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Network Working Group                                       A. BallardieRequest for Comments: 1949                     University College LondonCategory: Experimental                                          May 1996Scalable Multicast Key DistributionStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Abstract   The benefits of multicasting are becoming ever-more apparent, and its   use much more widespread. This is evident from the growth of the   MBONE [1]. Providing security services for multicast, such as traffic   integrity, authentication, and confidentiality, is particularly   problematic since it requires securely distributing a group (session)   key to each of a group's receivers.  Traditionally, the key   distribution function has been assigned to a central network entity,   or Key Distribution Centre (KDC), but this method does not scale for   wide-area multicasting, where group members may be widely-distributed   across the internetwork, and a wide-area group may be densely   populated.   Even more problematic is the scalable distribution of sender-specific   keys. Sender-specific keys are required if data traffic is to be   authenticated on a per-sender basis.   This memo provides a scalable solution to the multicast key   distribution problem.   NOTE: this proposal requires some simple support mechanisms, which,   it is recommended here, be integrated into version 3 of IGMP. This   support is described inAppendix B.1.  Introduction   Growing concern about the integrity of Internet communication [13]   (routing information and data traffic) has led to the development of   an Internet Security Architecture, proposed by the IPSEC working   group of the IETF [2]. The proposed security mechanisms are   implemented at the network layer - the layer of the protocol stack at   which networking resources are best protected [3].Ballardie                     Experimental                      [Page 1]

RFC 1949          Scalable Multicast Key Distribution           May 1996   Unlike many network layer protocols, the Core Based Tree (CBT)   multicast protocol [4] makes explicit provision for security; it has   its own protocol header, unlike existing IP multicast schemes   [10,11], and other recently proposed schemes [12].   In this document we describe how the CBT multicast protocol can   provide for the secure joining of a CBT group tree, and how this same   process can provide a scalable solution to the multicast key   distribution problem.  These security services are an integral part   of the CBT protocol [4]. Their use is optional, and is dependent on   each individual group's requirements for security. Furthermore, the   use of the CBT multicast protocol for multicast key distribution does   not preclude the use of other multicast protocols for the actual   multicast communication itself, that is, CBT need only be the vehicle   with which to distribute keys.   Secure joining implies the provision for authentication, integrity,   and optionally, confidentiality, of CBT join messages. The scheme we   describe provides for the authentication of tree nodes (routers) and    receivers (end-systems) as part of the tree joining process. Key   distribution (optional) is an integral part of secure joining.   Network layer multicast protocols, such as DVMRP [7] and M-OSPF [9],   do not have their own protocol header(s), and so cannot provision for   security in themselves; they must rely on whatever security is   provided by IP itself. Multicast key distribution is not addressed to   any significant degree by the new IP security architecture [2].   The CBT security architecture is independent of any particular   cryptotechniques, although many security services, such as   authentication, are easier if public-key cryptotechniques are   employed.   What follows is an overview of the CBT multicasting. The description   of our proposal insection 6.1 assumes the reader is reasonably   familiar with the CBT protocol. Details of the CBT architecture and   protocol can be found in [7] and [4], respectively.2.  Overview of BCT Multicasting   CBT is a new architecture for local and wide-area IP multicasting,   being unique in its utilization of just one shared delivery tree per   group, as opposed to the source-based delivery tree approach of   existing IP multicast schemes, such as DVMRP and MOSPF.   A shared multicast delivery tree is built around several so-called   core routers. A group receiver's local multicast router is required   to explicitly join the corresponding delivery tree after receiving anBallardie                     Experimental                      [Page 2]

RFC 1949          Scalable Multicast Key Distribution           May 1996   IGMP [8] group membership report over a directly connected interface.   A CBT join message is targeted at one of the group's core routers.   The resulting acknowledgement traverses the reverse-path of the join,   resulting in the creation of a tree branch. Routers along these   branches are called non-core routers for the group, and there exists   a parent-child relationship between adjacent routers along a branch   of the same tree (group).3.  How the CBT Architecture Complements Security   The CBT architecture requires "leaf" routers to explicitly join a CBT   tree. Hence, CBT is not data driven; the ack associated with a join   "fixes" tree state in the routers that make up the tree. This so-   called "hard state" remains until the tree re-configures, for   example, due to receivers leaving the group, or because an upstream   failure has occurred. The CBT protocol incorporates mechanisms   enabling a CBT tree to repair itself in the event of the latter.   As far as the establishment of an authenticated multicast   distribution tree is concerned, DVMRP, M-OSPF, and PIM, are at a   disadvan- tage; the nature of their "soft state" means a delivery   tree only exists as long as there is data flow.  Also, routers   implementing a multicast protocol that builds its delivery tree based   on a reverse-path check (like DVMRP and PIM dense mode) cannot be   sure of the previous-hop router, but only the interface a multicast   packet arrived on.   These problems do not occur in the CBT architecture. CBT's hard state   approach means that all routers that make up a delivery tree know who   their on-tree neighbours are; these neighbours can be authenticated   as part of delivery tree set-up. As part of secure tree set-up,   neighbours could exchange a secret packet handle for inclusion in the   CBT header of data packets exchanged between those neighbours,   allowing for the simple and efficient hop-by-hop authentication of   data packets (on-tree).   The presence of tree focal points (i.e. cores) provides CBT trees   with natural authorization points (from a security viewpoint) -- the   formation of a CBT tree requires a core to acknowledge at least one   join in order for a tree branch to be formed. Thereafter,   authorization and key distribution capability can be passed on to   joining nodes that are authenticated.   In terms of security, CBT's hard state approach offers several   additional advantages: once a multicast tree is established, tree   state maintained in the routers that make up the tree does not time   out or change necessarily to reflect underlying unicast topology.   The security implications of this are that nodes need not be subjectBallardie                     Experimental                      [Page 3]

RFC 1949          Scalable Multicast Key Distribution           May 1996   to repeated authentication subsequent to a period of inactivity, and   tree nodes do not need to re-authenticate themselves as a result of   an underlying unicast topology change, unless of course, an network   (node) failure has occurred.   Hard-state protocol mechanisms are often thought of as being less   fault tolerant than soft-state schemes, but there are pros and cons   to both approaches; we see here that security is one of the pros.4.  The Multicast Key Distribution Problem   We believe that multicast key distribution needs to be combined with   group access control. Without group access control, there is no point   in employing multicast key distribution, since, if there are no group   restrictions, then it should not matter to whom multicast information   is divulged.   There are different ways of addressing group access control. The   group access control we describe requires identifying one group   member (we suggest in [14] that this should be the group initiator)   who has the ability to create, modify and delete all or part of a   group access control list. The enforcement of group access control   may be done by a network entity external to the group, or by a group   member.   The essential problem of distributing a session (or group) key to a   group of multicast receivers lies in the fact that some central key   management entity, such as a key distribution centre (KDC) (A Key   Distribution Centre (KDC) is a network entity, usually residing at a   well-known address. It is a third party entity whose responsibility   it to generate and distribute symmetric key(s) to peers, or group   receivers in the case of multicast, wishing to engage in a "secure"   communication. It must therefore be able to identify and reliably   authenticate requestors of symmetric keys.), must authenticate each   of a group's receivers, as well as securely distribute a session key   to each of them.  This involves encrypting the relevant message n   times, once with each secret key shared between the KDC and   corresponding receiver (or alternatively, with the public key of the   receiver), before multicasting it to the group. (Alternatively, the   KDC could send an encrypted message to each of the receivers   individually, but this does not scale either.)  Potentially, n may be   very large.  Encrypting the group key with the secret key (of a   secret-public key pair) of the KDC is not an option, since the group   key would be accessible to anyone holding the KDC's public key, and   public keys are either well-known or readily available.  In short,   existing multicast key distribution methods do not scale.Ballardie                     Experimental                      [Page 4]

RFC 1949          Scalable Multicast Key Distribution           May 1996   The scaling problem of secure multicast key distribution is   compounded for the case where sender-specific keys need to be   distributed to a group. This is required for sender-specific   authentication of data traffic. It is not possible to achieve per-   sender authentication, given only a group session key.   Recently a proposal has emerged, called the Group Key Management   Protocol (GKMP) [15]. This was designed for military networks, but   the authors have demonstrated how the architecture could be applied   to a network like the Internet, running receiver-oriented multicast   applications.   GKMP goes a considerable way to addressing the problems of multicast   key distribution: it does not rely on a centralised KDC, but rather   places the burden of key management on a group member(s). This is the   approach adopted by the CBT solution, but our solution can take this   distributed approach further, which makes our scheme that much more   scalable. Furthermore, our scheme is relatively simple.   The CBT model for multicast key distribution is unique in that it is   integrated into the CBT multicast protocol itself. It offers a   simple, low-cost, scalable solution to multicast key distribution. We   describe the CBT multicast key distribution approach below.5.  Multicast Security Associations   The IP security architecture [2] introduces the concept of "Security   Associations" (SAs), which must be negotiated in advance during the   key management phase, using a protocol such as Photuris [20], or   ISAKMP [21].  A Security Association is normally one-way, so if two-   way communication is to take place (e.g. a typical TCP connection),   then two Security Associations need to be negotiated.  During the   negotiation phase, the destination system normally assigns a Security   Parameter Index to the association, which is used, together with the   destination address (or, for the sender, the sender's user-id) to   index into a Security Association table, maintained by the   communicating parties.  This table enables those parties to index the   correct security parameters pertinent to an association.  The   security association parameters include authentication algorithm,   algorithm mode, cryptographic keys, key lifetime, sensitivity level,   etc.   The establishment of Security Associations (SA) for multicast   communication does not scale using protocols like Photuris, or   ISAKMP.  This is why it is often assumed that a multicast group will   be part of a single Security Association, and hence share a single   SPI. It is assumed that one entity (or a pair of entities) creates   the SPI "by some means" (which may be an SA negotiation protocol,Ballardie                     Experimental                      [Page 5]

RFC 1949          Scalable Multicast Key Distribution           May 1996   like [20] and [21]), which is then simply multicast, together with   the SA parameters, to the group for subsequent use. However, this   precludes multicast receivers from performing sender-specific origin   authentication; all a receiver can be sure of is that the sender is   part of the multicast Security Association.   We advocate that the primary core, either alone, or in conjunction   with the group initiator, establish the security parameters to be   used in the group communication. These are distributed as part of the   secure join process. Thereafter, individual senders can distribute   their own key and security parameters to the group.  In the case of   the latter, there are two cases to consider:   +    the sender is already a group member. In this case, the sender        can decide upon/generate its own security parameters, and multi-        cast them to the group using the current group session key.   +    the sender is not a group member. In this case, before the        sender begins sending, it must first negotiate the security        parameters with the primary core, using a protocol such as Pho-        turis [20] or ISAKMP [21].  Once completed, the primary core        multicasts (securely) the new sender's session key and security        parameters to the group.   Given that we assume the use of asymmetric cryptotechniques   throughout, this scheme provides a scalable solution to multicast   origin authentication.   Sender-specific keys are also discussed insection 8.6.  The CBT Multicast Key Distribution Model   The security architecture we propose allows not only for the secure   joining of a CBT multicast tree, but also provides a solution to the   multicast key distribution problem [16]. Multicast key distribution   is an optional, but integral, part of the secure tree joining   process; if a group session key is not required, its distribution may   be omitted.   The use of CBT for scalable multicast key distribution does not   preclude the use of other multicast protocols for the actual   multicast communication.  CBT could be used solely for multicast key   distribution -- any multicast protocol could be used for the actual   multicast communication itself.   The model that we propose does not rely on the presence of a   centralised KDC -- indeed, the KDC we propose need not be dedicated   to key distribution. We are proposing that each group have its ownBallardie                     Experimental                      [Page 6]

RFC 1949          Scalable Multicast Key Distribution           May 1996   group key distribution centre (GKDC), and that the functions it   provides should be able to be "passed on" to other nodes as they join   the tree.  Hence, our scheme involves truly distributed key   distribution capability, and is therefore scalable. It does not   require dedicated KDCs.  We are proposing that a CBT primary core   initially take on the role of a GKDC.6.1  Operational Overview   When a CBT group is created, it is the group initiator's   responsibility to create a multicast group access control list (ACL)   [14]. It is recommended that this list is a digitally signed   "document", the same as (or along the lines of) an X.509 certificate   [9], such that it can be authenticated.  The group initiator   subsequently unicasts the ACL to the primary core for the group. This   communication is not part of the CBT protocol. The ACL's digital   signature ensures that it cannot be modified in transit without   detection. If the group membership itself is sensitive information,   the ACL can be additionally encrypted with the public key of the   primary core before being sent.  The ACL can be an "inclusion" list   or an "exclusion" list, depending on whether group membership   includes relatively few, or excludes relatively few.   The ACL described above consists of group membership (inclusion or   exclusion) information, which can be at the granularity of hosts or   users. How these granularities are specified is outside the scope of   this document.  Additionally, it may be desirable to restrict key   distribution capability to certain "trusted" nodes (routers) in the   network, such that only those trusted nodes will be given key   distribution capability should they become part of a CBT delivery   tree. For this case, an additional ACL is required comprising   "trusted" network nodes.   The primary core creates a session key subsequent to receiving and   authenticating the message containing the access control list.  The   primary core also creates a key encrypting key (KEK) which is used   for re-keying the group just prior to an old key exceeding its life-   time.  This re-keying strategy means that an active key is less   likely to become compromised during its lifetime.   The ACL(s), group key, and KEK are distributed to secondary cores as   they become part of the distribution tree.   Any tree node with this information can authenticate a joining   member, and hence, secure tree joining and multicast session key   distribution are truly distributed across already authenticated tree   nodes.Ballardie                     Experimental                      [Page 7]

RFC 1949          Scalable Multicast Key Distribution           May 19966.2  Integrated Join Authentication and Multicast Key Distribution   For simplicity, in our example we assume the presence of an   internetwork-wide asymmetric key management scheme, such as that   proposed in [17].  However, we are not precluding the use of   symmetric cryptographic techniques -- all of the security services we   are proposing, i.e. integrity, authentication, and confidentiality,   can all be achieved using symmetric cryptography, albeit a greater   expense, e.g. negotiation with a third party to establish pairwise   secret keys. For these reasons, we assume that a public (asymmetric)   key management scheme is globally available, for example, through the   Domain Name System (DNS) [17] or World Wide Web [18].   NOTE: given the presence of asymmetric keys, we can assume digital   signatures provide integrity and origin authentication services   combined.   The terminology we use here is described inAppendix A. We formally   define some additional terms here:   +    grpKey: group key used for encrypting group data traffic.   +    ACL: group access control list.   +    KEK: key encrypting key, used for re-keying a group with a new        group key.   +    SAparams: Security Association parameters, including SPI.   +    group access package (grpAP): sent from an already verified tree        node to a joining node.        [token_sender, [ACL]^SK_core, {[grpKey, KEK,        SAparams]^SK_core}^PK_origin-host,        {[grpKey, KEK, SAparams]^SK_core}^PK_next-hop]^SK_sender        NOTE: SK_core is the secret key of the PRIMARY core.   As we have already stated, the elected primary core of a CBT tree   takes on the initial role of GKDC. In our example, we assume that a   group access control list has already been securely communicated to   the primary core. Also, it is assumed the primary core has already   participated in a Security Association estabishment protocol [20,21],   and thus, holds a group key, a key-encrypting key, and an SPI.      NOTE, there is a minor modification required to the CBT protocol      [4], which is as follows: when a secondary core receives a join,      instead of sending an ack followed by a re-join to the primary,Ballardie                     Experimental                      [Page 8]

RFC 1949          Scalable Multicast Key Distribution           May 1996      the secondary forwards the join to the primary; the ack travels      from the primary (or intermediate on-tree router) back to the join      origin. All routers (or only specific routers) become GKDCs after      they receive the ack.   We now demonstrate, by means of an example, how CBT routers join a   tree securely, and become GKDCs. For clarity, in the example, it is   assumed all routers are authorised to become GKDCs, i.e. there is no   trusted-router ACL.   In the diagram below, only one core (the primary) is shown. The   process of a secondary joining the primary follows exactly what we   describe here.   In the diagram, host h wishes to join multicast group G.  Its local   multicast router (router A) has not yet joined the CBT tree for the   group G.    b      b     b-----b     \     |     |      \    |     |       b---b     b------b      /     \  /              KEY....     /       \/    b         C               C = Core (Initial Group Key Dist'n Centre)             / \             A, B, b = non-core routers            /   \           /     \           ======= LAN where host h is located           B      b------b            \             \              NOTE: Only one core is shown, but typicallyhost h        A              a CBT tree is likely to comprise several.    o         |=====================       Figure 1: Example of Multicast Key Distribution using CBT   A branch is created as part of the CBT secure tree joining process,   as follows:   +    Immediately subsequent to a multicast application starting up on        host h, host h immediately sends an IGMP group membership        report, addressed to the group. This report is not suppressible        (seeAppendix B), like other IGMP report types, and it also        includes the reporting host's token, which is digitally signed        h --> DR (A): [[token_h]^SK_h, IGMP group membership report]Ballardie                     Experimental                      [Page 9]

RFC 1949          Scalable Multicast Key Distribution           May 1996        (A host's token differs in two respects compared with tokens        defined in [9]. To refresh, a token assists a recipient in the        verification process, and typically contains: recipient's        unique identity, a timestamp, and a pseudo-random number. A        token is also usually digitally signed by its originator.        Firstly, A host's token does not contain the intended        recipient's identity, since this token may need to traverse        several CBT routers before reaching a GKDC.  A host does not        actually know which router, i.e. GKDC, will actually        acknowledge the join that it invoked.  Secondly, the host's        token is digitally signed -- this is usual for a token.        However, tokens generated by routers need not be explicitly        digitally signed because the JOIN-REQUESTs and JOIN-ACKs that        carry them are themselves digitally signed.)   +    In response to receiving the IGMP report, the local designated        router (router A) authenticates the host's enclosed token. If        successful, router A formulates a CBT join-request, whose target        is core C (the primary core). Router A includes its own token in        the join, as well as the signed token received from host h. The        join is digitally signed by router A.        NOTE 1: router A, like all CBT routers, is configured with the        unicast addresses of a prioritized list of cores, for different        group sets, so that joins can be targeted accordingly.        NOTE 2: the host token is authenticated at most twice, once by        the host's local CBT router, and once by a GKDC. If the local        router is already a GKDC, then authentication only happens once.        If the local router is not already a GKDC, a failed authentica-        tion check removes the overhead of generating and sending a CBT        join-request.        Router A unicasts the join to the best next-hop router on the        path to core C (router B).            A --> B: [[token_A], [token_h]^SK_h, JOIN-REQUEST]^SK_A   +    B authenticates A's join-request. If successful, B repeats the        previous step, but now the join is sent from B to C (the pri-        mary, and target), and the join includes B's token. Host h's        token is copied to this new join.            B --> C: [[token_B], [token_h]^SK_h, JOIN-REQUEST]^SK_B   +    C authenticates B's join. As the tree's primary authorization        point (and GKDC), C also authenticates host h, which triggered        the join process. For this to be successful, host h must beBallardie                     Experimental                     [Page 10]

RFC 1949          Scalable Multicast Key Distribution           May 1996        included in the GKDC's access control list for the group.  If h        is not in the corresponding access control list, authentication        is redundant, and a join-nack is returned from C to B, which        eventually reaches host h's local DR, A.        Assuming successful authentication of B and h, C forms a group        access package (grpAP), encapsulates it in a join-ack, and digi-        tally signs the complete message. C's token, host h's signed        token, a signed ACL, and two (group key, KEK) pairs are included        in the group access package; one for the originating host, and        one for the next-hop CBT router to which the join-ack is des-        tined. Each key pair is digitally signed by the issuer, i.e. the        primary core for the group. The host key pair is encrypted using        the public key of the originating host, so as to be only deci-        pherable by the originating host, and the other key pair is        encrypted using the public key of the next-hop router to which        the ack is destined -- in this case, B.  Host h's token is used        by the router connected to the subnet where h resides so as to        be able to identify the new member.              C --> B: [[token^h]^SK_h, grpAP, JOIN-ACK]^SK_C   +    B authenticates the join-ack from C. B extracts its encrypted        key pair from the group access package, decrypts it, authenti-        cates the primary core, and stores the key pair in encrypted        form, using a local key.  B also verifies the digital signature        included with the access control list. It subsequently stores        the ACL in an appropriate table.  The originating host key pair        remains enciphered.        The other copy of router B's key pair is taken and deciphered        using its secret key, and immediately enciphered with the public        key of next-hop to which a join-ack must be passed, i.e. router        A. A group access package is formulated by B for A. It contains        B's token, the group ACL (which is digitally signed by the pri-        mary core), a (group key, KEK) pair encrypted using the public        key of A, and the originating host's key pair, already        encrypted.  The group access package is encapsulated in a join-        ack, the complete message is digitally signed by B, then for-        warded to A.                B --> A: [[token^h]^SK_h, grpAP, JOIN-ACK]^SK_B   +    A authenticates the join-ack received from B.  A copy of the        encrypted key pair that is for itself is extracted from the        group access package and deciphered, and the key issuer (primary        core) is authenticated.  If successful, the enciphered key pair        is stored by A.  The digital signature of the included accessBallardie                     Experimental                     [Page 11]

RFC 1949          Scalable Multicast Key Distribution           May 1996        control list is also verified, and stored in an appropriate        table.  The key pair encrypted for host h is extracted from the        group access package, and is forwarded directly to host h, which        is identified from the presence of its signed token.  On        receipt, host h decrypts the key pair for subsequent use, and        stores the SA parameters in its SA table.          A --> h: [[token^h]^SK_h, {grpKey, KEK, SAparams}^PK_h]   Going back to the initial step of the tree-joining procedure, if the   DR for the group being joined by host h were already established as   part of the corresponding tree, it would already be a GKDC. It would   therefore be able to directly pass the group key and KEK to host h   after receiving an IGMP group membership report from h:          A --> h: [[token^h]^SK_h, {grpKey, KEK, SAparams}^PK_h]   If paths, or nodes fail, a new route to a core is gleaned as normal   from the underlying unicast routing table, and the re-joining process   (see [4]) occurs in the same secure fashion.7.  A Question of Trust   The security architecture we have described, involving multicast key   distribution, assumes that all routers on a delivery tree are trusted   and do not misbehave. A pertinent question is: is it reasonable to   assume that network routers do not misbehave and are adequately   protected from malicious attacks?   Many would argue that this is not a reasonable assumption, and   therefore the level of security should be increased to discount the   threat of misbehaving routers. As we described above, routers   periodically decrypt key pairs in order to verify them, and/or re-   encrypt them to pass them on to joining neighbour routers.   In view of the above, we suggest that if more stringent security is   required, the model we presented earlier should be slightly amended   to accommodate this requirement.  However, depending on the security   requirement and perceived threat, the model we presented may be   acceptable.   We recommend the following change to the model already presented   above, to provide a higher level of security:   All join-requests must be authenticated by a core router, i.e. a join   arriving at an on-tree router must be forwarded upstream to a core if   the join is identified as being a "secure" join (as indicated by the   presence of a signed host token).Ballardie                     Experimental                     [Page 12]

RFC 1949          Scalable Multicast Key Distribution           May 1996   The implication of this is that key distribution capability remains   with the core routers and is not distributed to non-core routers   whose joins have been authenticated. Whilst this makes our model   somewhat less distributed than it was before, the concept of key   distribution being delegated to the responsibility of individual   groups remains.  Our scheme therefore retains its attractiveness over   centralized schemes.8.  The Multicast Distribution of Sender-Specific KeysSection 5, in part, discussed the scalable distribution of sender-   specific keys and sender-specific security parameters to a multicast   group, for both member-senders, and non-member senders. If asymmetric   cryptotechniques are employed, this allows for sender-specific origin   authentication.   For member-senders, the following message is multicast to the group,   encrypted using the current group session key, prior to the new   sender transmitting data:            {[sender_key, senderSAparams]^SK_sender}^group_key   Non-member senders must first negotiate (e.g. using Photuris or   ISAKMP) with the primary core, to establish the security association   parameters, and the session key, for the sender.  The sender, of   course, is subject to access control at the primary.  Thereafter, the   primary multicasts the sender-specific session key, together with   sender's security parameters to the group, using the group's current   session key.  Receivers are thus able to perform origin   authentication.                           Photuris or ISAKMP             1. sender <----------------------> primary core          2. {[sender_key, senderSAparams]^SK_primary}^group_key   For numerous reasons, it may be desirable to exclude certain group   members from all or part of a group's communication.  We cannot offer   any solution to providing this capability, other than requiring new   keys to be distributed via the establishment of a newly-formed group   (CBT tree).Ballardie                     Experimental                     [Page 13]

RFC 1949          Scalable Multicast Key Distribution           May 19969.  Summary   This memo has offered a scalable solution to the multicast key   distribution problem. Our solution is based on the CBT architecture   and protocol, but this should not preclude the use of other multicast   protocols for secure multicast communication subsequent to key   distribution. Furthermore, virtually all of the functionality present   in our solution is in-built in the secure version of the CBT   protocol, making multicast key distribution an optional, but integral   part, of the CBT protocol.Ballardie                     Experimental                     [Page 14]

RFC 1949          Scalable Multicast Key Distribution           May 1996Appendix A   The following terminology is used throughout this document:   +    PK_A indicates the public key of entity A.   +    SK_A indicates the secret key of entity A. The secret key can be        used by a sender to digitally sign a digest of the message,        which is computed using a strong, one-way hash function, such as        MD5 [19].   +    Unencrypted messages will appear enclosed within square brack-        ets, e.g. [X, Y, Z]. If a message is digitally signed, a super-        script will appear outside the right hand bracket, indicating        the message signer.  Encrypted messages appear enclosed within        curly braces, with a superscript on the top right hand side out-        side the closing curly brace indicating the encryption key, e.g.        {X, Y, Z}^{PK_A}.   +    a token is information sent as part of a strong authentication        exchange, which aids a receiver in the message verification pro-        cess. It consists of a timestamp, t (to demonstrate message        freshness), a random, non-repeating number, r (to demonstrate        message originality), and the unique name of the message        recipient (to demonstrate that the message is indeed intended        for the recipient).  A digital signature is appended to the        token by the sender (which allows the recipient to authenticate        the sender). The token is as follows:             [t_A, r_A, B]^{SK_A} --  token sent from A to B.   +    A --> B:  -- denotes a message sent from A to B.Appendix B   The group access controls described in this document require a few   simple support mechanisms, which, we recommend, be integrated into   version 3 of IGMP. This would be a logical inclusion to IGMP, given   that version 3 is expected to accommodate a variety of multicast   requirements, including security. Furthermore, this would remove the   need for the integration of a separate support protocol in hosts.   To refresh, IGMP [8] is a query/response multicast support protocol   that operates between a multicast router and attached hosts.   Whenever an multicast application starts on a host, that host   generates a small number of IGMP group membership reports in quick   succession (to overcome potential loss). Thereafter, a host onlyBallardie                     Experimental                     [Page 15]

RFC 1949          Scalable Multicast Key Distribution           May 1996   issues a report in response to an IGMP query (issued by the local   multicast router), but only if the host has not received a report for   the same group (issued by some other host on the same subnet) before   the host's IGMP random response timer expires. Hence, IGMP,   incorporates a report "suppression" mechanism to help avoid "IGMP   storms" on a subnet, and generally conserve bandwidth.   We propose that IGMP accommodate "secure joins" - IGMP reports that   indicate the presence of a digitally signed host (or user) token.   These report types must not be suppressible, as is typically the case   with IGMP reports; it must be possible for each host to independently   report its group presence to the local router, since a GKDC bases its   group access control decision on this information.   This functionality should not adversely affect backwards   compatibility with earlier versions of IGMP that may be present on   the same subnet; the new reports will simply be ignored by older IGMP   versions, which thus continue to operate normally.Security Considerations   Security issues are discussed throughout this memo.Author's Address   Tony Ballardie,   Department of Computer Science,   University College London,   Gower Street,   London, WC1E 6BT,   ENGLAND, U.K.   Phone: ++44 (0)71 419 3462   EMail: A.Ballardie@cs.ucl.ac.ukReferences   [1] MBONE, The Multicast BackbONE; M. Macedonia and D. Brutzman;   available fromhttp://www.cs.ucl.ac.uk/mice/mbone_review.html.   [2] R. Atkinson. Security Architecture for the Internet Protocol;RFC1825, SRI Network Information Center, August 1995.   [3] D. Estrin and G. Tsudik. An End-to-End Argument for Network Layer,   Inter-Domain Access Controls; Journal of Internetworking & Experience,   Vol 2, 71-85, 1991.Ballardie                     Experimental                     [Page 16]

RFC 1949          Scalable Multicast Key Distribution           May 1996   [4] A. Ballardie, S. Reeve, N. Jain. Core Based Tree (CBT) Multicast -   Protocol Specification; Work in Progress, 1996. Available from:ftp://cs.ucl.ac.uk/darpa/IDMR/draft-ietf-idmr-cbt-spec-XX.txt.   [5] R. Atkinson. IP Authentication Header;RFC 1826, SRI Network   Information Center, August 1995.   [6] R. Atkinson. IP Encapsulating Security Payload;RFC 1827, SRI Net-   work Information Center, August 1995.   [7] A. Ballardie. Core Based Tree (CBT) Multicast Architecture; Work   in progress, 1996. Available from:ftp://cs.ucl.ac.uk/darpa/IDMR/draft-ietf-idmr-cbt-arch-XX.txt   [8] W. Fenner. Internet Group Management Protocol, version 2 (IGMPv2),   Work in progress, 1996.   [9] CCITT Data Communication Networks Directory (Blue Book). Recommen-   dation X.509, Authentication Framework.   [10] T. Pusateri. Distance-Vector Multicast Routing Protocol (DVMRP)   version 3. Working draft, February 1996.   [11] J. Moy. Multicast Extensions to OSPF;RFC 1584, SRI Network   Information Center, March 1994.   [12] D. Estrin et al. Protocol Independent Multicast, protocol specif-   ication; Work in progress, January 1996.   [13] R. Braden, D. Clark, S. Crocker and C. Huitema. Security in the   Internet Architecture.RFC 1636, June 1994.   [14] A. Ballardie and J. Crowcroft. Multicast-Specific Security   Threats and Counter-Measures. In ISOC Symposium on Network and Distri-   buted System Security, February 1995.   [15] H. Harney, C. Muckenhirn, and T. Rivers. Group Key Management   Protocol (GKMP) Architecture. Working draft, 1994.   [16] N. Haller and R. Atkinson.RFC 1704, On Internet Authentication.   SRI Network Information Center, October 1994.   [17] C. Kaufman and D. Eastlake. DNS Security Protocol Extensions.   Working draft, January 1996.   [18] T. Berners-Lee, R. Cailliau, A. Luotonen, H. Frystyk Nielsen, A.   Secret.  The World Wide Web. Communications of the ACM, 37(8):76-82,   August 1994.Ballardie                     Experimental                     [Page 17]

RFC 1949          Scalable Multicast Key Distribution           May 1996   [19] R. Rivest.RFC 1321, The MD-5 Message Digest Algorithm, SRI Net-   work Information Center, 1992.   [20] P. Karn, W. Simpson. The Photuris Session Key Management Proto-   col; Working draft, January 1996.   [21] D. Maughan, M. Schertler. Internet Security Association and Key   Management Protocol; Working draft, November 1995.Ballardie                     Experimental                     [Page 18]