Internet Engineering Task Force (IETF) Z. Cao Request for Comments: 6696 China Mobile Obsoletes: 5296 B. He Category: Standards Track CATR ISSN: 2070-1721 Y. Shi Q. Wu, Ed. Huawei G. Zorn, Ed. Network Zen July 2012 EAP Extensions for the EAP Re-authentication Protocol (ERP)
AbstractThe Extensible Authentication Protocol (EAP) is a generic framework supporting multiple types of authentication methods. In systems where EAP is used for authentication, it is desirable to avoid repeating the entire EAP exchange with another authenticator. This document specifies extensions to EAP and the EAP keying hierarchy to support an EAP method-independent protocol for efficient re- authentication between the peer and an EAP re-authentication server through any authenticator. The re-authentication server may be in the home network or in the local network to which the peer is connecting. This memo obsoletes RFC 5296. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6696.
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1. Introduction ....................................................4 1.1. Changes from RFC 5296 ......................................5 2. Terminology .....................................................5 3. ERP Description .................................................7 3.1. ERP with the Home ER Server ...............................10 3.2. ERP with a Local ER Server ................................11 4. ER Key Hierarchy ...............................................13 4.1. rRK Derivation ............................................13 4.2. rRK Properties ............................................14 4.3. rIK Derivation ............................................14 4.4. rIK Properties ............................................15 4.5. rIK Usage .................................................16 4.6. rMSK Derivation ...........................................16 4.7. rMSK Properties ...........................................17 5. Protocol Details ...............................................17 5.1. ERP Bootstrapping .........................................17 5.2. Steps in ERP ..............................................20 5.2.1. Multiple Simultaneous Runs of ERP ..................23 5.2.2. ERP Failure Handling ...............................23 5.3. EAP Codes .................................................25 5.3.1. EAP-Initiate/Re-auth-Start Packet ..................26 220.127.116.11. Authenticator Operation ...................27 18.104.22.168. Peer Operation ............................27 5.3.2. EAP-Initiate/Re-auth Packet ........................28 5.3.3. EAP-Finish/Re-auth Packet ..........................30 5.3.4. TV and TLV Attributes ..............................32 5.4. Replay Protection .........................................33 5.5. Channel Binding ...........................................34 6. Lower-Layer Considerations .....................................35 7. AAA Transport of ERP Messages ..................................36 8. Security Considerations ........................................36 9. IANA Considerations ............................................41 10. Contributors ..................................................41 11. Acknowledgments ...............................................42 12. References ....................................................42 12.1. Normative References .....................................42 12.2. Informative References ...................................42 Appendix A. RFC 5296 Acknowledgments ..............................45 Appendix B. Sample ERP Exchange ...................................46
RFC4187]), but method-specific re-authentication takes at least 2 round trips with the original EAP server in most cases. It is also important to note that several methods do not offer support for re-authentication. Key sharing across authenticators is sometimes used as a practical solution to lower handover times. In that case, however, the compromise of one authenticator results in the compromise of key material established via other authenticators. Other solutions for fast re-authentication exist in the literature: for example, see Lopez, et al. [MSKHierarchy]; Clancy, et al. have described the EAP re-authentication problem statement in detail [RFC5169]. In conclusion, to achieve low latency handovers, there is a need for a method-independent re-authentication protocol that completes in less than 2 round trips, preferably with a local server. This document specifies EAP Re-authentication Extensions (ERXs) for efficient re-authentication using EAP. The protocol that uses these extensions is itself referred to as the EAP Re-authentication Protocol (ERP). It supports EAP method-independent re-authentication
for a peer that has valid, unexpired key material from a previously performed EAP authentication. The protocol and the key hierarchy required for EAP re-authentication are described in this document. Note that to support ERP, lower-layer specifications may need to be revised to allow carrying EAP messages that have a code value higher than 4 and to accommodate the peer-initiated nature of ERP. Specifically, the Internet Key Exchange (IKE) protocol [RFC5996] must be updated to carry ERP messages; work is in progress on this project [IKE-EXT-for-ERP]. RFC 5296 but is fully backward compatible with that document. The changes introduced in this document focus on fixing issues that have surfaced since the publication of the original ERP specification [RFC5296]. An overview of some of the major changes is given below. o Co-location of the home EAP Re-authentication (ER) and EAP servers is no longer required (see the "ER Server" entry in Section 2). o The behavior of the authenticator and local ER server during the bootstrapping process has been clarified (Section 5.1); in particular, the authenticator and/or local ER server is now required to check for current possession of the root keys. o The authenticator is now recommended, rather than just allowed, to initiate the ERP conversation by means of the EAP-Initiate/ Re-auth-Start message (Section 22.214.171.124). In addition, many editorial changes have been made to improve the clarity of the document and to eliminate perceived ambiguities. A comprehensive list of changes is not given here for practical reasons. RFC2119].
This document uses the basic EAP terminology [RFC3748] and EMSK keying hierarchy terminology [RFC5295]. In addition, this document uses the following terms: ER Peer - An EAP peer that supports the EAP Re-authentication Protocol. All references to "peer" in this document imply an ER peer, unless specifically noted otherwise. ER Authenticator - An entity that supports the authenticator functionality for EAP re-authentication described in this document. All references to "authenticator" in this document imply an ER authenticator, unless specifically noted otherwise. ER Server - An entity that performs the server portion of ERP described here. This entity may or may not be an EAP server. All references to "server" in this document imply an ER server, unless specifically noted otherwise. An ER server is a logical entity; it may not necessarily be co-located with, or physically part of, a full EAP server. ERX - EAP re-authentication extensions. ERP - EAP Re-authentication Protocol. Uses the re-authentication extensions. rRK - re-authentication Root Key, derived from the EMSK or the Domain-Specific Root Key (DSRK). rIK - re-authentication Integrity Key, derived from the rRK. rMSK - re-authentication MSK. This is a per-authenticator key, derived from the rRK. keyName-NAI - ERP messages are integrity protected with the rIK or the DS-rIK. The use of rIK or DS-rIK for integrity protection of ERP messages is indicated by the EMSKname [RFC5295]; the protocol, which is ERP; and the realm, which indicates the domain name of the ER server. The EMSKname is copied into the username part of the Network Access Identifier (NAI). Domain - Refers to a "key management domain" as defined in Salowey, et al. [RFC5295]. For simplicity, it is referred to as "domain" in this document. The terms "home domain" and "local domain" are used to differentiate between the originating key management domain that performs the full EAP exchange with the peer and the local domain to which a peer may be attached at a given time.
RFC3748]. ERP is a single round-trip exchange between the peer and the server; it is independent of the lower layer and the EAP method used during the full EAP exchange. The ER server may be in the home domain or in the same (visited) domain as the peer and the authenticator (i.e., the local domain). Figure 1 shows the protocol exchange. The first time the peer attaches to any network, it performs a full EAP exchange (shown in Figure 2) with the EAP server; as a result, an MSK is distributed to the EAP authenticator. The MSK is then used by the authenticator and the peer to establish TSKs as needed. At the time of the initial EAP exchange, the peer and the server also derive an EMSK, which is used to derive an rRK. More precisely, an rRK is derived from the EMSK or from a DSRK, which is itself derived from the EMSK. The rRK is only available to the peer and the ER server and is never handed out to any other entity. Further, an rIK is derived from the rRK; the peer and the ER server use the rIK to provide proof of possession while performing an ERP exchange. The rIK is also never handed out to any entity and is only available to the peer and server. Peer ER Authenticator ER Server ==== ================ ========= <-- EAP-Initiate/ ----- Re-auth-Start [<-- EAP-Request/ ------ Identity] ---- EAP-Initiate/ ----> ----AAA(EAP-Initiate/ ----------> Re-auth/ Re-auth/ [Bootstrap] [Bootstrap]) <--- EAP-Finish/ ------> <---AAA(rMSK,EAP-Finish/--------- Re-auth/ Re-auth/ [Bootstrap] [Bootstrap]) Note:  brackets indicate optionality. Figure 1: ERP Exchange
EAP Peer EAP Authenticator EAP Server ======== ================= ========== <--- EAP-Request/ ------ Identity ----- EAP Response/ ---> Identity ---AAA(EAP Response/Identity)--> <--- EAP Method -------> <------ AAA(EAP Method --------> exchange exchange) <----AAA(MSK, EAP-Success)------ <---EAP-Success--------- Figure 2: EAP Authentication Two EAP codes -- EAP-Initiate and EAP-Finish -- are specified in this document for the purpose of EAP re-authentication. When the peer identifies a target authenticator that supports EAP re-authentication, it performs an ERP exchange, as shown in Figure 1; the exchange itself may happen when the peer attaches to a new authenticator supporting EAP re-authentication, or prior to attachment. The peer initiates ERP by itself; it may also do so in response to an EAP-Initiate/Re-auth-Start message from the new authenticator. The EAP-Initiate/Re-auth-Start message allows the authenticator to trigger the ERP exchange. The EAP-Finish message also can be used by the authenticator to announce the local domain name. It is plausible that the authenticator does not know whether the peer supports ERP and whether the peer has performed a full EAP authentication through another authenticator. The authenticator MAY initiate the ERP exchange by sending the EAP-Initiate/Re-auth-Start message and if there is no response MAY send the EAP-Request/Identity message. Note that this avoids having two EAP messages in flight at the same time [RFC3748]. The authenticator may send the EAP-Initiate/Re-auth-Start message and wait for a short, locally configured amount of time. This message indicates to the peer that the authenticator supports ERP. In response to this trigger from the authenticator, the peer can initiate the ERP exchange by sending an EAP-Initiate/Re-auth message. If there is no response from the peer after the necessary number of retransmissions (see Section 6), the authenticator MUST initiate EAP by sending an EAP-Request message, typically the EAP-Request/Identity message. Note that the authenticator may receive an EAP-Initiate/Re-auth message after it has sent an EAP-Request/Identity message. If the authenticator
supports ERP, it MUST proceed with the ERP exchange. When the EAP-Request/Identity times out, the authenticator MUST NOT close the connection if an ERP exchange is in progress or has already succeeded in establishing a re-authentication MSK. If the authenticator does not support ERP, it will silently discard EAP-Initiate/Re-auth messages (Section 5.3.2), since the EAP code of those packets is greater than 4 ([RFC3748], Section 4). An ERP- capable peer will exhaust the EAP-Initiate/Re-auth message retransmissions and fall back to EAP authentication by responding to EAP-Request/Identity messages from the authenticator. If the peer does not support ERP or if it does not have unexpired key material from a previous EAP authentication, it drops EAP-Initiate/ Re-auth-Start messages. If there is no response to the EAP-Initiate/ Re-auth-Start message, the authenticator SHALL send an EAP-Request message (typically EAP-Request/Identity) to start EAP authentication. From this point onward, RFC 3748 rules apply. Note that this may introduce some delay in starting EAP. In some lower layers, the delay can be minimized or even avoided by the peer initiating EAP by sending messages such as EAPoL-Start [IEEE_802.1X]. The peer sends an EAP-Initiate/Re-auth message that contains the keyName-NAI to identify the ER server's domain and the rIK used to protect the message, and a sequence number for replay protection. The EAP-Initiate/Re-auth message is integrity protected with the rIK. The authenticator uses the realm in the keyName-NAI field to send the message to the appropriate ER server. The server uses the keyName to look up the rIK. The server, after verifying proof of possession of the rIK and freshness of the message, derives an rMSK from the rRK using the sequence number as an input to the key derivation. The server then updates the expected sequence number to the received sequence number plus one. In response to the EAP-Initiate/Re-auth message, the server sends an EAP-Finish/Re-auth message; this message is integrity protected with the rIK. The server transports the rMSK along with this message to the authenticator. The rMSK is transported in a manner similar to that of the MSK along with the EAP-Success message in a full EAP exchange. Hoeper, et al. [RFC5749] discuss an additional key distribution protocol that can be used to transport the rRK from an EAP server to one of many different ER servers that share a trust relationship with the EAP server. The peer MAY request the rMSK lifetime from the server. If so, the ER server sends the rMSK lifetime in the EAP-Finish/Re-auth message.
In an ERP bootstrap exchange, the peer MAY ask the server for the rRK lifetime. If so, the ER server sends the rRK lifetime in the EAP-Finish/Re-auth message. The peer verifies the sequence number and the integrity of the message. It then uses the sequence number in the EAP-Finish/Re-auth message to compute the rMSK. The lower-layer security association protocol is ready to be triggered after this point. The ER server is located either in the home domain or in the visited domain. When the ER server is in the home domain and there is no local ER server in the visited domain, the peer and the server use the rIK and rRK derived from the EMSK; and when the ER server is in the local domain, they use the DS-rIK and DS-rRK corresponding to the local domain. The domain of the ER server is identified by the realm portion of the keyName-NAI in ERP messages. Figure 1. Peer ER Authenticator Home ER Server ==== ================ ============== <-- EAP-Initiate/ ----- Re-auth-Start [<-- EAP-Request/ ------ Identity] ---- EAP-Initiate/ ----> ----AAA(EAP-Initiate/ ----------> Re-auth/ Re-auth/ Bootstrap Bootstrap) <--- EAP-Finish/ ------> <---AAA(rMSK,EAP-Finish/--------- Re-auth/ Re-auth/ Bootstrap Bootstrap) Figure 3: ER Explicit Bootstrapping Exchange/ERP with the Home ER Server
Figure 4 shows the ER implicit bootstrapping exchange through a local ER server; Figure 5 shows ERP with a local ER server. EAP Authenticator Local AAA Agent Peer /ER Authenticator /Local ER Server Home EAP Server ==== ================== ================ =============== <-- EAP-Request/ -- Identity -- EAP Response/--> Identity --AAA(EAP Response/--> Identity, --AAA(EAP Response/ --> [domain name]) Identity, [DSRK Request, domain name]) <------------------------ EAP Method exchange------------------> <---AAA(MSK, DSRK, ---- EMSKname, EAP-Success) <--- AAA(MSK, ----- EAP-Success) <---EAP-Success----- Figure 4: Implicit Bootstrapping ERP Exchange, Initial EAP Exchange
Peer ER Authenticator Local ER Server ==== ================ =============== <-- EAP-Initiate/ -------- Re-auth-Start [<-- EAP-Request/ --------- Identity] ---- EAP-Initiate/ -------> ----AAA(EAP-Initiate/ --------> Re-auth Re-auth) <--- EAP-Finish/ ---------- <---AAA(rMSK,EAP-Finish/------- Re-auth Re-auth) Figure 5: Local ERP Exchange As shown in Figure 4, the local ER server may be present in the path of the full EAP exchange (e.g., this may be one of the AAA entities, such as AAA proxies, in the path between the EAP authenticator and the home EAP server of the peer). In that case, the local ER server requests the DSRK by sending the domain name to the home EAP server by means of a AAA message. In response, the home EAP server computes the DSRK by following the procedure specified in RFC 5295 and sends the DSRK and the key name, EMSKname, to the ER server in the claimed domain (i.e., the local ER server). The local domain is responsible for announcing that same domain name to the peer via a lower layer (for example, through DHCP-based local domain name discovery [RFC6440] or through the EAP-Initiate/Re-auth-Start message with the local ER server). After receiving the DSRK and the EMSKname, the local ER server computes the DS-rRK and the DS-rIK from the DSRK as defined in Sections 4.1 and 4.3 below. After receiving the domain name, the peer also derives the DSRK, the DS-rRK, and the DS-rIK. These keys are referred to by a keyName-NAI formed as follows: the username part of the NAI is the EMSKname, and the realm portion of the NAI is the domain name. Both parties also maintain a sequence number (initialized to zero) corresponding to the specific keyName-NAI. If the peer subsequently attaches to an authenticator within the local domain, it may perform an ERP exchange with the local ER server to obtain an rMSK for the new authenticator. ERP with the local ER server is similar to the ERP exchange illustrated in Figure 1.
Section 4.1. For the purpose of rRK derivation, this document specifies derivation of a Usage-Specific Root Key (USRK) or a Domain- Specific USRK (DSUSRK) [RFC5295] for re-authentication. The USRK designated for re-authentication is the rRK. A DSUSRK designated for re-authentication is the DS-rRK available to a local ER server in a particular domain. For simplicity, the keys are referred to without the DS label in the rest of the document. However, the scope of the various keys is limited to just the respective domains for which they are derived, in the case of the domain-specific keys. Based on the ER server with which the peer performs the ERP exchange, it knows the corresponding keys that must be used. The rRK is used to derive an rIK and rMSKs for one or more authenticators. The figure below shows the key hierarchy with the rRK, rIK, and rMSKs. rRK | +--------+--------+ | | | rIK rMSK1 ...rMSKn Figure 6: Re-authentication Key Hierarchy The derivations in this document are from RFC 5295. Key derivations and field encodings, where unspecified, default to that document. RFC 5295.
rRK = KDF (K, S), where K = EMSK or K = DSRK and S = rRK Label | "\0" | length The rRK Label is an IANA-assigned 8-bit ASCII string: EAP Re-authentication Root Key@ietf.org assigned from the "USRK Key Labels" name space in accordance with the policy stated in RFC 5295. The Key Derivation Function (KDF) and algorithm agility for the KDF are as defined in RFC 5295. An rRK derived from the DSRK is referred to as a DS-rRK in the rest of the document. All of the key derivation and properties specified in this section remain the same.
The rIK is derived as follows: rIK = KDF (K, S), where K = rRK and S = rIK Label | "\0" | cryptosuite | length The rIK Label is the 8-bit ASCII string: Re-authentication Integrity Key@ietf.org The length field refers to the length of the rIK in octets and is encoded as specified in RFC 5295. The cryptosuite and length of the rIK are part of the input to the KDF to ensure cryptographic separation of keys if different rIKs of different lengths (for example, for use with different Message Authentication Code (MAC) algorithms) are derived from the same rRK. The cryptosuite is encoded as an 8-bit number; see Section 5.3.2 for the cryptosuite specification. The rIK is referred to by the EMSKname-NAI within the context of ERP messages. The username part of the EMSKname-NAI is the EMSKname; the realm is the domain name of the ER server. In the case of ERP with the home ER server, the peer uses the realm from its original NAI; in the case of a local ER server, the peer uses the domain name received at the lower layer or through an ERP bootstrapping exchange. An rIK derived from a DS-rRK is referred to as a DS-rIK in the rest of the document. All of the key derivation and properties specified in this section remain the same.
o The rIK is cryptographically separate from any other keys derived from the rRK. o The lifetime of the rIK is never greater than that of its parent key. The rIK MUST be expired when the EMSK expires and MUST be removed from use at that time. RFC2104]. In the case of cipher-based MAC algorithms, if the required key length is less than 32 octets, the rIK is hashed using HMAC-SHA256 and the first k octets of the output are used, where k is the key length required by the algorithm. If the required key length is more than 32 octets, the first k octets of the rIK are used by the cipher-based MAC algorithm.
The rMSK Label is the 8-bit ASCII string: Re-authentication Master Session Key@ietf.org The length field refers to the length of the rMSK in octets and is encoded as specified in RFC 5295. SEQ is the sequence number sent by the peer in the EAP-Initiate/ Re-auth message. This field is encoded as a 16-bit number in network byte order (see Section 5.3.2). An rMSK derived from a DS-rRK is referred to as a DS-rIK in the rest of the document. The key derivation and properties specified in this section remain the same.
if not, the ER-capable authenticator or local ER server SHOULD include its domain name in the AAA message encapsulating the first EAP Response message sent by the peer and request the DSRK from the home EAP server during the initial EAP exchange. If such an EAP exchange is successful, the home EAP server sends the DSRK for the specified local AAA client or agent (derived using the EMSK and the domain name as specified in RFC 5295), EMSKname, and DSRK lifetime along with the EAP-Success message. The local AAA client or agent MUST extract the DSRK, EMSKname, and DSRK lifetime (if present) before forwarding the EAP-Success message to the peer. Note that the MSK (also present with the EAP-Success message) is extracted by the EAP authenticator as usual. The peer learns the domain name through the EAP-Initiate/Re-auth-Start message or by means of a lower-layer announcement (for example, DHCP [RFC6440]). When the domain name is available to the peer during or after the full EAP authentication, it attempts to use ERP when it associates with a new authenticator. If the peer knows there is no local ER server present in the visited domain, it SHOULD initiate ERP explicit bootstrapping (ERP exchange with the bootstrap flag turned on) with the home ER server to obtain the rRK. The peer MAY also initiate bootstrapping to fetch information such as the rRK lifetime from the AAA server. The following steps describe the ERP explicit bootstrapping process: o The peer sends the EAP-Initiate/Re-auth message with the bootstrapping flag set (1). The bootstrap message is always sent to the home ER server, and the keyName-NAI attribute in the bootstrap message is constructed as follows: the username portion of the NAI contains the EMSKname, and the realm portion contains the home domain name. o In addition, the message MUST contain a sequence number for replay protection, a cryptosuite, and an integrity checksum. The cryptosuite indicates the authentication algorithm. The integrity checksum indicates that the message originated at the claimed entity, the peer indicated by the Peer-ID, or the rIKname. o The peer MAY additionally set the lifetime flag to request the key lifetimes. o Upon receipt of the EAP-Initiate/Re-auth message from a peer, the ERP-capable authenticator verifies whether it has the local domain name and valid key material corresponding to the peer. If it knows the local domain name and has valid key material corresponding to the peer, it MUST be able to respond directly in the same way as the home ER does, with the local domain name included. If not, it copies the contents of the keyName-NAI into
the appropriate AAA attribute and may include its domain name in the AAA message encapsulating the EAP-Initiate/Re-auth message sent by the peer. o Upon receipt of an EAP-Initiate/Re-auth message, the home ER server verifies whether the message is fresh or is a replay by evaluating whether the received sequence number is equal to or greater than the expected sequence number for that rIK. The home ER server then verifies that the cryptosuite used by the peer is acceptable. Next, it verifies the integrity of the message by looking up the rIK and checking the integrity checksum contained in the Authentication Tag field. If any of the checks fail, the home ER server sends an EAP-Finish/Re-auth message with the Result flag set to '1'. Please refer to Section 5.2.2 for details on failure handling. This error MUST NOT have any correlation to any EAP-Success message that may have been received by the EAP authenticator and the peer earlier. If the EAP-Initiate/Re-auth message is well formed and valid, the server prepares the EAP-Finish/Re-auth message. The bootstrap flag MUST be set to indicate that this is a bootstrapping exchange. The message contains the following fields: * A sequence number for replay protection. * The same keyName-NAI as in the EAP-Initiate/Re-auth message. * If the lifetime flag was set in the EAP-Initiate/Re-auth message, the ER server SHOULD include the rRK lifetime and the rMSK lifetime in the EAP-Finish/Re-auth message. The server may have a local policy for the network to maintain and enforce lifetime unilaterally. In such cases, the server need not respond to the peer's request for the lifetime. * If the bootstrap flag is set, the ER server MUST include the domain name to which the DSRK is being sent along with the EAP-Finish/Re-auth message. * If the ER server verifies the authorization of a local ER server, it MAY include the Authorization Indication TLV to indicate to the peer that the server that received the DSRK and that is advertising the domain included in the Domain name TLV is authorized. * An authentication tag MUST be included to prove that the EAP-Finish/Re-auth message originates at a server that possesses the rIK corresponding to the EMSKname-NAI.
o If the home ER server is involved in the ERP exchange and the ERP exchange is successful, the home ER server SHOULD request the DSRK from the home EAP server; the home EAP server MUST provide the DSRK for the home ER server (derived using the EMSK and the domain name as specified in RFC 5295), EMSKname, and DSRK lifetime for inclusion in the AAA message. The home ER server SHOULD obtain them before sending the EAP-Finish/Re-auth message. o In addition, the rMSK is sent along with the EAP-Finish/Re-auth message in a AAA attribute (for an example, see Bournelle, et al. [DIAMETER-ERP]). o The authenticator receives the rMSK. o When the peer receives an EAP-Finish/Re-auth message with the bootstrap flag set, if a local domain name is present, it MUST use that name to derive the appropriate DSRK, DS-rRK, DS-rIK, and keyName-NAI, and initialize the replay counter for the DS-rIK. If not, the peer SHOULD derive the domain-specific keys using the domain name it learned via the lower layer or from the EAP-Initiate/Re-auth-Start message. If the peer does not know the domain name, it must assume that there is no local ER server available. o The peer MAY also verify the Authorization Indication TLV. o The procedures for encapsulating ERP and obtaining relevant keys using Diameter are specified in Bournelle, et al. [DIAMETER-ERP]. Since the ER bootstrapping exchange is typically done immediately following the full EAP exchange, it is feasible that the process is completed through the same entity that served as the EAP authenticator for the full EAP exchange. In this case, the lower layer may already have established TSKs based on the MSK received earlier. The lower layer may then choose to ignore the rMSK that was received with the ER bootstrapping exchange. Alternatively, the lower layer may choose to establish a new TSK using the rMSK. In either case, the authenticator and the peer know which key is used based on whether or not a TSK establishment exchange is initiated. The bootstrapping exchange may also be carried out via a new authenticator, in which case, the rMSK received SHOULD trigger a lower-layer TSK establishment exchange.
consisting of an EAP-Initiate/Re-auth and an EAP-Finish/Re-auth message. The ERP exchange may be performed with a local ER server (when one is present) or with the original EAP server. It is plausible for the network to trigger the EAP re-authentication process, however. An ERP-capable authenticator SHOULD send an EAP-Initiate/Re-auth-Start message to indicate support for ERP. The peer may or may not wait for these messages to arrive to initiate the EAP-Initiate/Re-auth message. The EAP-Initiate/Re-auth-Start message SHOULD be sent by an ERP- capable authenticator. The authenticator may retransmit it a few times until it receives an EAP-Initiate/Re-auth message in response from the peer. The EAP-Initiate/Re-auth message from the peer may have originated before the peer receives either an EAP-Request/ Identity or an EAP-Initiate/Re-auth-Start message from the authenticator. Hence, the Identifier value in the EAP-Initiate/ Re-auth message is independent of the Identifier value in the EAP-Initiate/Re-auth-Start or EAP-Request/Identity messages. Operational Considerations at the Peer: ERP requires that the peer maintain retransmission timers for reliable transport of EAP re-authentication messages. The reliability considerations of Section 4.3 of RFC 3748 apply with the peer as the retransmitting entity. ERP has the following steps: o The ERP-capable authenticator sends the EAP-Initiate/Re-auth-Start message to trigger the ERP exchange. o The peer sends an EAP-Initiate/Re-auth message. At a minimum, the message SHALL include the following fields: * a 16-bit sequence number for replay protection. * keyName-NAI as a TLV attribute to identify the rIK used to integrity protect the message. * cryptosuite to indicate the authentication algorithm used to compute the integrity checksum. * authentication tag computed over the message. o When the peer is performing ERP with a local ER server, it MUST use the corresponding DS-rIK it shares with the local ER server. The peer SHOULD set the lifetime flag to request the key lifetimes
from the server. The peer can use the rRK lifetime to know when to trigger an EAP method exchange and the rMSK lifetime to know when to trigger another ERP exchange. o The authenticator copies the contents of the value field of the keyName-NAI TLV into an appropriate attribute (e.g., User-Name [RFC2865]) in the AAA message to the ER server. o The ER server uses the keyName-NAI to look up the rIK. It MUST first verify whether the sequence number is equal to or greater than the expected sequence number. If the ER server supports a sequence number window size greater than 1, it MUST verify whether the sequence number falls within the window and has not been received before. The ER server MUST then verify that the cryptosuite used by the peer is acceptable. The ER server then proceeds to verify the integrity of the message using the rIK, thereby verifying proof of possession of that key by the peer. If any of these verifications fail, the ER server MUST send an EAP-Finish/Re-auth message with the Result flag set to '1' (Failure). Please refer to Section 5.2.2 for details on failure handling. Otherwise, it MUST compute an rMSK from the rRK using the sequence number as the additional input to the key derivation. o In response to a well-formed EAP-Initiate/Re-auth message, the ER server MUST send an EAP-Finish/Re-auth message with the following contents: * a 16-bit sequence number for replay protection, which MUST be the same as the received sequence number. The local copy of the sequence number MUST be incremented by 1. If the ER server supports multiple simultaneous ERP exchanges, it MUST instead update the sequence number window. * keyName-NAI as a TLV attribute to identify the rIK used to integrity protect the message. * cryptosuite to indicate the authentication algorithm used to compute the integrity checksum. * authentication tag computed over the message. * If the lifetime flag was set in the EAP-Initiate/Re-auth message, the ER server SHOULD include the rRK lifetime and the rMSK lifetime.
o The ER server causes the rMSK along with this message to be transported to the authenticator. The rMSK is transported in a manner similar to the MSK and the EAP-Success message in a regular EAP exchange. o The peer looks up the sequence number to verify whether it is expecting an EAP-Finish/Re-auth message with that sequence number protected by the keyName-NAI. It then verifies the integrity of the message. If the verifications fail, the peer logs an error and stops the process; otherwise, it proceeds to the next step. o The peer uses the sequence number to compute the rMSK. o The lower-layer security association protocol can be triggered at this point. Section 4.3 using the new cryptosuite. If the server does not have a valid rIK for the peer, the EAP-Finish/ Re-auth message indicating a failure will be unauthenticated; the server MAY include a list of acceptable cryptosuites in the message.
The peer, upon receiving an EAP-Finish/Re-auth message with the Result flag set to '1', MUST verify the sequence number and, if possible, the authentication tag to determine the validity of the message. If the peer supports the cryptosuite, it MUST verify the integrity of the received EAP-Finish/Re-auth message. If the EAP-Finish message contains a TLV of Type 5, the peer SHOULD retry the ERP exchange with a cryptosuite picked from the list included by the server. The peer MUST use the appropriate rIK for the subsequent ERP exchange by computing it with the corresponding cryptosuite, as specified in Section 4.3. If the Pseudo-Random Function (PRF) in the chosen cryptosuite is different from the PRF originally used by the peer, it MUST derive a new DSRK (if required), rRK, and rIK before proceeding with the subsequent ERP exchange. If the peer cannot verify the integrity of the received message, it MAY choose to retry the ERP exchange with one of the cryptosuites in the list of acceptable cryptosuites (in a TLV of Type 5), after a failure has been clearly determined following the procedure in the next paragraph. If the replay or integrity checks fail, the failure message may have been sent by an attacker. It may also mean that the server and peer do not support the same cryptosuites; however, the peer cannot determine if that is the case. Hence, the peer SHOULD continue the ERP exchange per the retransmission timers before declaring a failure. When the peer runs explicit bootstrapping (ERP with the bootstrapping flag on), there may not be a local ER server available to send a DSRK Request and the domain name. In that case, the server cannot send the DSRK and MUST NOT include the Domain name TLV. When the peer receives a response in the bootstrapping exchange without a Domain name TLV, it assumes that there is no local ER server. The home ER server sends an rMSK to the ER authenticator, however, and the peer SHALL run the TSK establishment protocol as usual.
RFC3748]. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Type-Data ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Figure 7: EAP Packet Code Two code values are defined for the purpose of ERP: 5 Initiate 6 Finish Identifier The Identifier field is one octet. The Identifier field MUST be the same if an EAP-Initiate packet is retransmitted due to a timeout while waiting for an EAP-Finish message. Any new (non-retransmission) EAP-Initiate message MUST use a new Identifier field. The Identifier field of the EAP-Finish message MUST match that of the currently outstanding EAP-Initiate message. A peer or authenticator receiving an EAP-Finish message whose Identifier value does not match that of the currently outstanding EAP-Initiate message MUST silently discard the packet. In order to avoid confusion between new EAP-Initiate messages and retransmissions, the peer must choose an Identifier value that is different from the previous EAP-Initiate message, especially if that exchange has not finished. It is RECOMMENDED that the authenticator clear EAP Re-auth state after 300 seconds.
Type This field indicates that this is an ERP exchange. Two type values are defined in this document for this purpose -- Re-auth-Start (Type 1) and Re-auth (Type 2). Type-Data The Type-Data field varies according to the value of the Type field in the re-authentication packet. Figure 8. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Reserved | 1 or more TVs or TLVs ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: EAP-Initiate/Re-auth-Start Packet Type = 1. Reserved: MUST be zero. Set to zero on transmission and ignored on reception. One or more Type/Values (TVs) or TLVs are used to convey information to the peer; for instance, the authenticator may send the domain name to the peer. TVs or TLVs: In the TV payloads, there is a 1-octet type payload and a value with type-specific length. In the TLV payloads, there is a 1-octet type payload and a 1-octet length payload. The length field indicates the length of the value expressed in number of octets. Domain name: This is a TLV payload. The Type is 4. The domain name is to be used as the realm in an NAI [RFC4282]. The Domain name TLV SHOULD be present in an EAP-Initiate/ Re-auth-Start message.
In addition, channel binding information MAY be included; see Section 5.5 for discussion. See Figure 12 for parameter specification.
Figure 9. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |R|B|L| Reserved| SEQ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 or more TVs or TLVs ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cryptosuite | Authentication Tag ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: EAP-Initiate/Re-auth Packet Type = 2. Flags 'R' - The R flag is set to 0 and ignored upon reception. 'B' - The B flag is used as the bootstrapping flag. If the flag is turned on, the message is a bootstrap message. 'L' - The L flag is used to request the key lifetimes from the server. The remaining 5 bits are set to 0 on transmission and ignored on reception. SEQ: An unsigned 16-bit sequence number is used for replay protection. The SEQ field is initialized to 0 every time a new rRK is derived. The field is encoded in network byte order. TVs or TLVs: In the TV payloads, there is a 1-octet type payload and a value with type-specific length. In the TLV payloads, there is a 1-octet type payload and a 1-octet length payload. The length field indicates the length of the value expressed in number of octets. keyName-NAI: This is carried in a TLV payload. The Type is 1. The NAI is variable in length, not exceeding 253 octets. The EMSKname is in the username part of the NAI and is encoded in hexadecimal values. The EMSKname is 64 bits in
length, and so the username portion takes up 16 octets. If the rIK is derived from the EMSK, the realm part of the NAI is the home domain name, and if the rIK is derived from a DSRK, the realm part of the NAI is the domain name used in the derivation of the DSRK. The NAI syntax is specified in Aboba, et al. [RFC4282]. Exactly one keyName-NAI attribute SHALL be present in an EAP-Initiate/Re-auth packet. In addition, channel binding information MAY be included; see Section 5.5 for discussion. See Figure 12 for parameter specification. Cryptosuite: This field indicates the integrity algorithm used for ERP. Key lengths and output lengths are either indicated or are obvious from the cryptosuite name. We specify some cryptosuites below: * 0 RESERVED * 1 HMAC-SHA256-64 * 2 HMAC-SHA256-128 * 3 HMAC-SHA256-256 HMAC-SHA256-128 is mandatory to implement and SHOULD be enabled in the default configuration. Authentication Tag: This field contains the integrity checksum over the ERP packet, excluding the Authentication Tag field itself. The length of the field is indicated by the cryptosuite.
Figure 10. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |R|B|L| Reserved | SEQ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 or more TVs or TLVs ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cryptosuite | Authentication Tag ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: EAP-Finish/Re-auth Packet Type = 2. Flags 'R' - The R flag is used as the Result flag. When set to 0, it indicates success, and when set to '1', it indicates a failure. 'B' - The B flag is used as the bootstrapping flag. If the flag is turned on, the message is a bootstrap message. 'L' - The L flag is used to indicate the presence of the rRK lifetime TLV. The remaining 5 bits are set to 0 on transmission and ignored on reception. SEQ: An unsigned 16-bit sequence number is used for replay protection. The SEQ field is initialized to 0 every time a new rRK is derived. The field is encoded in network byte order.
TVs or TLVs: In the TV payloads, there is a 1-octet type payload and a value with type-specific length. In the TLV payloads, there is a 1-octet type payload and a 1-octet length payload. The length field indicates the length of the value expressed in number of octets. keyName-NAI: This is carried in a TLV payload. The Type is 1. The NAI is variable in length, not exceeding 253 octets. EMSKname is in the username part of the NAI and is encoded in hexadecimal values. The EMSKname is 64 bits in length, and so the username portion takes up 16 octets. If the rIK is derived from the EMSK, the realm part of the NAI is the home domain name, and if the rIK is derived from a DSRK, the realm part of the NAI is the domain name used in the derivation of the DSRK. The NAI syntax is specified in [RFC4282]. Exactly one instance of the keyName-NAI attribute SHALL be present in an EAP-Finish/Re-auth message. rRK Lifetime: This is a TV payload. The Type is 2. The value field contains an unsigned 32-bit integer in network byte order representing the lifetime of the rRK in seconds. If the 'L' flag is set, the rRK Lifetime attribute SHOULD be present. rMSK Lifetime: This is a TV payload. The Type is 3. The value field contains an unsigned 32-bit integer in network byte order representing the lifetime of the rMSK in seconds. If the 'L' flag is set, the rMSK Lifetime attribute SHOULD be present. Domain name: This is a TLV payload. The Type is 4. The domain name is to be used as the realm in an NAI [RFC4282]. The Domain name attribute MUST be present in an EAP-Finish/ Re-auth message if the bootstrapping flag is set and if the local ER server sent a DSRK Request. List of cryptosuites: This is a TLV payload. The Type is 5. The value field contains a list of cryptosuites, each of size 1 octet. The cryptosuite values are as specified in Figure 9. The server SHOULD include this attribute if the cryptosuite used in the EAP-Initiate/Re-auth message was not acceptable and the message is being rejected. The server MAY include this attribute in other cases. The server MAY use this attribute to signal its cryptographic algorithm capabilities to the peer.
Authorization Indication: This is a TLV payload. The Type is 6. This attribute MAY be included in the EAP-Finish/ Re-auth message when a DSRK is delivered to a local ER server and if the home EAP server can verify the authorization of the local ER server to advertise the domain name included in the domain TLV in the same message. The value field in the TLV contains an authentication tag computed over the entire packet, starting from the first bit of the code field to the last bit of the Cryptosuite field, with the value field of the Authorization Indication TLV filled with all 0s for the computation. The key used for the computation MUST be derived from the EMSK with key label "DSRK Delivery Authorized Key@ietf.org" and optional data containing an ASCII string representing the key management domain for which the DSRK is being derived. In addition, channel binding information MAY be included: see Section 5.5 for discussion. See Figure 12 for parameter specification. The server sends this information so that the peer can verify the information seen at the lower layer, if channel binding is to be supported. Cryptosuite: This field indicates the integrity algorithm and the PRF used for ERP. Key lengths and output lengths are either indicated or are obvious from the cryptosuite name. Authentication Tag: This field contains the integrity checksum over the ERP packet, excluding the Authentication Tag field itself. The length of the field is indicated by the cryptosuite. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: TV Attribute Format
The TLV attributes that may be present in the EAP-Initiate or EAP-Finish messages are of the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: TLV Attribute Format The following Types are defined in this document: '1' - keyName-NAI: This is a TLV payload. '2' - rRK Lifetime: This is a TV payload. '3' - rMSK Lifetime: This is a TV payload. '4' - Domain name: This is a TLV payload. '5' - Cryptosuite list: This is a TLV payload. '6' - Authorization Indication: This is a TLV payload. The TLV type range of 128-191 is reserved to carry channel binding information in the EAP-Initiate/Re-auth and EAP-Finish/Re-auth messages. Below are the current assignments (all of them are TLVs): '128' - Called-Station-Id [RFC2865] '129' - Calling-Station-Id [RFC2865] '130' - NAS-Identifier [RFC2865] '131' - NAS-IP-Address [RFC2865] '132' - NAS-IPv6-Address [RFC3162] The length field indicates the length of the value part of the attribute in octets.
uses a sequence number value of zero or higher. Note that when the sequence number wraps back to zero, the rIK MUST be changed by running a full EAP authentication. The server expects a sequence number of zero or higher. When the server receives an EAP-Initiate/ Re-auth message, it uses the same sequence number in the EAP-Finish/ Re-auth message. The server then sets the expected sequence number to the received sequence number plus 1. The server MUST accept sequence numbers greater than or equal to the expected sequence number. If the peer sends an EAP-Initiate/Re-auth message but does not receive a response, it retransmits the request (with no changes to the message itself) a preconfigured number of times before giving up. However, it is plausible that the server itself may have responded to the message and the response was lost in transit. Thus, the peer MUST increment the sequence number and use the new sequence number to send subsequent EAP re-authentication messages. The peer SHOULD increment the sequence number by 1; however, it may choose to increment by a larger number. If the sequence number wraps back to zero, the peer MUST run full EAP authentication. Section 7.15 of [RFC3748]). The TLV type range of 128-191 is reserved to carry CB information in the EAP-Initiate/ Re-auth and EAP-Finish/Re-auth messages. Called-Station-Id, Calling-Station-Id, NAS-Identifier, NAS-IP-Address, and NAS-IPv6-Address are some examples of channel binding information listed in RFC 3748, and they are assigned values 128-132. Additional values are managed by IANA, based on IETF Review (formerly called "IETF Consensus") [RFC5226]. The authenticator MAY provide CB information to the peer via the EAP-Initiate/Re-auth-Start message. The peer sends the information to the server in the EAP-Initiate/Re-auth message; the server verifies whether the authenticator identity available via AAA attributes is the same as the identity provided to the peer. If the peer does not include the CB information in the EAP-Initiate/ Re-auth message, and if the local ER server's policy requires channel binding support, it SHALL send the CB attributes for the peer's verification. The peer attempts to verify the CB information if the authenticator has sent the CB parameters, and it proceeds with the lower-layer security association establishment if the attributes match. Otherwise, the peer SHALL NOT proceed with the lower-layer security association establishment.
RFC3748]. Where the EAP-Initiate message is sent over a reliable lower layer, the retransmission timer SHOULD be set to an infinite value so that retransmissions do not occur at the EAP layer. Please refer to RFC 3748 for additional guidance on setting timers. The Identifier value in the EAP-Finish/Re-auth packet is the same as the Identifier value in the EAP-Initiate/Re-auth packet. If an authenticator receives a valid duplicate EAP-Initiate/Re-auth message for which it has already sent an EAP-Finish/Re-auth message, it MUST resend the EAP-Finish/Re-auth message without reprocessing the EAP-Initiate/Re-auth message. To facilitate this, the authenticator SHALL store a copy of the EAP-Finish/Re-auth message for a finite amount of time. The actual value of time is a local matter; this specification recommends a value of 100 milliseconds. The lower layer may provide facilities for exchanging information between the peer and the authenticator about support for ERP, for the authenticator to send the domain name information and channel binding information to the peer.
Note that to support ERP, lower-layer specifications may need to be revised. Specifically, RFC 5996 must be updated to include EAP code values higher than 4 in order to use ERP with Internet Key Exchange Protocol version 2 (IKEv2). IKEv2 may also be updated to support peer-initiated ERP for optimized operation. Other lower layers may need similar revisions. Our analysis indicates that some EAP implementations are not RFC 3748 compliant in that instead of silently dropping EAP packets with code values higher than 4, they may consider it an error. To accommodate such non-compliant EAP implementations, additional guidance has been provided below. Furthermore, it may not be easy to upgrade all the peers in some cases. In such cases, authenticators may be configured to not send EAP-Initiate/Re-auth-Start messages; peers may learn whether an authenticator supports ERP via configuration or from advertisements at the lower layer. In order to accommodate implementations that are not compliant to RFC 3748, such lower layers SHOULD ensure that both parties support ERP; this is trivial, for instance, when using a lower layer that is known to always support ERP. For lower layers where ERP support is not guaranteed, ERP support may be indicated through signaling (e.g., piggybacked on a beacon) or through negotiation. Alternatively, clients may recognize environments where ERP is available based on preconfiguration. Other similar mechanisms may also be used. When ERP support cannot be verified, lower layers may mandate falling back to full EAP authentication to accommodate EAP implementations that are not compliant to RFC 3748. RFC5749] and Bournelle, et al. [DIAMETER-ERP]. RFC4962]. Cryptographic algorithm independence ERP satisfies this requirement. The algorithm chosen by the peer for the MAC generation is indicated in the EAP-Initiate/ Re-auth message. If the chosen algorithm is unacceptable, the EAP server returns an EAP-Finish/Re-auth message indicating a failure. Algorithm agility for the KDF is specified in Salowey, et al. [RFC5295]. Only when the algorithms used are
deemed acceptable does the server proceed with the derivation of keys and verification of the proof of possession of relevant key material presented by the peer. A full-blown negotiation of algorithms cannot be provided in a single round-trip protocol. Hence, while the protocol provides algorithm agility, it does not provide true negotiation. Strong, fresh session keys ERP results in the derivation of strong, fresh keys that are unique for the given session. An rMSK is always derived on demand when the peer requires a key with a new authenticator. The derivation ensures that the compromise of one rMSK does not result in the compromise of another rMSK at any time. Limited key scope The scope of all the keys derived by ERP is well defined. The rRK and rIK are never shared with any entity and always remain on the peer and the server. The rMSK is provided only to the authenticator through which the peer performs the ERP exchange. No other authenticator is authorized to use that rMSK. Replay detection mechanism For replay protection of ERP messages, a sequence number associated with the rIK is used. The sequence number is maintained by the peer and the server and is initialized to zero when the rIK is generated. The peer increments the sequence number by one after it sends an ERP message. The server sets the expected sequence number to the received sequence number plus one after verifying the validity of the received message and responds to the message. Authenticating all parties ERP provides mutual authentication of the peer and the server. Both parties need to possess the key material that resulted from a previous EAP exchange in order to successfully derive the required keys. Also, both the EAP re-authentication Response and the EAP re-authentication Information messages are integrity protected so that the peer and the server can verify each other. When the ERP exchange is executed with a local ER server, the peer and the local server mutually authenticate each other via that exchange in the same manner. The peer and the authenticator authenticate each other in the secure association protocol executed by the lower layer, just as in the case of a regular EAP exchange.
Peer and authenticator authorization The peer and authenticator demonstrate possession of the same key material without disclosing it, as part of the lower-layer secure association protocol. Channel binding with ERP may be used to verify consistency of the identities exchanged, when the identities used in the lower layer differ from those exchanged within the AAA protocol. Key material confidentiality The peer and the server derive the keys independently using parameters known to each entity. The AAA server sends the DSRK of a domain to the corresponding local ER server via the AAA protocol. Likewise, the ER server sends the rMSK to the authenticator via the AAA protocol. Note that compromise of the DSRK results in compromise of all keys derived from it. Moreover, there is no forward secrecy within ERP. Thus, compromise of a DSRK retroactively compromises all ERP keys. It is RECOMMENDED that the AAA protocol be protected using IPsec or Transport Layer Security (TLS) so that the keys are protected in transit. Note, however, that keys may be exposed to AAA proxies along the way, and compromise of any of those proxies may result in compromise of keys being transported through them. The home EAP server MUST NOT hand out a given DSRK to a local domain server more than once, unless it can verify that the entity receiving the DSRK after the first time is the same entity that received the DSRK originally. If the home EAP server verifies authorization of a local domain server, it MAY hand out the DSRK to that domain more than once. In this case, the home EAP server includes the Authorization Indication TLV to assure the peer that DSRK delivery is secure. Confirming cryptosuite selection Cryptographic algorithms for integrity and key derivation in the context of ERP MAY be the same as that used by the EAP method. In that case, the EAP method is responsible for confirming the cryptosuite selection. Furthermore, the cryptosuite is included in the ERP exchange by the peer and confirmed by the server. The protocol allows the server to reject the cryptosuite selected by the peer and provide alternatives. When a suitable rIK is not available for the
peer, the alternatives may be sent in an unprotected fashion. The peer is allowed to retry the exchange using one of the allowed cryptosuites. However, in this case, any en route modifications to the list sent by the server will go undetected. If the server does have an rIK available for the peer, the list will be provided in a protected manner and this issue does not apply. Uniquely named keys All keys produced within the context of ERP can be referred to uniquely as specified in this document. Also, the key names do not reveal any part of the key material. Preventing the domino effect The compromise of one peer does not result in the compromise of key material held by any other peer in the system. Also, the rMSK is meant for a single authenticator and is not shared with any other authenticator. Hence, the compromise of one authenticator does not lead to the compromise of sessions or keys held by any other authenticator in the system, and ERP thereby allows prevention of the domino effect by appropriately defining key scope. However, if keys are transported using hop-by-hop protection, compromise of a proxy may result in compromise of key material, e.g., the DSRK being sent to a local ER server. Binding a key to its context All the keys derived for ERP are bound to the appropriate context using appropriate key labels. The lifetime of a child key is less than or equal to that of its parent key as specified in RFC 4962 [RFC4962]. The key usage, lifetime, and the parties that have access to the keys are specified. Confidentiality of identity Deployments where privacy is a concern may find that the use of the rIKname-NAI to route ERP messages serves their privacy requirements. Note that it is plausible to associate multiple runs of ERP messages, since the rIKname is not changed as part of ERP. There was no consensus for that requirement at the time of development of this specification. If the rIKname is not used and the Peer-ID is used instead, the ERP exchange will reveal the Peer-ID over the wire.
Authorization restriction All the derived keys are limited in lifetime by that of the parent key or by server policy. Any domain-specific keys are further restricted for use only in the domain for which the keys are derived. All the keys specified in this document are meant for use in ERP only. Other restrictions on the use of session keys may be imposed by the specific lower layer but are out of scope for this specification. Preventing a DoS attack A denial-of-service (DoS) attack on the peer may be possible when using the EAP-Initiate/Re-auth message. An attacker may send a bogus EAP-Initiate/Re-auth message, which may be carried by the authenticator in a AAA-Request to the server; in response, the server may send in a AAA reply an EAP-Finish/ Re-auth message indicating failure. Note that such attacks may be possible with the EAPoL-Start capability of IEEE 802.11 and other similar facilities in other link layers and where the peer can initiate EAP authentication. An attacker may use such messages to start an EAP method run, which fails and may result in the server sending a rejection message, thus resulting in the link-layer connections being terminated. To prevent such DoS attacks, an ERP failure should not result in deletion of any authorization state established by a full EAP exchange. Alternatively, the lower layers and AAA protocols may define mechanisms to allow two link-layer Security Associations (SAs) derived from different EAP key material for the same peer to exist so that smooth migration from the current link-layer SA to the new one is possible during rekey. These mechanisms prevent the link-layer connections from being terminated when a re-authentication procedure fails due to a bogus EAP-Initiate/Re-auth message. Key material transport When a DSRK is sent from the home EAP server to a local domain server or when an rMSK is sent from an ER server to an authenticator, in the absence of end-to-end security between the entity that is sending the key and the entity receiving the key, it is plausible for other entities to get access to keys being sent to an ER server in another domain. This mode of key transport is similar to that of MSK transport in the context of EAP authentication. We further observe that ERP is for access authentication and does not support end-to-end data security. In typical implementations, the traffic is in the clear beyond
the access control enforcement point (the authenticator or an entity delegated by the authenticator for access control enforcement). The model works as long as entities in the middle of the network do not use keys intended for other parties to steal service from an access network. If that is not achievable, key delivery must be protected in an end-to-end manner. RFC5296] -- performed the following IANA [IANA] actions: 1. It registered Packet Codes "Initiate" and "Finish" in the EAP Registry. Those codes are referred to as "EAP-Initiate" and "EAP-Finish" throughout this document. 2. It created a Message Types table in the EAP Registry and registered several items in that table. Those items are referred to as "Re-auth-start" and "Re-auth" throughout this document. 3. It created an EAP-Initiate and Finish Attributes table in the EAP registry and registered several items in that table. Those items are recorded in this document in Section 5.3.4. 4. It created a Re-authentication Cryptosuites table in the EAP registry and registered several items in that table. Those items are recorded in this document at the end of Section 5.3.2. 5. It registered two items in the USRK Key Labels registry: * Re-auth usage label "EAP Re-authentication Root Key@ietf.org", recorded in this document in Section 4.1. * DSRK-authorized delivery key "DSRK Delivery Authorized Key@ietf.org", recorded in this document in the description of "Authorization Indication" in Section 5.3.3. Section 9 and, as Applications Area Director, insisted upon its inclusion as a condition of publication.
RFC 5296; thanks to all who participated in that effort (see Appendix A). In addition, thanks to Yaron Sheffer, Sebastien Decugis, Ralph Droms, Stephen Farrell, Charlie Kaufman, and Yoav Nir for (mostly) useful comments and review. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The Network Access Identifier", RFC 4282, December 2005. [RFC5295] Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri, "Specification for the Derivation of Root Keys from an Extended Master Session Key (EMSK)", RFC 5295, August 2008. [DIAMETER-ERP] Bournelle, J., Morand, L., Decugis, S., Wu, Q., and G. Zorn, "Diameter Support for the EAP Re-authentication Protocol (ERP)", Work in Progress, June 2012. [IANA] "Internet Assigned Numbers Authority", <http://www.iana.org/>. [IEEE_802.1X] Institute of Electrical and Electronics Engineers, "IEEE Standard for Local and Metropolitan Area Networks: Port-Based Network Access Control", IEEE Std 802.1X-2010, February 2010.
[IKE-EXT-for-ERP] Nir, Y. and Q. Wu, "An IKEv2 Extension for Supporting ERP", Work in Progress, May 2012. [MSKHierarchy] Lopez, R., Skarmeta, A., Bournelle, J., Laurent- Maknavicus, M., and J. Combes, "Improved EAP keying framework for a secure mobility access service", IWCMC '06, Proceedings of the 2006 International Conference on Wireless Communications and Mobile Computing, New York, NY, USA, 2006. [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC 3162, August 2001. [RFC4187] Arkko, J. and H. Haverinen, "Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA)", RFC 4187, January 2006. [RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication, Authorization, and Accounting (AAA) Key Management", BCP 132, RFC 4962, July 2007. [RFC5169] Clancy, T., Nakhjiri, M., Narayanan, V., and L. Dondeti, "Handover Key Management and Re-Authentication Problem Statement", RFC 5169, March 2008. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5296] Narayanan, V. and L. Dondeti, "EAP Extensions for EAP Re-authentication Protocol (ERP)", RFC 5296, August 2008. [RFC5749] Hoeper, K., Ed., Nakhjiri, M., and Y. Ohba, Ed., "Distribution of EAP-Based Keys for Handover and Re-Authentication", RFC 5749, March 2010.