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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 in Appendix 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 (see Appendix 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⌨️ 快捷键说明
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