Internet Draft Mark Baugher (Cisco) IETF MSEC WG Ran Canetti (IBM) Expires: September 2003 Lakshminath Dondeti (Nortel) Fredrik Lindholm (Ericsson) March 03, 2003 Group Key Management Architecture Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document presents a group key-management architecture for MSEC. The purpose of this document is to define the common architecture for MSEC group key-management protocols that support a variety of application, transport, and internetwork security protocols. To address these diverse uses, MSEC may need to standardize two or more group key management protocols that have common requirements, abstractions, overall design, and messages. The framework and guidelines in this document allow for a modular and flexible design of group key management protocols for a variety different settings that are specialized to application needs. Comments on this document should be sent to msec@securemulticast.org. Baugher, Canetti, Dondeti, Lindholm February 2002 Table of Contents Status of this Memo...............................................1 Abstract..........................................................1 1.0 Introduction: Purpose of this Document........................3 2.0 Requirements for a group key management protocol..............3 3.0 Overall Design................................................6 3.1 Overview.....................................................6 3.2 Detailed description.........................................8 3.3 Properties of the design....................................10 3.4 Implementation Diagram......................................10 4.0 Registration Protocol........................................12 4.1 Registration Protocol Message Exchange......................12 4.2 Properties of Alternative Registration Exchange Types.......13 4.3 Infrastructure for Alternative Registration Exchange Types..14 4.2 De-Registration Exchange....................................15 5.0 Rekey protocol...............................................15 5.1 Goals of the Rekey protocol.................................16 5.2 Rekey messages..............................................17 5.3 Reliable transport of rekey messages........................17 5.4 Implosion...................................................18 5.5 Issues in incorporating group key management algorithms....19 5.5.1 Stateless vs. stateful rekeying..........................19 5.6 Interoperability of a GKMA..................................19 6.0 Group Security Association...................................20 6.1 Group policy................................................21 6.2 Contents of the Re-key SA...................................22 6.2.1 Re-key SA policy.........................................22 6.2.2 Group identity...........................................22 6.2.3 Key encrypting key(s)....................................23 6.2.4 Authentication key.......................................23 6.2.5 Replay protection information............................23 6.2.6 Security Parameter Index (SPI)...........................23 6.3 Contents of the Data SA.....................................23 6.3.1 Group identity...........................................24 6.3.2 Source identity..........................................24 6.3.3 Traffic encrypting key...................................24 6.3.4 Authentication key.......................................24 6.3.5 Sequence numbers.........................................24 6.3.6 Security Parameter Index (SPI)...........................24 6.3.7 Data SA policy...........................................24 7.0 Scalability Considerations...................................24 8.0 Security Considerations......................................27 9.0 References and Bibliography..................................28 10.0 Authors' Addresses..........................................31 Appendix: MSEC Security Documents Roadmap........................32 Internet Draft Group Key Management Architecture [PAGE 2] Baugher, Canetti, Dondeti, Lindholm February 2002 1.0 Introduction: Purpose of this Document Group and multicast applications have diverse requirements in IP networks [CP00]. Their key-management requirements, which are briefly reviewed below (see "Requirements"), include support for internetwork, transport, and application-layer protocols. In particular, while Internet-standard ISAKMP and IKE protocols purport to manage keys for any and all services in a host, some applications may achieve simpler operation by running key-management messaging over TLS or IPsec security services. Other security protocols may benefit from a key management protocol that can run over SIP or RTSP [MIKEY]. For these reasons, application, transport, and internetwork-layer security protocols such as SRTP, IPsec, and AMESP may benefit from using different group key management systems. Some security protocols will benefit from a key management protocol that can run over IPsec or TLS [GSAKMP]. Other security protocols may run over SIP or RTSP [KMMS]. Extensions to IKE may be the best solution for running IPsec protocols over IP multicast services [GDOI]. The purpose of this document is to define a common architecture and design for these different group key-management protocols for internet, transport, and application services. Indeed, key-management protocols are difficult to design and validate. The common architecture described in this document eases this burden by defining common abstractions and overall design that can be specialized for different uses. This document builds on and extends the Group Key Management Building Block document of the IRTF SMuG research group [HBH01] and is part of the MSEC document roadmap. To correctly place the current document in the context of the MSEC literature we include a copy of the MSEC draft tree in the appendix. Section 2 discusses the security, performance and architectural requirements for a group key management protocol. Section 3 presents the overall architectural design principles. Section 4 describes the Registration protocol in detail and Section 5 does the same for Rekey protocol. Section 6 considers the interface to the Group Security Association (GSA) using the standard keywords of RFC 2119. Section 7 reviews the scalability issues for group key management protocols and Section 8 discusses Security Considerations. 2.0 Requirements for a group key management protocol A group key management protocol supports multicast applications that need a secure group. A "secure group" is a collection of principals, called "members," who may be senders, receivers or both receivers and senders to other members of the group. (Note that group membership may vary over time.) A "group key management protocol" helps to ensure that only members of a secure group gain access to group data Internet Draft Group Key Management Architecture [PAGE 3] Baugher, Canetti, Dondeti, Lindholm February 2002 (by gaining access to group keys) and can authenticate group data. The goal of a group key management protocol is to provide legitimate group members with the up-to-date cryptographic state they need for their secrecy and authenticity requirements. Multicast applications, such as video broadcast and multicast file transfer, have the following key-management requirements (see also [CP00]). 1. The group members receive "security associations" including encryption keys, authentication/integrity keys, cryptographic policy that describes the keys, and attributes such as an index for referencing the security association (SA) or particular objects contained in the SA. 2. Keys will have a predetermined lifetime and will be periodically refreshed. 3. Key material are delivered securely to members of the group so that they are secret, integrity-protected, and can be verified as coming from an authorized source. 4. The key-management protocol is also secure against replay attacks and Denial of Service (DoS) attacks (see the Security Considerations section of this memo). 5. The protocol adds and removes group members so that members who are added may optionally be denied access to the key material used before they joined the group, and that removed members lose access to the key material following their departure. 6. The protocol supports a scalable group re-key operation without unicast exchange between members and a group controller/key server, which might overwhelm a GCKS when the group is large. 7. The protocol is compatible with the infrastructure and performance needs of the data-security application, such as IPsec security protocols, AH and ESP, and/or application-layer security protocols, AMESP and SRTP. (Note: needs for further clarification) 8. The key management protocol offers a framework for replacing or renewing transforms, authorization infrastructure and authentication systems. 9. The key management protocol must be secure against collusions among excluded members and non-members. Specifically, collusions must not result in gaining any additional group secrets than the colluding entities themselves are privy to. Internet Draft Group Key Management Architecture [PAGE 4] Baugher, Canetti, Dondeti, Lindholm February 2002 10. The key management protocol must provide a mechanism to securely recover from a compromise of some or all of the key material. Although it is not a requirement for a multicast security protocol, the group key management protocol may also be useful to unicast applications that share many of the requirements of multicast applications. In other words group key management protocols may be used for protecting multicast communications, or communications in groups where members communicate among themselves mainly via unicast. There are other requirements for small group operation where there will be many senders or in which all members may potentially be senders. In this case, the group setup time may need to be optimized to support a small, highly interactive group environment [RFC2627]. A single group controller (or GCKS) may not be the best design for small, interactive groups. However, large single-source multicast groups generally may benefit from the use of a specialized GCKS. Large distributed simulations, moreover, may combine the need for large-group operation with many senders. We also take as a requirement the support of large single-sender groups, such as source-specific (single-source) multicast groups. Thus, group key management should support high-capacity operation to large groups that have one or very few senders. Nonetheless, scalable operation to a range of group sizes is a desirable feature, and a better group key management protocol will support large, single-sender groups as well as groups that have many senders. It may be that no single key management protocol can satisfy the scalability requirements of all group-security applications. The group key management architecture allows two or more key management protocols, where each protocol is suitable to a different scenario such large single-source groups or small interactive groups. In addition to these requirements, it is useful to emphasize two non- requirements, namely, technical protection measures (TPM) and broadcast key management. TPM are used for such things as copy protection by preventing the user of a device to get easy access to the group keys. Although we should expect that a device under the control of an attacker would lose its secrets to that attacker, some TPM advocates see tamper-resistant technologies as a means to keep honest people honest [MT] and want TPM for that purpose. There is no reason why a group key management protocol cannot be used in an environment where the keys are kept in a "tamper-resistant" store using various types of hardware or software to implement TPM. The group key management architecture described in this document, however, is for key management protocols and not a design for technical protection measures, which are outside the scope of this document. Internet Draft Group Key Management Architecture [PAGE 5] Baugher, Canetti, Dondeti, Lindholm February 2002 The second non-requirement is broadcast key management where there is no back channel [FN93, JKKV94] or the device is not on a network, such as a digital videodisk player. We assume IP network operation where there is two-way communication, however asymmetric, and that authenticated key-exchange procedures can be used for member registration. It is possible that broadcast applications can make use of a one-way Internet group key management protocol message, and a one-way Re-key message is described below. 3.0 Overall Design This section describes the overall structure of a group key management protocol, and provides a reference implementation diagram for group key management. This design is based upon a Ÿgroup controller÷ model [RFC2093, RFC2094, RFC2627, OFT, GSAKMP, GDOI] with a single group owner as the root-of-trust. The group owner designates a group controller for member registration and re-key. 3.1 Overview The main goal of a group key management protocol is to securely provide the group members with an up-to-date security association (SA), which contains the needed information for securing group communication (i.e., the group data). We call this SA the "Data Security Protocol SA", or "Data SA" for short. In order to obtain this goal, the Group Key Management Architecture consists of the following protocols. (1) Registration protocol. ===================== This is a two-way unicast protocol between the group controller/key server (GCKS) and a joining group member. In this protocol the GCKS and joining member mutually authenticate each other. If the authentication succeeds and the GCKS finds that the joining member is authorized, then the GCKS supplies the joining member with the following information: (a) Sufficient information to initialize a "Re-key Protocol SA" within the joining member (see more details about this SA below). This information is given only in case that the group security policy calls for using a Re-key protocol. (b) Sufficient information to initialize the Data Security Protocol SA within the joining member. This information is given only in the case that the group security policy calls for initializing the Data Security Protocol SA at Registration, instead of or in addition to at Re-key. The Registration Protocol must ensure that the transfer of information from GCKS to member is done in an authenticated and confidential manner over a security association. We call this SA the Internet Draft Group Key Management Architecture [PAGE 6] Baugher, Canetti, Dondeti, Lindholm February 2002 "Registration Protocol SA". A complementary "De-registration protocol" serves to explicitly remove Registration Protocol SA state. (2) Re-key protocol. ================ This is an optional protocol where a GCKS periodically sends re- key information to the group members. Re-key messages may result from group membership changes, the creation of new traffic-protection keys (TPKs, see next section) for the particular Group, or from key expiration. Re-key messages are protected by the Re-key protocol SA, which is initialized in the Registration protocol. The Re-key message includes information for updating both the Re-key protocol SA and/or the Data Security Protocol SA. The Re-key messages can be sent via multicast to group members or unicast from the GCKS to a particular group member. The Re-key protocol is optional as there are other means for managing (e.g. expiring or refreshing) the keys locally without interaction between the GCKS and member [MARKS]. The Re-key SA that is established includes authentication data for the re-key. There are two cases. o The first and primary option is to use source authentication. That is, each group member verifies that Re-key data originates with the GCKS and none other. o The second option is to use only group-based authentication using a symmetric key, such as a message authentication code. Members can only be assured that the Re-key messages originated within the group. Therefore, this is applicable only when all members of the group are trusted not to impersonate the GCKS. Group authentication for Re-key messages is typically used when public- key cryptography is not suitable for the particular group. The Re-key protocol ensures that all members receive the re-key information in a timely manner. In addition, the Re-key protocol specifies mechanisms for the parties to contact the GCKS and "re- synch" in case that their keys expired and an updated key has not yet been received. The Re-key protocol for large-scale groups offers mechanisms to avoid implosion problems and ensure the needed reliability in its delivery of keying material. The Re-key message is protected by a Re-key SA, which is established by the Registration Protocol. It is a recommended practice that a member who leaves the group destroys the Re-key SA, one or more Data SAs, and the Registration SA to which these SAs belong. Use of a De- Registration message is often an efficient mechanisms for a member to inform the GCKS that it has destroyed it SAs, or is about to destroy them. Such a message may prompt the GCKS to cryptographically Internet Draft Group Key Management Architecture [PAGE 7] Baugher, Canetti, Dondeti, Lindholm February 2002 remove the member from the group (i.e., to prevent the member from having access to future group communication). In large-scale multicast applications, however, De-registration has the potential to cause implosion at the GCKS. 3.2 Detailed description Figure 1 depicts the overall design [HBH01]. Each group member, sender or receiver, uses the Registration Protocol to get authorized, authenticated access to a particular Group, its policies, and its keys. The two types of group keys are the KEK (key-encrypting key) and the Traffic Protection Keys or TPKs (TPKs refer to both Traffic Encryption Keys or TPKs, and Traffic integrity protection keys). The KEK may be a single key that encrypts the TPKs or it may be a vector of keys in a group key membership algorithm [RFC2627, OFT, CP00, LNN01, SD] that encrypts the TPKs and other KEKs. The KEK is used by the Re-key protocol. The TPKs are used by the Data Security Protocol to protect streams, files, or other data sent and received by the Data Security Protocol. Thus the Registration Protocol and/or the Re-key Protocol establish the KEK and TPKs. There are a few, distinct outcomes to a successful Registration Protocol exchange. o If the GCKS uses Re-key messages, then the admitted member receives the Group KEK; if it uses a group key management algorithm, then the member receives a set of KEKs according to the particular algorithm. o If Re-key messages are not used for the Group, then the admitted member will receive TPKs (in SAs) that are passed to the member's Data Security Protocol (as IKE does for IPsec). o The GCKS may pass one or more TPKs to the member even if Re- key messages are used, for efficiency reasons according to group policy. The GCKS creates the KEK and TPKs and downloads them to each member - as the KEK and TPKs are common to the entire Group. The GCKS is a separate, logical entity that performs member authentication and authorization according to the Group policy that is set by the Group Owner. The GCKS MAY present a credential to the Group member that is signed by the Group Owner so the member can check the GCKS's authorization. The GCKS, which may be co-located with a member or be a separate physical entity, runs the Re-key Protocol to push Re-key messages of refreshed KEKs, new TPKs, and refreshed TPKs to members. Alternatively, the sender may forward Re-key messages on behalf of the GCKS when it uses a credential mechanism that supports delegation. Thus, it is possible for the sender or other member to source keying material (TPKs encrypted in the Group KEK) as it sources multicast or unicast data. As mentioned above, the Re-key message can be sent using unicast or multicast delivery. Upon Internet Draft Group Key Management Architecture [PAGE 8] Baugher, Canetti, Dondeti, Lindholm February 2002 receipt of TPKs from a Re-key Message or a Registration protocol exchange, the member's group key management will provide a security association (SA) to a Data Security Protocol for the data sent from sender to receiver. +------------------------------------------------------------------+ | +-----------------+ +-----------------+ | | | POLICY | | AUTHORIZATION | | | | INFRASTRUCTURE | | INFRASTRUCTURE | | | +-----------------+ +-----------------+ | | ^ ^ | | | | | | v v | | +--------------------------------------------------------------+ | | | | | | | +--------------------+ | | | | +------>| GCKS |<------+ | | | | | +--------------------+ | | | | | REGISTRATION or | REGISTRATION or | | | | DE-REGISTRATION | DE-REGISTRATION | | | | PROTOCOL | PROTOCOL | | | | | | | | | | | v RE-KEY v | | | | +-----------------+ PROTOCOL +-----------------+ | | | | | | (OPTIONAL) | | | | | | | SENDER(S) |<-------+-------->| RECEIVER(S) | | | | | | | | | | | | | +-----------------+ +-----------------+ | | | | | ^ | | | | v | | | | | +-------DATA SECURITY PROTOCOL-------+ | | | | | | | +--------------------------------------------------------------+ | | | +------------------------------------------------------------------+ FIGURE 1: Group Security Association Model The "Security Protocol SA" protects the data sent on the arc labeled "DATA SECURITY PROTOCOL" in Figure 1. A second SA, the "Re-key SA," is optionally established by the key-management protocol for Re- key messages, and the arc labeled "RE-KEY PROTOCOL" in Figure 1 depicts this. The Re-key message is optional because all keys, KEK and TPKs, can be delivered by the Registration Protocol exchanges shown in Figure 1, and those keys may not need to be updated. The Registration Protocol is protected by a third, symmetric, unicast SA between the GCKS and each member; this is called the "Registration Protocol SA." There may be no need for the Registration Protocol SA to remain in place after the completion of the Registration Protocol exchanges. The De-registration protocol is also optional and is used when explicit teardown or the SA is desirable (such as when a phone Internet Draft Group Key Management Architecture [PAGE 9] Baugher, Canetti, Dondeti, Lindholm February 2002 call or conference terminates). The three SAs comprise the Group Security Association. Only one SA is optional and that is the Re-key SA. Figure 1 shows two blocks that are external to the group key management protocol: The Policy and Authorization Infrastructures are discussed in Section 6.1. 3.3 Properties of the design The design of Section 3.2 achieves scalable operation by (1) allowing the de-coupling of authenticated key exchange in a "Registration Protocol" from a "Re-key Protocol," (2) allowing the Re-key Protocol to use unicast push or multicast distribution of group and data keys as an option, and (3) allowing all keys to be obtained by the unicast Registration Protocol and (4) delegating the functionality of the GCKS among multiple entities, i.e., permit distributed operation of the GCKS. High-capacity operation is obtained by (1) amortizing computationally-expensive asymmetric cryptography over multiple data keys used by data security protocols, (2) supporting unicast push or multicast distribution of symmetric group and data keys, and (3) supporting key revocation algorithms such as LKH [RFC2627, OFT, LNN01] that allow members to be added or removed at logarithmic rather than linear space/time complexity. The Registration protocol may use asymmetric cryptography to authenticate joining members and optionally establish the group KEK. Asymmetric cryptography such as Diffie-Hellman key agreement and/or digital signatures are amortized over the life of the group KEK: A Data Security SA can be established without the use of asymmetric cryptography - the TPKs are simply encrypted in the symmetric KEK and sent unicast or multicast in the Re-key protocol. The design of the Registration and Re-key Protocols is flexible. The Registration protocol establishes one KEK or multiple TPKs or both KEK and TPKs. The TPKs (or data keys) are associated with a data security protocol SA; there may in fact be multiple keys pushed with or derived from the TPKs. The Re-key Protocol establishes KEKs or TPKs or both. 3.4 Implementation Diagram In the block diagram of Figure 2, group key management protocols run between a GCKS and member principal to establish a Group Security Association (GSA). The GSA consists of a Security Protocol SA, an optional Re-key SA, and a Registration Protocol SA. The GCKS MAY use a delegated principal, such as an SRTP [SRTP] sender, which has a delegation credential signed by the GCKS. The "Member" of Figure 2 Internet Draft Group Key Management Architecture [PAGE 10] Baugher, Canetti, Dondeti, Lindholm February 2002 may be a sender or receiver of multicast or unicast data [HCBD]. There are two functional blocks in Figure 2 labeled "GKM," and there are two arcs between them depicting the group key-management Registration ("reg") and Re-key ("rek") protocols. The message exchanges are the GSA establishment protocols, which are the Registration Protocol and the Re-key Protocol described above. +----------------------------------------------------------+ | | | +-------------+ +------------+ | | | CONTROL | | CONTROL | | | +------^------+ +------|-----+ +--------+ | | | | +-----| CRED | | | | | | +--------+ | | +----v----+ +----v--v-+ +--------+ | | | <-----Reg-----> |<->| SAD | | | | GKM -----Rek-----> GKM | +--------+ | | | | | | +--------+ | | | ------+ | |<->| SPD | | | +---------+ | +-^-------+ +--------+ | | +--------+ | | | | | | | CRED |----->+ | | +-------------------+ | | +--------+ | | +--------------------+ | | | +--------+ | +-V-------+ +--------+ | | | | | SAD <----->+ | |<->| SAD <-+ | | | +--------+ | |SECURITY | +--------+ | | | +--------+ | |PROTOCOL | +--------+ | | | | SPD <----->+ | |<->| SPD <----+ | | +--------+ +---------+ +--------+ | | | | (A) GCKS (B) MEMBER | +----------------------------------------------------------+ Figure 2: Group key management block diagram for a host computer Figure 2 shows that a complete group-key management functional specification includes much more than the message exchange. Some of these functional blocks and the arcs between them are peculiar to an operating system (OS) or vendor product, such as vendor specifications for products that support updates to the IPsec Security Association Database (SAD) and Security Policy Database (SPD) [RFC2367]. Various vendors also define the functions and interface of credential stores, "CRED" in Figure 2. The CONTROL function directs the GCKS to establish a group, admit a member or remove a member, or it directs a member to join or leave a group. CONTROL includes authorization, which is subject to Group Policy [HH], but how this is done is specific to the GCKS implementation. CONTROL may be a telephony signaling protocol such Internet Draft Group Key Management Architecture [PAGE 11] Baugher, Canetti, Dondeti, Lindholm February 2002 as SIP with the GCKS function operating on a caller's phone. For large-scale multicast sessions, CONTROL could perform session announcement functions to inform a potential group member that it may join a group or receive group data (e.g. a stream of file transfer protected by a Data Security protocol). Announcements notify group members to establish multicast SAs in advance of secure multicast data transmission. Session Description Protocol (SDP) is one form that the announcements might take [RFC2327]. The announcement function may be implemented in a session-directory tool, an electronic program guide (EPG), or by other means. The Data Security or the announcement function directs group key management using an application-programming interface (API), which is peculiar to the host OS in its specifics. A generic API for group key management is for further study, but this function is necessary to allow Group (KEK) and Data key (TPKs) establishment to be done in a way that is scalable to the particular application. A GCKS application program will use the API to initiate the procedures to establish SAs on behalf of a Security Protocol in which members join secure groups and receive keys for streams, files or other data. The goal of the exchanges is to establish a GSA through updates to the SAD of a key-management implementation and particular Security Protocol. The "Security Protocol" of Figure 2 may span internetwork and application layers [AMESP] or operate at the internetwork layer, such as AH and ESP. 4.0 Registration Protocol The design of the Registration is flexible. The Registration protocol establishes one Rekey SA or multiple Data Security SAs or both. The TPKs (or "data key") are associated with a Data Security SA; there may in fact be multiple keys pushed with or derived from the TPKs. A particular group key management protocol MAY restrict these many options according to its particular requirements. Each registration protocol supports different scenarios. The chosen registration protocol solution reflects the specific requirements of specific scenarios. In principle, it is possible to base a registration protocol on any secure-channel protocol, such as IPsec and TLS, which is the case in GSAKMP [GSAKMP]. However, registration protocols that address other scenarios, such as GDOI [GDOI] and MIKEY [MIKEY], use other methods to secure SA establishment. Some of the different solutions that arise from specific scenarios are discussed in the sections below. This document also refers in more detail to the specific registration protocols GDOI and MIKEY, and shows how these fit within the general architecture. Internet Draft Group Key Management Architecture [PAGE 12] Baugher, Canetti, Dondeti, Lindholm February 2002 4.1 Registration Protocol Message Exchange Some registration protocols need "tunnel" through a data-signaling protocol. The reason may e.g. be to take advantage of already existing (security) functionality, and/or to optimize the total session setup time. For example, a telephone call has strict bounds for delay in setup time; we donËt like to wait a second longer than we have to. It is not feasible to run security exchanges in parallel with call setup since the latter often resolves the address: Call setup must complete before the caller knows the address of the callee. A better solution is to tunnel the key exchange procedures inside call establishment [H.235, MIKEY] so both can complete (or fail, see below) at the same time. The registration protocol has different requirements depending on the particular integration/tunneling approach. These requirements are not necessarily security requirements, but will have an impact on the chosen security solution. For example, the security association will certainly fail if the call setup fails in the case of IP telephony. Conversely, the registration protocol imposes requirements on the protocol that tunnels it. In the case of IP telephony, the call setup usually will fail when the security association is not successfully established. In the case of video-on-demand, protocols such as RTSP that convey key management data will fail when a needed security association cannot be established. Both GDOI and MIKEY use this approach, but in different ways. MIKEY can be tunneled in SIP and RTSP. It takes advantage of the session information contained in these protocols and the possibility to optimize the setup time for the registration procedure. SIP requires that a tunneled protocol must use at most one roundtrip (i.e. two messages). This is also desirable requirement from RTSP as well. The GDOI approach takes advantage of the already defined ISAKMP phase 1 exchange [RFC2409], and extends the phase 2 exchange for the registration. This is a good example of reusing security functionality, where the defined phase 2 exchange is protected by the SA created by phase 1. The GDOI will also inherent other functionality of the ISAKMP. This may e.g. make the solution very suitable for running IPsec protocols over IP multicast services. 4.2 Properties of Alternative Registration Exchange Types The required design properties of a registration protocol has different tradeoffs. A protocol that provides perfect forward secrecy and identity protection trades security for performance or efficiency, while a protocol that completes in one or two messages may trade security functionality (e.g. identity protection) for efficiency. Internet Draft Group Key Management Architecture [PAGE 13] Baugher, Canetti, Dondeti, Lindholm February 2002 In a one- or two-message protocol, replay protection generally uses either a timestamp or a sequence number. The first requires synchronized clocks, while the latter requires that it is possible to keep state. In a timestamp-based protocol, a replay cache is needed to store the messages (or the hashes of the messages) received within the allowable "clock skew". The size of the replay cache depends on the number of messages received during the allowable clock skew. During a DoS attack, the replay cache might become overloaded. One solution is to over provision the replay cache. However, this may lead to a large replay cache. Another solution is to let the allowable clock skew be changed dynamically during runtime. During a suspected DoS attack, the allowable clock skew is then decreased so that the replay cache becomes manageable. A challenge-response mechanism (using Nonce) obviates the need for synchronized clocks for replay protection when the exchange uses three or more messages [MVV]. This does not guarantee the replay protection of individual messages (unless the protocols record all Nonce), but on the exchange itself. "Cookies", such as stateless cookies are means to protect against the replay of individual messages [Photuris]. Additional security functions become possible as the number of allowable messages in the registration protocol increase. ISAKMP offers identity protection, for example, as part of a six-message exchange. With additional security features, however, comes added complexity: Identity protection, for example, not only requires additional messages, but may result in DoS vulnerabilities since authentication is performed in a late stage of the exchange after resources already have been devoted. In all cases, there are tradeoffs with the number of message exchanged, the desired security services, and the amount of infrastructure that is needed to support the group key management service. Whereas protocols that use two or even one-message setup have low latency and computation requirements, they may require more infrastructure such as secure time or offer less security such as the absence of identity protection. What tradeoffs are acceptable and what are not is very much dictated by the application and application environment. 4.3 Infrastructure for Alternative Registration Exchange Types The registration protocols need external infrastructures to be able to handle authentication, replay protection, protocol-run integrity, authorization and potentially other security services such as secure, synchronized clocks. These may be solved by e.g. deploying a PKI (with either authorization-based certificates or a separate Internet Draft Group Key Management Architecture [PAGE 14] Baugher, Canetti, Dondeti, Lindholm February 2002 management for this). Other existing solutions may be employed such as AAA infrastructure. Depending on the registration protocol and its application, other external infrastructures may also be needed e.g. timestamp-based protocols may need an infrastructure to synchronize the clocks. However, external infrastructures may not always be needed. This could be the case when e.g. pre-shared keys are used and the subscription base is very small. In a conversational multimedia scenario (e.g. a VoIP call between two or more people), it may very well be the end user who handles the authorization by manually accepting/rejecting the incoming calls. In general, protocols that use fewer messages require more infrastructure (such as synchronized clocks) or fewer security features such as PFS or identity protection. 4.2 De-Registration Exchange The session-establishment protocol (e.g. SIP, RTSP) that conveys a Registration exchange often has a session-disestablishment protocol such as RTSP TEARDOWN [RFC2326] or SIP BYE [RFC2543]. The session- disestablishment exchange between endpoints offers an opportunity to signal the end of the GSA state at the endpoints. This "exchange" need only be a uni-directional notification by one side that the GSA is to be destroyed. Authentication of this notification can use a proof-of-possession of the group key(s) by one side to the other. Some applications benefit from acknowledgement in a mutual, two- message exchange signaling disestablishment of the GSA concomitant with disestablishment of the session, e.g. RTSP or SIP session. In this case, a two-way proof-of-possession might serve for mutual acknowledgement of the GSA disestablishment. 5.0 Rekey protocol Group Rekey protocol is for transport of keys and SAs between a GCKS and the members of a secure communications group. The GCKS sends Rekey messages to update a Rekey SA, or initialize/update a Data Security SA or both. Rekey messages are protected by a Rekey SA. The GCKS may update the Rekey SA when group membership changes or when KEKs or TPKs expire. Recall that KEKs correspond to a Rekey SA and TPKs correspond to a Data Security SA. The following are some desirable properties of the Rekey protocol: o Rekey protocol ensures that all members receive the rekey information in a timely manner. Internet Draft Group Key Management Architecture [PAGE 15] Baugher, Canetti, Dondeti, Lindholm February 2002 o Rekey protocol specifies mechanisms for the parties involved, to contact the GCKS and re-sync when their keys expire and no updates have been received. o Rekey protocol avoids implosion problems and ensures the needed reliability in delivering Rekey information. We further note that the Rekey protocol is primarily responsible for scalability of the group key management architecture. Hence it is imperative that we provide the above listed properties in a scalable manner. Note that solutions exist in the literature (both IETF standards and research articles) for parts of the problem. For instance, the Rekey protocol may use a scalable group key management algorithm (GKMA) to reduce the number of keys sent in a rekey message. Examples of a GKMA include LKH, OFT, Subset difference based schemes etc. 5.1 Goals of the Rekey protocol The goals of the Rekey protocol are: o to synchronize a GSA o to provide privacy and (symmetric or asymmetric) authentication, o efficient rekeying after changes in group membership, or when keys (KEKs) expire, o (optional) reliable delivery of rekey messages o high throughput and low latency, and o to use IP Multicast or multi-unicast. We identify five major issues in the design of a rekey protocol: 1. rekey message format 2. reliable transport of rekey messages 3. implosion 4. incorporating GKMAs in rekey messages 5. interoperability of GKMAs Internet Draft Group Key Management Architecture [PAGE 16] Baugher, Canetti, Dondeti, Lindholm February 2002 Note that for a GCKS to successfully rekey a group, it is not sufficient that Rekey protocol implementations interoperate. We also need to ensure that the GKMA also interoperates. In the rest of this section we discuss these issues in detail. 5.2 Rekey messages Rekey messages are at the core of the rekey protocol. They contain Rekey and/or Data Security SAs along with KEKs and TPKs. These messages need to be confidential, authenticated, and protected against replay attacks. Rekey messages contain group key updates corresponding to a single[LKH,OFT] or multiple membership changes[Subset, BatchRekey] and often contain group key initialization messages [OFT]. 5.3 Reliable transport of rekey messages The GCKS needs to ensure that all members have the current Data Security and Rekey SAs. Otherwise, authorized members may be inadvertently excluded from receiving group communications. Thus, the GCKS needs to use a rekey algorithm that is inherently reliable or employ some reliable transport mechanism to send rekey messages. There are two dimensions to the problem: Messages that update group keys may be lost in transit or may be missed by a host when it is offline. LKH and OFT group key management algorithms rely on past history of updates being received by the host. If the host is offline, then it will need to resynchronize its group-key state, which probably requires a unicast exchange with the GCKS. The Subset Difference algorithm, however, conveys all needed state in its re-key message and does not need members to be always on nor always connected. Subset difference does not require a backchannel and can operate on a broadcast network. Subset difference, however, does need to have its key management message received by the member. Thus Subset difference, LKH and OFT are not inherently reliable. Reliable multicasting is a hard problem, but there are several solutions in the literature. We discuss reliable transport of rekey messages in this section. Rekey messages are typically short (for single membership change as well as for small groups) which makes it easy to design a reliable delivery protocol. On the other hand, the security requirements may add an additional dimension to address. Also there are some special cases where membership changes are processed as a batch, which reduces the frequency of rekey messages, but increases their size. Furthermore, among all the KEKs sent in a rekey message, Internet Draft Group Key Management Architecture [PAGE 17] Baugher, Canetti, Dondeti, Lindholm February 2002 as many as half the members need only a single KEK. We need to take advantage of these properties in designing a rekey message(s) and a protocol for their reliable delivery. Three categories of solutions have been proposed: 1. Repeatedly transmit the rekey message: Recall that in many cases rekey messages translate to only one or two IP packets. 2. Use an existing reliable multicast protocol/infrastructure 3. Use FEC for encoding rekey packets (with NACKs as feedback) [BatchRekey] Note that for small messages, category 3 is essentially the same as category 1. 5.4 Implosion Implosion may occur due to one of two reasons. First, recall that one of the goals of the rekey protocol is to "synchronize a GSA." When a rekey or data security SA expires, members may contact the GCKS for an update. If all or even many members contact the GCKS at about the same time, the GCKS cannot handle all those messages. We refer to this as an "out-of-sync implosion." The second case is in the reliable delivery of rekey messages. Reliable multicast protocols use feedback (NACK or ACK) to determine which packets must be retransmitted. Packet losses may result in many members sending NACKs to the GCKS. We refer to this as feedback implosion. The implosion problem has been studied extensively in the context of reliable multicasting. Some of the proposed solutions viz., feedback suppression and aggregation, might be useful in this context as well. The GCKS might send each receiver a random number to be used as time to wait before sending a NACK or out-of-sync message. Meanwhile, members might receive the key updates they need and therefore will not send a feedback message. An alternative solution is to have the members contact one of several registration servers when they are out-of-sync. This results in repetition of the registration process for those members. Furthermore, there is the need to setup multiple registration servers and synchronize them. Internet Draft Group Key Management Architecture [PAGE 18] Baugher, Canetti, Dondeti, Lindholm February 2002 Feedback aggregation and local recovery employed by some reliable multicast protocols are not easily adaptable to transport of rekey messages. There are authentication issues to address in aggregation. Local recovery is more complex in that members need to establish SAs with the local repair server. 5.5 Issues in incorporating group key management algorithms Group key management algorithms make re-keying scalable. Large group re-keying without employing GKMAs is prohibitively expensive. First we list some requirements to consider in selecting a GKMA: o Collusion: Members (or non members) should not be able to collaborate to deduce keys that they are not privileged (following the GKMA key distribution rules) to. o Forward access control: Ensure that departing members cannot get access to future group data. o Backward access control: Ensure that joining members cannot decrypt past data. 5.5.1 Stateless vs. stateful rekeying We classify group key management algorithms into two categories, viz., stateful and stateless algorithms. Stateful algorithms use KEKs from the ith rekeying instance to encrypt (protect) KEKS corresponding to the i+1st rekeying instance. The main disadvantage in these schemes is that if a member was offline or otherwise fails to receive KEKs from a rekeying instance i, it can no longer synchronize its GSA even though it can receive KEKs from all future rekeying instances starting at i+1. The only solution is to contact the GCKS explicitly for resynchronization. Note that the KEKs for the first rekeying instance are protected by the registration SA. Recall that communication in that phase is one to one, and therefore it is easy to ensure reliable delivery. Stateless GKMAs encrypt rekey messages with KEKs sent during the registration protocol. Since rekey messages are independent of any past rekey messages (i.e. not protected by KEKs therein), a member may go offline, but continue to be able to decipher future communications. The catch however is that members can never decrypt any messages sent while they were offline, even though there are eligible to (i.e. paid for that content as well). Stateless rekeying may be relatively inefficient, particularly for immediate (in contrast to batch) rekeying in highly dynamic groups. Internet Draft Group Key Management Architecture [PAGE 19] Baugher, Canetti, Dondeti, Lindholm February 2002 5.6 Interoperability of a GKMA Most GKMA specifications do not specify packet formats although any group key management algorithms needs to for the purposes of interoperability. In particular there are several alternative ways to managing key trees and numbering nodes within key trees. The following information is generally needed during initialization of a rekey SA or included with each GKMA packet. o GKMA name (e.g. LKH, OFT, Subset difference) o GKMA version number (implementation specific). Version may imply several things such as the degree of a key tree, proprietary enhancements, and qualify another field such as a key id. o Number of keys or Largest ID o Version specific data o Per key information - Key ID - Key lifetime (creation/expiration data) - Encrypted key - encryption key's ID (optional) Key IDs may change in some implementations in which case we need to send: o List of 6.0 Group Security Association The GKM Architecture defines the interfaces between the Registration, Re-key, and Data Security protocols in terms of the Security Associations (SAs) of those protocols. By isolating these protocols behind a uniform interface, our architecture allows implementations to use protocols best suited to their needs. For example, a Re-key protocol for a small group could use multiple unicast transmissions with symmetric authentication, while that for a large group could use IP Multicast with packet-level Forward Error Correction and source authentication. The Group Key Management Architecture provides an interface between the security protocols and the group SA (GSA), which consists of three SAs, viz., Registration SA, Re-key SA and Data SA. The Re-key Internet Draft Group Key Management Architecture [PAGE 20] Baugher, Canetti, Dondeti, Lindholm February 2002 SA is optional. There are two cases in defining the relationships between the three SAs. In both cases, the Registration SA protects the Registration protocol. In Case 1, Group key management is done WITHOUT using a Re-key SA. The Registration protocol initializes and updates one or more Data SAs (having TPKs to protect files or streams). Each Data SA corresponds to a single group “ and a group may have more than one data SA. In Case 2, group key management USES a Re-key SA to protect the Re- key protocol. The Registration protocol initializes the Re-key SAs (one or more) as well as zero or more Data SAs upon successful completion. When a Data SA is not initialized in the Registration protocol, this is done in the Re-key protocol. The Re-key protocol updates Re-key SA(s) AND establishes Data SA(s). 6.1 Group policy Group-policy is currently being defined [GSPT]. It can be distributed through announcement, key management protocols, and other means. The group key management carries cryptographic policies of the SA keys it establishes as well as additional policies for the group as well. The acceptable cryptographic policies for the Registration Protocol, which may run over TLS, IPsec, or IKE, are not conveyed in the group key-management protocol since they precede any of the key management exchanges. Thus, a security policy repository having some access protocol may need to be queried prior to key-management session establishment to determine what the initial cryptographic policies are for that establishment. This document assumes the existence of such a repository and protocol for GCKS and member policy queries. Thus group security policy will be represented in a policy repository and accessible using a policy protocol. This memo assumes that at least the following group-policy information is externally managed. o Group owner, authentication method, and delegation method for identifying a GCKS for the group o Group GCKS, authentication method, and any method used for delegating other GCKSs for the group o Group membership rules or list and authentication method There are also two additional policy-related requirements external to group key management. o There is an authorization and authentication infrastructure such as X.509, SPKI, or pre-shared key scheme in accordance with the group policy for a particular group. Internet Draft Group Key Management Architecture [PAGE 21] Baugher, Canetti, Dondeti, Lindholm February 2002 o There is an announcement mechanism for secure groups and events that operates according to group policy for a particular group. Group policy determines how the Registration and Re-key protocols initialize or update Re-key and Data SAs. The following sections describe the information that is sent by the GCKS for the Re-key and Data SAs. A member needs to have the information specified in the next sections to establish Re-key and Data SAs. 6.2 Contents of the Re-key SA The Re-key SA protects the Re-key protocol. It contains cryptographic policy, Security Parameter Index (SPI) [RFC2401] to uniquely identify an SA, replay protection information, and secret keys. 6.2.1 Re-key SA policy The MEMBERSHIP MANAGEMENT ALGORITHM represents the group key revocation algorithm that enforces forward and backward access control. Examples of key revocation algorithms include LKH, LKH+, OFT, OFC and Subset Difference [RFC2627, OFT, CP00, LNN01]. The key revocation algorithm could also be NULL. In that case, the Re-key SA contains only one KEK, which serves as the group KEK. The Re-key messages initialize or update Data SAs as usual. But, the Re-key SA itself can be updated (group KEK can be re-keyed) when members join or the KEK is about to expire. Leave re-keying is done by re- initializing the Re-key SA through the Re-key Protocol. The KEK ENCRYPTION ALGORITHM uses a standard encryption algorithm such as 3DES or AES. The KEK KEY LENGTH is also specified. The AUTHENTICATION ALGORITHM uses digital signatures for GCKS authentication (since all shared secrets are known to some or all members of the group), or some symmetric secret in computing MACs for group authentication. Symmetric authentication provides weaker authentication in that any group member can impersonate a particular source. The AUTHENTICATION KEY LENGTH is also be specified. The CONTROL GROUP ADDRESS is used for multicast transmission of Re- key messages. This information is sent over the control channel such as in an ANNOUNCEMENT protocol or call setup message. The degree to which the control group address is protected is a matter of group policy. The REKEY SERVER ADDRESS allows the registration server to be a different entity from the server used for re-key, such as for future invocations of the Registration and Re-key protocols. If the registration server and the re-key server are two different entities, the registration server sends the re-key server's address as part of the Re-key SA. 6.2.2 Group identity Internet Draft Group Key Management Architecture [PAGE 22] Baugher, Canetti, Dondeti, Lindholm February 2002 The Group identity accompanies the SA (payload) information as an identifier if the specific group key management protocol allows multiple groups to be initialized in a single invocation of the Registration protocol or multiple groups to be updated in a single Re-key message. It is often much simpler to restrict each Registration invocation to a single group, this Group Key Management Architecture mandates no such restriction. There is always a need to identify the group when establishing a Re-key SA either implicitly through an SPI or explicitly as an SA parameter. 6.2.3 Key encrypting key(s) Corresponding to the key management algorithm, the Re-key SA contains one or more KEKs. The GCKS holds the key encrypting keys of the group, while the members receive keys following the specification of the key-management algorithm. When there are multiple KEKs for a group (as in an LKH tree), each KEK needs to be associated with a Key ID, which is used to identify the key needed to decrypt it. Each KEK has a LIFETIME associated with it, after which the KEK expires. 6.2.4 Authentication key The GCKS provides a symmetric or public key for authentication of its Re-key messages. Symmetric-key authentication is appropriate only when all group members can be trusted not to impersonate the GCKS. The architecture does not rule out methods for deriving symmetric authentication keys at the member [RFC2409] rather than being pushed from the GCKS. 6.2.5 Replay protection information Re-key messages need to be protected from replay/reflection attacks. Sequence numbers are used for this purpose and the Re-key SA (or protocol) contains this information. 6.2.6 Security Parameter Index (SPI) The triple (Group identity, SPI, an identifier for "Re-key SA") uniquely identifies an SA. The SPI changes each time the KEKs change. 6.3 Contents of the Data SA The GCKS specifies the Data Security protocol used for secure transmission of data from sender(s) to receiving members. Examples Internet Draft Group Key Management Architecture [PAGE 23] Baugher, Canetti, Dondeti, Lindholm February 2002 of Data Security protocols include IPsec ESP, SRTP, MESP, and AMESP. While the content of each of these protocols is out of the scope of this document, we list the information sent by the Registration protocol (or the Re-key Protocol) to initialize or update the Data SA. 6.3.1 Group identity The Group identity accompanies SA information when Data SAs are initialized or re-keyed for multiple groups in a single invocation of the Registration protocol or in a single Re-key message (see 4.2.2). 6.3.2 Source identity The SA includes source identity information when the Group Owner chooses to reveal Source identity to authorized members only. A public channel such as announcement protocol is only appropriate when there is no need to protect source or group identities. 6.3.3 Traffic protection keys Irrespective of the Data Security Protocol used, the GCKS supplies the TPKs (or information to derive TPKs) used in secure data transmission, source and group authentication. 6.3.4 Authentication key Depending on the data-authentication method used by the Data Security protocol, group key management may pass one or more keys, functions (e.g., TESLA), or other parameters used for authenticating streams or files. 6.3.5 Sequence numbers The GCKS passes sequence numbers when needed by the Data Security protocol, for replay protection. 6.3.6 Security Parameter Index (SPI) The GCKS sends provides an identifier as part of the Data SA contents for data security protocols that use an SPI or similar mechanism to identify an SA or keys within an SA. 6.3.7 Data SA policy The Data SA parameters are specific to the Data Security Protocol but generally include encryption algorithm and parameters, the source authentication algorithm and parameters, the group authentication algorithm and parameters, and/or replay protection information. Internet Draft Group Key Management Architecture [PAGE 24] Baugher, Canetti, Dondeti, Lindholm February 2002 Generally, specification of source or group authentication is mutually exclusive. 7.0 Scalability Considerations Group communications is quite diverse. In commercial teleconferencing, a multipoint control unit (MCU) may be used to aggregate a number of teleconferencing members into a single session; MCUs may be hierarchically organized as well. A "loosely coupled" teleconferencing session [RFC 1889] has no central controller but is fully distributed and end-to-end. Teleconferencing sessions tend to have at most dozens of participants whereas video broadcast, which uses multicast communications, and media on demand, which uses unicast, are large-scale groups numbering hundreds to millions of participants. As described in the Requirements section above, the group key management architecture supports source-specific multicast. One-to- many (single-sender) applications are well suited to source-specific multicast, which tend to have large numbers of participants and problems with synchronization among the participants. ŸFlash crowds÷ are one manifestation of the problem with synchronized participants who make concurrent request for group data with concomitant requests for secure group keys. Thus, a group key management protocol designed for single-source multicast applications must support large- scale operation. The architecture described in this paper supports large-scale operation through the following features. 1. There is no need for a unicast exchange to provide data keys to a security protocol for members who have previously-registered in the particular group; data keys can be pushed in the Re-key protocol. 2. The Registration and Re-key protocols are separable to allow flexibility in how members get group secrets. A group can use a smart-card based system in place of the Registration protocol, for example, to allow the Re-key protocol to be used with no back channel for broadcast applications such as television conditional access systems. 3. The Registration and Re-key protocols support new keys, algorithms, authorization infrastructures and authentication mechanisms in the architecture. When the authorization infrastructure supports delegation, as does X.509 and SPKI, the GCKS function can be distributed as shown in Figure 3. Internet Draft Group Key Management Architecture [PAGE 25] Baugher, Canetti, Dondeti, Lindholm February 2002 +----------------------------------------+ | +-------+ | | | GCKS | | | +-------+ | | | ^ | | | | | | | +---------------+ | | | ^ ^ | | | | ... | | | | +--------+ +--------+ | | | | MEMBER | | MEMBER | | | | +--------+ +--------+ | | v | | +-------------+ | | | | | | v ... v | | +-------+ +-------+ | | | GCKS | | GCKS | | | +-------+ +-------+ | | | ^ | | | | | | | +---------------+ | | | ^ ^ | | | | ... | | | | +--------+ +--------+ | | | | MEMBER | | MEMBER | | | | +--------+ +--------+ | | v | | ... | +----------------------------------------+ Figure 3: Hierarchically-organized Key Distribution The first feature in the list allows fast keying of Data Security protocols when the member already belongs to the group. While this is realistic for subscriber groups and customers of service providers who offer content events, it may be too restrictive for applications that allow member enrollment at the time of the event. The recourse for handling member registration in the context of a Ÿflash crowd÷ is Figure 3, which will require the use of many GCKSs to accommodate the load. The Figure 3 configuration may be needed when conventional clustering and load-balancing solutions of a central GCKS site cannot meet customer requirements. Unlike conventional caching and content- distribution networks, however, the configuration shown in Figure 3 has additional security ramifications for physical security of a GCKS. More analysis and work needs to be done on the protocol instantiations of the Group Key Management architecture to determine how effectively and securely the architecture can operate in large- scale environments such as source-specific multicast and video on Internet Draft Group Key Management Architecture [PAGE 26] Baugher, Canetti, Dondeti, Lindholm February 2002 demand. Specifically, the requirements for a Figure 3 configuration must be determined such as the need for additional protocols between the GCKS designated by the Group Owner and GCKSs that have been delegated to serve keys on behalf of the designated GCKS. 8.0 Security Considerations This memo describes an architecture for group key management. This architecture will be instantiated in one or more group key management protocols, which must be protected against man-in-the-middle, connection hijacking, replay or reflection of past messages, and denial of service attacks. Authenticated key exchange [STS, SKEME, RFC2408, RFC2412, RFC2409] techniques limit the effects of man-in-the-middle and connection- hijacking attacks. Sequence numbers and low-computation message authentication techniques can be effective against replay and reflection attacks. Cookies [RFC2522], when properly implemented, provide an efficient means to reduce the effects of denial of service attacks. This memo does not address attacks against key management or security protocol implementations such as so-called Ÿtype attacks÷ that aim to disrupt an implementation by such means as buffer overflow. The focus of this memo is on securing the protocol, not an implementation of the protocol. While classical techniques of authenticated key exchange can be applied to group key management, new problems arise with the sharing of secrets among a group of members: Group secrets may be disclosed by a member of the group and group senders may be impersonated by other members of the group. Key management messages from the GCKS should not be authenticated using shared symmetric secrets unless all members of the group can be trusted not to impersonate the GCKS. Similarly, members who disclose group secrets undermine the security of the entire group. Group Owners and GCKS administrators must be aware of these inherent limitations of group key management. Another limitation of group key management is policy complexity: Whereas peer-to-peer security policy is an intersection of the policy of the individual peers, a Group Owner sets group security policy externally in secure groups. This document assumes there is no negotiation of cryptographic or other security parameters in group key management. Group security policy, therefore, poses new risks to members who send and receive data from secure groups. Security administrators, GCKS operators, and users need to determine minimal acceptable levels of trust, authenticity and confidentiality when joining secure groups. Internet Draft Group Key Management Architecture [PAGE 27] Baugher, Canetti, Dondeti, Lindholm February 2002 Given the limitations and risks of group security, the security of the group key management Registration protocol should be as good as the base protocols on which it is developed such as IKE, IPsec, TLS, or SSL. The particular instantiations of this Group Key Management architecture must ensure that the high standards for authenticated key exchange are preserved in their protocol specifications, which will be Internet standards-track documents that are subject to review, analysis and testing. The second protocol, the group key management Re-key protocol, is new and has unknown risks associated with it. The source-authentication risks describe above are obviated by the use of public-key cryptography. The use of multicast delivery may raise additional security issues such as reliability, implosion, and denial of service attacks based upon the use of multicast. The Re-key protocol specification (see Appendix A for the drafts roadmap) needs to offer secure solutions to these problems. Each instantiation of the Re-key protocol, such as the GSAKMP Re-key or the GDOI Groupkey-push operations, need to validate the security of their Re-key specifications. Novelty and complexity are the biggest risks to group key management protocols. Much more analysis and experience are needed to ensure that the architecture described in this document can provide a well- articulate standard for security and risks of group key management. 9.0 References and Bibliography [AMESP] R. Canetti, P. Rohatgi, Pau-Chen Cheng, Multicast Data Security Transformations: Requirements, Considerations, and Prominent Choices, http://search.ietf.org/internet-drafts/draft-irtf-smug-data- transforms.txt, Work In Progress, 2000. [CP00] R. Canetti, B. Pinkas, A taxonomy of multicast security issues, http://www.ietf.org/internet-drafts/draft-irtf-smug- taxonomy-01.txt, Work in Progress, August 2000. [FN93]A. Fiat, M. Naor, Broadcast Encryption, Advances in Cryptology - CRYPTO Ë93 Proceedings, Lecture Notes in Computer Science, Vol. 773, 1994, pp. 480“491. [FS00] N. Ferguson and B. Schneier, A Cryptographic Evaluation of IPsec, CounterPane, http://www.counterpane.com/ipsec.html. [GDOI] M. Baugher, T. Hardjono, H. Harney, B. Weis, The Group Domain of Interpretation, http://www.ietf.org/internet-drafts/draft-ietf- msec-gdoi-00.txt, February 2001, Work in Progress. Internet Draft Group Key Management Architecture [PAGE 28] Baugher, Canetti, Dondeti, Lindholm February 2002 [GSAKMP] H.Harney, A.Colegrove, E.Harder, U.Meth, R.Fleischer, Group Secure Association Key Management Protocol, http://www.ietf.org/internet-drafts/draft-ietf-msec-gsakmp-sec- 00.txt, March 2001, Work in Progress. [H.235] ITU, Security and encryption for H-Series (H.323 and other H.245-based) multimedia terminals, ITU-T Recommendation H.235 Version 3, 2001, Work in progress. [JKKV94] M. Just, E. Kranakis, D. Krizanc, P. van Oorschot, On Key Distribution via True Broadcasting, On Key Distribution via True Broadcasting. In Proceedings of 2nd ACM Conference on Computer and Communications Security, November 1994, pp. 81--88. [MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. Norrman, "MIKEY: Multimedia Internet KEYing", Internet Draft, IETF, Work in progress. [MARKS] B. Briscoe, MARKS: Zero Side Effect Multicast Key Management using Arbitrarily Revealed Key Sequences, Proceedings of NGC'99, rbriscoe@bt.co.uk. [MT] D.S. Marks, B.H. Turnbull, Technical protection measures: The intersection of technology, law, and commercial licenses, Workshop on Implementation Issues of the WIPO Copyright Treaty (WCT) and the WIPO Performances and Phonograms Treaty (WPPT), World Intellectual Property Organization, Geneva, December 6 and 7, 1999 (http://www.wipo.org/eng/meetings/1999/wct_wppt/pdf/imp99_3.pdf). [MVV] A.J.Menzes, P.C.van Oorschot, S.A. Vanstone, Handbook of Applied Cryptography, CRC Press, 1996. [OFT] D. Balenson, D. McGrew, A. Sherman, Key Management for Large Dynamic Groups: One-Way Function Trees and Amortized Initialization, http://www.ietf.org/internet-drafts/draft-balenson-groupkeymgmt-oft- 00.txt, February 1999, Work in Progress. [RFC1889] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP: A Transport Protocol for Real-Time Applications, January 1996. [RFC2093] Harney, H., and Muckenhirn, C., "Group Key Management Protocol (GKMP) Specification," RFC 2093, July 1997. [RFC2094] Harney, H., and Muckenhirn, C., "Group Key Management Protocol (GKMP) Architecture," RFC 2094, July 1997. [RFC2326] ftp://ftp.isi.edu/in-notes/rfc2326.txt Internet Draft Group Key Management Architecture [PAGE 29] Baugher, Canetti, Dondeti, Lindholm February 2002 [RFC2327] M. Handley, V. Jacobson, SDP: Session Description Protocol, April 1998. [RFC2367] D. McDonald, C. Metz, B. Phan, PF_KEY Key Management API, Version 2, July 1998. [RFC2401] S. Kent, R. Atkinson, Security Architecture for the Internet Protocol, November 1998 [RFC2406] S. Kent, R. Atkinson, IP Encapsulating Security Payload (ESP), November 1998. [RFC2407] D. Piper, The Internet IP Domain of Interpretation for ISAKMP, November 1998. [RFC2408] D. Maughan, M. Shertler, M. Schneider, J. Turner, Internet Security Association and Key Management Protocol, November 1998. [RFC2409] D. Harkins, D. Carrel, The Internet Key Exchange (IKE), November, 1998. [RFC2412] H. Orman, The OAKLEY Key Determination Protocol, November 1998. [RFC2522] P. Karn, W. Simpson, Photuris: Session-Key Management Protocol, March 1999. [RFC2543] ftp://ftp.isi.edu/in-notes/rfc2543.txt [RFC2627] D. M. Wallner, E. Harder, R. C. Agee, Key Management for Multicast: Issues and Architectures, September 1998. [SKEME] H. Krawczyk, SKEME: A Versatile Secure Key Exchange Mechanism for Internet, ISOC Secure Networks and Distributed Systems Symposium, San Diego, 1996. [STS] Diffie, P. van Oorschot, M. J. Wiener, Authentication and Authenticated Key Exchanges, Designs, Codes and Cryptography, 2, 107-125 (1992), Kluwer Academic Publishers. [SRTP] R.Blom, E.Carrara, D.McGrew, M.Nasland, K.Norrman, D. Oran, The Secure Real Time Transport Protocol, http://www.ietf.org/internet-drafts/draft-ietf-avt-srtp-00.txt, February 2001, Work in Progress. Internet Draft Group Key Management Architecture [PAGE 30] Baugher, Canetti, Dondeti, Lindholm February 2002 10.0 Authors' Addresses Mark Baugher Cisco Systems 5510 SW Orchid St. Portland, OR 97219, USA +1 408-853-4418 mbaugher@cisco.com Ran Canetti IBM Research 30 Saw Mill River Road Hawthorne, NY 10532, USA +1 914-784-7076 canetti@watson.ibm.com Lakshminath R. Dondeti Nortel Networks 600 Technology Park Drive Billerica, MA 01821, USA +1 978-288-6406 ldondeti@nortelnetworks.com Fredrik Lindholm Ericsson Research SE-16480 Stockholm, Sweden +46 8 58531705 fredrik.lindholm@era.ericsson.se Internet Draft Group Key Management Architecture [PAGE 31] Baugher, Canetti, Dondeti, Lindholm February 2002 Appendix: MSEC Security Documents Roadmap +--------------+ | MSEC | | Requirements | +--------------+ : : +--------------+ | MSEC | | Architecture | +--------------+ : .....................:....................... : : : +--------------+ +--------------+ +--------------+ | Policy | | GKM | | Data Security| | Architecture | | Architecture | | Architecture | +--------------+ +--------------+ +--------------+ : : : : : : . +------------+ : +------------+ : . | GDOI | : |TESLA/MESP | : | Resolution |-: | |-: | | : | | : +------------+ : +------------+ : : : : : +------------+ : +------------+ : | GSAKMP- | : | | : | Resolution |-: | TBD |-: | | : | | : +------------+ : +------------+ : : : : : +------------+ : +------------+ : | | : | | : | RE-KEY |-: | TBD |-: | | : | | : +------------+ : +------------+ : : : . . . . FIGURE A: Graphic rendition of the inter-relations between the I-D's of MSEC. Note that some of these drafts are still in the process of being written. Internet Draft Group Key Management Architecture [PAGE 32]