Network Working Group B. Aboba Request for Comments: 5247 D. Simon Updates: 3748 Microsoft Corporation Category: Standards Track P. Eronen Nokia August 2008 Extensible Authentication Protocol (EAP) Key Management Framework Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract The Extensible Authentication Protocol (EAP), defined in RFC 3748, enables extensible network access authentication. This document specifies the EAP key hierarchy and provides a framework for the transport and usage of keying material and parameters generated by EAP authentication algorithms, known as "methods". It also provides a detailed system-level security analysis, describing the conditions under which the key management guidelines described in RFC 4962 can be satisfied.
Table of Contents 1. Introduction ....................................................3 1.1. Requirements Language ......................................3 1.2. Terminology ................................................3 1.3. Overview ...................................................7 1.4. EAP Key Hierarchy .........................................10 1.5. Security Goals ............................................15 1.6. EAP Invariants ............................................16 2. Lower-Layer Operation ..........................................20 2.1. Transient Session Keys ....................................20 2.2. Authenticator and Peer Architecture .......................22 2.3. Authenticator Identification ..............................23 2.4. Peer Identification .......................................27 2.5. Server Identification .....................................29 3. Security Association Management ................................31 3.1. Secure Association Protocol ...............................32 3.2. Key Scope .................................................35 3.3. Parent-Child Relationships ................................35 3.4. Local Key Lifetimes .......................................37 3.5. Exported and Calculated Key Lifetimes .....................37 3.6. Key Cache Synchronization .................................40 3.7. Key Strength ..............................................40 3.8. Key Wrap ..................................................41 4. Handoff Vulnerabilities ........................................41 4.1. EAP Pre-Authentication ....................................43 4.2. Proactive Key Distribution ................................44 4.3. AAA Bypass ................................................46 5. Security Considerations ........................................50 5.1. Peer and Authenticator Compromise .........................51 5.2. Cryptographic Negotiation .................................53 5.3. Confidentiality and Authentication ........................54 5.4. Key Binding ...............................................59 5.5. Authorization .............................................60 5.6. Replay Protection .........................................63 5.7. Key Freshness .............................................64 5.8. Key Scope Limitation ......................................66 5.9. Key Naming ................................................66 5.10. Denial-of-Service Attacks ................................67 6. References .....................................................68 6.1. Normative References ......................................68 6.2. Informative References ....................................68 Acknowledgments ...................................................74 Appendix A - Exported Parameters in Existing Methods ..............75
1. Introduction The Extensible Authentication Protocol (EAP), defined in [RFC3748], was designed to enable extensible authentication for network access in situations in which the Internet Protocol (IP) protocol is not available. Originally developed for use with Point-to-Point Protocol (PPP) [RFC1661], it has subsequently also been applied to IEEE 802 wired networks [IEEE-802.1X], Internet Key Exchange Protocol version 2 (IKEv2) [RFC4306], and wireless networks such as [IEEE-802.11] and [IEEE-802.16e]. EAP is a two-party protocol spoken between the EAP peer and server. Within EAP, keying material is generated by EAP authentication algorithms, known as "methods". Part of this keying material can be used by EAP methods themselves, and part of this material can be exported. In addition to the export of keying material, EAP methods can also export associated parameters such as authenticated peer and server identities and a unique EAP conversation identifier, and can import and export lower-layer parameters known as "channel binding parameters", or simply "channel bindings". This document specifies the EAP key hierarchy and provides a framework for the transport and usage of keying material and parameters generated by EAP methods. It also provides a detailed security analysis, describing the conditions under which the requirements described in "Guidance for Authentication, Authorization, and Accounting (AAA) Key Management" [RFC4962] can be satisfied. 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 [RFC2119]. 1.2. Terminology The terms "Cryptographic binding", "Cryptographic separation", "Key strength" and "Mutual authentication" are defined in [RFC3748] and are used with the same meaning in this document, which also frequently uses the following terms: 4-Way Handshake A pairwise Authentication and Key Management Protocol (AKMP) defined in [IEEE-802.11], which confirms mutual possession of a Pairwise Master Key by two parties and distributes a Group Key.
AAA Authentication, Authorization, and Accounting AAA protocols with EAP support include "RADIUS Support for EAP" [RFC3579] and "Diameter EAP Application" [RFC4072]. In this document, the terms "AAA server" and "backend authentication server" are used interchangeably. AAA-Key The term AAA-Key is synonymous with Master Session Key (MSK). Since multiple keys can be transported by AAA, the term is potentially confusing and is not used in this document. Authenticator The entity initiating EAP authentication. 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]. Channel Binding A secure mechanism for ensuring that a subset of the parameters transmitted by the authenticator (such as authenticator identifiers and properties) are agreed upon by the EAP peer and server. It is expected that the parameters are also securely agreed upon by the EAP peer and authenticator via the lower layer if the authenticator advertised the parameters. Derived Keying Material Keys derived from EAP keying material, such as Transient Session Keys (TSKs). EAP Keying Material Keys derived by an EAP method; this includes exported keying material (MSK, Extended MSK (EMSK), Initialization Vector (IV)) as well as local keying material such as Transient EAP Keys (TEKs). EAP Pre-Authentication The use of EAP to pre-establish EAP keying material on an authenticator prior to arrival of the peer at the access network managed by that authenticator. EAP Re-Authentication EAP authentication between an EAP peer and a server with whom the EAP peer shares valid unexpired EAP keying material.
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. Exported Keying Material The EAP Master Session Key (MSK), Extended Master Session Key (EMSK), and Initialization Vector (IV). 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. The EMSK MUST be at least as long as the MSK in size. 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 (Transport Layer Security) [RFC5216], 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 it is OPTIONAL for EAP methods to generate it. However, when it is generated, it MUST be unpredictable. Keying Material Unless otherwise qualified, the term "keying material" refers to EAP keying material as well as derived keying material. Key Scope The parties to whom a key is available. Key Wrap The encryption of one symmetric cryptographic key in another. The algorithm used for the encryption is called a key wrap algorithm or a key encryption algorithm. The key used in the encryption process is called a key-encryption key (KEK). 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.
Lower Layer The lower layer is responsible for carrying EAP frames between the peer and authenticator. Lower-Layer Identity A name used to identify the EAP peer and authenticator within the lower layer. 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. Network Access Server (NAS) A device that provides an access service for a user to a network. Pairwise Master Key (PMK) Lower layers use the MSK in a lower-layer dependent manner. For instance, in IEEE 802.11 [IEEE-802.11], Octets 0-31 of the MSK are known as the Pairwise Master Key (PMK); the Temporal Key Integrity Protocol (TKIP) and Advanced Encryption Standard Counter Mode with CBC-MAC Protocol (AES CCMP) ciphersuites derive their Transient Session Keys (TSKs) solely from the PMK, whereas the Wired Equivalent Privacy (WEP) ciphersuite, as noted in "IEEE 802.1X RADIUS Usage Guidelines" [RFC3580], derives its TSKs from both halves of the MSK. In [IEEE-802.16e], the MSK is truncated to 20 octets for PMK and 20 octets for PMK2. Peer The entity that responds to the authenticator. In [IEEE-802.1X], this entity is known as the Supplicant. Security Association A set of policies and cryptographic state used to protect information. Elements of a security association include cryptographic keys, negotiated ciphersuites and other parameters, counters, sequence spaces, authorization attributes, etc. Secure Association Protocol An exchange that occurs between the EAP peer and authenticator in order to manage security associations derived from EAP exchanges. The protocol establishes unicast and (optionally) multicast security associations, which include symmetric keys and a context for the use of the keys. An example of a Secure Association Protocol is the 4-way handshake defined within [IEEE-802.11].
Session-Id The EAP Session-Id uniquely identifies an EAP authentication exchange between an EAP peer (as identified by the Peer-Id(s)) and server (as identified by the Server-Id(s)). For more information, see Section 1.4. Transient EAP Keys (TEKs) Session keys that 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. The TEKs are stored locally by the EAP method and are not exported. 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. Transient Session Keys (TSKs) Keys used to protect data exchanged after EAP authentication has successfully completed using the ciphersuite negotiated between the EAP peer and authenticator. 1.3. Overview Where EAP key derivation is supported, the conversation typically takes place in three phases: Phase 0: Discovery Phase 1: Authentication 1a: EAP authentication 1b: AAA Key Transport (optional) Phase 2: Secure Association Protocol 2a: Unicast Secure Association 2b: Multicast Secure Association (optional) Of these phases, phase 0, 1b, and 2 are handled external to EAP. phases 0 and 2 are handled by the lower-layer protocol, and phase 1b is typically handled by a AAA protocol. In the discovery phase (phase 0), peers locate authenticators and discover their capabilities. A peer can locate an authenticator providing access to a particular network, or a peer can locate an authenticator behind a bridge with which it desires to establish a Secure Association. Discovery can occur manually or automatically, depending on the lower layer over which EAP runs.
The authentication phase (phase 1) can begin once the peer and authenticator discover each other. This phase, if it occurs, always includes EAP authentication (phase 1a). Where the chosen EAP method supports key derivation, in phase 1a, EAP keying material is derived on both the peer and the EAP server. An additional step (phase 1b) is needed in deployments that include a backend authentication server, in order to transport keying material from the backend authentication server to the authenticator. In order to obey the principle of mode independence (see Section 1.6.1), where a backend authentication server is present, all keying material needed by the lower layer is transported from the EAP server to the authenticator. Since existing TSK derivation and transport techniques depend solely on the MSK, in existing implementations, this is the only keying material replicated in the AAA key transport phase 1b. 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). The Secure Association Protocol exchange (phase 2) 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 between the parties is shown in Figure 1. 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 1: Conversation Overview 1.3.1. Examples Existing EAP lower layers implement phase 0, 2a, and 2b in different ways: PPP The Point-to-Point Protocol (PPP), defined in [RFC1661], does not support discovery, nor does it include a Secure Association Protocol. PPPoE PPP over Ethernet (PPPoE), defined in [RFC2516], includes support for a Discovery stage (phase 0). In this step, the EAP peer sends a PPPoE Active Discovery Initiation (PADI) packet to the broadcast address, indicating the service it is requesting. The Access Concentrator replies with a PPPoE Active Discovery Offer (PADO) packet containing its name, the service name, and an indication of the services offered by the concentrator. The discovery phase is not secured. PPPoE, like PPP, does not include a Secure Association Protocol. IKEv2 Internet Key Exchange v2 (IKEv2), defined in [RFC4306], includes support for EAP and handles the establishment of unicast security associations (phase 2a). However, the establishment of multicast security associations (phase 2b) typically does not involve EAP and needs to be handled by a group key management protocol such as Group Domain of Interpretation (GDOI) [RFC3547], Group Secure Association Key Management Protocol (GSAKMP) [RFC4535], Multimedia Internet KEYing (MIKEY) [RFC3830], or Group Key Distribution Protocol (GKDP) [GKDP]. Several mechanisms have been proposed for the discovery of IPsec security gateways. [RFC2230] discusses the use of Key eXchange (KX) Resource Records (RRs) for IPsec gateway discovery; while KX RRs are supported by many Domain Name Service (DNS) server implementations, they have not yet been widely deployed. Alternatively, DNS SRV RRs [RFC2782] can be used for this purpose. Where DNS is used for gateway location, DNS security mechanisms such as DNS Security (DNSSEC) ([RFC4033], [RFC4035]), TSIG [RFC2845], and Simple Secure Dynamic Update [RFC3007] are available. IEEE 802.11 IEEE 802.11, defined in [IEEE-802.11], handles discovery via the Beacon and Probe Request/Response mechanisms. IEEE 802.11 Access Points (APs) periodically announce their Service Set Identifiers (SSIDs) as well as capabilities using Beacon frames. Stations can
query for APs by sending a Probe Request. Neither Beacon nor Probe Request/Response frames are secured. The 4-way handshake defined in [IEEE-802.11] enables the derivation of unicast (phase 2a) and multicast/broadcast (phase 2b) secure associations. Since the group key exchange transports a group key from the AP to the station, two 4-way handshakes can be needed in order to support peer-to-peer communications. A proof of the security of the IEEE 802.11 4-way handshake, when used with EAP-TLS, is provided in [He]. IEEE 802.1X IEEE 802.1X-2004, defined in [IEEE-802.1X], does not support discovery (phase 0), nor does it provide for derivation of unicast or multicast secure associations. 1.4. EAP Key Hierarchy As illustrated in Figure 2, the EAP method key derivation 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 as well as additional information. This information is typically used outside of the EAP method to determine whether to grant access to a service. 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 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, methods derive two types of EAP keying material: (a) Keying material calculated locally by the EAP method but not exported, such as the Transient EAP Keys (TEKs). (b) Keying material exported by the EAP method: Master Session Key (MSK), Extended Master Session Key (EMSK), Initialization Vector (IV). As noted in [RFC3748] Section 7.10: 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 also MAY export the IV; however, the use of the IV is deprecated. The EMSK MUST NOT be provided to an entity outside the EAP server or peer, nor is it permitted to pass any quantity to an entity outside the EAP server or peer from which the EMSK could be computed without breaking some cryptographic assumption, such as inverting a one-way function. EAP methods supporting key derivation and mutual authentication SHOULD export a method-specific EAP conversation identifier known as the Session-Id, as well as one or more method-specific peer identifiers (Peer-Id(s)) and MAY export one or more method-specific server identifiers (Server-Id(s)). EAP methods MAY also support the import and export of channel binding parameters. EAP method specifications developed after the publication of this document MUST define the Peer-Id, Server-Id, and Session-Id. The Peer-Id(s) and Server-Id(s), when provided, identify the entities involved in generating EAP keying material. For existing EAP methods, the Peer-Id, Server-Id, and Session-Id are defined in Appendix A.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+ | | ^ | EAP Method | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | | | | | | | | | | | EAP Method Key |<->| Long-Term | | | | | Derivation | | Credential | | | | | | | | | | | | | +-+-+-+-+-+-+-+ | Local to | | | | | EAP | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Method | | | | | | | | | | | | | | | | | | | | | | | | | | | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | | | | | TEK | |MSK, EMSK | |IV | | | | | |Derivation | |Derivation | |Derivation | | | | | | | | | |(Deprecated) | | | | | +-+-+-+-+-+-+ +-+-+-+-+-+-+ +-+-+-+-+-+-+-+ | | | | ^ | | | | | | | | | | V +-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+ ---+ | | | | ^ | | | | Exported | | Peer-Id(s), | channel | MSK (64+B) | IV (64B) by | | Server-Id(s), | bindings | EMSK (64+B) | (Optional) EAP | | Session-Id | & Result | | Method | V V V V V Figure 2: EAP Method Parameter Import/Export Peer-Id If an EAP method that generates keys authenticates one or more method-specific peer identities, those identities are exported by the method as the Peer-Id(s). It is possible for more than one Peer-Id to be exported by an EAP method. Not all EAP methods provide a method-specific peer identity; where this is not defined, the Peer-Id is the null string. In EAP methods that do not support key generation, the Peer-Id MUST be the null string. Where an EAP method that derives keys does not provide a Peer-Id, the EAP server will not authenticate the identity of the EAP peer with which it derived keying material.
Server-Id If an EAP method that generates keys authenticates one or more method-specific server identities, those identities are exported by the method as the Server-Id(s). It is possible for more than one Server-Id to be exported by an EAP method. Not all EAP methods provide a method-specific server identity; where this is not defined, the Server-Id is the null string. If the EAP method does not generate keying material, the Server-Id MUST be the null string. Where an EAP method that derives keys does not provide a Server-Id, the EAP peer will not authenticate the identity of the EAP server with which it derived EAP keying material. Session-Id The Session-Id uniquely identifies an EAP session between an EAP peer (as identified by the Peer-Id) and server (as identified by the Server-Id). Where non-expanded EAP Type Codes are used (EAP Type Code not equal to 254), the EAP Session-Id is the concatenation of the single octet EAP Type Code and a temporally unique identifier obtained from the method (known as the Method-Id): Session-Id = Type-Code || Method-Id Where expanded EAP Type Codes are used, the EAP Session-Id consists of the Expanded Type Code (including the Type, Vendor-Id (in network byte order) and Vendor-Type fields (in network byte order) defined in [RFC3748] Section 5.7), concatenated with a temporally unique identifier obtained from the method (Method-Id): Session-Id = 0xFE || Vendor-Id || Vendor-Type || Method-Id The Method-Id is typically constructed from nonces or counters used within the EAP method exchange. The inclusion of the Type Code or Expanded Type Code in the EAP Session-Id ensures that each EAP method has a distinct Session-Id space. Since an EAP session is not bound to a particular authenticator or specific ports on the peer and authenticator, the authenticator port or identity are not included in the Session-Id.
Channel Binding Channel binding is the process by which lower-layer parameters are verified for consistency between the EAP peer and server. In order to avoid introducing media dependencies, EAP methods that transport channel binding parameters MUST treat this data as opaque octets. See Section 5.3.3 for further discussion. 1.4.1. Key Naming Each key created within the EAP key management framework has a name (a unique identifier), as well as a scope (the parties to whom the key is available). The scope of exported keying material and TEKs is defined by the authenticated method-specific peer identities (Peer-Id(s)) and the authenticated server identities (Server-Id(s)), where available. MSK and EMSK Names The MSK and EMSK are exported by the EAP peer and EAP server, and MUST be named using the EAP Session-Id and a binary or textual indication of the EAP keying material being referred to. PMK Name This document does not specify a naming scheme for the Pairwise Master Key (PMK). The PMK is only identified by the name of the key from which it is derived. Note: IEEE 802.11 names the PMK for the purposes of being able to refer to it in the Secure Association Protocol; the PMK name (known as the PMKID) is based on a hash of the PMK itself as well as some other parameters (see [IEEE-802.11] Section 188.8.131.52). TEK Name Transient EAP Keys (TEKs) MAY be named; their naming is specified in the EAP method specification. TSK Name Transient Session Keys (TSKs) are typically named. Their naming is specified in the lower layer so that the correct set of TSKs can be identified for processing a given packet.
1.5. Security Goals The goal of the EAP conversation is to derive fresh session keys between the EAP peer and authenticator that are known only to those parties, and for both the EAP peer and authenticator to demonstrate that they are authorized to perform their roles either by each other or by a trusted third party (the backend authentication server). Completion of an EAP method exchange (phase 1a) supporting key derivation results in the derivation of EAP keying material (MSK, EMSK, TEKs) known only to the EAP peer (identified by the Peer-Id(s)) and EAP server (identified by the Server-Id(s)). Both the EAP peer and EAP server know this keying material to be fresh. The Peer-Id and Server-Id are discussed in Sections 1.4, 2.4, and 2.5 as well as in Appendix A. Key freshness is discussed in Sections 3.4, 3.5, and 5.7. Completion of the AAA exchange (phase 1b) results in the transport of keying material from the EAP server (identified by the Server-Id(s)) to the EAP authenticator (identified by the NAS-Identifier) without disclosure to any other party. Both the EAP server and EAP authenticator know this keying material to be fresh. Disclosure issues are discussed in Sections 3.8 and 5.3; security properties of AAA protocols are discussed in Sections 5.1 - 5.9. The backend authentication server is trusted to transport keying material only to the authenticator that was established with the peer, and it is trusted to transport that keying material to no other parties. In many systems, EAP keying material established by the EAP peer and EAP server are combined with publicly available data to derive other keys. The backend authentication server is trusted to refrain from deriving these same keys or acting as a man-in-the-middle even though it has access to the keying material that is needed to do so. The authenticator is also a trusted party. The authenticator is trusted not to distribute keying material provided by the backend authentication server to any other parties. If the authenticator uses a key derivation function to derive additional keying material, the authenticator is trusted to distribute the derived keying material only to the appropriate party that is known to the peer, and no other party. When this approach is used, care must be taken to ensure that the resulting key management system meets all of the principles in [RFC4962], confirming that keys used to protect data are to be known only by the peer and authenticator.
Completion of the Secure Association Protocol (phase 2) results in the derivation or transport of Transient Session Keys (TSKs) known only to the EAP peer (identified by the Peer-Id(s)) and authenticator (identified by the NAS-Identifier). Both the EAP peer and authenticator know the TSKs to be fresh. Both the EAP peer and authenticator demonstrate that they are authorized to perform their roles. Authorization issues are discussed in Sections 4.3.2 and 5.5; security properties of Secure Association Protocols are discussed in Section 3.1. 1.6. EAP Invariants Certain basic characteristics, known as "EAP Invariants", hold true for EAP implementations: Mode independence Media independence Method independence Ciphersuite independence 1.6.1. Mode Independence EAP is typically deployed to support extensible network access authentication in situations where a peer desires network access via one or more authenticators. Where authenticators are deployed standalone, the EAP conversation occurs between the peer and authenticator, and the authenticator locally implements one or more EAP methods. However, when utilized in "pass-through" mode, EAP enables the deployment of new authentication methods without requiring the development of new code on the authenticator. While the authenticator can implement some EAP methods locally and use those methods to authenticate local users, it can 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. 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 [RFC4072]. It is a fundamental property of EAP that at the EAP method layer, the conversation between the EAP peer and server is unaffected by whether the EAP authenticator is operating in "pass-through" mode. EAP methods operate identically in all aspects, including key derivation and parameter import/export, regardless of whether or not the authenticator is operating as a pass-through.
The successful completion of an EAP method that supports key derivation results in the export of EAP keying material and parameters on the EAP peer and server. Even though the EAP peer or server can import channel binding parameters that can include the identity of the EAP authenticator, this information is treated as opaque octets. As a result, within EAP, the only relevant identities are the Peer-Id(s) and Server-Id(s). Channel binding parameters are only interpreted by the lower layer. Within EAP, the primary function of the AAA protocol is to maintain the principle of mode independence. As far as the EAP peer is concerned, its conversation with the EAP authenticator, and all consequences of that conversation, are identical, regardless of the authenticator mode of operation. 1.6.2. 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 [IEEE-802.1X], and wireless networks such as 802.11 [IEEE-802.11] and 802.16 [IEEE-802.16e]. In order to maintain media independence, it is necessary for EAP to avoid consideration 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 be restricted to identifiers associated with a particular usage environment (e.g., Medium Access Control (MAC) addresses). Note that media independence can 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. Channel binding parameters are treated as opaque octets by EAP methods so that handling them does not require media-specific knowledge.
1.6.3. 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 having to implement the EAP methods configured for use by peers. In fact, since a pass-through authenticator need not implement any EAP methods at all, it cannot be assumed to support any EAP method-specific code. As noted in [RFC3748] Section 2.3: Compliant pass-through authenticator implementations MUST by default forward EAP packets of any Type. 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 Local Area Network (WLAN) authentication [RFC4017]. However, despite this, it is possible for the peer and authenticator to interoperate as long as a suitable EAP method is supported both on the EAP peer and server. 1.6.4. Ciphersuite Independence Ciphersuite Independence is a requirement for media independence. Since lower-layer ciphersuites vary between media, media independence requires that exported EAP keying material be large enough (with sufficient entropy) to handle any ciphersuite. While EAP methods can negotiate the ciphersuite used in protection of the EAP conversation, the ciphersuite used for the protection of the data exchanged after EAP authentication has completed is negotiated between the peer and authenticator within the lower layer, outside of EAP. For example, within PPP, the ciphersuite is negotiated within the Encryption Control Protocol (ECP) defined in [RFC1968], after EAP authentication is completed. Within [IEEE-802.11], the AP ciphersuites are advertised in the Beacon and Probe Responses prior to EAP authentication and are securely verified during a 4-way handshake exchange.
Since the ciphersuites used to protect data depend on the lower layer, requiring that EAP methods have knowledge of lower-layer ciphersuites would compromise the principle of media independence. As a result, methods export EAP keying material that is ciphersuite independent. Since ciphersuite negotiation occurs in the lower layer, there is no need for lower-layer ciphersuite negotiation within EAP. In order to allow a ciphersuite to be usable within the EAP keying framework, the ciphersuite specification needs to describe how TSKs suitable for use with the ciphersuite are derived from exported EAP keying material. To maintain method independence, algorithms for deriving TSKs MUST NOT depend on the EAP method, although algorithms for TEK derivation MAY be specific to the EAP method. Advantages of ciphersuite-independence include: Reduced update requirements Ciphersuite independence enables EAP methods to be used with new ciphersuites without requiring the methods to be updated. If EAP methods were to specify how to derive transient session keys 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. Reduced EAP method complexity Ciphersuite independence enables EAP methods to avoid having to include ciphersuite-specific code. Requiring each EAP method to include ciphersuite-specific code for transient session key derivation would increase method complexity and result in duplicated effort. Simplified configuration Ciphersuite independence enables EAP method implementations on the peer and server to avoid having to configure ciphersuite-specific parameters. The ciphersuite is negotiated between the peer and authenticator outside of EAP. Where the authenticator operates in "pass-through" mode, the EAP server is not a party to this negotiation, nor is it involved in the data flow between the EAP peer and authenticator. As a result, the EAP server does not have knowledge of the ciphersuites and negotiation policies implemented by the peer and authenticator, nor is it aware of the ciphersuite negotiated between them. For example, since Encryption Control Protocol (ECP) negotiation occurs after authentication, when run over PPP, the EAP peer and
server cannot anticipate the negotiated ciphersuite, and therefore, this information cannot be provided to the EAP method.