2. Lower-Layer Operation On completion of EAP authentication, EAP keying material and parameters exported by the EAP method are provided to the lower layer and AAA layer (if present). These include the Master Session Key (MSK), Extended Master Session Key (EMSK), Peer-Id(s), Server-Id(s), and Session-Id. The Initialization Vector (IV) is deprecated, but might be provided. In order to preserve the security of EAP keying material derived within methods, lower layers MUST NOT export keys passed down by EAP methods. This implies that EAP keying material passed down to a lower layer is for the exclusive use of that lower layer and MUST NOT be used within another lower layer. This prevents compromise of one lower layer from compromising other applications using EAP keying material. EAP keying material provided to a lower layer MUST NOT be transported to another entity. For example, EAP keying material passed down to the EAP peer lower layer MUST NOT leave the peer; EAP keying material passed down or transported to the EAP authenticator lower layer MUST NOT leave the authenticator. On the EAP server, keying material and parameters requested by and passed down to the AAA layer MAY be replicated to the AAA layer on the authenticator (with the exception of the EMSK). On the authenticator, the AAA layer provides the replicated keying material and parameters to the lower layer over which the EAP authentication conversation took place. This enables mode independence to be maintained. The EAP layer, as well as the peer and authenticator layers, MUST NOT modify or cache keying material or parameters (including channel bindings) passing in either direction between the EAP method layer and the lower layer or AAA layer. 2.1. Transient Session Keys Where explicitly supported by the lower layer, lower layers MAY cache keying material, including exported EAP keying material and/or TSKs; the structure of this key cache is defined by the lower layer. So as to enable interoperability, new lower-layer specifications MUST describe key caching behavior. Unless explicitly specified by the lower layer, the EAP peer, server, and authenticator MUST assume that
peers and authenticators do not cache keying material. Existing EAP lower layers and AAA layers handle the generation of transient session keys and caching of EAP keying material in different ways: IEEE 802.1X-2004 When used with wired networks, IEEE 802.1X-2004 [IEEE-802.1X] does not support link-layer ciphersuites, and as a result, it does not provide for the generation of TSKs or caching of EAP keying material and parameters. Once EAP authentication completes, it is assumed that EAP keying material and parameters are discarded; on IEEE 802 wired networks, there is no subsequent Secure Association Protocol exchange. Perfect Forward Secrecy (PFS) is only possible if the negotiated EAP method supports this. PPP PPP, defined in [RFC1661], does not include support for a Secure Association Protocol, nor does it support caching of EAP keying material or parameters. PPP ciphersuites derive their TSKs directly from the MSK, as described in [RFC2716] Section 3.5. This is NOT RECOMMENDED, since if PPP were to support caching of EAP keying material, this could result in TSK reuse. As a result, once the PPP session is terminated, EAP keying material and parameters MUST be discarded. Since caching of EAP keying material is not permitted within PPP, there is no way to handle TSK re-key without EAP re-authentication. Perfect Forward Secrecy (PFS) is only possible if the negotiated EAP method supports this. IKEv2 IKEv2, defined in [RFC4306], only uses the MSK for authentication purposes and not key derivation. The EMSK, IV, Peer-Id, Server-Id or Session-Id are not used. As a result, the TSKs derived by IKEv2 are cryptographically independent of the EAP keying material and re-key of IPsec SAs can be handled without requiring EAP re-authentication. Within IKEv2, it is possible to negotiate PFS, regardless of which EAP method is negotiated. IKEv2 as specified in [RFC4306] does not cache EAP keying material or parameters; once IKEv2 authentication completes, it is assumed that EAP keying material and parameters are discarded. The Session-Timeout Attribute is therefore interpreted as a limit on the VPN session time, rather than an indication of the MSK key lifetime. IEEE 802.11 IEEE 802.11 enables caching of the MSK, but not the EMSK, IV, Peer-Id, Server-Id, or Session-Id. More details about the structure of the cache are available in [IEEE-802.11]. In IEEE
802.11, TSKs are derived from the MSK using a Secure Association Protocol known as the 4-way handshake, which includes a nonce exchange. This guarantees TSK freshness even if the MSK is reused. The 4-way handshake also enables TSK re-key without EAP re-authentication. PFS is only possible within IEEE 802.11 if caching is not enabled and the negotiated EAP method supports PFS. IEEE 802.16e IEEE 802.16e, defined in [IEEE-802.16e], supports caching of the MSK, but not the EMSK, IV, Peer-Id, Server-Id, or Session-Id. IEEE 802.16e supports a Secure Association Protocol in which TSKs are chosen by the authenticator without any contribution by the peer. The TSKs are encrypted, authenticated, and integrity protected using the MSK and are transported from the authenticator to the peer. TSK re-key is possible without EAP re-authentication. PFS is not possible even if the negotiated EAP method supports it. AAA Existing implementations and specifications for RADIUS/EAP [RFC3579] or Diameter EAP [RFC4072] do not support caching of keying material or parameters. In existing AAA clients, proxy and server implementations, exported EAP keying material (MSK, EMSK, and IV), as well as parameters and derived keys are not cached and MUST be presumed lost after the AAA exchange completes. In order to avoid key reuse, the AAA layer MUST delete transported keys once they are sent. The AAA layer MUST NOT retain keys that it has previously sent. For example, a AAA layer that has transported the MSK MUST delete it, and keys MUST NOT be derived from the MSK from that point forward. 2.2. Authenticator and Peer Architecture This specification does not impose constraints on the architecture of the EAP authenticator or peer. For example, any of the authenticator architectures described in [RFC4118] can be used. As a result, lower layers need to identify EAP peers and authenticators unambiguously, without incorporating implicit assumptions about peer and authenticator architectures.
For example, it is possible for multiple base stations and a "controller" (e.g., WLAN switch) to comprise a single EAP authenticator. In such a situation, the "base station identity" is irrelevant to the EAP method conversation, except perhaps as an opaque blob to be used in channel binding. Many base stations can share the same authenticator identity. An EAP authenticator or peer: (a) can contain one or more physical or logical ports; (b) can advertise itself as one or more "virtual" authenticators or peers; (c) can utilize multiple CPUs; (d) can support clustering services for load balancing or failover. Both the EAP peer and authenticator can have more than one physical or logical port. A peer can simultaneously access the network via multiple authenticators, or via multiple physical or logical ports on a given authenticator. Similarly, an authenticator can offer network access to multiple peers, each via a separate physical or logical port. When a single physical authenticator advertises itself as multiple virtual authenticators, it is possible for a single physical port to belong to multiple virtual authenticators. An authenticator can be configured to communicate with more than one EAP server, each of which is configured to communicate with a subset of the authenticators. The situation is illustrated in Figure 3. 2.3. Authenticator Identification The EAP method conversation is between the EAP peer and server. The authenticator identity, if considered at all by the EAP method, is treated as an opaque blob for the purpose of channel binding (see Section 5.3.3). However, the authenticator identity is important in two other exchanges - the AAA protocol exchange and the Secure Association Protocol conversation. The AAA conversation is between the EAP authenticator and the backend authentication server. From the point of view of the backend authentication server, keying material and parameters are transported to the EAP authenticator identified by the NAS-Identifier Attribute. Since an EAP authenticator MUST NOT share EAP keying material or parameters with another party, if the EAP peer or backend authentication server detects use of EAP keying material and parameters outside the scope defined by the NAS-Identifier, the keying material MUST be considered compromised.
The Secure Association Protocol conversation is between the peer and the authenticator. For lower layers that support key caching, it is particularly important for the EAP peer, authenticator, and backend server to have a consistent view of the usage scope of the transported keying material. In order to enable this, it is RECOMMENDED that the Secure Association Protocol explicitly communicate the usage scope of the EAP keying material passed down to the lower layer, rather than implicitly assuming that this is defined by the authenticator and peer endpoint addresses. +-+-+-+-+ | EAP | | Peer | +-+-+-+-+ | | | Peer Ports / | \ / | \ / | \ / | \ / | \ / | \ / | \ / | \ Authenticator | | | | | | | | | Ports +-+-+-+-+ +-+-+-+-+ +-+-+-+-+ | | | | | | | Auth1 | | Auth2 | | Auth3 | | | | | | | +-+-+-+-+ +-+-+-+-+ +-+-+-+-+ \ | \ | \ | \ | \ | \ | EAP over AAA \ | \ | (optional) \ | \ | \ | \ | \ | \ | \ | \ | +-+-+-+-+-+ +-+-+-+-+-+ Backend | EAP | | EAP | Authentication | Server1 | | Server2 | Servers +-+-+-+-+-+ +-+-+-+-+-+ Figure 3: Relationship between EAP Peer, Authenticator, and Server Since an authenticator can have multiple ports, the scope of the authenticator key cache cannot be described by a single endpoint address. Similarly, where a peer can have multiple ports and sharing of EAP keying material and parameters between peer ports of the same
link type is allowed, the extent of the peer key cache cannot be communicated by using a single endpoint address. Instead, it is RECOMMENDED that the EAP peer and authenticator consistently identify themselves utilizing explicit identifiers, rather than endpoint addresses or port identifiers. AAA protocols such as RADIUS [RFC3579] and Diameter [RFC4072] provide a mechanism for the identification of AAA clients; since the EAP authenticator and AAA client MUST be co-resident, this mechanism is applicable to the identification of EAP authenticators. RADIUS [RFC2865] requires that an Access-Request packet contain one or more of the NAS-Identifier, NAS-IP-Address, and NAS-IPv6-Address attributes. Since a NAS can have more than one IP address, the NAS-Identifier Attribute is RECOMMENDED for explicit identification of the authenticator, both within the AAA protocol exchange and the Secure Association Protocol conversation. Problems that can arise where the peer and authenticator implicitly identify themselves using endpoint addresses include the following: (a) It is possible that the peer will not be able to determine which authenticator ports are associated with which authenticators. As a result, the EAP peer will be unable to utilize the authenticator key cache in an efficient way, and will also be unable to determine whether EAP keying material has been shared outside its authorized scope, and therefore needs to be considered compromised. (b) It is possible that the authenticator will not be able to determine which peer ports are associated with which peers, preventing the peer from communicating with it utilizing multiple peer ports. (c) It is possible that the peer will not be able to determine with which virtual authenticator it is communicating. For example, multiple virtual authenticators can share a MAC address, but utilize different NAS-Identifiers. (d) It is possible that the authenticator will not be able to determine with which virtual peer it is communicating. Multiple virtual peers can share a MAC address, but utilize different Peer-Ids. (e) It is possible that the EAP peer and server will not be able to verify the authenticator identity via channel binding.
For example, problems (a), (c), and (e) occur in [IEEE-802.11], which utilizes peer and authenticator MAC addresses within the 4-way handshake. Problems (b) and (d) do not occur since [IEEE-802.11] only allows a virtual peer to utilize a single port. The following steps enable lower-layer identities to be securely verified by all parties: (f) Specify the lower-layer parameters used to identify the authenticator and peer. As noted earlier, endpoint or port identifiers are not recommended for identification of the authenticator or peer when it is possible for them to have multiple ports. (g) Communicate the lower-layer identities between the peer and authenticator within phase 0. This allows the peer and authenticator to determine the key scope if a key cache is utilized. (h) Communicate the lower-layer authenticator identity between the authenticator and backend authentication server within the NAS- Identifier Attribute. (i) Include the lower-layer identities within channel bindings (if supported) in phase 1a, ensuring that they are communicated between the EAP peer and server. (j) Support the integrity-protected exchange of identities within phase 2a. (k) Utilize the advertised lower-layer identities to enable the peer and authenticator to verify that keys are maintained within the advertised scope. 2.3.1. Virtual Authenticators When a single physical authenticator advertises itself as multiple virtual authenticators, if the virtual authenticators do not maintain logically separate key caches, then by authenticating to one virtual authenticator, the peer can gain access to the other virtual authenticators sharing a key cache.
For example, where a physical authenticator implements "Guest" and "Corporate Intranet" virtual authenticators, an attacker acting as a peer could authenticate with the "Guest" virtual authenticator and derive EAP keying material. If the "Guest" and "Corporate Intranet" virtual authenticators share a key cache, then the peer can utilize the EAP keying material derived for the "Guest" network to obtain access to the "Corporate Intranet" network. The following steps can be taken to mitigate this vulnerability: (a) Authenticators are REQUIRED to cache associated authorizations along with EAP keying material and parameters and to apply authorizations to the peer on each network access, regardless of which virtual authenticator is being accessed. This ensures that an attacker cannot obtain elevated privileges even where the key cache is shared between virtual authenticators, and a peer obtains access to one virtual authenticator utilizing a key cache entry created for use with another virtual authenticator. (b) It is RECOMMENDED that physical authenticators maintain separate key caches for each virtual authenticator. This ensures that a cache entry created for use with one virtual authenticator cannot be used for access to another virtual authenticator. Since a key cache entry can no longer be shared between virtual authentications, this step provides protection beyond that offered in (a). This is valuable in situations where authorizations are not used to enforce access limitations. For example, where access is limited using a filter installed on a router rather than using authorizations provided to the authenticator, a peer can gain unauthorized access to resources by exploiting a shared key cache entry. (c) It is RECOMMENDED that each virtual authenticator identify itself consistently to the peer and to the backend authentication server, so as to enable the peer to verify the authenticator identity via channel binding (see Section 5.3.3). (d) It is RECOMMENDED that each virtual authenticator identify itself distinctly, in order to enable the peer and backend authentication server to tell them apart. For example, this can be accomplished by utilizing a distinct value of the NAS- Identifier Attribute. 2.4. Peer Identification As described in [RFC3748] Section 7.3, the peer identity provided in the EAP-Response/Identity can be different from the peer identities authenticated by the EAP method. For example, the identity provided
in the EAP-Response/Identity can be a privacy identifier as described in "The Network Access Identifier" [RFC4282] Section 2. As noted in [RFC4284], it is also possible to utilize a Network Access Identifier (NAI) for the purposes of source routing; an NAI utilized for source routing is said to be "decorated" as described in [RFC4282] Section 2.7. When the EAP peer provides the Network Access Identity (NAI) within the EAP-Response/Identity, as described in [RFC3579], the authenticator copies the NAI included in the EAP-Response/Identity into the User-Name Attribute included within the Access-Request. As the Access-Request is forwarded toward the backend authentication server, AAA proxies remove decoration from the NAI included in the User-Name Attribute; the NAI included within the EAP-Response/Identity encapsulated in the Access-Request remains unchanged. As a result, when the Access-Request arrives at the backend authentication server, the EAP-Response/Identity can differ from the User-Name Attribute (which can have some or all of the decoration removed). In the absence of a Peer-Id, the backend authentication server SHOULD use the contents of the User-Name Attribute, rather than the EAP-Response/Identity, as the peer identity. It is possible for more than one Peer-Id to be exported by an EAP method. For example, a peer certificate can contain more than one peer identity; in a tunnel method, peer identities can be authenticated within both an outer and inner exchange, and these identities could be different in type and contents. For example, an outer exchange could provide a Peer-Id in the form of a Relative Distinguished Name (RDN), whereas an inner exchange could identify the peer via its NAI or MAC address. Where EAP keying material is determined solely from the outer exchange, only the outer Peer-Id(s) are exported; where the EAP keying material is determined from both the inner and outer exchanges, then both the inner and outer Peer-Id(s) are exported by the tunnel method.
2.5. Server Identification It is possible for more than one Server-Id to be exported by an EAP method. For example, a server certificate can contain more than one server identity; in a tunnel method, server identities could be authenticated within both an outer and inner exchange, and these identities could be different in type and contents. For example, an outer exchange could provide a Server-Id in the form of an IP address, whereas an inner exchange could identify the server via its Fully-Qualified Domain Name (FQDN) or hostname. Where EAP keying material is determined solely from the outer exchange, only the outer Server-Id(s) are exported by the EAP method; where the EAP keying material is determined from both the inner and outer exchanges, then both the inner and outer Server-Id(s) are exported by the EAP method. As shown in Figure 3, an authenticator can be configured to communicate with multiple EAP servers; the EAP server that an authenticator communicates with can vary according to configuration and network and server availability. While the EAP peer can assume that all EAP servers within a realm have access to the credentials necessary to validate an authentication attempt, it cannot assume that all EAP servers share persistent state. Authenticators can be configured with different primary or secondary EAP servers, in order to balance the load. Also, the authenticator can dynamically determine the EAP server to which requests will be sent; in the event of a communication failure, the authenticator can fail over to another EAP server. For example, in Figure 3, Authenticator2 can be initially configured with EAP server1 as its primary backend authentication server, and EAP server2 as the backup, but if EAP server1 becomes unavailable, EAP server2 can become the primary server. In general, the EAP peer cannot direct an authentication attempt to a particular EAP server within a realm, this decision is made by AAA clients, nor can the peer determine with which EAP server it will be communicating, prior to the start of the EAP method conversation. The Server-Id is not included in the EAP-Request/Identity, and since the EAP server may be determined dynamically, it typically is not possible for the authenticator to advertise the Server-Id during the discovery phase. Some EAP methods do not export the Server-Id so that it is possible that the EAP peer will not learn with which server it was conversing after the EAP conversation completes successfully. As a result, an EAP peer, on connecting to a new authenticator or reconnecting to the same authenticator, can find itself communicating with a different EAP server. Fast reconnect, defined in [RFC3748]
Section 7.2, can fail if the EAP server with which the peer communicates is not the same one with which it initially established a security association. For example, an EAP peer attempting an EAP-TLS session resume can find that the new EAP-TLS server will not have access to the TLS Master Key identified by the TLS Session-Id, and therefore the session resumption attempt will fail, requiring completion of a full EAP-TLS exchange. EAP methods that export the Server-Id MUST authenticate the server. However, not all EAP methods supporting mutual authentication provide a non-null Server-Id; some methods only enable the EAP peer to verify that the EAP server possesses a long-term secret, but do not provide the identity of the EAP server. In this case, the EAP peer will know that an authenticator has been authorized by an EAP server, but will not confirm the identity of the EAP server. Where the EAP method does not provide a Server-Id, the peer cannot identify the EAP server with which it generated keying material. This can make it difficult for the EAP peer to identify the location of a key possessed by that EAP server. As noted in [RFC5216] Section 5.2, EAP methods supporting authentication using server certificates can determine the Server-Id from the subject or subjectAltName fields in the server certificate. Validating the EAP server identity can help the EAP peer to decide whether a specific EAP server is authorized. In some cases, such as where the certificate extensions defined in [RFC4334] are included in the server certificate, it can even be possible for the peer to verify some channel binding parameters from the server certificate. It is possible for problems to arise in situations where the EAP server identifies itself differently to the EAP peer and authenticator. For example, it is possible that the Server-Id exported by EAP methods will not be identical to the Fully Qualified Domain Name (FQDN) of the backend authentication server. Where certificate-based authentication is used within RADIUS or Diameter, it is possible that the subjectAltName used in the backend authentication server certificate will not be identical to the Server-Id or backend authentication server FQDN. This is not normally an issue in EAP, as the authenticator will be unaware of the identities used between the EAP peer and server. However, this can be an issue for key caching, if the authenticator is expected to locate a backend authentication server corresponding to a Server-Id provided by an EAP peer. Where the backend authentication server FQDN differs from the subjectAltName in the backend authentication server certificate, it is possible that the AAA client will not be able to determine whether it is talking to the correct backend authentication server. Where
the Server-Id and backend authentication server FQDN differ, it is possible that the combination of the key scope (Peer-Id(s), Server- Id(s)) and EAP conversation identifier (Session-Id) will not be sufficient to determine where the key resides. For example, the authenticator can identify backend authentication servers by their IP address (as occurs in RADIUS), or using a Fully Qualified Domain Name (as in Diameter). If the Server-Id does not correspond to the IP address or FQDN of a known backend authentication server, then it may not be possible to locate which backend authentication server possesses the key. 3. Security Association Management EAP, as defined in [RFC3748], supports key derivation, but does not provide for the management of lower-layer security associations. Missing functionality includes: (a) Security Association negotiation. EAP does not negotiate lower-layer unicast or multicast security associations, including cryptographic algorithms or traffic profiles. EAP methods only negotiate cryptographic algorithms for their own use, not for the underlying lower layers. EAP also does not negotiate the traffic profiles to be protected with the negotiated ciphersuites; in some cases the traffic to be protected can have lower-layer source and destination addresses different from the lower-layer peer or authenticator addresses. (b) Re-key. EAP does not support the re-keying of exported EAP keying material without EAP re-authentication, although EAP methods can support "fast reconnect" as defined in [RFC3748] Section 7.2.1. (c) Key delete/install semantics. EAP does not synchronize installation or deletion of keying material on the EAP peer and authenticator. (d) Lifetime negotiation. EAP does not support lifetime negotiation for exported EAP keying material, and existing EAP methods also do not support key lifetime negotiation. (e) Guaranteed TSK freshness. Without a post-EAP handshake, TSKs can be reused if EAP keying material is cached. These deficiencies are typically addressed via a post-EAP handshake known as the Secure Association Protocol.
3.1. Secure Association Protocol Since neither EAP nor EAP methods provide for establishment of lower-layer security associations, it is RECOMMENDED that these facilities be provided within the Secure Association Protocol, including: (a) Entity Naming. A basic feature of a Secure Association Protocol is the explicit naming of the parties engaged in the exchange. Without explicit identification, the parties engaged in the exchange are not identified and the scope of the EAP keying parameters negotiated during the EAP exchange is undefined. (b) Mutual proof of possession of EAP keying material. During the Secure Association Protocol, the EAP peer and authenticator MUST demonstrate possession of the keying material transported between the backend authentication server and authenticator (e.g., MSK), in order to demonstrate that the peer and authenticator have been authorized. Since mutual proof of possession is not the same as mutual authentication, the peer cannot verify authenticator assertions (including the authenticator identity) as a result of this exchange. Authenticator identity verification is discussed in Section 2.3. (c) Secure capabilities negotiation. In order to protect against spoofing during the discovery phase, ensure selection of the "best" ciphersuite, and protect against forging of negotiated security parameters, the Secure Association Protocol MUST support secure capabilities negotiation. This includes the secure negotiation of usage modes, session parameters (such as security association identifiers (SAIDs) and key lifetimes), ciphersuites and required filters, including confirmation of security-relevant capabilities discovered during phase 0. The Secure Association Protocol MUST support integrity and replay protection of all capability negotiation messages. (d) Key naming and selection. Where key caching is supported, it is possible for the EAP peer and authenticator to share more than one key of a given type. As a result, 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. Use of the key naming mechanism described in Section 1.4.1 is RECOMMENDED. 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. (e) Generation of fresh transient session keys (TSKs). Where the lower layer supports caching of keying material, the EAP peer lower layer can initiate a new session using keying material that was derived in a previous session. Were the TSKs to be derived solely from a portion of the exported EAP keying material, this would result in reuse of the session keys that could expose the underlying ciphersuite to attack. In lower layers where caching of keying material is supported, the Secure Association Protocol phase is REQUIRED, and MUST support the derivation of fresh unicast and multicast TSKs, even when the transported keying material provided by the backend authentication server is not fresh. This is typically supported via the exchange of nonces or counters, which are then mixed with the keying material in order to generate fresh unicast (phase 2a) and possibly multicast (phase 2b) session keys. By not using exported EAP keying material directly to protect data, the Secure Association Protocol protects it against compromise. (f) Key lifetime management. This includes explicit key lifetime negotiation or seamless re-key. EAP does not support the re-keying of EAP keying material without re-authentication, and existing EAP methods do not support key lifetime negotiation. As a result, the Secure Association Protocol MAY handle the re-key and determination of the key lifetime. Where key caching is supported, secure negotiation of key lifetimes is RECOMMENDED. Lower layers that support re-key, but not key caching, may not require key lifetime negotiation. For example, a difference between IKEv1 [RFC2409] and IKEv2 [RFC4306] is that in IKEv1 SA lifetimes were negotiated; in IKEv2, each end of the SA is responsible for enforcing its own lifetime policy on the SA and re-keying the SA when necessary. (g) Key state resynchronization. It is possible for the peer or authenticator to reboot or reclaim resources, clearing portions or all of the key cache. Therefore, key lifetime negotiation cannot guarantee that the key cache will remain synchronized, and it may not be possible for the peer to determine before attempting to use a key whether it exists within the authenticator cache. It is therefore RECOMMENDED for the EAP lower layer to provide a mechanism for key state
resynchronization, either via the SAP or using a lower layer indication (see [RFC3748] Section 3.4). Where the peer and authenticator do not jointly possess a key with which to protect the resynchronization exchange, secure resynchronization is not possible, and alternatives (such as an initiation of EAP re-authentication after expiration of a timer) are needed to ensure timely resynchronization. (h) Key scope synchronization. To support key scope determination, the Secure Association Protocol SHOULD provide a mechanism by which the peer can determine the scope of the key cache on each authenticator and by which the authenticator can determine the scope of the key cache on a peer. This includes negotiation of restrictions on key usage. (i) Traffic profile negotiation. The traffic to be protected by a lower-layer security association will not necessarily have the same lower-layer source or destination address as the EAP peer and authenticator, and it is possible for the peer and authenticator to negotiate multiple security associations, each with a different traffic profile. Where this is the case, the profile of protected traffic SHOULD be explicitly negotiated. For example, in IKEv2 it is possible for an Initiator and Responder to utilize EAP for authentication, then negotiate a Tunnel Mode Security Association (SA), which permits passing of traffic originating from hosts other than the Initiator and Responder. Similarly, in IEEE 802.16e, a Subscriber Station (SS) can forward traffic to the Base Station (BS), which originates from the Local Area Network (LAN) to which it is attached. To enable this, Security Associations within IEEE 802.16e are identified by the Connection Identifier (CID), not by the EAP peer and authenticator MAC addresses. In both IKEv2 and IEEE 802.16e, multiple security associations can exist between the EAP peer and authenticator, each with their own traffic profile and quality of service parameters. (j) 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. (k) 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. See [RFC3748] Section 2.4 for more discussion. 3.2. Key Scope Absent explicit specification within the lower layer, after the completion of phase 1b, transported keying material, and parameters are bound to the EAP peer and authenticator, but are not bound to a specific peer or authenticator port. While EAP keying material passed down to the lower layer is not intrinsically bound to particular authenticator and peer ports, TSKs MAY be bound to particular authenticator and peer ports by the Secure Association Protocol. However, a lower layer MAY also permit TSKs to be used on multiple peer and/or authenticator ports, provided that TSK freshness is guaranteed (such as by keeping replay counter state within the authenticator). In order to further limit the key scope, the following measures are suggested: (a) The lower layer MAY specify additional restrictions on key usage, such as limiting the use of EAP keying material and parameters on the EAP peer to the port over which the EAP conversation was conducted. (b) The backend authentication server and authenticator MAY implement additional attributes in order to further restrict the scope of keying material. For example, in IEEE 802.11, the backend authentication server can provide the authenticator with a list of authorized Called or Calling-Station-Ids and/or SSIDs for which keying material is valid. (c) Where the backend authentication server provides attributes restricting the key scope, it is RECOMMENDED that restrictions be securely communicated by the authenticator to the peer. This can be accomplished using the Secure Association Protocol, but also can be accomplished via the EAP method or the lower layer. 3.3. Parent-Child Relationships When an EAP re-authentication takes place, new EAP keying material is exported by the EAP method. In EAP lower layers where EAP re-authentication eventually results in TSK replacement, the maximum
lifetime of derived keying material (including TSKs) can be less than or equal to that of EAP keying material (MSK/EMSK), but it cannot be greater. Where TSKs are derived from or are wrapped by exported EAP keying material, compromise of that exported EAP keying material implies compromise of TSKs. Therefore, if EAP keying material is considered stale, not only SHOULD EAP re-authentication be initiated, but also replacement of child keys, including TSKs. Where EAP keying material is used only for entity authentication but not for TSK derivation (as in IKEv2), compromise of exported EAP keying material does not imply compromise of the TSKs. Nevertheless, the compromise of EAP keying material could enable an attacker to impersonate an authenticator, so that EAP re-authentication and TSK re-key are RECOMMENDED. With respect to IKEv2, Section 5.2 of [RFC4718], "IKEv2 Clarifications and Implementation Guidelines", states: Rekeying the IKE_SA and reauthentication are different concepts in IKEv2. Rekeying the IKE_SA establishes new keys for the IKE_SA and resets the Message ID counters, but it does not authenticate the parties again (no AUTH or EAP payloads are involved)... This means that reauthentication also establishes new keys for the IKE_SA and CHILD_SAs. Therefore while rekeying can be performed more often than reauthentication, the situation where "authentication lifetime" is shorter than "key lifetime" does not make sense. Child keys that are used frequently (such as TSKs that are used for traffic protection) can expire sooner than the exported EAP keying material on which they are dependent, so that it is advantageous to support re-key of child keys prior to EAP re-authentication. Note that deletion of the MSK/EMSK does not necessarily imply deletion of TSKs or child keys. Failure to mutually prove possession of exported EAP keying material during the Secure Association Protocol exchange need not be grounds for deletion of keying material by both parties; rate-limiting Secure Association Protocol exchanges could be used to prevent a brute force attack.
3.4. 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. TEKs are typically created during an EAP conversation, used until the end of the conversation and then discarded. However, methods can re-key TEKs during an EAP conversation. When using TEKs within an EAP conversation or across conversations, it is necessary to ensure that replay protection and key separation requirements are fulfilled. For instance, if a replay counter is used, TEK re-key MUST occur prior to wrapping of the counter. Similarly, TSKs MUST remain cryptographically separate from TEKs despite TEK re-keying or caching. This prevents TEK compromise from leading directly to compromise of the TSKs and vice versa. EAP methods MAY cache local EAP keying material (TEKs) that can persist for multiple EAP conversations when fast reconnect is used [RFC3748]. For example, EAP methods based on TLS (such as EAP-TLS [RFC5216]) derive and cache the TLS Master Secret, typically for substantial time periods. The lifetime of other local EAP keying material calculated within the EAP method is defined by the method. Note that in general, when using fast reconnect, there is no guarantee that the original long-term credentials are still in the possession of the peer. For instance, a smart-card holding the private key for EAP-TLS may have been removed. EAP servers SHOULD also verify that the long-term credentials are still valid, such as by checking that certificate used in the original authentication has not yet expired. 3.5. Exported and Calculated Key Lifetimes The following mechanisms are available for communicating the lifetime of keying material between the EAP peer, server, and authenticator: AAA protocols (backend authentication server and authenticator) Lower-layer mechanisms (authenticator and peer) EAP method-specific negotiation (peer and server) Where the EAP method does not support the negotiation of the lifetime of exported EAP keying material, and a key lifetime negotiation mechanism is not provided by the lower layer, it is possible that there will not be a way for the peer to learn the lifetime of keying material. This can leave the peer uncertain of how long the authenticator will maintain keying material within the key cache. In this case the lifetime of keying material can be managed as a system parameter on the peer and authenticator; a default lifetime of 8 hours is RECOMMENDED.
3.5.1. AAA Protocols AAA protocols such as RADIUS [RFC2865] and Diameter [RFC4072] can be used to communicate the maximum key lifetime from the backend authentication server to the authenticator. The Session-Timeout Attribute is defined for RADIUS in [RFC2865] and for Diameter in [RFC4005]. Where EAP is used for authentication, [RFC3580] Section 3.17, indicates that a Session-Timeout Attribute sent in an Access-Accept along with a Termination-Action value of RADIUS-Request specifies the maximum number of seconds of service provided prior to EAP re-authentication. However, there is also a need to be able to specify the maximum lifetime of cached keying material. Where EAP pre-authentication is supported, cached keying material can be pre-established on the authenticator prior to session start and will remain there until expiration. EAP lower layers supporting caching of keying material MAY also persist that material after the end of a session, enabling the peer to subsequently resume communication utilizing the cached keying material. In these situations it can be desirable for the backend authentication server to specify the maximum lifetime of cached keying material. To accomplish this, [IEEE-802.11] overloads the Session-Timeout Attribute, assuming that it represents the maximum time after which transported keying material will expire on the authenticator, regardless of whether transported keying material is cached. An IEEE 802.11 authenticator receiving transported keying material is expected to initialize a timer to the Session-Timeout value, and once the timer expires, the transported keying material expires. Whether this results in session termination or EAP re-authentication is controlled by the value of the Termination-Action Attribute. Where EAP re-authentication occurs, the transported keying material is replaced, and with it, new calculated keys are put in place. Where session termination occurs, transported and derived keying material is deleted. Overloading the Session-Timeout Attribute is problematic in situations where it is necessary to control the maximum session time and key lifetime independently. For example, it might be desirable to limit the lifetime of cached keying material to 5 minutes while permitting a user once authenticated to remain connected for up to an hour without re-authenticating. As a result, in the future, additional attributes can be specified to control the lifetime of cached keys; these attributes MAY modify the meaning of the Session-Timeout Attribute in specific circumstances.
Since the TSK lifetime is often determined by authenticator resources, and the backend authentication server has no insight into the TSK derivation process by the principle of ciphersuite independence, it is not appropriate for the backend authentication server to manage any aspect of the TSK derivation process, including the TSK lifetime. 3.5.2. Lower-Layer Mechanisms Lower-layer mechanisms can be used to enable the lifetime of keying material to be negotiated between the peer and authenticator. This can be accomplished either using the Secure Association Protocol or within the lower-layer transport. Where TSKs are established as the result of a Secure Association Protocol exchange, it is RECOMMENDED that the Secure Association Protocol include support for TSK re-key. Where the TSK is taken directly from the MSK, there is no need to manage the TSK lifetime as a separate parameter, since the TSK lifetime and MSK lifetime are identical. 3.5.3. EAP Method-Specific Negotiation 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 deriving keys MUST provide for mutual authentication between the EAP peer and the EAP Server. However, EAP does not itself support the negotiation of lifetimes for exported EAP keying material such as the MSK, EMSK, and IV. While EAP itself does not support lifetime negotiation, it would be possible to specify methods that do. However, systems that rely on key lifetime negotiation within EAP methods would only function with these methods. Also, there is no guarantee that the key lifetime negotiated within the EAP method would be compatible with backend authentication server policy. In the interest of method independence and compatibility with backend authentication server implementations, management of the lifetime of keying material SHOULD NOT be provided within EAP methods.
3.6. Key Cache Synchronization Key lifetime negotiation alone cannot guarantee key cache synchronization. Even where a lower-layer exchange is run immediately after EAP in order to determine the lifetime of keying material, it is still possible for the authenticator to purge all or part of the key cache prematurely (e.g., due to reboot or need to reclaim memory). The lower layer can utilize the Discovery phase 0 to improve key cache synchronization. For example, if the authenticator manages the key cache by deleting the oldest key first, the relative creation time of the last key to be deleted could be advertised within the Discovery phase, enabling the peer to determine whether keying material had been prematurely expired from the authenticator key cache. 3.7. Key Strength As noted in Section 2.1, EAP lower layers determine TSKs in different ways. Where exported EAP keying material is utilized in the derivation, encryption or authentication of TSKs, it is possible for EAP key generation to represent the weakest link. In order to ensure that methods produce EAP keying material of an appropriate symmetric key strength, it is RECOMMENDED that EAP methods utilizing public key cryptography choose a public key that has a cryptographic strength providing the required level of attack resistance. This is typically provided by configuring EAP methods, since there is no coordination between the lower layer and EAP method with respect to minimum required symmetric key strength. Section 5 of BCP 86 [RFC3766] offers advice on the required RSA or DH module and DSA subgroup size in bits, for a given level of attack resistance in bits. The National Institute for Standards and Technology (NIST) also offers advice on appropriate key sizes in [SP800-57].
3.8. Key Wrap The key wrap specified in [RFC2548], which is based on an MD5-based stream cipher, has known problems, as described in [RFC3579] Section 4.3. RADIUS uses the shared secret for multiple purposes, including per-packet authentication and 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 concerns exist relating to the security of this hash [MD5Collision]. 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, [RFC3579] Section 4.2 recommends running RADIUS over IPsec. The same approach is taken in Diameter EAP [RFC4072], which in Section 4.1.3 defines the EAP-Master-Session-Key Attribute-Value Pair (AVP) in clear-text, to be protected by IPsec or TLS.