   SE-16480 Stockholm   Sweden   Phone: +46 8 4044502   EMail: karl.norrman@ericsson.comCarrara, et al.             Standards Track                     [Page 9]

RFC 4563          Key ID for General Extension Payload         June 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Carrara, et al.             Standards Track                    [Page 10]

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