EAP Working Group Bernard Aboba INTERNET-DRAFT Dan Simon Category: Informational Microsoft <draft-ietf-eap-keying-02.txt> J. Arkko 26 June 2004 Ericsson P. Eronen Nokia H. Levkowetz, Ed. ipUnplugged Extensible Authentication Protocol (EAP) Key Management Framework Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract The Extensible Authentication Protocol (EAP), defined in [RFC3748], enables extensible network access authentication. This document provides a framework for the generation, transport and usage of keying material generated by EAP authentication algorithms, known as "methods". Aboba, et al. Informational [Page 1]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Table of Contents 1. Introduction .......................................... 4 1.1 Requirements Language ........................... 4 1.2 Terminology ..................................... 4 1.3 Overview ........................................ 5 1.4 EAP Invariants .................................. 11 2. EAP Key Hierarchy ..................................... 13 2.1 Key Terminology ................................. 13 2.2 Key Hierarchy ................................... 15 2.3 Key Lifetimes ................................... 17 2.4 AAA-Key Scope ................................... 24 2.5 Fast Handoff Support ............................ 26 3. Security associations ................................. 30 3.1 EAP Method SA ................................... 31 3.2 EAP-Key SA ...................................... 33 3.3 AAA SA(s) ....................................... 33 3.4 Service SA(s) ................................... 34 3.5 SA Naming ....................................... 37 4. Security Considerations .............................. 39 4.1 Security Terminology ............................ 39 4.2 Threat Model .................................... 39 4.3 Security Analysis ............................... 41 4.4 Man-in-the-middle Attacks ....................... 45 4.5 Denial of Service Attacks ....................... 45 4.6 Impersonation ................................... 46 4.7 Channel Binding ................................. 47 4.8 Key Strength .................................... 48 4.9 Key Wrap ........................................ 48 Aboba, et al. Informational [Page 2]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 5. Security Requirements ................................. 49 5.1 EAP Method Requirements ......................... 49 5.2 AAA Protocol Requirements ....................... 52 5.3 Secure Association Protocol Requirements ........ 54 5.4 Ciphersuite Requirements ........................ 55 6. IANA Considerations ................................... 56 7. References ............................................ 56 7.1 Normative References ............................ 56 7.2 Informative References .......................... 57 Acknowledgments .............................................. 60 Author's Addresses ........................................... 61 Appendix A - Ciphersuite Keying Requirements ................. 62 Appendix B - Transient EAP Key (TEK) Hierarchy ............... 63 Appendix C - EAP Key Hierarchy ............................... 64 Appendix D - Transient Session Key (TSK) Derivation .......... 66 Appendix E - AAA-Key Derivation .............................. 67 Intellectual Property Statement .............................. 68 Full Copyright Statement ..................................... 68 Aboba, et al. Informational [Page 3]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 1. Introduction The Extensible Authentication Protocol (EAP), defined in [RFC3748], was designed to enable extensible authentication for network access in situations in which the IP protocol is not available. Originally developed for use with PPP [RFC1661], it has subsequently also been applied to IEEE 802 wired networks [IEEE8021X]. This document provides a framework for the generation, transport and usage of keying material generated by EAP authentication algorithms, known as "methods". Since in EAP keying material is generated by EAP methods, transported by AAA protocols, transformed into session keys by Secure Association Protocols and used by lower layer ciphersuites, it is necessary to describe each of these elements and provide a system-level security analysis. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119]. 1.2. Terminology This document frequently uses the following terms: authenticator The end of the link initiating EAP authentication. The term Authenticator is used in [IEEE-802.1X], and authenticator has the same meaning in this document. peer The end of the link that responds to the authenticator. In [IEEE-802.1X], this end is known as the Supplicant. Supplicant The end of the link that responds to the authenticator in [IEEE-802.1X]. In this document, this end of the link is called the peer. backend authentication server A backend authentication server is an entity that provides an authentication service to an authenticator. When used, this server typically executes EAP methods for the authenticator. This terminology is also used in [IEEE-802.1X]. AAA Authentication, Authorization and Accounting. AAA protocols with EAP support include RADIUS [RFC3579] and Diameter [I-D.ietf-aaa- eap]. In this document, the terms "AAA server" and "backend Aboba, et al. Informational [Page 4]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 authentication server" are used interchangeably. EAP server The entity that terminates the EAP authentication method with the peer. In the case where no backend authentication server is used, the EAP server is part of the authenticator. In the case where the authenticator operates in pass-through mode, the EAP server is located on the backend authentication server. security association A set of policies and key(s) used to protect information. This information in the security association is stored by each party of the security association and must be consistent among the parties. Elements of a security association include cryptographic keys, negotiated ciphersuites and other parameters, counters, sequence spaces, authorization attributes, etc. 1.3. Overview EAP is typically deployed in order to support extensible network access authentication in situations where a peer desires network access via one or more authenticators. The situation is illustrated in Figure 1. Since both the peer and authenticator may have more than one physical or logical port, a given peer may simultaneously access the network via multiple authenticators, or via multiple physical or logical ports on a given authenticator. Similarly, an authenticator may offer network access to multiple peers, each via a separate physical or logical port. The peer may be stationary, in which case it may establish communications with one or more authenticators while remaining in one location. Alternatively, the peer may be mobile, changing its point of attachment from one authenticator to another, or moving between points of attachment on a single authenticator. Where authenticators are deployed standalone, the EAP conversation occurs between the peer and authenticator, and the authenticator must locally implement an EAP method acceptable to the peer. However, one of the advantages of EAP is that it enables deployment of new authentication methods without requiring development of new code on the authenticator. While the authenticator may implement some EAP methods locally and use those methods to authenticate local users, it may at the same time act as a pass-through for other users and methods, forwarding EAP packets back and forth between the backend authentication server and the peer. Aboba, et al. Informational [Page 5]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 +-+-+-+-+ | | | EAP | | Peer | | | +-+-+-+-+ | | | Peer Ports / | \ / | \ Phase 0: Discovery / | \ Phase 1: Authentication / | \ Phase 2: Secure / | \ Association / | \ / | \ / | \ | | | | | | | | | Authenticator Ports +-+-+-+-+ +-+-+-+-+ +-+-+-+-+ | | | | | | | Auth. | | Auth. | | Auth. | | | | | | | +-+-+-+-+ +-+-+-+-+ +-+-+-+-+ \ | / \ | / \ | / EAP over AAA \ | / (optional) \ | / \ | / \ | / \ | / +-+-+-+-+ | | | AAA | |Server | | | +-+-+-+-+ Figure 1: Relationship between peer, authenticator and backend server This is accomplished by encapsulating EAP packets within the Authentication, Authorization and Accounting (AAA) protocol, spoken between the authenticator and backend authentication server. AAA protocols supporting EAP include RADIUS [RFC3579] and Diameter [I- D.ietf-aaa-eap]. Aboba, et al. Informational [Page 6]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Where EAP key derivation is supported, the conversation between the peer and the authenticator typically takes place in three phases: Phase 0: Discovery Phase 1: Authentication 1a: EAP authentication 1b: AAA-Key Transport (optional) Phase 2: Secure Association Establishment 2a: Unicast Secure Association 2b: Multicast Secure Association (optional) In the discovery phase (phase 0), peers locate authenticators and discover their capabilities. For example, a peer may locate an authenticator providing access to a particular network, or a peer may locate an authenticator behind a bridge with which it desires to establish a Secure Association. The authentication phase (phase 1) may begin once the peer and authenticator discover each other. This phase always includes EAP authentication (phase 1a). Where the chosen EAP method supports key derivation, in phase 1a keying material is derived on both the peer and the EAP server. This keying material may be used for multiple purposes, including protection of the EAP conversation and subsequent data exchanges. An additional step (phase 1b) is required in deployments which include a backend authentication server, in order to transport keying material (known as the AAA-Key) from the backend authentication server to the authenticator. A Secure Association exchange (phase 2) then occurs between the peer and authenticator in order to manage the creation and deletion of unicast (phase 2a) and multicast (phase 2b) security associations between the peer and authenticator. The conversation phases and relationship between the parties is shown in Figure 2. Aboba, et al. Informational [Page 7]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 EAP peer Authenticator Auth. Server -------- ------------- ------------ |<----------------------------->| | | Discovery (phase 0) | | |<----------------------------->|<----------------------------->| | EAP auth (phase 1a) | AAA pass-through (optional) | | | | | |<----------------------------->| | | AAA-Key transport | | | (optional; phase 1b) | |<----------------------------->| | | Unicast Secure association | | | (phase 2a) | | | | | |<----------------------------->| | | Multicast Secure association | | | (optional; phase 2b) | | | | | Figure 2: Conversation Overview 1.3.1. Discovery Phase In the discovery phase (phase 0), the EAP peer and authenticator locate each other and discover each other's capabilities. Discovery can occur manually or automatically, depending on the lower layer over which EAP runs. Since discovery is handled outside of EAP, there is no need to provide this functionality within EAP. For example, where EAP runs over PPP, the EAP peer might be configured with a phone book providing phone numbers of authenticators and associated capabilities such as supported rates, authentication protocols or ciphersuites. In contrast, PPPoE [RFC2516] provides support for a Discovery Stage to allow a peer to identify the Ethernet MAC address of one or more authenticators and establish a PPPoE SESSION_ID. IEEE 802.11 [IEEE80211] also provides integrated discovery support utilizing Beacon and/or Probe Request/Response frames, allowing the peer (known as the station or STA) to determine the MAC address and capabilities of one or more authenticators (known as Access Point or APs). 1.3.2. Authentication Phase Once the peer and authenticator discover each other, they exchange EAP packets. Typically, the peer desires access to the network, and Aboba, et al. Informational [Page 8]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 the authenticators provide that access. In such a situation, access to the network can be provided by any authenticator attaching to the desired network, and the EAP peer is typically willing to send data traffic through any authenticator that can demonstrate that it is authorized to provide access to the desired network. An EAP authenticator may handle the authentication locally, or it may act as a pass-through to a backend authentication server. In the latter case the EAP exchange occurs between the EAP peer and a backend authenticator server, with the authenticator forwarding EAP packets between the two. The entity which terminates EAP authentication with the peer is known as the EAP server. Where pass- through is supported, the backend authentication server functions as the EAP server; where authentication occurs locally, the EAP server is the authenticator. Where a backend authentication server is present, at the successful completion of an authentication exchange, the AAA-Key is transported to the authenticator (phase 1b). EAP may also be used when it is desired for two network devices (e.g. two switches or routers) to authenticate each other, or where two peers desire to authenticate each other and set up a secure association suitable for protecting data traffic. Some EAP methods exist which only support one-way authentication; however, EAP methods deriving keys are required to support mutual authentication. In either case, it can be assumed that the parties do not utilize the link to exchange data traffic unless their authentication requirements have been met. For example, a peer completing mutual authentication with an EAP server will not send data traffic over the link until the EAP server has authenticated successfully to the peer, and a Secure Association has been negotiated. Since EAP is a peer-to-peer protocol, an independent and simultaneous authentication may take place in the reverse direction. Both peers may act as authenticators and authenticatees at the same time. Successful completion of EAP authentication and key derivation by a peer and EAP server does not necessarily imply that the peer is committed to joining the network associated with an EAP server. Rather, this commitment is implied by the creation of a security association between the EAP peer and authenticator, as part of the Secure Association Protocol (phase 2). As a result, EAP may be used for "pre-authentication" in situations where it is necessary to pre- establish EAP security associations in order to decrease handoff or roaming latency. Aboba, et al. Informational [Page 9]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 1.3.3. Secure Association Phase The Secure Association phase (phase 2) always occurs after the completion of EAP authentication (phase 1a) and key transport (phase 1b), and typically supports the following features: [1] Entity Naming. A basic feature of a Secure Association Protocol is the naming of the parties engaged in the exchange. As illustrated in Figure 1, it is possible for both the peer and NAS to have more than one physical or virtual port. For the purposes of identification, it is therefore not possible to identify either peers or NAS devices using port identifiers. Proper identification of the parties is critical to the Secure Association phase, since without this the parties engaged in the exchange are not identified and the scope of the transient session keys (TSKs) generated during the exchange is undefined. [2] Secure capabilities negotiation. This provides for the secure negotiation of usage modes, session parameters (such as key lifetimes), ciphersuites, and required filters, including confirmation of the capabilities discovered during phase 0. By securely negotiating session parameters, the secure Association Protocol protects against spoofing during the discovery phase and ensures that the peer and authenticator are in agreement about how data is to be secured. [3] Generation of fresh transient session keys (TSKs). The Secure Association Protocol typically guarantees the freshness of session keys by exchanging nonces between both parties and then mixing the nonces with the AAA-Key in order to generate fresh unicast (phase 2a) and multicast (phase 2b) session keys. By not using the AAA- Key directly to protect data, the secure Association Protocol protects against compromise of the AAA-Key, and by guaranteeing the freshness of transient session keys, assures that they are not reused. [4] Key activation and deletion. In order for the peer and authenticator to communicate securely, it is necessary for both sides to derive the same session keys, and remain in sync with respect to key state going forward. One of the functions of the Secure Association Protocol is to synchronize the activation and deletion of keys so as to enable seamless rekey, or recovery from partial or complete loss of key state by the peer or authenticator. [5] Mutual proof of possession of the AAA-Key. This demonstrates that both the peer and authenticator have been authenticated and authorized by the backend authentication server. Since mutual proof of possession is not the same as mutual authentication, the Aboba, et al. Informational [Page 10]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 peer cannot verify authenticator assertions (including the authenticator identity) as a result of this exchange. 1.4. EAP Invariants By utilizing a three phase exchange, the EAP key management framework guarantees that certain basic characteristics, known as the "EAP Invariants" hold true for all implementations of EAP. These include: Media independence Method independence Ciphersuite independence 1.4.1. Media Independence One of the goals of EAP is to allow EAP methods to function on any lower layer meeting the criteria outlined in [RFC3748], Section 3.1. For example, as described in [RFC3748], EAP authentication can be run over PPP [RFC1661], IEEE 802 wired networks [IEEE8021X], and IEEE 802.11 wireless LANs [IEEE80211i]. In order to maintain media independence, it is necessary for EAP to avoid inclusion of media-specific elements. For example, EAP methods cannot be assumed to have knowledge of the lower layer over which they are transported, and cannot utilize identifiers associated with a particular usage environment (e.g. MAC addresses). The need for media independence has also motivated the development of the three phase exchange. Since discovery is typically media- specific, this function is handled outside of EAP, rather than being incorporated within it. Similarly, the Secure Association Protocol often contains media dependencies such as negotiation of media- specific ciphersuites or session parameters, and as a result this functionality also cannot be incorporated within EAP. Note that media independence may be retained within EAP methods that support channel binding or method-specific identification. An EAP method need not be aware of the content of an identifier in order to use it. This enables an EAP method to use media-specific identifiers such as MAC addresses without compromising media independence. To support channel binding, an EAP method can pass binding parameters to the AAA server in the form of an opaque blob, and receive confirmation of whether the parameters match, without requiring media-specific knowledge. Aboba, et al. Informational [Page 11]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 1.4.2. Method Independence By enabling pass-through, authenticators can support any method implemented on the peer and server, not just locally implemented methods. This allows the authenticator to avoid implementing code for each EAP method required by peers. In fact, since a pass-through authenticator is not required to implement any EAP methods at all, it cannot be assumed to support any EAP method-specific code. As a result, as noted in [RFC3748], authenticators must by default be capable of supporting any EAP method. Since the Discovery and Secure Association exchanges are also method independent, an authenticator can carry out the three phase exchange without having an EAP method in common with the peer. This is useful where there is no single EAP method that is both mandatory-to-implement and offers acceptable security for the media in use. For example, the [RFC3748] mandatory-to-implement EAP method (MD5-Challenge) does not provide dictionary attack resistance, mutual authentication or key derivation, and as a result is not appropriate for use in wireless LAN authentication [WLANREQ]. However, despite this it is possible for the peer and authenticator to interoperate as long as a suitable EAP method is supported on the EAP server. 1.4.3. Ciphersuite Independence While EAP methods may negotiate the ciphersuite used in protection of the EAP conversation, the ciphersuite used for the protection of data is negotiated within the Secure Association Protocol, out-of-band of EAP. The backend authentication server is not a party to this negotiation nor is it an intermediary in the data flow between the EAP peer and authenticator. The backend authentication server may not even have knowledge of the ciphersuites implemented by the peer and authenticator, or be aware of the ciphersuite negotiated between them, and therefore does not implement ciphersuite-specific code. Since ciphersuite negotiation occurs in the Secure Association protocol, not in EAP, ciphersuite-specific key generation, if implemented within an EAP method, would potentially conflict with the transient session key derivation occurring in the Secure Association protocol. As a result, EAP methods generate keying material that is ciphersuite-independent. Additional advantages of ciphersuite- independence include: Update requirements If EAP methods were to specify how to derive transient session keys Aboba, et al. Informational [Page 12]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 for each ciphersuite, they would need to be updated each time a new ciphersuite is developed. In addition, backend authentication servers might not be usable with all EAP-capable authenticators, since the backend authentication server would also need to be updated each time support for a new ciphersuite is added to the authenticator. EAP method complexity Requiring each EAP method to include ciphersuite-specific code for transient session key derivation would increase method complexity and result in duplicated effort. Knowledge of capabilities In practice, an EAP method may not have knowledge of the ciphersuite that has been negotiated between the peer and authenticator, since this negotiation typically occurs within the Secure Association Protocol. For example, PPP ciphersuite negotiation occurs in the Encryption Control Protocol (ECP) [RFC1968]. Since ECP negotiation occurs after authentication, unless an EAP method is utilized that supports ciphersuite negotiation, the peer, authenticator and backend authentication server may not be able to anticipate the negotiated ciphersuite and therefore this information cannot be provided to the EAP method. Since ciphersuite negotiation is assumed to occur out-of-band, there is no need for ciphersuite negotiation within EAP. 2. EAP Key Hierarchy 2.1. Key Terminology The EAP Key Hierarchy makes use of the following types of keys: Long Term Credential EAP methods frequently make use of long term secrets in order to enable authentication between the peer and server. In the case of a method based on pre-shared key authentication, the long term credential is the pre-shared key. In the case of a public-key based method, the long term credential is the corresponding private key. Master Session Key (MSK) Keying material that is derived between the EAP peer and server and exported by the EAP method. The MSK is at least 64 octets in length. Aboba, et al. Informational [Page 13]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Extended Master Session Key (EMSK) Additional keying material derived between the peer and server that is exported by the EAP method. The EMSK is at least 64 octets in length, and is never shared with a third party. AAA-Key A key derived by the peer and EAP server, used by the peer and authenticator in the derivation of Transient Session Keys (TSKs). Where a backend authentication server is present, the AAA-Key is transported from the backend authentication server to the authenticator, wrapped within the AAA-Token; it is therefore known by the peer, authenticator and backend authentication server. However, despite the name, the AAA-Key is computed regardless of whether a backend authentication server is present. AAA-Key derivation is discussed in Appendix E; in existing implementations the MSK is used as the AAA-Key. Application-specific Master Session Keys (AMSKs) Keys derived from the EMSK which are cryptographically separate from each other and may be subsequently used in the derivation of Transient Session Keys (TSKs) for extended uses. AMSK derivation is discussed in Appendix E. AAA-Token Where a backend server is present, the AAA-Key and one or more attributes is transported between the backend authentication server and the authenticator within a package known as the AAA-Token. The format and wrapping of the AAA-Token, which is intended to be accessible only to the backend authentication server and authenticator, is defined by the AAA protocol. Examples include RADIUS [RFC2548] and Diameter [I-D.ietf-aaa-eap]. Initialization Vector (IV) A quantity of at least 64 octets, suitable for use in an initialization vector field, that is derived between the peer and EAP server. Since the IV is a known value in methods such as EAP- TLS [RFC2716], it cannot be used by itself for computation of any quantity that needs to remain secret. As a result, its use has been deprecated and EAP methods are not required to generate it. However, when it is generated it MUST be unpredictable. Pairwise Master Key (PMK) The AAA-Key is divided into two halves, the "Peer to Authenticator Encryption Key" (Enc-RECV-Key) and "Authenticator to Peer Encryption Key" (Enc-SEND-Key) (reception is defined from the point of view of the authenticator). Within [IEEE80211i] Octets 0-31 of the AAA-Key (Enc-RECV-Key) are known as the Pairwise Master Key (PMK). In [IEEE80211i] the TKIP and AES CCMP ciphersuites derive Aboba, et al. Informational [Page 14]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 their Transient Session Keys (TSKs) solely from the PMK, whereas the WEP ciphersuite as noted in [RFC3580], derives its TSKs from both halves of the AAA-Key. Transient EAP Keys (TEKs) Session keys which are used to establish a protected channel between the EAP peer and server during the EAP authentication exchange. The TEKs are appropriate for use with the ciphersuite negotiated between EAP peer and server for use in protecting the EAP conversation. Note that the ciphersuite used to set up the protected channel between the EAP peer and server during EAP authentication is unrelated to the ciphersuite used to subsequently protect data sent between the EAP peer and authenticator. An example TEK key hierarchy is described in Appendix C. Transient Session Keys (TSKs) Session keys used to protect data which are appropriate for the ciphersuite negotiated between the EAP peer and authenticator. The TSKs are derived from AAA-Key during the Secure Association Protocol. In the case of [IEEE80211i] the Secure Association Protocol consists of the 4-way handshake and group key derivation. An example TSK derivation is provided in Appendix D. 2.2. Key Hierarchy The EAP Key Hierarchy, illustrated in Figure 3, has at the root the long term credential utilized by the selected EAP method. If authentication is based on a pre-shared key, the parties store the EAP method to be used and the pre-shared key. The EAP server also stores the peer's identity and/or other information necessary to decide whether access to some service should be granted. The peer stores information necessary to choose which secret to use for which service. If authentication is based on proof of possession of the private key corresponding to the public key contained within a certificate, the parties store the EAP method to be used and the trust anchors used to validate the certificates. The EAP server also stores the peer's identity and/or other information necessary to decide whether access to some service should be granted. The peer stores information necessary to choose which certificate to use for which service. Based on the long term credential established between the peer and the server, EAP derives four types of keys: [1] Keys calculated locally by the EAP method but not exported by the EAP method, such as the TEKs. [2] Keys exported by the EAP method: MSK, EMSK, IV Aboba, et al. Informational [Page 15]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 [3] Keys calculated from exported quantities: AAA-Key, AMSKs. [4] Keys calculated by the Secure Association Protocol: TSKs. In order to protect the EAP conversation, methods supporting key derivation typically negotiate a ciphersuite and derive Transient EAP Keys (TEKs) for use with that ciphersuite. The TEKs are stored locally by the EAP method and are not exported. As noted in [RFC3748] Section 7.10, EAP methods generating keys are required to calculate and export the MSK and EMSK, which must be at least 64 octets in length. EAP methods also may export the IV; however, the use of the IV is deprecated. On both the peer and EAP server, the exported MSK and EMSK are utilized in order to calculate the AAA-Key, as described in Appendix E. Where a backend authentication server is present, the AAA-Key is transported from the backend authentication server to the authenticator within the AAA-Token, using the AAA protocol. Once EAP authentication completes and is successful, the peer and authenticator obtain the AAA-Key and the Secure Association Protocol is run between the peer and authenticator in order to securely negotiate the ciphersuite, derive fresh TSKs used to protect data, and provide mutual proof of possession of the AAA-Key. When the authenticator acts as an endpoint of the EAP conversation rather than a pass-through, EAP methods are implemented on the authenticator as well as the peer. If the EAP method negotiated between the EAP peer and authenticator supports mutual authentication and key derivation, the EAP Master Session Key (MSK) and Extended Master Session Key (EMSK) are derived on the EAP peer and authenticator and exported by the EAP method. In this case, the MSK and EMSK are known only to the peer and authenticator and no other parties. The TEKs and TSKs also reside solely on the peer and authenticator. This is illustrated in Figure 4. As demonstrated in [I-D.ietf-roamops-cert], in this case it is still possible to support roaming between providers, using certificate-based authentication. Where a backend authentication server is utilized, the situation is illustrated in Figure 5. Here the authenticator acts as a pass- through between the EAP peer and a backend authentication server. In this model, the authenticator delegates the access control decision to the backend authentication server, which acts as a Key Distribution Center (KDC). In this case, the authenticator encapsulates EAP packet with a AAA protocol such as RADIUS [RFC3579] or Diameter [I-D.ietf-aaa-eap], and forwards packets to and from the Aboba, et al. Informational [Page 16]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 backend authentication server, which acts as the EAP server. Since the authenticator acts as a pass-through, EAP methods reside only on the peer and EAP server As a result, the TEKs, MSK and EMSK are derived on the peer and EAP server. On completion of EAP authentication, EAP methods on the peer and EAP server export the Master Session Key (MSK) and Extended Master Session Key (EMSK). The peer and EAP server then calculate the AAA- Key from the MSK and EMSK, and the backend authentication server sends an Access-Accept to the authenticator, providing the AAA-Key within a protected package known as the AAA-Token. The AAA-Key is then used by the peer and authenticator within the Secure Association Protocol to derive Transient Session Keys (TSKs) required for the negotiated ciphersuite. The TSKs are known only to the peer and authenticator. 2.3. Key Lifetimes As noted earlier, the EAP Key Management framework includes several types of keys, including: [1] Keys calculated locally by the EAP method but not exported by the EAP method, such as the TEKs. [2] Keys exported by the EAP method: MSK, EMSK, IV [3] Keys calculated from exported quantities: AAA-Key, AMSKs. [4] Keys calculated by the Secure Association Protocol: TSKs. Key lifetime issues associated with each type of key are discussed in the sections that follow. Challenges include: [a] Security. Where key lifetimes cannot be assumed, it may be necessary to negotiate them. While key lifetimes may be announced or negotiated in the clear, a protected lifetime negotiation is RECOMMENDED. Aboba, et al. Informational [Page 17]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ | | ^ | EAP Method | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | | | | | | | | | | | EAP Method Key |<->| Long-Term | | | | | Derivation | | Credential | | | | | | | | | | | | | +-+-+-+-+-+-+-+ | Local to | | | | | EAP | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Method | | | | | | | | | | | | | | | | | | | | | | | | | | V | | | | | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | | | | TEK | | MSK | |EMSK | |IV | | | | |Derivation | |Derivation | |Derivation | |Derivation | | | | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+ | | | | | | | | | | | | | V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ | | | ^ | | | | | MSK (64B) | EMSK (64B) | IV (64B) | | | | Exported| | | | by | V V V EAP | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ Method| | AAA Key Derivation, | | Known | | | Naming & Binding | |(Not Secret) | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ V | ---+ | Transported | | AAA-Key by AAA | | Protocol | V V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ | | ^ | TSK | Ciphersuite | | Derivation | Specific | | | V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ Figure 3: EAP Key Hierarchy Aboba, et al. Informational [Page 18]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 +-+-+-+-+-+ +-+-+-+-+-+ | | | | | | | | | Cipher- | | Cipher- | | Suite | | Suite | | | | | +-+-+-+-+-+ +-+-+-+-+-+ ^ ^ | | | | | | V V +-+-+-+-+-+ +-+-+-+-+-+ | | | | | |===============| | | |EAP, TEK Deriv.|Authenti-| | |<------------->| cator | | | | | | | Secure Assoc. | | | peer |<------------->| (EAP | | |===============| server) | | | Link layer | | | | (PPP,IEEE802) | | | | | | |MSK,EMSK | |MSK,EMSK | | AAA-Key | | AAA-Key | | (TSKs) | | (TSKs) | | | | | +-+-+-+-+-+ +-+-+-+-+-+ ^ ^ | | | MSK, EMSK | MSK, EMSK | | | | +-+-+-+-+-+ +-+-+-+-+-+ | | | | | EAP | | EAP | | Method | | Method | | | | | | (TEKs) | | (TEKs) | | | | | +-+-+-+-+-+ +-+-+-+-+-+ Figure 4: Relationship between EAP peer and authenticator (acting as an EAP server), where no backend authentication server is present. Aboba, et al. Informational [Page 19]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 +-+-+-+-+-+ +-+-+-+-+-+ | | | | | | | | | Cipher- | | Cipher- | | Suite | | Suite | | | | | +-+-+-+-+-+ +-+-+-+-+-+ ^ ^ | | | | | | V V +-+-+-+-+-+ +-+-+-+-+-+ +-+-+-+-+-+ | |===============| |========| | | |EAP, TEK Deriv.| | | | | |<-------------------------------->| backend | | | | | | | | | Secure Assoc. | | AAA-Key| | | peer |<------------->|Authenti-|<-------| auth | | |===============| cator |========| server | | | Link Layer | | AAA | (EAP | | | (PPP,IEEE 802)| |Protocol| server) | |MSK,EMSK | | | | | | AAA-Key | | AAA-Key | |MSK,EMSK,| | (TSKs) | | (TSKs) | | AAA-Key | | | | | | | +-+-+-+-+-+ +-+-+-+-+-+ +-+-+-+-+-+ ^ ^ | | | MSK, EMSK | MSK, EMSK | | | | +-+-+-+-+-+ +-+-+-+-+-+ | | | | | EAP | | EAP | | Method | | Method | | | | | | (TEKs) | | (TEKs) | | | | | +-+-+-+-+-+ +-+-+-+-+-+ Figure 5: Pass-through relationship between EAP peer, authenticator and backend authentication server. [b] Resource reclaimation. While key lifetimes may be securely negotiated, it is possible for the NAS or peer to reboot or reclaim resources, and therefore not be able to cache keys for their full Aboba, et al. Informational [Page 20]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 lifetime. As a result, lifetime negotiation does not guarantee that the key cache will remain sychronized. It is therefore RECOMMENDED for the lower layer to provide a mechanism for key state resynchronization. Note that securing this mechanism may be difficult since in this situation one or more of the parties initially do not possess a key with which to protect the resynchronization exchange. 2.3.1. Local Key Lifetimes The Transient EAP Keys (TEKs) are session keys used to protect the EAP conversation. The TEKs are internal to the EAP method and are not exported. They remain valid only for the duration of the EAP conversation, and are lost once the EAP conversation completes. EAP methods may also implement a cache for other local keying material which may persist for multiple EAP conversations. For example, EAP methods based on TLS (such as EAP-TLS [RFC2716]) derive and cache the TLS Master Secret, typically for substantial time periods. The lifetime of other local keying material calculated within the EAP method is defined by the method. 2.3.2. Exported Key Lifetimes All EAP methods generating keys are required to generate the MSK and EMSK, and may optionally generate the IV. However, although new exported keys are generated during reauthentication, the lifetime of exported keys is conceptually distinct from the reauthentication time, since while reauthentication causes new exported keys to be derived, exported keys may be cached on the peer and server after a session completes and therefore their lifetime may be greater than the reauthentication time. Although exported keys are generated by the EAP method, most existing EAP methods do not negotiate the lifetime of the exported keys. EAP, defined in [RFC3748], also does not support the negotiation of lifetimes for exported keying material such as the MSK, EMSK and IV. Several mechanisms exist for managing the lifetime of exported EAP keys. Exported EAP keys may be cached on the EAP server as well as on the peer. On the EAP server, it is RECOMMENDED that the lifetime of exported keys be managed as a system parameter. Where the EAP method does not support the negotiation of the exported key lifetime, and where a negotiation mechanism is not provided by the lower lower, it is RECOMMENDED that the peer assume a default value of the exported key lifetime. A value of 8 hours is suggested. Managing the lifetime of exported keys using a AAA attribute is NOT Aboba, et al. Informational [Page 21]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 RECOMMENDED. This is problematic because although this would ensure transport of the exported key lifetime between the AAA server and the authenticator, the goal is to synchronize the exported key lifetime between the peer and EAP server. Providing the the exported key lifetime on an per-session basis to the authenticator results in requiring the authenticator to maintain EAP-Key SA state. As a described in Section 3, EAP-Key SA state is typically only maintained on the peer and server, so that this represents a substantial additional burden. 2.3.3. Calculated Key Lifetimes When keying material exported by EAP methods is replaced, new calculated keys are also put in place. Similarly, when the keying material exported by EAP methods expires, so do the calculated keys. As a result, the lifetime of keys calculated from material exported by EAP methods can be no larger than the lifetime of the keying material they are calculated from. Since the lifetime of calculated keys can be less than that of the exported keys they are derived from, calculated key lifetimes are conceptually distinct from exported key lifetimes and reauthentication times, and need to be managed as a separate parameter. Note that just as the reauthentication time and the exported key lifetime are conceptually distinct parameters, so too are calculated key lifetimes conceptually distinct from the reauthentication time. Today AAA protocols such as RADIUS [RFC2865] support the Session- Timeout attribute. As described in [RFC3580], this may be used to determine the maximum session time prior to reauthentication. Since reauthentication results in the derivation of new exported keys and the transport of a new AAA-Key, while a session is in progress the maximum session time prior to reauthentication places an upper bound on the AAA-Key lifetime. However, after the session has terminated, it is possible for the AAA-Key to be cached on the authenticator. Therefore the AAA-Key lifetime may be larger than the reauthentication time. As a result, the AAA-Key lifetime needs to be managed as a separate parameter. Since the lifetime of the AAA-Key within the authenticator key cache is in part determined by authenticator resources, the AAA-Key lifetime is typically managed as a system parameter on the authenticator. Since the authenticator may have considerably fewer resources than either the EAP peer or server, it is possible that AAA-Key lifetime on the authenticator may be less than exported key lifetime maintained by the server, or that the authenticator may need to reclaim AAA-Key resources prior to expiration of the AAA-Key Aboba, et al. Informational [Page 22]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 lifetime. As a result, the primary issue with managing the AAA-Key lifetime is the determination by the peer whether a particular AAA-Key exists within the key cache of a given authenticator. Transmitting the AAA- Key lifetime from the AAA server to the authenticator is not helpful in solving this problem in several important scenarios. Where the AAA-key lifetime is negotiated between the authenticator and the peer within the Secure Association Protocol, this may be used by the peer to manage the lifetime of the AAA-Key once the Secure Association Protocol has completed. However, should a time gap may exist between the time of completion of the EAP method and the initiation of the Secure Association Protocol, the lifetime of the AAA-Key cannot be determined by the peer during this period. As a result, unless the Secure Association Protocol always follos the completion of the EAP method exchange without a gap in time, it may not be possible for the peer and authenticator to negotiate session-specific value of the AAA-Key lifetime. For example, where EAP pre-authentication is used, the AAA-Key may be derived and remain resident on the peer and authenticator prior to initiation of the Secure Association Protocol. However, if the AAA-Key lifetime is managed as an authenticator system parameter, it may be possible for lower layer solutions to bridge the gap. For example, the lower layer may utilize Discovery mechanisms to ensure AAA-Key cache synchronization between the peer and authenticator. If the authenticator manages the AAA-Key cache by deleting the oldest AAA-Key first (LIFO), the relative creation time of the last AAA-Key to be deleted could be advertised with the Discovery phase, enabling the peer to determine whether a given AAA-Key had been expired from the authenticator key cache. 2.3.4. TSK Key Lifetimes Since the TSKs depend on the AAA-Key, replacement of the AAA-Key implies replacement of the TSKs. However, replacement of the TSKs only implies replacement of the AAA-Key when the TSKs are taken from a portion of the AAA-Key. Therefore while the lifetime of the TSKs may be shorter than or equal to the AAA-Key lifetime, the TSK lifetime cannot exceed the AAA-Key lifetime. Where a Secure Association Protocol exists, it is possible for TSKs to be refreshed prior to reauthentication, and so the TSK Aboba, et al. Informational [Page 23]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Key Lifetime may also be shorter than or equal to the reauthentication timeout. It is therefore RECOMMENDED that the TSK Key lifetime be managed parameter distinct from the reauthentication timeout and the AAA-Key lifetime (except where the TSK is taken from the AAA-Key). Where TSKs are established as the result of a Secure Association Protocol exchange, it is RECOMMENDED that the Secure Association Protocol include secure negotiation of the TSK lifetime between the peer and authenticator. Where the TSK is taken from the AAA-Key, there is no need to manage the TSK lifetime as a separate parameter, since the TSK lifetime and AAA-Key lifetime are identical. As described in Section 3, TSKs are part of Service SAs which reside on the peer and authenticator and as with the AAA-Key lifetime, the TSK lifetime is often determined by authenticator resources. As a result, the AAA server has no insight into the TSK derivation process, and by the principle of ciphersuite independence, it is not appropriate for the AAA server to manage any aspect of the TSK derivation process, including the TSK lifetime. 2.4. AAA-Key Scope As described in Appendix E, the AAA-Key is calculated from the EMSK and MSK by the EAP peer and server, and is used as the root of the ciphersuite-specific key hierarchy. Where a backend authentication server is present, the AAA-Key is transported from the EAP server to the authenticator; where it is not present, the AAA-Key is calculated on the authenticator. The AAA-Key is restricted to use between the EAP peer that calculates it, and the authenticator that either calculates it (where no backend authenticator is present) or receives it from the server (where a backend authenticator server is present). However, in practice difficulties arise in ensuring that the AAA-Key is used only within the defined scope. A wide variety of authenticator and peer designs need to be accomodated within the EAP key management framework. An authenticator may contain multiple physical ports; a single physical authenticator may, for the purpose of peer discovery, advertise itself as multiple "virtual authenticators"; authenticators may be compromised of multiple CPUs; authenticators may utilize clustering in order to provide load balancing or failover. Similarly, a peer may support multiple ports; may support multiple CPUs; or may support clustering. As illustrated in Figure 1, an EAP peer with multiple ports may be Aboba, et al. Informational [Page 24]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 attached to one or more authenticators, each with multiple ports. Where an authenticator identifies itself to the peer only via use of a port identifer (such as a link layer address), it may not be obvious to the peer which authenticator ports are associated with which authenticators. Similarly, where an EAP peer identifies itself using a port identifier (such as a link layer address), it may not be obvious to the authenticator which peer ports are associated with which peers. In such situations, the peer and authenticator may not be able to determine the appropriate AAA-Key scope. Additional issues arise when a single physical authenticator advertises itself as multiple "virtual authenticators". In such a situation, the EAP peer may act as though each "virtual authenticator" represented a distinct physical authenticator, thereby restricting the AAA-Key to use with the "virtual authenticator" that it interacts with. However, depending on the architecture of the physical AP, it may or may not share AAA-Keys between "virtual authenticators". Once again, the peer and authenticator may not be in agreement on the AAA-key scope. This lack of synchronization may create security vulnerabilities. For example, where the AAA-Key is shared between "virtual authenticators" an EAP peer could authenticate with the "Guest" "virtual authenticator" and derive a AAA-Key. The peer could then use that AAA-Key within the Secure Association Protocol in order to connect to the "Corporate Intranet" "virtual authenticator" within the same physical authenticator. If the "virtual authenticators" share a AAA-Key cache, then the attempt will be successful. Several measures are recommended to address these issues: [a] Authenticators are REQUIRED to cache associated authorizations along with the AAA-Key and apply authorizations consistently. This ensures that an attacker cannot obtain elevated privileges even where the AAA-Key cache is shared between "virtual authenticators". [b] It is RECOMMENDED that Secure Association Protocols utilize peer and authenticator identities that are unambiguous and do not incorporate implicit assumptions about peer and authenticator architectures. For example, using port-specific MAC addresses as identifiers is a particularly poor choice, given that peers and authenticators may have multiple ports. Aboba, et al. Informational [Page 25]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 [c] It is RECOMMENDED that physical authenticators maintain separate AAA-Key caches for each "virtual authenticator". [d] Where a "virtual authenticator" is implemented, the AAA client MAY also be virtualized. Where a "virtual AAA client" is implemented, each "virtual authenticator" identifies itself distinctly to the AAA server. Where the AAA client and server communicate directly, this enables the AAA server to authenticate each "virtual AAA client" distinctly. [e] The AAA server and authenticator MAY implement additional attributes in order to further restrict the AAA-Key scope. When this is done, it is RECOMMENDED that the Secure Association Protocol be extended to enable the restrictions to be communicated between the authenticator and the peer. For example, in 802.11, the AAA server may provide the authenticator with a list of authorized Called-Station-Ids and/or SSIDs for which the AAA-Key is valid, restricting the use of the AAA-Key by the peer. Similarly, the authenticator may provide the peer with a list of Calling-Station-Ids for which the AAA-Key is valid. 2.5. Fast Handoff Support Within EAP, "fast handoff" is defined as a conversation in which the EAP exchange (phase 1a) and associated AAA passthrough is bypassed, so as to reduce latency. Depending on the fast handoff mechanism, AAA-Key transport (phase 1b) may also be bypassed or it may be provided in a pre-emptive manner as in [IEEE-03-084] and [I-D.irtf- aaaarch-handoff]. The introduction of fast handoff creates a new class of security vulnerabilities as well as requirements for the secure handling of authorization context. 2.5.1. Authorization Issues In a typical network access scenario (dial-in, wireless LAN, etc.) access control mechanisms are typically applied. These mechanisms include user authentication as well as authorization for the offered service. As a part of the authentication process, the AAA network determines the user's authorization profile. The user authorizations are transmitted by the backend authentication server to the EAP authenticator (also known as the Network Access Server or authenticator) included with the AAA-Token, which also contains the AAA-Key, in Phase 1b of the EAP conversation. Typically, the profile is determined based on the user identity, but a certificate presented Aboba, et al. Informational [Page 26]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 by the user may also provide authorization information. The backend authentication server is responsible for making a user authorization decision, answering the following questions: [a] Is this a legitimate user for this particular network? [b] Is this user allowed the type of access he or she is requesting? [c] Are there any specific parameters (mandatory tunneling, bandwidth, filters, and so on) that the access network should be aware of for this user? [d] Is this user within the subscription rules regarding time of day? [e] Is this user within his limits for concurrent sessions? [f] Are there any fraud, credit limit, or other concerns that indicate that access should be denied? While the authorization decision is in principle simple, the process is complicated by the distributed nature of AAA decision making. Where brokering entities or proxies are involved, all of the AAA devices in the chain from the authenticator to the home AAA server are involved in the decision. For instance, a broker can disallow access even if the home AAA server would allow it, or a proxy can add authorizations (e.g., bandwidth limits). Decisions can be based on static policy definitions and profiles as well as dynamic state (e.g. time of day or limits on the number of concurrent sessions). In addition to the Accept/Reject decision made by the AAA chain, parameters or constraints can be communicated to the authenticator. The criteria for Accept/Reject decisions or the reasons for choosing particular authorizations are typically not communicated to the authenticator, only the final result. As a result, the authenticator has no way to know what the decision was based on. Was a set of authorization parameters sent because this service is always provided to the user, or was the decision based on the time/day and the capabilities of the requesting authenticator device? 2.5.2. Correctness in Fast Handoff Bypassing all or portions of the AAA conversation creates challenges in ensuring that authorization is properly handled. These include: Aboba, et al. Informational [Page 27]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 [a] Consistent application of session time limits. A fast handoff should not automatically increase the available session time, allowing a user to endlessly extend their network access by changing the point of attachment. [b] Avoidance of privilege elevation. A fast handoff should not result in a user being granted access to services which they are not entitled to. [c] Consideration of dynamic state. In situations in which dynamic state is involved in the access decision (day/time, simultaneous session limit) it should be possible to take this state into account either before or after access is granted. Note that consideration of network-wide state such as simultaneous session limits can typically only be taken into account by the backend authentication server. [d] Encoding of restrictions. Since a authenticator may not be aware of the criteria considered by a backend authentication server when allowing access, in order to ensure consistent authorization during a fast handoff it may be necessary to explicitly encode the restrictions within the authorizations provided in the AAA-Token. [e] State validity. The introduction of fast handoff should not render the authentication server incapable of keeping track of network- wide state. A fast handoff mechanism capable of addressing these concerns is said to be "correct". One condition for correctness is as follows: For a fast handoff to be "correct" it MUST establish on the new device the same context as would have been created had the new device completed a AAA conversation with the authentication server. A properly designed fast handoff scheme will only succeed if it is "correct" in this way. If a successful fast handoff would establish "incorrect" state, it is preferable for it to fail, in order to avoid creation of incorrect context. Some backend authentication server and authenticator configurations are incapable of meeting this definition of "correctness". For example, if the old and new device differ in their capabilities, it may be difficult to meet this definition of correctness in a fast handoff mechanism that bypasses AAA. Backend authentication servers often perform conditional evaluation, in which the authorizations returned in an Access-Accept message are contingent on the authenticator or on dynamic state such as the time of day or number of simultaneous sessions. For example, in a heterogeneous deployment, the backend authentication server might return Aboba, et al. Informational [Page 28]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 different authorizations depending on the authenticator making the request, in order to make sure that the requested service is consistent with the authenticator capabilities. If differences between the new and old device would result in the backend authentication server sending a different set of messages to the new device than were sent to the old device, then if the fast handoff mechanism bypasses AAA, then the fast handoff cannot be carried out correctly. For example, if some authenticator devices within a deployment support dynamic VLANs while others do not, then attributes present in the Access-Request (such as the authenticator-IP-Address, authenticator-Identifier, Vendor-Identifier, etc.) could be examined to determine when VLAN attributes will be returned, as described in [RFC3580]. VLAN support is defined in [IEEE8021Q]. If a fast handoff bypassing the backend authentication server were to occur between a authenticator supporting dynamic VLANs and another authenticator which does not, then a guest user with access restricted to a guest VLAN could be given unrestricted access to the network. Similarly, in a network where access is restricted based on the day and time, Service Set Identifier (SSID), Calling-Station-Id or other factors, unless the restrictions are encoded within the authorizations, or a partial AAA conversation is included, then a fast handoff could result in the user bypassing the restrictions. In practice, these considerations limit the situations in which fast handoff mechanisms bypassing AAA can be expected to be successful. Where the deployed devices implement the same set of services, it may be possible to do successful fast handoffs within such mechanisms. However, where the supported services differ between devices, the fast handoff may not succeed. For example, [RFC2865], section 1.1 states: "A authenticator that does not implement a given service MUST NOT implement the RADIUS attributes for that service. For example, a authenticator that is unable to offer ARAP service MUST NOT implement the RADIUS attributes for ARAP. A authenticator MUST treat a RADIUS access-accept authorizing an unavailable service as an access-reject instead." Note that this behavior only applies to attributes that are known, but not implemented. For attributes that are unknown, section of 5 of [RFC2865] states: "A RADIUS server MAY ignore Attributes with an unknown Type. A Aboba, et al. Informational [Page 29]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 RADIUS client MAY ignore Attributes with an unknown Type." In order to perform a correct fast handoff, if a new device is provided with RADIUS context for a known but unavailable service, then it MUST process this context the same way it would handle a RADIUS Access-Accept requesting an unavailable service. This MUST cause the fast handoff to fail. However, if a new device is provided with RADIUS context that indicates an unknown attribute, then this attribute MAY be ignored. Although it may seem somewhat counter-intuitive, failure is indeed the "correct" result where a known but unsupported service is requested. Presumably a correctly configured backend authentication server would not request that a device carry out a service that it does not implement. This implies that if the new device were to complete a AAA conversation that it would be likely to receive different service instructions. In such a case, failure of the fast handoff is the desired result. This will cause the new device to go back to the AAA server in order to receive the appropriate service definition. In practice, this implies that fast handoff mechanisms which bypass AAA are most likely to be successful within a homogeneous device deployment within a single administrative domain. For example, it would not be advisable to carry out a fast handoff bypassing AAA between a authenticator providing confidentiality and another authenticator that does not support this service. The correct result of such a fast handoff would be a failure, since if the handoff were blindly carried out, then the user would be moved from a secure to an insecure channel without permission from the backend authentication server. Thus the definition of a "known but unsupported service" MUST encompass requests for unavailable security services. This includes vendor-specific attributes related to security, such as those described in [RFC2548]. 3. Security Associations During EAP authentication and subsequent exchanges, four types of security associations (SAs) are created: [1] EAP method SA. This SA is between the peer and EAP server. It stores state that can be used for "fast resume" or other functionality in some EAP methods. Not all EAP methods create such an SA. [2] EAP-Key SA. This is an SA between the peer and EAP server, which is used to store the keying material exported by the EAP method. Current EAP server implementations do not retain this SA after the Aboba, et al. Informational [Page 30]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 EAP conversation completes, but future implementations could use this SA for purposes such as pre-emptive key distribution. [3] AAA SA(s). These SAs are between the authenticator and the backend authentication server. They permit the parties to mutually authenticate each other and protect the communications between them. [4] Service SA(s). These SAs are between the peer and authenticator, and they are created as a result of phases 1-2 of the conversation (see Section 1.3). 3.1. EAP Method SA (peer - EAP server) An EAP method may store some state on the peer and EAP server even after phase 1a has completed. Typically, this is used for "fast resume": the peer and EAP server can confirm that they are still talking to the same party, perhaps using fewer roundtrips or less computational power. In this case, the EAP method SA is essentially a cache for performance optimization, and either party may remove the SA from its cache at any point. An EAP method may also keep state in order to support pseudonym-based identity protection. This is typically a cache as well (the information can be recreated if the original EAP method SA is lost), but may be stored for longer periods of time. The EAP method SA is not restricted to a particular service or authenticator and is most useful when the peer accesses many different authenticators. An EAP method is responsible for specifying how the parties select if an existing EAP method SA should be used, and if so, which one. Where multiple backend authentication servers are used, EAP method SAs are not typically synchronized between them. EAP method implementations should consider the appropriate lifetime for the EAP method SA. "Fast resume" assumes that the information required (primarily the keys in the EAP method SA) hasn't been compromised. In case the original authentication was carried out using, for instance, a smart card, it may be easier to compromise the EAP method SA (stored on the PC, for instance), so typically the EAP method SAs have a limited lifetime. Contents: Aboba, et al. Informational [Page 31]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 o Implicitly, the EAP method this SA refers to o One or more internal (non-exported) keys o EAP method SA name o SA lifetime 3.1.1. Example: EAP-TLS In EAP-TLS [RFC2716], after the EAP authentication the client (peer) and server can store the following information: o Implicitly, the EAP method this SA refers to (EAP-TLS) o Session identifier (a value selected by the server) o Certificate of the other party (server stores the clients's certificate and vice versa) o Ciphersuite and compression method o TLS Master secret (known as the EAP-TLS Master Key or MK) o SA lifetime (ensuring that the SA is not stored forever) o If the client has multiple different credentials (certificates and corresponding private keys), a pointer to those credentials When the server initiates EAP-TLS, the client can look up the EAP-TLS SA based on the credentials it was going to use (certificate and private key), and the expected credentials (certificate or name) of the server. If an EAP-TLS SA exists, and it is not too old, the client informs the server about the existence of this SA by including its Session-Id in the TLS ClientHello message. The server then looks up the correct SA based on the Session-Id (or detects that it doesn't yet have one). 3.1.2. Example: EAP-AKA In EAP-AKA [I-D.arkko-pppext-eap-aka], after EAP authentication the client and server can store the following information: o Implicitly, the EAP method this SA refers to (EAP-AKA) o A re-authentication pseudonym o The client's permanent identity (IMSI) (server) o Replay protection counter o Authentication key (K_aut) o Encryption key (K_encr) o Original Master Key (MK) o SA lifetime (ensuring that the SA is not stored forever) When the server initiates EAP-AKA, the client can look up the EAP-AKA SA based on the credentials it was going to use (permanent identity). If an EAP-AKA SA exists, and it is not too old, the client informs the server about the existence of this SA by sending its re- authentication pseudonym as its identity in EAP Identity Response Aboba, et al. Informational [Page 32]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 message, instead of its permanent identity. The server then looks up the correct SA based on this identity. 3.2. EAP-key SA This is an SA between the peer and EAP server, which is used to store the keying material exported by the EAP method. Current EAP server implementations do not retain this SA after the EAP conversation completes, but future implementations could use this SA for pre- emptive key distribution. Contents: o Name/identifier for this SA o Identities of the parties o MSK and EMSK o SA lifetime 3.3. AAA SA(s) (authenticator - backend authentication server) In order for the authenticator and backend authentication server to authenticate each other, they need to store some information. In case the authenticator and backend authentication server are colocated, and they communicate using local procedure calls or shared memory, this SA need not necessarily contain any information. 3.3.1. Example: RADIUS In RADIUS, where shared secret authentication is used, the client and server store each other's IP address and the shared secret, which is used to calculate the Response Authenticator [RFC2865] and Message- Authenticator [RFC3579] values, and to encrypt some attributes (such as the AAA-Key [RFC2548]). Where IPsec is used to protect RADIUS [RFC3579] and IKE is used for key management, the parties store information necessary to authenticate and authorize the other party (e.g. certificates, trust anchors and names). The IKE exchange results in IKE Phase 1 and Phase 2 SAs containing information used to protect the conversation (session keys, selected ciphersuite, etc.) 3.3.2. Example: Diameter with TLS When using Diameter protected by TLS, the parties store information necessary to authenticate and authorize the other party (e.g. certificates, trust anchors and names). The TLS handshake results in a short-term TLS SA that contains information used to protect the Aboba, et al. Informational [Page 33]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 actual communications (session keys, selected TLS ciphersuite, etc.). 3.4. Service SA(s) (peer - authenticator) The service SA stores information about the service being provided. This includes: o Service parameters (or at least those parameters that are still needed) o On the authenticator, service authorization information received from the backend authentication server (or necessary parts of it) o On the peer, usually locally configured service authorization information. o Transient Session Keys used to protect the communication o The AAA-Key, if it can be needed again (to refresh and/or resynchronize other keys or for another reason) o AAA-Key lifetime The information in the service SA can be grouped into several different SAs. This would make sense if, for instance, the service provided is naturally divided into several different subconversations with different parameters. How exactly the relevant service SA is chosen at each point depends on the protocol; see below for examples. 3.4.1. Example: 802.11i [IEEE802.11i] Section 8.4.1.1 defines the security associations used within IEEE 802.11. A summary follows; the standard should be consulted for details. o Pairwise Master Key Security Association (PMKSA) The PMKSA is a bi-directional SA, used by both parties for sending and receiving. It is created on the peer when EAP authentication completes successfully or a pre-shared key is configured. The PMKSA is created on the authenticator when the PMK is received or created on the authenticator or a pre-shared key is configured. The PMKSA is used to create the PTKSA. PMKSAs are cached for their lifetimes. The PMKSA consists of the following elements: - PMKID (security association identifier) - Authenticator MAC address - PMK - Lifetime - Authenticated Key Management Protocol (AKMP) Aboba, et al. Informational [Page 34]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 - Authorization parameters specified the AAA server or by local configuration. This can include parameters such as the peer's authorized SSID. On the peer, this information can be locally configured. - Key replay counters (for EAPOL-Key messages) - Reference to PTKSA (if any), needed to: o delete it (e.g. AAA server initiated disconnect) o replace it when a new four-way handshake is done - Reference to accounting context (the details of which depend on the accounting protocol used, and various implementation and administrative details. In RADIUS, this could include (e.g. packet and octet counters, and Acct-Multi-Session-Id). o Pairwise Transient Key Security Association (PTKSA) The PTKSA is a bi-directional SA created as the result of a successful four-way handshake. There may only be one PTKSA between a pair of peer and authenticator MAC addresses. PTKSAs are cached for the lifetime of the PMKSA. Since the PTKSA is tied to the PMKSA, it only has the addititional information from the 4-way handshake. The PTKSA consists of the following: - Key (PTK) - Selected ciphersuite - MAC addresses of the parties - Replay counters, and ciphersuite specific state - Reference to PMKSA: This is needed when: o A new four-way handshake is needed (lifetime, TKIP countermeasures), and we need to know which PMKSA to use o Group Transient Key Security Association (GTKSA) The GTKSA is a uni-directional SA created based on the four-way handshake or the group key handshake. A GTKSA consists of the following: - Direction vector (whether the GTK is used for transmit or receive) - Group cipher suite selector - Key (GTK) - Authenticator MAC addres - Via reference to PMKSA, or copied here: o Authorization parameters o Reference to accounting context Aboba, et al. Informational [Page 35]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 3.4.2. Example: IKEv2/IPsec Note that this example is intended to be informative, and it does not necessarily include all information stored. o IKEv2 SA - Protocol version - Identitities of the parties - IKEv2 SPIs - Selected ciphersuite - Replay protection counters (Message ID) - Keys for protecting IKEv2 messages (SK_ai/SK_ar/SK_ei/SK_er) - Key for deriving keys for IPsec SAs (SK_d) - Lifetime information - On the authenticator, service authorization information received from the backend authentication server. When processing an incoming message, the correct SA is looked up based on the SPIs. o IPsec SAs/SPD - Traffic selectors - Replay protection counters - Selected ciphersuite - IPsec SPI - Keys - Lifetime information - Protocol mode (tunnel or transport) The correct SA is looked up based on SPI (for inbound packets), or SPD traffic selectors (for outbound traffic). A separate IPsec SA exists for each direction. 3.4.3. Sharing service SAs A single service may be provided by multiple logical or physical service elements. Each service is responsible for specifying how changing service elements is handled. Some approaches include: Transparent sharing If the service parameters visible to the other party (either peer or authenticator) do not change, the service can be moved without requiring cooperation from the other party. Whether such a move should be supported or used depends on implementation and administrative considerations. For instance, an Aboba, et al. Informational [Page 36]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 administrator may decide to configure a group of IKEv2/IPsec gateways in a cluster for high-availability purposes, if the implementation used supports this. The peer does not necessarily have any way of knowing when the change occurs. No sharing If the service parameters require changing, some changes may require terminating the old service, and starting a new conversation from phase 0. This approach is used by all services for at least some parameters, and it doesn't require any protocol for transferring the service SA between the service elements. The service may support keeping the old service element active while the new conversation takes phase, to decrease the time the service is not available. Some sharing The service may allow changing some parameters by simply agreeing about the new values. This may involve a similar exchange as in phase 2, or perhaps a shorter conversation. This option usually requires some protocol for transferring the service SA between the elements. An administrator may decide not to enable this feature at all, and typically the sharing is restricted to some particular service elements (defined either by a service parameter, or simple administrative decision). If the old and new service element do not support such "context transfer", this approach falls back to the previous option (no transfer). Services supporting this feature should also consider what changes require new authorization from the backend authentication server (see Section 1.7). Note that these considerations are not limited to service parameters related to the authenticator--they apply to peer's parameters as well. 3.5. SA Naming In order to support the correct processing of phase 2 security associations, the Secure Association (phase 2) protocol supports the naming of phase 2 security associations and associated transient session keys, so that the correct set of transient session keys can be identified for processing a given packet. Explicit creation and deletion operations are also typically supported so that establishment and re-establishment of transient session keys can be synchronized between the parties. Aboba, et al. Informational [Page 37]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 In order to securely bind the AAA SA (phase 1b) to its child phase 2 security associations, the phase 2 Secure Association Protocol allows the EAP peer and authenticator to mutually prove possession of the AAA-Key. In order to avoid confusion in the case where an EAP peer has more than one AAA-Key (phase 1b) applicable to establishment of a phase 2 security association, it is necessary for the secure Association Protocol (phase 2) to support key selection, so that the appropriate phase 1b keying material can be utilized by both parties in the Secure Association Protocol exchange. For example, a peer might be pre-configured with policy indicating the ciphersuite to be used in communicating with a given authenticator. Within PPP, the ciphersuite is negotiated within the Encryption Control Protocol (ECP), after EAP authentication is completed. Within [IEEE80211i], the AP ciphersuites are advertised in the Beacon and Probe Responses, and are securely verified during a 4-way exchange after EAP authentication has completed. As part of the Secure Association Protocol (phase 2), it is necessary to bind the Transient Session Keys (TSKs) to the keying material provided in the AAA-Token. This ensures that the EAP peer and authenticator are both clear about what key to use to provide mutual proof of possession. Keys within the EAP key hierarchy are named as follows: EAP SA name The EAP security association is negotiated between the EAP peer and EAP server, and is uniquely named as follows <EAP peer name, EAP server name, EAP Method Type, EAP peer nonce, EAP server nonce>. Here the EAP peer name and EAP server name are the identifiers securely exchanged within the EAP method. Since multiple EAP SAs may exist between an EAP peer and EAP server, the EAP peer nonce and EAP server nonce allow EAP SAs to be differentiated. The inclusion of the Method Type in the EAP SA name ensures that each EAP method has a distinct EAP SA space. AAA-Key Name The AAA-Key is named by the concatenation of the EAP SA name, "AAA- Key" and the authenticator name, since the AAA-Key is bound to a particular authenticator. For the purpose of identification, the NAS-Identifier attribute is recommended. In order to ensure that all parties can agree on the NAS name this requires the NAS to advertise its name (typically using a media-specific mechanism, such as the 802.11 Beacon/Probe Response)." Aboba, et al. Informational [Page 38]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 4. Security considerations 4.1. Security Terminology Cryptographic binding The demonstration of the EAP peer to the EAP server that a single entity has acted as the EAP peer for all methods executed within a tunnel method. Binding MAY also imply that the EAP server demonstrates to the peer that a single entity has acted as the EAP server for all methods executed within a tunnel method. If executed correctly, binding serves to mitigate man-in-the-middle vulnerabilities. Cryptographic separation Two keys (x and y) are "cryptographically separate" if an adversary that knows all messages exchanged in the protocol cannot compute x from y or y from x without "breaking" some cryptographic assumption. In particular, this definition allows that the adversary has the knowledge of all nonces sent in cleartext as well as all predictable counter values used in the protocol. Breaking a cryptographic assumption would typically require inverting a one- way function or predicting the outcome of a cryptographic pseudo- random number generator without knowledge of the secret state. In other words, if the keys are cryptographically separate, there is no shortcut to compute x from y or y from x, but the work an adversary must do to perform this computation is equivalent to performing exhaustive search for the secret state value. Key strength If the effective key strength is N bits, the best currently known methods to recover the key (with non-negligible probability) require on average an effort comparable to 2^(N-1) operations of a typical block cipher. Mutual authentication This refers to an EAP method in which, within an interlocked exchange, the authenticator authenticates the peer and the peer authenticates the authenticator. Two independent one-way methods, running in opposite directions do not provide mutual authentication as defined here. 4.2. Threat Model The EAP threat model is described in [RFC3748], Section 7.1. In order to address these threats, EAP relies on the security properties of EAP methods (known as "security claims", described in [RFC3784], Section 7.2.1). EAP method requirements for application such as Wireless LAN authentication are described in [WLANREQ]. Aboba, et al. Informational [Page 39]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 The RADIUS threat model is described in [RFC3579] Section 4.1, and responses to these threats are described in [RFC3579] Sections 4.2 and 4.3. Among other things, [RFC3579] Section 4.2 recommends the use of IPsec ESP with non-null transform to provide per-packet authentication and confidentiality, integrity and replay protection for RADIUS/EAP. Given the existing documentation of EAP and AAA threat models and responses, there is no need to duplicate that material here. However, there are many other system-level threats no covered in these document which have not been described or analyzed elsewhere. These include: [1] An attacker may try to modify or spoof Secure Association Protocol packets. [2] An attacker compromising an authenticator may provide incorrect information to the EAP peer and/or server via out-of-band mechanisms (such as via a AAA or lower layer protocol). This includes impersonating another authenticator, or providing inconsistent information to the peer and EAP server. [3] An attacker may attempt to perform downgrading attacks on the ciphersuite negotiation within the Secure Association Protocol in order to ensure that a weaker ciphersuite is used to protect data. Depending on the lower layer, these attacks may be carried out without requiring physical proximity. In order to address these threats, [Housley56] describes the mandatory system security properties: Algorithm independence Wherever cryptographic algorithms are chosen, the algorithms must be negotiable, in order to provide resilient against compromise of a particular algorithm. Algorithm independence must be demonstrated within all aspects of the system, including within EAP, AAA and the Secure Association Protocol. However, for interoperability, at least one suite of algorithms MUST be implemented. Strong, fresh session keys Session keys must be demonstrated to be strong and fresh in all circumstances, while at the same time retaining algorithm independence. Replay protection All protocol exchanges must be replay protected. This includes Aboba, et al. Informational [Page 40]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 exchanges within EAP, AAA, and the Secure Association Protocol. Authentication All parties need to be authenticated. The confidentiality of the authenticator must be maintained. No plaintext passwords are allowed. Authorization EAP peer and authenticator authorization must be performed. Session keys Confidentiality of session keys must be maintained. Ciphersuite negotiation The selection of the "best" ciphersuite must be securely confirmed. Unique naming Session keys must be uniquely named. Domino effect Compromise of a single authenticator cannot compromise any other part of the system, including session keys and long-term secrets. Key binding The key must be bound to the appropriate context. 4.3. Security Analysis Figure 6 illustrates the relationship between the peer, authenticator and backend authentication server. EAP peer /\ / \ Protocol: EAP / \ Protocol: Secure Association Auth: Mutual / \ Auth: Mutual Unique keys: / \ Unique keys: TSKs TEKs,EMSK / \ / \ EAP server +--------------+ Authenticator Protocol: AAA Auth: Mutual Unique key: AAA session key Figure 6: Relationship between peer, authenticator and auth. server The peer and EAP server communicate using EAP [RFC3748]. The Aboba, et al. Informational [Page 41]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 security properties of this communication are largely determined by the chosen EAP method. Method security claims are described in [RFC3748] Section 7.2. These include the key strength, protected ciphersuite negotiation, mutual authentication, integrity protection, replay protection, confidentiality, key derivation, key strength, dictionary attack resistance, fast reconnect, cryptographic binding, session independence, fragmentation and channel binding claims. At a minimum, methods claiming to support key derivation must also support mutual authentication. As noted in [RFC3748] Section 7.10: EAP Methods deriving keys MUST provide for mutual authentication between the EAP peer and the EAP Server. Ciphersuite independence is also required: Keying material exported by EAP methods MUST be independent of the ciphersuite negotiated to protect data. In terms of key strength and freshness, [RFC3748] Section 10 says: EAP methods SHOULD ensure the freshness of the MSK and EMSK even in cases where one party may not have a high quality random number generator.... In order to preserve algorithm independence, EAP methods deriving keys SHOULD support (and document) the protected negotiation of the ciphersuite used to protect the EAP conversation between the peer and server... In order to enable deployments requiring strong keys, EAP methods supporting key derivation SHOULD be capable of generating an MSK and EMSK, each with an effective key strength of at least 128 bits. The authenticator and backend authentication server communicate using a AAA protocol such as RADIUS [RFC3579] or Diameter [I-D.ietf-aaa- eap]. As noted in [RFC3588] Section 13, Diameter must be protected by either IPsec ESP with non-null transform or TLS. As a result, Diameter requires per-packet integrity and confidentiality. Replay protection must be supported. For RADIUS, [RFC3579] Section 4.2 recommends that RADIUS be protected by IPsec ESP with a non-null transform, and where IPsec is implemented replay protection must be supported. The peer and authenticator communicate using the Secure Association Protocol. As noted in the figure, each party in the exchange mutually authenticates with each of the other parties, and derives a unique key. All parties in the diagram have access to the AAA-Key. The EAP peer and backend authentication server mutually authenticate Aboba, et al. Informational [Page 42]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 via the EAP method, and derive the TEKs and EMSK which are known only to them. The TEKs are used to protect some or all of the EAP conversation between the peer and authenticator, so as to guard against modification or insertion of EAP packets by an attacker. The degree of protection afforded by the TEKs is determined by the EAP method; some methods may protect the entire EAP packet, including the EAP header, while other methods may only protect the contents of the Type-Data field, defined in [RFC3748]. Since EAP is spoken only between the EAP peer and server, if a backend authentication server is present then the EAP conversation does not provide mutual authentication between the peer and authenticator, only between the EAP peer and EAP server (backend authentication server). As a result, mutual authentication between the peer and authenticator only occurs where a Secure Association protocol is used, such the unicast and group key derivation handshake supported in [IEEE80211i]. This means that absent use of a secure Association Protocol, from the point of view of the peer, EAP mutual authentication only proves that the authenticator is trusted by the backend authentication server; the identity of the authenticator is not confirmed. Utilizing the AAA protocol, the authenticator and backend authentication server mutually authenticate and derive session keys known only to them, used to provide per-packet integrity and replay protection, authentication and confidentiality. The AAA-Key is distributed by the backend authentication server to the authenticator over this channel, bound to attributes constraining its usage, as part of the AAA-Token. The binding of attributes to the AAA-Key within a protected package is important so the authenticator receiving the AAA-Token can determine that it has not been compromised, and that the keying material has not been replayed, or mis-directed in some way. The security properties of the EAP exchange are dependent on each leg of the triangle: the selected EAP method, AAA protocol and the Secure Association Protocol. Assuming that the AAA protocol provides protection against rogue authenticators forging their identity, then the AAA-Token can be assumed to be sent to the correct authenticator, and where it is wrapped appropriately, it can be assumed to be immune to compromise by a snooping attacker. Where an untrusted AAA intermediary is present, the AAA-Token must not be provided to the intermediary so as to avoid compromise of the AAA-Token. This can be avoided by use of re-direct as defined in [RFC3588]. Aboba, et al. Informational [Page 43]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 When EAP is used for authentication on PPP or wired IEEE 802 networks, it is typically assumed that the link is physically secure, so that an attacker cannot gain access to the link, or insert a rogue device. EAP methods defined in [RFC3748] reflect this usage model. These include EAP MD5, as well as One-Time Password (OTP) and Generic Token Card. These methods support one-way authentication (from EAP peer to authenticator) but not mutual authentication or key derivation. As a result, these methods do not bind the initial authentication and subsequent data traffic, even when the the ciphersuite used to protect data supports per-packet authentication and integrity protection. As a result, EAP methods not supporting mutual authentication are vulnerable to session hijacking as well as attacks by rogue devices. On wireless networks such as IEEE 802.11 [IEEE80211], these attacks become easy to mount, since any attacker within range can access the wireless medium, or act as an access point. As a result, new ciphersuites have been proposed for use with wireless LANs [IEEE80211i] which provide per-packet authentication, integrity and replay protection. In addition, mutual authentication and key derivation, provided by methods such as EAP-TLS [RFC2716] are required [IEEE80211i], so as to address the threat of rogue devices, and provide keying material to bind the initial authentication to subsequent data traffic. If the selected EAP method does not support mutual authentication, then the peer will be vulnerable to attack by rogue authenticators and backend authentication servers. If the EAP method does not derive keys, then TSKs will not be available for use with a negotiated ciphersuite, and there will be no binding between the initial EAP authentication and subsequent data traffic, leaving the session vulnerable to hijack. If the backend authentication server does not protect against authenticator masquerade, or provide the proper binding of the AAA- Key to the session within the AAA-Token, then one or more AAA-Keys may be sent to an unauthorized party, and an attacker may be able to gain access to the network. If the AAA-Token is provided to an untrusted AAA intermediary, then that intermediary may be able to modify the AAA-Key, or the attributes associated with it, as described in [RFC2607]. If the Secure Association Protocol does not provide mutual proof of possession of the AAA-Key material, then the peer will not have assurance that it is connected to the correct authenticator, only that the authenticator and backend authentication server share a trust relationship (since AAA protocols support mutual authentication). This distinction can become important when multiple Aboba, et al. Informational [Page 44]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 authenticators receive AAA-Keys from the backend authentication server, such as where fast handoff is supported. If the TSK derivation does not provide for protected ciphersuite and capabilities negotiation, then downgrade attacks are possible. 4.4. Man-in-the-middle Attacks As described in [I-D.puthenkulam-eap-binding], EAP method sequences and compound authentication mechanisms may be subject to man-in-the- middle attacks. When such attacks are successfully carried out, the attacker acts as an intermediary between a victim and a legitimate authenticator. This allows the attacker to authenticate successfully to the authenticator, as well as to obtain access to the network. In order to prevent these attacks, [I-D.puthenkulam-eap-binding] recommends derivation of a compound key by which the EAP peer and server can prove that they have participated in the entire EAP exchange. Since the compound key must not be known to an attacker posing as an authenticator, and yet must be derived from quantities that are exported by EAP methods, it may be desirable to derive the compound key from a portion of the EMSK. In order to provide proper key hygiene, it is recommended that the compound key used for man-in- the-middle protection be cryptographically separate from other keys derived from the EMSK, such as fast handoff keys, discussed in Appendix E. 4.5. Denial of Service Attacks The caching of security associations may result in vulnerability to denial of service attacks. Since an EAP peer may derive multiple EAP SAs with a given EAP server, and creation of a new EAP SA does not implicitly delete a previous EAP SA, EAP methods that result in creation of persistent state may be vulnerable to denial of service attacks by a rogue EAP peer. As a result, EAP methods creating persistent state may wish to limit the number of cached EAP SAs (Phase 1a) corresponding to an EAP peer. For example, an EAP server may choose to only retain a few EAP SAs for each peer. This prevents a rogue peer from denying access to other peers. Similarly, an authenticator may have multiple AAA-Key SAs corresponding to a given EAP peer; to conserve resources an authenticator may choose to limit the number of cached AAA-Key (Phase 1 b) SAs for each peer. Depending on the media, creation of a new unicast Secure Association SA may or may not imply deletion of a previous unicast secure Aboba, et al. Informational [Page 45]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 association SA. Where there is no implied deletion, the authenticator may choose to limit Phase 2 (unicast and multicast) Secure Association SAs for each peer. 4.6. Impersonation Both the RADIUS and Diameter protocols are potentially vulnerable to impersonation by a rogue authenticator. While AAA protocols such as RADIUS [RFC2865] or Diameter [RFC3588] support mutual authentication between the authenticator (known as the AAA client) and the backend authentication server (known as the AAA server), the security mechanisms vary according to the AAA protocol. In RADIUS, the shared secret used for authentication is determined by the source address of the RADIUS packet. As noted in [RFC3579] Section 4.3.7, it is highly desirable that the source address be checked against one or more NAS identification attributes so as to detect and prevent impersonation attacks. When RADIUS requests are forwarded by a proxy, the NAS-IP-Address or NAS-IPv6-Address attributes may not correspond to the source address. Since the NAS-Identifier attribute need not contain an FQDN, it also may not correspond to the source address, even indirectly. [RFC2865] Section 3 states: A RADIUS server MUST use the source IP address of the RADIUS UDP packet to decide which shared secret to use, so that RADIUS requests can be proxied. This implies that it is possible for a rogue authenticator to forge NAS-IP-Address, NAS-IPv6-Address or NAS-Identifier attributes within a RADIUS Access-Request in order to impersonate another authenticator. Among other things, this can result in messages (and MSKs) being sent to the wrong authenticator. Since the rogue authenticator is authenticated by the RADIUS proxy or server purely based on the source address, other mechanisms are required to detect the forgery. In addition, it is possible for attributes such as the Called-Station-Id and Calling-Station-Id to be forged as well. As recommended in [RFC3579], this vulnerability can be mitigated by having RADIUS proxies check authenticator identification attributes against the source address. To allow verification of session parameters such as the Called- Station- Id and Calling-Station-Id, these can be sent by the EAP peer to the server, protected by the TEKs. The RADIUS server can then check the parameters sent by the EAP peer against those claimed by Aboba, et al. Informational [Page 46]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 the authenticator. If a discrepancy is found, an error can be logged. While [RFC3588] requires use of the Route-Record AVP, this utilizes FQDNs, so that impersonation detection requires DNS A/AAAA and PTR RRs to be properly configured. As a result, it appears that Diameter is as vulnerable to this attack as RADIUS, if not more so. To address this vulnerability, it is necessary to allow the backend authentication server to communicate with the authenticator directly, such as via the redirect functionality supported in [RFC3588]. 4.7. Channel binding It is possible for a compromised or poorly implemented EAP authenticator to communicate incorrect information to the EAP peer and/or server. This may enable an authenticator to impersonate another authenticator or communicate incorrect information via out- of-band mechanisms (such as via a AAA or lower layer protocol). Where EAP is used in pass-through mode, the EAP peer typically does not verify the identity of the pass-through authenticator, it only verifies that the pass-through authenticator is trusted by the EAP server. This creates a potential security vulnerability, described in Section 7.15 of [RFC2284bis]. Section 4.3.7 of [RFC3579] describes how an EAP pass-through authenticator acting as a AAA client can be detected if it attempts to impersonate another authenticator (such by sending incorrect NAS- Identifier [RFC2865], NAS-IP-Address [RFC2865] or NAS-IPv6-Address [RFC3162] attributes via the AAA protocol). However, it is possible for a pass-through authenticator acting as a AAA client to provide correct information to the AAA server while communicating misleading information to the EAP peer via a lower layer protocol. For example, it is possible for a compromised authenticator to utilize another authenticator's Called-Station-Id or NAS-Identifier in communicating with the EAP peer via a lower layer protocol, or for a pass-through authenticator acting as a AAA client to provide an incorrect peer Calling-Station-Id [RFC2865][RFC3580] to the AAA server via the AAA protocol. As noted in Section 7.15 of [RFC3748] this vulnerability can be addressed by use of EAP methods that support a protected exchange of channel properties such as endpoint identifiers, including (but not limited to): Called-Station-Id [RFC2865][RFC3580], Calling-Station-Id [RFC2865][RFC3580], NAS-Identifier [RFC2865], NAS-IP-Address [RFC2865], and NAS-IPv6-Address [RFC3162]. Aboba, et al. Informational [Page 47]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Using such a protected exchange, it is possible to match the channel properties provided by the authenticator via out-of-band mechanisms against those exchanged within the EAP method. 4.8. Key Strength In order to guard against brute force attacks, EAP methods deriving keys need to be capable of generating keys with an appropriate effective symmetric key strength. In order to ensure that key generation is not the weakest link, it is necessary for EAP methods utilizing public key cryptography to choose a public key that has a cryptographic strength meeting the symmetric key strength requirement. As noted in Section 5 of [RFC3766], this results in the following required RSA or DH module and DSA subgroup size in bits, for a given level of attack resistance in bits: Attack Resistance RSA or DH Modulus DSA subgroup (bits) size (bits) size (bits) ----------------- ----------------- ------------ 70 947 128 80 1228 145 90 1553 153 100 1926 184 150 4575 279 200 8719 373 250 14596 475 4.9. Key Wrap As described in [RFC3579], Section 4.3, known problems exist in the key wrap specified in [RFC2548]. Where the same RADIUS shared secret is used by a PAP authenticator and an EAP authenticator, there is a vulnerability to known plaintext attack. Since RADIUS uses the shared secret for multiple purposes, including per-packet authentication, attribute hiding, considerable information is exposed about the shared secret with each packet. This exposes the shared secret to dictionary attacks. MD5 is used both to compute the RADIUS Response Authenticator and the Message-Authenticator attribute, and some concerns exist relating to the security of this hash [MD5Attack]. As discussed in [RFC3579], Section 4.3, the security vulnerabilities of RADIUS are extensive, and therefore development of an alternative key wrap technique based on the RADIUS shared secret would not substantially improve security. As a result, [RFC3759], Section 4.2 Aboba, et al. Informational [Page 48]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 recommends running RADIUS over IPsec. The same approach is taken in Diameter EAP [I-D.ietf-aaa-eap], which defines cleartext key attributes, to be protected by IPsec or TLS. Where an untrusted AAA intermediary is present (such as a RADIUS proxy or a Diameter agent), and data object security is not used, the AAA-Key may be recovered by an attacker in control of the untrusted intermediary. Possession of the AAA-Key enables decryption of data traffic sent between the peer and a specific authenticator; however where key separation is implemented, compromise of the AAA-Key does not enable an attacker to impersonate the peer to another authenticator, since that requires possession of the MK or EMSK, which are not transported by the AAA protocol. This vulnerability may be mitigated by implementation of redirect functionality, as provided in [RFC3588]. 5. Security Requirements This section summarizes the security requirements that must be met by EAP methods, AAA protocols, Secure Association Protocols and Ciphersuites in order to address the security threats described in this document. These requirements MUST be met by specifications requesting publication as an RFC. Each requirement provides a pointer to the sections of this document describing the threat that it mitigates. 5.1. EAP Method Requirements It is possible for the peer and EAP server to mutually authenticate and derive keys. In order to provide keying material for use in a subsequently negotiated ciphersuite, an EAP method supporting key derivation MUST export a Master Session Key (MSK) of at least 64 octets, and an Extended Master Session Key (EMSK) of at least 64 octets. EAP Methods deriving keys MUST provide for mutual authentication between the EAP peer and the EAP Server. The MSK and EMSK MUST NOT be used directly to protect data; however, they are of sufficient size to enable derivation of a AAA-Key subsequently used to derive Transient Session Keys (TSKs) for use with the selected ciphersuite. Each ciphersuite is responsible for specifying how to derive the TSKs from the AAA-Key. The AAA-Key is derived from the keying material exported by the EAP method (MSK and EMSK). This derivation occurs on the AAA server. In many existing protocols that use EAP, the AAA-Key and MSK are equivalent, but more complicated mechanisms are possible (see Appendix E for details). Aboba, et al. Informational [Page 49]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 EAP methods SHOULD ensure the freshness of the MSK and EMSK even in cases where one party may not have a high quality random number generator. A RECOMMENDED method is for each party to provide a nonce of at least 128 bits, used in the derivation of the MSK and EMSK. EAP methods export the MSK and EMSK and not Transient Session Keys so as to allow EAP methods to be ciphersuite and media independent. Keying material exported by EAP methods MUST be independent of the ciphersuite negotiated to protect data. Depending on the lower layer, EAP methods may run before or after ciphersuite negotiation, so that the selected ciphersuite may not be known to the EAP method. By providing keying material usable with any ciphersuite, EAP methods can used with a wide range of ciphersuites and media. It is RECOMMENDED that methods providing integrity protection of EAP packets include coverage of all the EAP header fields, including the Code, Identifier, Length, Type and Type-Data fields. In order to preserve algorithm independence, EAP methods deriving keys SHOULD support (and document) the protected negotiation of the ciphersuite used to protect the EAP conversation between the peer and server. This is distinct from the ciphersuite negotiated between the peer and authenticator, used to protect data. The strength of Transient Session Keys (TSKs) used to protect data is ultimately dependent on the strength of keys generated by the EAP method. If an EAP method cannot produce keying material of sufficient strength, then the TSKs may be subject to brute force attack. In order to enable deployments requiring strong keys, EAP methods supporting key derivation SHOULD be capable of generating an MSK and EMSK, each with an effective key strength of at least 128 bits. Methods supporting key derivation MUST demonstrate cryptographic separation between the MSK and EMSK branches of the EAP key hierarchy. Without violating a fundamental cryptographic assumption (such as the non-invertibility of a one-way function) an attacker recovering the MSK or EMSK MUST NOT be able to recover the other quantity with a level of effort less than brute force. Non-overlapping substrings of the MSK MUST be cryptographically separate from each other. That is, knowledge of one substring MUST NOT help in recovering some other substring without breaking some hard cryptographic assumption. This is required because some existing ciphersuites form TSKs by simply splitting the AAA-Key to pieces of appropriate length. Likewise, non-overlapping substrings Aboba, et al. Informational [Page 50]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 of the EMSK MUST be cryptographically separate from each other, and from substrings of the MSK. The EMSK MUST remain on the EAP peer and EAP server where it is derived; it MUST NOT be transported to, or shared with, additional parties, or used to derive any other keys. Since EAP does not provide for explicit key lifetime negotiation, EAP peers, authenticators and authentication servers MUST be prepared for situations in which one of the parties discards key state which remains valid on another party. The development and validation of key derivation algorithms is difficult, and as a result EAP methods SHOULD reuse well established and analyzed mechanisms for key derivation (such as those specified in IKE [RFC2409] or TLS [RFC2246]), rather than inventing new ones. EAP methods SHOULD also utilize well established and analyzed mechanisms for MSK and EMSK derivation. 5.1.1. Requirements for EAP methods In order for an EAP method to meet the guidelines for EMSK usage it must meet the following requirements: o It must specify how to derive the EMSK o The key material used for the EMSK MUST be computationally independent of the MSK and TEKs. o The EMSK MUST NOT be used for any other purpose than the key derivation described in this document. o The EMSK MUST be secret and not known to someone observing the authentication mechanism protocol exchange. o The EMSK MUST be maintained within the EAP server. Only keys (AMSKs) derived according to this specification may be exported from the EAP server. o The EMSK MUST be unique for each session. o The EAP mechanism SHOULD provide a way of naming the EMSK. Implementations of EAP frameworks on the EAP-Peer and EAP-Server SHOULD provide an interface to obtain AMSKs. The implementation MAY restrict which callers can obtain which keys. Aboba, et al. Informational [Page 51]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 5.1.2. Requirements for EAP applications In order for an application to meet the guidelines for EMSK usage it must meet the following requirements: o The application MAY use the MSK transmitted to the NAS in any way it chooses. This is required for backward compatibility. New applications following this specification SHOULD NOT use the MSK. If more than one application uses the MSK, then the cryptographic separation is not achieved. Implementations SHOULD prevent such combinations. o The application MUST NOT use the EMSK in any other way except to derive Application Master Session Keys (AMSK) using the key derivation specified in this document. It MUST NOT use the EMSK directly for cryptographic protection of data. o Applications MUST define distinct key labels, application specific data, length of derived key material used in the key derivation described in section 2.4.3. o Applications MUST define how they use their AMSK to derive TSKs for their use. 5.2. AAA Protocol Requirements AAA protocols suitable for use in transporting EAP MUST provide the following facilities: Security services AAA protocols used for transport of EAP keying material MUST implement and SHOULD use per-packet integrity and authentication, replay protection and confidentiality. These requirements are met by Diameter EAP [I-D.ietf-aaa-eap], as well as RADIUS over IPsec [RFC3579]. Session Keys AAA protocols used for transport of EAP keying material MUST implement and SHOULD use dynamic key management in order to derive fresh session keys, as in Diameter EAP [I-D.ietf-aaa-eap] and RADIUS over IPsec [RFC3579], rather than using a static key, as originally defined in RADIUS [RFC2865]. Mutual authentication AAA protocols used for transport of EAP keying material MUST provide for mutual authentication between the authenticator and backend authentication server. These requirements are met by Diameter EAP [I-D.ietf-aaa-eap] as well as by RADIUS EAP [RFC3579]. Aboba, et al. Informational [Page 52]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Authorization AAA protocols used for transport of EAP keying material SHOULD provide protection against rogue authenticators masquerading as other authenticators. This can be accomplished, for example, by requiring that AAA agents check the source address of packets against the origin attributes (Origin-Host AVP in Diameter, NAS-IP- Address, NAS-IPv6-Address, NAS-Identifier in RADIUS). For details, see Section 4.3.7 of [RFC3579]. Key transport Since EAP methods do not export Transient Session Keys (TSKs) in order to maintain media and ciphersuite independence, the AAA server MUST NOT transport TSKs from the backend authentication server to authenticator. Key transport specification In order to enable backend authentication servers to provide keying material to the authenticator in a well defined format, AAA protocols suitable for use with EAP MUST define the format and wrapping of the AAA-Token. EMSK transport Since the EMSK is a secret known only to the backend authentication server and peer, the AAA-Token MUST NOT transport the EMSK from the backend authentication server to the authenticator. AAA-Token protection To ensure against compromise, the AAA-Token MUST be integrity protected, authenticated, replay protected and encrypted in transit, using well-established cryptographic algorithms. Session Keys The AAA-Token SHOULD be protected with session keys as in Diameter [RFC3588] or RADIUS over IPsec [RFC3579] rather than static keys, as in [RFC2548]. Key naming In order to ensure against confusion between the appropriate keying material to be used in a given Secure Association Protocol exchange, the AAA-Token SHOULD include explicit key names and context appropriate for informing the authenticator how the keying material is to be used. Key Compromise Where untrusted intermediaries are present, the AAA-Token SHOULD NOT be provided to the intermediaries. In Diameter, handling of keys by intermediaries can be avoided using Redirect functionality [RFC3588]. Aboba, et al. Informational [Page 53]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 5.3. Secure Association Protocol Requirements The Secure Association Protocol supports the following: Entity Naming The peer and authenticator SHOULD identify themselves in a manner that is independent of their attached ports. Mutual proof of possession The peer and authenticator MUST each demonstrate possession of the keying material transported between the backend authentication server and authenticator (AAA-Key). Key Naming The Secure Association Protocol MUST explicitly name the keys used in the proof of possession exchange, so as to prevent confusion when more than one set of keying material could potentially be used as the basis for the exchange. Creation and Deletion In order to support the correct processing of phase 2 security associations, the Secure Association (phase 2) protocol MUST support the naming of phase 2 security associations and associated transient session keys, so that the correct set of transient session keys can be identified for processing a given packet. The phase 2 Secure Association Protocol also MUST support transient session key activation and SHOULD support deletion, so that establishment and re-establishment of transient session keys can be synchronized between the parties. Integrity and Replay Protection The Secure Association Protocol MUST support integrity and replay protection of all messages. Direct operation Since the phase 2 Secure Association Protocol is concerned with the establishment of security associations between the EAP peer and authenticator, including the derivation of transient session keys, only those parties have "a need to know" the transient session keys. The Secure Association Protocol MUST operate directly between the peer and authenticator, and MUST NOT be passed-through to the backend authentication server, or include additional parties. Derivation of transient session keys The Secure Association Protocol negotiation MUST support derivation of unicast and multicast transient session keys suitable for use with the negotiated ciphersuite. Aboba, et al. Informational [Page 54]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 TSK freshness The Secure Association (phase 2) Protocol MUST support the derivation of fresh unicast and multicast transient session keys, even when the keying material provided by the backend authentication server is not fresh. This is typically supported by including an exchange of nonces within the Secure Association Protocol. Bi-directional operation While some ciphersuites only require a single set of transient session keys to protect traffic in both directions, other ciphersuites require a unique set of transient session keys in each direction. The phase 2 Secure Association Protocol SHOULD provide for the derivation of unicast and multicast keys in each direction, so as not to require two separate phase 2 exchanges in order to create a bi-directional phase 2 security association. Secure capabilities negotiation The Secure Association Protocol MUST support secure capabilities negotiation. This includes security parameters such as the security association identifier (SAID) and ciphersuites, as well as negotiation of the lifetime of the TSKs, AAA-Key and exported EAP keys. Secure capabilities negotiation also includes confirmation of the capabilities discovered during the discovery phase (phase 0), so as to ensure that the announced capabilities have not been forged. Key Scoping The Secure Association Protocol MUST ensure the synchronization of key scope between the peer and authenticator. This includes negotiation of restrictions on key usage. 5.4. Ciphersuite Requirements Ciphersuites suitable for keying by EAP methods MUST provide the following facilities: TSK derivation In order to allow a ciphersuite to be usable within the EAP keying framework, a specification MUST be provided describing how transient session keys suitable for use with the ciphersuite are derived from the AAA-Key. EAP method independence Algorithms for deriving transient session keys from the AAA-Key MUST NOT depend on the EAP method. However, algorithms for deriving TEKs MAY be specific to the EAP method. Aboba, et al. Informational [Page 55]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Cryptographic separation The TSKs derived from the AAA-Key MUST be cryptographically separate from each other. Similarly, TEKs MUST be cryptographically separate from each other. In addition, the TSKs MUST be cryptographically separate from the TEKs. 6. IANA Considerations This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to EAP key management, in accordance with BCP 26, [RFC2434]. The following terms are used here with the meanings defined in BCP 26: "name space", "assigned value", "registration". The following policies are used here with the meanings defined in BCP 26: "Private Use", "First Come First Served", "Expert Review", "Specification Required", "IETF Consensus", "Standards Action". For registration requests where a Designated Expert should be consulted, the responsible IESG area director should appoint the Designated Expert. The intention is that any allocation will be accompanied by a published RFC. But in order to allow for the allocation of values prior to the RFC being approved for publication, the Designated Expert can approve allocations once it seems clear that an RFC will be published. The Designated expert will post a request to the EAP WG mailing list (or a successor designated by the Area Director) for comment and review, including an Internet-Draft. Before a period of 30 days has passed, the Designated Expert will either approve or deny the registration request and publish a notice of the decision to the EAP WG mailing list or its successor, as well as informing IANA. A denial notice must be justified by an explanation and, in the cases where it is possible, concrete suggestions on how the request can be modified so as to become acceptable. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. Aboba, et al. Informational [Page 56]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H. Lefkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. 7.2. Informative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1968] Meyer, G. and K. Fox, "The PPP Encryption Control Protocol (ECP)", RFC 1968, June 1996. [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998. [RFC2420] Kummert, H., "The PPP Triple-DES Encryption Protocol (3DESE)", RFC 2420, September 1998. [RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D. and R. Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)", RFC 2516, February 1999. [RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC 2548, March 1999. [RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy Implementation in Roaming", RFC 2607, June 1999. [RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication Protocol", RFC 2716, October 1999. Aboba, et al. Informational [Page 57]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 [RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [RFC3078] Pall, G. and G. Zorn, "Microsoft Point-To-Point Encryption (MPPE) Protocol", RFC 3078, March 2001. [RFC3079] Zorn, G., "Deriving Keys for use with Microsoft Point-to-Point Encryption (MPPE)", RFC 3079, March 2001. [RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial In User Service) Support For Extensible Authentication Protocol (EAP)", RFC 3579, September 2003. [RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G. and J. Roese, "IEEE 802.1X Remote Authentication Dial In User Service (RADIUS) Usage Guidelines", RFC 3580, September 2003. [RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko, "Diameter Base Protocol", RFC 3588, September 2003. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", RFC 3766, April 2004. [FIPSDES] National Institute of Standards and Technology, "Data Encryption Standard", FIPS PUB 46, January 1977. [DESMODES] National Institute of Standards and Technology, "DES Modes of Operation", FIPS PUB 81, December 1980, <http:// www.itl.nist.gov/fipspubs/fip81.htm>. [IEEE802] Institute of Electrical and Electronics Engineers, "IEEE Standards for Local and Metropolitan Area Networks: Overview and Architecture", ANSI/IEEE Standard 802, 1990. [IEEE80211] Institute of Electrical and Electronics Engineers, "Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", IEEE IEEE Standard 802.11-1999, 1999. [IEEE8021X] Institute of Electrical and Electronics Engineers, "Local and Metropolitan Area Networks: Port-Based Network Access Aboba, et al. Informational [Page 58]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Control", IEEE Standard 802.1X-2004, September 2004. [IEEE8021Q] Institute of Electrical and Electronics Engineers, "IEEE Standards for Local and Metropolitan Area Networks: Draft Standard for Virtual Bridged Local Area Networks", IEEE Standard 802.1Q/D8, January 1998. [IEEE80211F] Institute of Electrical and Electronics Engineers, "Recommended Practice for Multi-Vendor Access Point Interoperability via an Inter-Access Point Protocol Across Distribution Systems Supporting IEEE 802.11 Operation", IEEE 802.11F, July 2003. [IEEE80211i] Institute of Electrical and Electronics Engineers, "Draft Supplement to STANDARD FOR Telecommunications and Information Exchange between Systems - LAN/MAN Specific Requirements - Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Specification for Enhanced Security", IEEE Draft 802.11I/ D8, February 2004. [IEEE-02-758] Mishra, A., Shin, M., Arbaugh, W., Lee, I. and K. Jang, "Proactive Caching Strategies for IAPP Latency Improvement during 802.11 Handoff", IEEE 802.11 Working Group, IEEE-02-758r1-F Draft 802.11I/D5.0, November 2002. [IEEE-03-084] Mishra, A., Shin, M., Arbaugh, W., Lee, I. and K. Jang, "Proactive Key Distribution to support fast and secure roaming", IEEE 802.11 Working Group, IEEE-03-084r1-I, http://www.ieee802.org/11/Documents/DocumentHolder/ 3-084.zip, January 2003. [IEEE-03-155] Aboba, B., "Fast Handoff Issues", IEEE 802.11 Working Group, IEEE-03-155r0-I, http://www.ieee802.org/11/ Documents/DocumentHolder/3-155.zip, March 2003. [I-D.ietf-roamops-cert] Aboba, B., "Certificate-Based Roaming", draft-ietf-roamops- cert-02 (work in progress), April 1999. [I-D.ietf-aaa-eap] Eronen, P., Hiller, T. and G. Zorn, "Diameter Extensible Authentication Protocol (EAP) Application", draft-ietf-aaa- Aboba, et al. Informational [Page 59]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 eap-08 (work in progress), June 2004. [I-D.irtf-aaaarch-handoff] Arbaugh, W. and B. Aboba, "Handoff Extension to RADIUS", draft-irtf-aaaarch-handoff-04 (work in progress), October 2003. [I-D.puthenkulam-eap-binding] Puthenkulam, J., "The Compound Authentication Binding Problem", draft-puthenkulam-eap-binding-04 (work in progress), October 2003. [I-D.aboba-802-context] Aboba, B. and T. Moore, "A Model for Context Transfer in IEEE 802", draft-aboba-802-context-03 (work in progress), October 2003. [I-D.arkko-pppext-eap-aka] Arkko, J. and H. Haverinen, "EAP AKA Authentication", draft- arkko-pppext-eap-aka-11 (work in progress), October 2003. [IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", draft- ietf-ipsec-ikev2-12 (work in progress), March 2004. [8021XHandoff] Pack, S. and Y. Choi, "Pre-Authenticated Fast Handoff in a Public Wireless LAN Based on IEEE 802.1X Model", School of Computer Science and Engineering, Seoul National University, Seoul, Korea, 2002. [MD5Attack] Dobbertin, H., "The Status of MD5 After a Recent Attack", CryptoBytes, Vol.2 No.2, 1996. [WLANREQ] Stanley, D., Walker, J. and B. Aboba, "EAP Method Requirements for Wireless LANs", draft-walker-ieee802-req-02.txt (work in progress), July 2004. [Housley56] Housley, R., "Key Management in AAA", Presentation to the AAA WG at IETF 56, http://www.ietf.org/proceedings/03mar/slides/aaa-5/index.html, March 2003. Acknowledgments Thanks to Arun Ayyagari, Ashwin Palekar, and Tim Moore of Microsoft, Dorothy Stanley of Agere, Bob Moskowitz of TruSecure, and Russ Aboba, et al. Informational [Page 60]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Housley of Vigil Security for useful feedback. Author Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: bernarda@microsoft.com Phone: +1 425 706 6605 Fax: +1 425 936 7329 Dan Simon Microsoft Research Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: dansimon@microsoft.com Phone: +1 425 706 6711 Fax: +1 425 936 7329 Jari Arkko Ericsson Jorvas 02420 Finland Phone: EMail: jari.arkko@ericsson.com Pasi Eronen Nokia Research Center P.O. Box 407 FIN-00045 Nokia Group Finland EMail: pasi.eronen@nokia.com Henrik Levkowetz (editor) ipUnplugged AB Arenavagen 27 Stockholm S-121 28 SWEDEN Phone: +46 708 32 16 08 EMail: henrik@levkowetz.com Aboba, et al. Informational [Page 61]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Appendix A - Ciphersuite Keying Requirements To date, PPP and IEEE 802.11 ciphersuites are suitable for keying by EAP. This Appendix describes the keying requirements of common PPP and 802.11 ciphersuites. PPP ciphersuites include DESEbis [RFC2419], 3DES [RFC2420], and MPPE [RFC3078]. The DES algorithm is described in [FIPSDES], and DES modes (such as CBC, used in [RFC2419] and DES-EDE3-CBC, used in [RFC2420]) are described in [DESMODES]. For PPP DESEbis, a single 56-bit encryption key is required, used in both directions. For PPP 3DES, a 168-bit encryption key is needed, used in both directions. As described in [RFC2419] for DESEbis and [RFC2420] for 3DES, the IV, which is different in each direction, is "deduced from an explicit 64-bit nonce, which is exchanged in the clear during the [ECP] negotiation phase." There is therefore no need for the IV to be provided by EAP. For MPPE, 40-bit, 56-bit or 128-bit encryption keys are required in each direction, as described in [RFC3078]. No initialization vector is required. While these PPP ciphersuites provide encryption, they do not provide per-packet authentication or integrity protection, so an authentication key is not required in either direction. Within [IEEE80211], Transient Session Keys (TSKs) are required both for unicast traffic as well as for multicast traffic, and therefore separate key hierarchies are required for unicast keys and multicast keys. IEEE 802.11 ciphersuites include WEP-40, described in [IEEE80211], which requires a 40-bit encryption key, the same in either direction; and WEP-128, which requires a 104-bit encryption key, the same in either direction. These ciphersuites also do not support per-packet authentication and integrity protection. In addition to these unicast keys, authentication and encryption keys are required to wrap the multicast encryption key. Recently, new ciphersuites have been proposed for use with IEEE 802.11 that provide per-packet authentication and integrity protection as well as encryption [IEEE80211i]. These include TKIP, which requires a single 128-bit encryption key and a 128-bit authentication key (used in both directions); AES CCMP, which requires a single 128-bit key (used in both directions) in order to authenticate and encrypt data; and WRAP, which requires a single 128-bit key (used in both directions). As with WEP, authentication and encryption keys are also required to wrap the multicast encryption (and possibly, authentication) keys. Aboba, et al. Informational [Page 62]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Appendix B - Transient EAP Key (TEK) Hierarchy Figure B-1 illustrates the TEK key hierarchy for EAP-TLS [RFC2716], which is based on the TLS key hierarchy described in [RFC2246]. The TLS-negotiated ciphersuite is used to set up a protected channel for use in protecting the EAP conversation, keyed by the derived TEKs. The TEK derivation proceeds as follows: master_secret = TLS-PRF-48(pre_master_secret, "master secret", client.random || server.random) TEK = TLS-PRF-X(master_secret, "key expansion", server.random || client.random) Where: TLS-PRF-X = TLS pseudo-random function defined in [RFC2246], computed to X octets. master_secret = TLS term for the MK. | | | | | pre_master_secret | server| | | client Random| V | Random | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | | | | +---->| master_secret |<------+ | | (MK) | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | | | | | V V V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Key Block | | (TEKs) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | | client | server | client | server | client | server | MAC | MAC | write | write | IV | IV | | | | | | V V V V V V Figure B-1 - TLS [RFC2246] Key Hierarchy Aboba, et al. Informational [Page 63]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Appendix C - EAP Key Hierarchy In EAP-TLS [RFC2716], the MSK is divided into two halves, corresponding to the "Peer to Authenticator Encryption Key" (Enc- RECV-Key, 32 octets, also known as the PMK) and "Authenticator to Peer Encryption Key" (Enc-SEND-Key, 32 octets). In [RFC2548], the Enc-RECV-Key (the PMK) is transported in the MS-MPPE-Recv-Key attribute, and the Enc-SEND-Key is transported in the MS-MPPE-Send- Key attribute. The EMSK is also divided into two halves, corresponding to the "Peer to Authenticator Authentication Key" (Auth-RECV-Key, 32 octets) and "Authenticator to Peer Authentication Key" (Auth-SEND-Key, 32 octets). The IV is a 64 octet quantity that is a known value; octets 0-31 are known as the "Peer to Authenticator IV" or RECV-IV, and Octets 32-63 are known as the "Authenticator to Peer IV", or SEND-IV. In EAP-TLS, the MSK, EMSK and IV are derived from the MK via a one- way function. This ensures that the MK cannot be derived from the MSK, EMSK or IV unless the one-way function (TLS PRF) is broken. Since the MSK is derived from the MK, if the MK is compromised then the MSK is also compromised. As described in [RFC2716], the formula for the derivation of the MSK, EMSK and IV from the MK is as follows: MSK = TLS-PRF-64(MK, "client EAP encryption", client.random || server.random) EMSK = second 64 octets of: TLS-PRF-128(MK, "client EAP encryption", client.random || server.random) IV = TLS-PRF-64("", "client EAP encryption", client.random || server.random) AAA-Key(0,31) = Peer to Authenticator Encryption Key (Enc-RECV-Key) (MS-MPPE-Recv-Key in [RFC2548]). Also known as the PMK. AAA-Key(32,63)= Authenticator to Peer Encryption Key (Enc-SEND-Key) (MS-MPPE-Send-Key in [RFC2548]) EMSK(0,31) = Peer to Authenticator Authentication Key (Auth-RECV-Key) EMSK(32,63) = Authenticator to Peer Authentication Key (Auth-Send-Key) IV(0,31) = Peer to Authenticator Initialization Vector (RECV-IV) IV(32,63) = Authenticator to Peer Initialization vector (SEND-IV) Where: AAA-Key(W,Z) = Octets W through Z includes of the AAA-Key. Aboba, et al. Informational [Page 64]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 IV(W,Z) = Octets W through Z inclusive of the IV. MSK(W,Z) = Octets W through Z inclusive of the MSK. EMSK(W,Z) = Octets W through Z inclusive of the EMSK. MK = TLS master_secret TLS-PRF-X = TLS PRF function defined in [RFC2246] computed to X octets client.random = Nonce generated by the TLS client. server.random = Nonce generated by the TLS server. Figure C-1 describes the process by which the MSK,EMSK,IV and ultimately the TSKs, are derived from the MK. Note that in [RFC2716], the MK is referred to as the "TLS Master Secret". ---+ | ^ | TLS Master Secret (MK) | | | V | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | EAP | | Master Session Key (MSK) | Method | | Derivation | | | | V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ EAP ---+ | | | API ^ | MSK | EMSK | IV | | | | | V V V v +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ | | | | | | | backend authentication server | | | | | | | V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ | | ^ | AAA-Key(0,31) | AAA-Key(32,63) | | (PMK) | Transported | | | via AAA | | | | V V V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ | | ^ | Ciphersuite-Specific Transient Session | Auth.| | Key Derivation | | | | V +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ Figure C-1 - EAP TLS [RFC2716] Key hierarchy Aboba, et al. Informational [Page 65]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Appendix D - Transient Session Key (TSK) Derivation Within IEEE 802.11 RSN, the Pairwise Transient Key (PTK), a transient session key used to protect unicast traffic, is derived from the PMK (octets 0-31 of the MSK), known in [RFC2716] as the Peer to Authenticator Encryption Key. In [IEEE80211i], the PTK is derived from the PMK via the following formula: PTK = EAPOL-PRF-X(PMK, "Pairwise key expansion", Min(AA,SA) || Max(AA, SA) || Min(ANonce,SNonce) || Max(ANonce,SNonce)) Where: PMK = AAA-Key(0,31) SA = Station MAC address (Calling-Station-Id) AA = Access Point MAC address (Called-Station-Id) ANonce = Access Point Nonce SNonce = Station Nonce EAPOL-PRF-X = Pseudo-Random Function based on HMAC-SHA1, generating a PTK of size X octets. TKIP uses X = 64, while CCMP, WRAP, and WEP use X = 48. The EAPOL-Key Confirmation Key (KCK) is used to provide data origin authenticity in the TSK derivation. It utilizes the first 128 bits (bits 0-127) of the PTK. The EAPOL-Key Encryption Key (KEK) provides confidentiality in the TSK derivation. It utilizes bits 128-255 of the PTK. Bits 256-383 of the PTK are used by Temporal Key 1, and Bits 384-511 are used by Temporal Key 2. Usage of TK1 and TK2 is ciphersuite specific. Details are available in [IEEE80211i]. Aboba, et al. Informational [Page 66]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Appendix E - AAA-Key Derivation Where a AAA-Key is generated as the result of a successful EAP authentication, the AAA-Key is set to MSK(0,63). As discussed in [I-D.irtf-aaaarch-handoff], [IEEE-02-758], [IEEE-03-084], and [8021XHandoff], keying material may be required for use in fast handoff between authenticators. Where the backend authentication server provides keying material to multiple authenticators in order to facilitate fast handoff, it is highly desirable for the keying material used on different authenticators to be cryptographically separate, so that if one authenticator is compromised, it does not lead to the compromise of other authenticators. Where keying material is provided by the backend authentication server, a key hierarchy derived from the EMSK, can be used to provide cryptographically separate keying material for use in fast handoff: AAA-Key-A = MSK(0,63) AAA-Key-B = PRF(EMSK(0,63),"EAP AAA-Key derivation for multiple attachments", AAA-Key-A,B-Called-Station-Id, Calling-Station-Id,length) AAA-Key-E = PRF(EMSK(0,63),"EAP AAA-Key derivation for multiple attachments",AAA-Key-A,E-Called-Station-Id, Calling-Station-Id, length) Where: Calling-Station-Id = STA MAC address B-Called-Station-Id = AP B MAC address E-Called-Station-Id = AP E MAC address length = length of derived key material Here AAA-Key-A is the AAA-Key derived during the initial EAP authentication between the peer and authenticator A. Based on this initial EAP authentication, the EMSK is also derived, which can be used to derive AAA-Keys for fast authentication between the EAP peer and authenticators B and E. Since the EMSK is cryptographically separate from the MSK, each of these AAA-Keys is cryptographically separate from each other, and are guaranteed to be unique between the EAP peer (also known as the STA) and the authenticator (also known as the AP). Aboba, et al. Informational [Page 67]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards- related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Aboba, et al. Informational [Page 68]
INTERNET-DRAFT EAP Key Management Framework 26 June 2004 Open Issues Open issues relating to this specification are tracked on the following web site: http://www.drizzle.com/~aboba/EAP/eapissues.html Expiration Date This memo is filed as <draft-ietf-eap-keying-02.txt>, and expires December 22, 2004. Aboba, et al. Informational [Page 69]
