EAP-IKEv2 Method
draft-tschofenig-eap-ikev2-15.txt

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Abstract

This document specifies EAP-IKEv2, an Extensible Authentication Protocol (EAP) method that is based on the Internet Key Exchange (IKEv2) protocol. EAP-IKEv2 provides mutual authentication and session key establishment between an EAP peer and an EAP server. It supports authentication techniques that are based on passwords, high-entropy shared keys, and public key certificates. EAP-IKEv2 further provides support for cryptographic ciphersuite negotiation, hash function agility, identity confidentiality (in certain modes of operation), fragmentation, and an optional "fast reconnect" mode. EAP-IKEv2 has sucessfully passed Designated Expert Review as mandated by RFC 3748.

Table of Contents

1.  Introduction
2.  Terminology
3.  Protocol Overview
4.  Fast Reconnect
5.  Key Derivation
6.  Session ID, Peer ID, and Server ID
7.  Error Handling
8.  Specification of Protocol Fields
    8.1.  The Flags, Message Length, and Integrity Checksum Data fields
    8.2.  EAP-IKEv2 header
    8.3.  Security Association Payload
    8.4.  Key Exchange Payload
    8.5.  Nonce Payload
    8.6.  Identification Payload
    8.7.  Certificate Payload
    8.8.  Certificate Request Payload
    8.9.  Encrypted Payload
    8.10.  Authentication Payload
    8.11.  Notify Payload
    8.12.  Next Fast-ID Payload
9.  Payload Types and Extensibility
10.  Security Considerations
    10.1.  Protected Ciphersuite Negotiation
    10.2.  Mutual Authentication
    10.3.  Integrity Protection
    10.4.  Replay Protection
    10.5.  Confidentiality
    10.6.  Key Strength
    10.7.  Dictionary Attack Resistance
    10.8.  Fast Reconnect
    10.9.  Cryptographic Binding
    10.10.  Session Independence
    10.11.  Fragmentation
    10.12.  Channel Binding
    10.13.  Summary
11.  IANA Considerations
12.  Contributors
13.  Acknowledgements
14.  References
    14.1.  Normative References
    14.2.  Informative References
Appendix A.  EAP-IKEv2 Protocol Runs with Failed Authentication
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

This document specifies EAP-IKEv2, an EAP method that is based on the Internet Key Exchange Protocol version 2 (IKEv2) [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The IKEv2 protocol is also able to carry AP exchanges inside IKEv2 but this method does not inherit the capabilites to tunnel EAP methods inside this EAP method. EAP-IKEv2 provides mutual authentication and session key establishment between an EAP peer and an EAP server. It supports authentication techniques that are based on the following types of credential.

• Asymmetric key pairs: these are public/private key pairs where the public key is embedded into a digital certificate, and the corresponding private key is known only to a single party.
• Passwords: these are low-entropy bit strings that are known to both the server and the peer.
• Symmetric keys: these are high-entropy bit strings that known to both the server and the peer.

It is possible to use a different authentication credential (and thereby technique) for each direction, e.g., the EAP server may authenticate itself using a public/private key pair and the EAP client may authenticate itself using a symmetric key. In particular, the following combinations are expected to be used in practice. These are referred to as "use cases or modes" in the remainder of this document.

1. EAP server: asymmetric key pair, EAP peer: asymmetric key pair
2. EAP server: asymmetric key pair, EAP peer: symmetric key
3. EAP server: asymmetric key pair, EAP peer: password
4. EAP server: symmetric key, EAP peer: symmetric key

Note that, in use cases 2 and 4, a symmetric key is assumed to be chosen uniformly at random from its key space; it is therefore assumed that symmetric keys are not derived from passwords. Deriving a symmetric key from a password is insecure when used with mode 4 since the exchange is vulnerable to dictionary attacks, as described in more detail in Section 10.7 (Dictionary Attack Resistance). Also note that, in use case 3, the EAP server must either have access to all passwords in plaintext, or, alternatively, for each password store the value prf(password,"Key Pad for EAP-IKEv2") for all supported pseudorandom functions (also see Section 8.10 (Authentication Payload) below and Section 2.15 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.)). Other conceivable use cases are not expected to be used in practice due to key management issues, and have not been considered in this document.

The remainder of this document is structured as follows.

• Section 2 (Terminology) provides an overview of the terminology and the abbreviations used in this document.
• Section 3 (Protocol Overview) provides an overview of the full EAP-IKEv2 exchange and thereby specifies the protocol message composition.
• Section 4 (Fast Reconnect) specifies the optional EAP-IKEv2 "fast reconnect" mode of operation.
• Section 5 (Key Derivation) specifies how exportable session keys are derived.
• Section 6 (Session ID, Peer ID, and Server ID) specifies how the Session-ID, Peer-ID, and Server-ID elements are derived.
• Section 7 (Error Handling) specifies how errors that may potentially occur during protocol execution are handled.
• Section 8 (Specification of Protocol Fields) specifies the format of the EAP-IKEv2 data fields. Section 8.1 (The Flags, Message Length, and Integrity Checksum Data fields) describes how fragmentation is handled in EAP-IKEv2.
• Section 9 (Payload Types and Extensibility) specifies the payload type values and describes protocol extensibility.
• Section 10 (Security Considerations) provides a list of claimed security properties.

2.  Terminology

This document makes use of terms defined in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP) (RFC 3748),” June 2004.) and [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). In addition, the keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [3] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

A list of abbreviations that are used in this document follows.

AUTH:

Denotes a data field containing either a MAC or a signature. This field is in embedded into an Authentication payload, defined in Section 8.10 (Authentication Payload).

CERT:

Public key certificate or similar structure.

CERTREQ:

Certificate Request.

NFID:

Next Fast-ID payload (see Section 8.12 (Next Fast-ID Payload) and Section 4 (Fast Reconnect))

EMSK:

Extended Master Session Key, defined in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP) (RFC 3748),” June 2004.).

HDR:

EAP-IKEv2 header, defined in Section 8.2 (EAP-IKEv2 header).

I:

Initiator, the party that sends the first message of an EAP-IKEv2 protocol run. This is always the EAP server.

MAC:

Message Authentication Code. The result of a cryptographic operation that involves a symmetric key.

MSK:

Master Session Key, defined in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP) (RFC 3748),” June 2004.).

prf:

Pseudorandom function: a cryptographic function whose output is assumed to be indistinguishable from that of a truly random function.

R:

Responder, the party that sends the second message of an EAP-IKEv2 protocol run. This is always the EAP peer.

SA:

Security Association. In this document SA denotes a type of payload that is used for the negotiation of the cryptographic algorithms that are to be used within an EAP-IKEv2 protocol run. Specifically, SAi denotes a set of choices that are accepted by an initiator, and SAr denotes the choice of the responder.

Signature:

The result of a cryptographic operation that involves an asymmetric key. In particular, it involves the private part of a public/private key pair.

SK:

Session Key. In this document, the notation SK{x} denotes that x is embedded within an Encrypted payload, i.e.,, that x is encrypted and integrity-protected using EAP-IKEv2 internal keys. These keys are different in each direction.

SK_xx:

EAP-IKEv2 internal key, defined in Section 2.14 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

SKEYSEED:

Keying material, defined in Section 2.14 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

3.  Protocol Overview

In this section, the full EAP-IKEv2 protocol run is specified. All messages are sent between two parties, namely an EAP peer and an EAP server. In EAP-IKEv2, the EAP server always assumes the role of the initiator (I), and the EAP peer that of the responder (R) of an exchange.

The semantics and formats of EAP-IKEv2 messages are similar, albeit not identical, to those specified in IKEv2 [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) for the establishment of an IKE Security Association. The full EAP-IKEv2 protocol run consists of two roundtrips that are followed by either an EAP-Success or an EAP-Failure message. An optional roundtrip for exchanging EAP identities may precede the two exchanges.

1. R<-I: EAP-Request/Identity

2. R->I: EAP-Response/Identity(Id)

3. R<-I: EAP-Req (HDR, SAi, KEi, Ni)

4. R->I: EAP-Res (HDR, SAr, KEr, Nr, [CERTREQ], [SK{IDr}])

5. R<-I: EAP-Req (HDR, SK{IDi, [CERT], [CERTREQ], [NFID], AUTH})

6. R->I: EAP-Res (HDR, SK{IDr, [CERT], AUTH})

7. R<-I: EAP-Success

Figure 1 (EAP-IKEv2 full, successful protocol run) shows the full EAP-IKEv2 protocol run, including the optional EAP identity exchange (messages 1 and 2). A detailed specification of the message composition follows.

Messages 1 and 2 are a standard EAP Identity Request and Response, as defined in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP) (RFC 3748),” June 2004.). Message 3 is the first EAP-IKEv2-specific message. With this, the server starts the actual EAP authentication exchange. It contains the initiator SPI in the EAP-IKEv2 header (HDR) (the initiator selects this value on a per-protocol run basis), the set of cryptographic algorithms the server is willing to accept for the protection of EAP-IKEv2 traffic (encryption and integrity protection) and the derivation of the session key. This set is encoded in the Security Association payload (SAi). It also contains a Diffie-Hellman payload (KEi), and a Nonce payload (Ni).

When the peer receives message 3, it selects a set of cryptographic algorithms from the ones that are proposed in the message. In this overview, it is assumed that an acceptable such set exists (and is thus selected), and that the Diffie-Hellman value KEi belongs to an acceptable group. The peer then generates a non-zero Responder SPI value for this protocol run, its own Diffie-Hellman value (KEr) and nonce (Nr), and calculates the keys SKEYSEED, SK_d, SK_ai, SK_ar, SK_ei, SK_er, SK_pi, and SK_pr according to Section 2.14 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The peer then constructs message 4. In the context of use cases 1, 2 and 3, the peer's local policy MAY dictate the inclusion of the optional CERTREQ payload in that message, which gives a hint to the server to include a certificate for its public key in its next message. In the context of use case 4, the peer MUST include the optional SK{IDr} payload, which contains its EAP-IKEv2 identifier, encrypted and integrity-protected within an Encrypted payload. The keys used to construct this Encrypted payload are SK_er (for encryption) and SK_ar (for integrity protection), in accordance with [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The responder's EAP-IKEv2 identifier (IDr) is likely to be needed in these use cases by the server in order to select the correct symmetric key or password for the construction of the AUTH payload of message 5.

Upon reception of message 4, the server also computes SKEYSEED, SK_d, SK_ai, SK_ar, SK_ei, SK_er, SK_pi, and SK_pr according to Section 2.14 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). If an SK{IDr} payload is included, the server decrypts it and verifies its integrity with the corresponding keys. In this overview, decryption and verification is assumed to succeed. The server then constructs message 5 which contains only the EAP-IKEv2 header followed by a single Encrypted payload. The keys used to generate the encrypted payload MUST be SK_ei (for encryption) and SK_ai (for integrity protection), in accordance with [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The initiator MUST embed at least two payloads in the Encrypted Payload, as follows. An Identification payload with the initiator's EAP-IKEv2 identifer MUST be embedded in the Encrypted payload. The Authentication payload MUST be embedded in the Encrypted payload. A Certificate payload, and/or a Certificate Request payload MAY also be embedded in the Encrypted payload. Moreover, a Next Fast-Reconnect Identifier payload MAY also be embedded in the Encrypted payload. Message 5 is sent to the responder.

Upon reception of message 5, the responder (EAP peer) authenticates the initiator (EAP server). The checks that are performed to this end depend on the use case, local policies, and are specified in [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). These checks include (but may not be limited to) decrypting the Encrypted payload, verifying its integrity, and checking that the Authentication payload contains the expected value. If all checks succeed (which is assumed in this overview), then the responder constructs message 6. That message MUST contain the EAP-IKEv2 header followed by a single Encrypted payload, in which at least two further payloads MUST be embedded, as shown in Figure 1 (EAP-IKEv2 full, successful protocol run).

Upon reception of message 6, the initiator (EAP server) authenticates the responder (EAP peer). As above, the checks that are performed to this end depend on the use case, local policies, and MUST include decryption and verification of the Encrypted payload, as well as checking that the Authentication payload contains the expected value. If the optional SK{IDr} payload was included in message 4, the EAP server MUST also ensure that the IDr payload in message 6 is identical to that in message 4.

If authentication succeeds, an EAP-Success message is sent to the responder as message 7. The EAP server and the EAP peer generate a Master Session Key (MSK) and an Extended Master Session Key (EMSK) after a successful EAP-IKEv2 protocol run, according to Section 5 (Key Derivation).

4.  Fast Reconnect

This section specifies a "fast reconnect" mode of operation for EAP-IKEv2. This mode is mandatory to implement, but optional to use. The purpose of fast reconnect is to enable an efficient re-authentication procedure that also results in a fresh MSK and EMSK. The "fast reconnect" mode can only be used where an EAP-IKEv2 security context already exists at both the server and the peer, and its usage is subject to the local policies. In other words, it can only be used by an EAP server/EAP peer pair that has already performed mutual authentication in a previous EAP-IKEv2 protocol run.

The fast reconnect mode makes use of dedicated "fast reconnect EAP identifiers". The idea is that the server indicates its willingness to engage in "fast reconnect" protocol runs in the future by including the optional "Next Fast-ID" (NFID) payload in message 5 of a "full" protocol run (see Figure 1 (EAP-IKEv2 full, successful protocol run)), or in message 3 of a "fast reconnect" protocol run (see Figure 2 (Fast Reconnect Protocol Run)). This NFID payload contains a special EAP identity, denoted Fast Reconnect Identity (FRID) as the NAI in the EAP-Response/Identity message. The FRID contains an obfuscated username part and a realm part. When generating a FRID the following aspects should be considered:

The FRID and therefore the pseudonym usernames are generated by the EAP server. The EAP server produces pseudonym usernames in an implementation-dependent manner. Only the EAP server needs to be able to map the pseudonym username to the permanent identity.

EAP-IKEv2 includes no provisions to ensure that the same EAP server that generated a pseudonym username will be used on the authentication exchange when the pseudonym username is used. It is recommended that the EAP servers implement some centralized mechanism to allow all EAP servers of the home operator to map pseudonyms generated by other severs to the permanent identity. If no such mechanism is available, then the EAP server, failing to understand a pseudonym issued by another server, can request the peer to send the permanent identity.

When generating FRIDs, the server SHOULD choose a fresh and unique FRID that is different from the previous ones that were used after the same full authentication exchange. The FRID SHOULD include a random component in the username part. The random component works as a reference to the security context. Regardless of the construction method, the pseudonym username MUST conform to the grammar specified for the username portion of an NAI. Also, the FRID MUST conform to the NAI grammar [4] (Aboba, B., Beadles, M., Arkko, J., and P. Eronen, “The Network Access Identifier,” December 2005.). The EAP servers, which subscribers of an operator can use, MUST ensure that the username part of a FRIDs that they generate are unique.

The peer MAY use the FRID to indicate to start a "fast reconnect" protocol run. The EAP Identity Response MUST be sent at the beginning of a "fast reconnect" protocol run. If, in the previous successful "full" EAP-IKEv2 protocol execution, the server had not included an NFID payload in message 5 (resp. 3), then the peer MUST NOT start a fast reconnect protocol run. On reception of FRID, the server maps it to an existing EAP-IKEv2 security context. Depending on local policy, the server either proceeds with the "fast reconnect" protocol run, or proceeds with message 3 of a "full" protocol run. If the server had advertised the FRID in the previous EAP-IKEv2 protocol execution, it SHOULD proceed with a "fast reconnect" protocol run. The peer MUST be able to correctly handle a message 3 of a "full" protocol run, even if it indicated a FRID in its EAP Identity Response.

Because the peer may fail to save a FRID that was sent in the NFID payload (for example, due to malfunction), the EAP server SHOULD maintain, at least, the most recently used FRID in addition to the most recently issued FRID. If the authentication exchange is not completed successfully, then the server MUST NOT overwrite the FRID that was issued during the most recent successful authentication exchange.

The EAP-IKEv2 fast reconnect exchange is similar to the IKE-SA rekeying procedure as specified in Section 2.18 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). Thus, it uses a CREATE_CHILD_SA request and response. The SPIs on those two messages would be the SPI's negotiated on the previous exchange. During fast reconnect, the server and the peer MAY exchange fresh Diffie-Hellman values.

1. R<-I: EAP-Request/Identity

2. R->I: EAP-Response/Identity(FRID)

3. R<-I: EAP-Req(HDR, SK{SA, Ni, [KEi], [NFID]})

4. R->I: EAP-Res(HDR, SK{SA, Nr, [KEr]})

5. R<-I: EAP-Success

Figure 2 (Fast Reconnect Protocol Run) shows the message exchange for the EAP-IKEv2 fast reconnect mode. As in the full mode, the EAP server is the initiator and the EAP peer is the responder. The first two messages constitute the standard EAP identity exchange. Note that, in order to use the "fast reconnect" mode, message 2 MUST be sent. This is in order to enable the peer to indicate its "fast reconnect" identity FRID in message 2. If the server can map the FRID to an existing EAP-IKEv2 context it proceeds with message 3. Note that, in this message, the server MAY embed a NFID payload into the encrypted payload to provide a new FRID to the peer. The server MAY choose to perform a full EAP-IKEv2 run, in which case it would respond with a message that conforms to the format of message 3 in Figure 1 (EAP-IKEv2 full, successful protocol run).

Messages 3 and 4 establish a new EAP-IKEv2 security context. In message 3, the initiator MUST select a new (non-zero) value for the SPI field in each proposal substructure in the SA payload (see Section 3.3 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.)). The value of the IKE_SA Responder's SPI field in HDR MUST be the one from the previous successful EAP-IKEv2 protocol run. The nonce inside the Nonce payload (Ni) MUST be fresh, and the Diffie-Hellman value inside the Diffie-Hellman payload (if present, KEi) MUST also be fresh. If present, the Diffie-Hellman value MUST be drawn from the same group as the Diffie-Hellman value in the previous successful full EAP-IKEv2 protocol run. Note that the algorithms and keys that are used to construct the Encrypted payload in message 3, are the same as in the previous successful EAP-IKEv2 protocol run.

Upon reception of message 3, the responder (EAP peer) decrypts and verifies the Encrypted payload. If successful (as assumed in Figure 2 (Fast Reconnect Protocol Run)), it constructs message 4 in a fashion similar to the construction of message 3. The responder MUST choose a new (non-zero) value for the SPI field in each proposal substructure. Upon reception of message 4, the initiator (EAP server) decrypts and verifies the Encrypted payload. If a correct message 4 is received, then this protocol run is deemed successful, and the server responds with an EAP-Success message (message 5).

After successful EAP-IKEv2 fast reconnect protocol run, both the initiator and the responder generate fresh keying material, that is used for the protection of subsequent EAP-IKEv2 traffic. Furthermore, both the initiator and the responder MUST generate a fresh MSK and EMSK and export them.

The new EAP-IKEv2-specific keying material is computed in the same way as in the full EAP-IKEv2 protocol run, and in accordance with Section 2.18 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). That is, SKEYSEED is computed as SKEYSEED = prf(SK_d (old), [g^ir (new)] | Ni | Nr), where SK_d (old) is the key SK_d from the previous successful EAP-IKEv2 protocol run, Ni and Nr are the nonces (without the Nonce payload headers) that were exchanged in messages 3 and 4, and g^ir (new) is the newly computed Diffie-Hellman key, if both the values KEi and KEr were present in messages 3 and 4. The remaining EAP-IKEv2-specific keys (SK_d, SK_ai, SK_ar, SK_ei, SK_er, SK_pi, and SK_pr) are generated as in the full EAP-IKEv2 protocol run.

The generation of a fresh MSK and EMSK follows the generation of the EAP-IKEv2-specific keys and adheres to the rules in Section 5 (Key Derivation).

Note 1: In EAP-IKEv2, the EAP server initiates the fast reconnect mode and thereby causes fresh session keys to be established.

Note 2: It is conceivable that an adversary tries to launch a replay attack against the EAP-IKEv2 fast reconnect mode of operation. In particular, the adversary may try to send a previously captured message 3 in a subsequent fast reconnect protocol run. This replay attempt will, however, fail because the keys that the responder will use to verify and decrypt the Encrypted payload are changed with every successful reconnect protocol run.

5.  Key Derivation

This section describes how the Master Session Key (MSK) and the Extended Master Session Key (EMSK) are derived in EAP-IKEv2. It is expected that the MSK and the EMSK are exported by the EAP-IKEv2 process and be used in accordance with the EAP keying framework [7] (Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” November 2007.).

During an EAP-IKEv2 protocol run, the initiator and the responder generate a number of keys, as described above and in accordance with Section 2.14 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The generation of these keys is based on a pseudorandom function (prf) that both parties have agreed to use and which is applied in an iterative fashion. This iterative fashion is specified in Section 2.13 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) and is denoted by prf+.

In particular, following a successful EAP-IKEv2 protocol run, both parties generate 128 octets of keying material, denoted KEYMAT, as KEYMAT = prf+(SK_d, Ni | Nr), where Ni and Nr are the nonces (just payload without headers) from messages 3 and 4 shown in Figure 1 (EAP-IKEv2 full, successful protocol run) (in the context of a full EAP-IKEv2 protocol run) or Figure 2 (Fast Reconnect Protocol Run) (in the context of a fast reconnect EAP-IKEv2 protocol run). Note that only the nonces are used, i.e., not the entire Nonce payload that contains them.

The first 64 octets of KEYMAT are exported as the EAP MSK, and the second 64 octets are exported as the EMSK.

The MSK and EMSK MUST NOT be generated unless an EAP-IKEv2 protocol run completes successfully. Note that the EAP-IKEv2 method does not produce an initialisation vector [7] (Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” November 2007.).

6.  Session ID, Peer ID, and Server ID

The EAP key management framework [7] (Aboba, B., Simon, D., and P. Eronen, “Extensible Authentication Protocol (EAP) Key Management Framework,” November 2007.) requires that EAP methods export three information elements, called the Session-ID, the Peer-ID, and the Server-ID. In EAP-IKEv2 these elements are derived as follows.

• The Session-ID is constructed and exported as the concatenation of the following three elements, in this order: (a) the EAP Code Type for EAP-IKEv2 (to be defined by IANA), (b) the contents of the Nonce Data field of the Nonce Payload Ni from message 3, (c) the contents of the Nonce Data field of the Nonce Payload Nr from message 4.
• In case of a full EAP-IKEv2 protocol run, the Peer-ID is constructed and exported as the content of the Identification Data field of the Identification Payload IDr from message 6. Note that only the "actual" identification data is exported, as indicated in the Payload Length field; if the Identification Data field contains any padding, this padding is ignored. In case of a "fast reconnect" protocol run, the Peer-ID field is constructed in exactly the same manner, where message 6 refers to the full EAP-IKEv2 protocol run that originally established the security context between the EAP peer and EAP server.
• In case of a full EAP-IKEv2 protocol run, the Server-ID is constructed and exported as the contents of the Identification Data field of the Identification Payload IDi from message 5. Note that only the "actual" identification data is exported, as indicated in the Payload Length field; if the Identification Data field contains any padding, this padding is ignored. In case of a "fast reconnect" protocol run, the Server-ID field is constructed in exactly the same manner, where message 5 refers to the full EAP-IKEv2 protocol run that originally established the security context between the EAP peer and EAP server.

7.  Error Handling

This section specifies how errors are handled within EAP-IKEv2. For conveying error information from one party to the other, the Notify payload is defined and used (see Section 8.11 (Notify Payload)).

If, in a full EAP-IKEv2 protocol run, authentication fails (i.e., the verification of the AUTH field fails at the server or the peer), but no other errors have occurred, the message flow deviates from that described in Section 3 (Protocol Overview). The message flows in the presence of authentication failures are specified in Appendix A (EAP-IKEv2 Protocol Runs with Failed Authentication).

If, in message 3 of a full EAP-IKEv2 protocol run (see Figure 1 (EAP-IKEv2 full, successful protocol run)), the responder receives a Diffie-Hellman value (KEi) that belongs to a group that is not supported (and in the absence of other errors), then the responder MUST send a message of the form shown in Figure 3 (Error handling in case of unsupported D-H value) to the initiator. This effectively becomes message 4 in the full protocol run.

1. R<-I: EAP-Request/Identity

2. R->I: EAP-Response/Identity(Id)

3. R<-I: EAP-Req (HDR, SAi, KEi, Ni)

4. R->I: EAP-Res (HDR, N(INVALID_KE_PAYLOAD))

The above message consists of the EAP-IKEv2 header and a Notification payload with the value of the Notify Message Type field value set to 17 (INVALID_KE_PAYLOAD). There are two octets of data associated with this notification: the number of the selected DH Group in big endian order, as specified in Section 3.10.1 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). This number MUST represent a DH group that is supported by both the initiator and the responder.

If, during a full EAP-IKEv2 protocol run (see Figure 1 (EAP-IKEv2 full, successful protocol run)), the initiator receives a message conforming to Figure 3 (Error handling in case of unsupported D-H value) instead of the usual message 4, then it MUST check whether or not the indicated DH group was proposed in message 3. If it was not, then the initiator MUST silently discard the message. Otherwise, the protocol continues with a new message 3 that the initiator sends to the peer. In this new message 3 the initiator MUST use a Diffie-Hellman value that is drawn from the group that is indicated in the Notify payload of message 4 in Figure 3 (Error handling in case of unsupported D-H value).

If, in the context of use case 4 and during a full EAP-IKEv2 protocol run (see Figure 1 (EAP-IKEv2 full, successful protocol run)), the initiator receives, in message 4, an SK{IDr} payload that decrypts to a non-existent or unauthorised EAP-IKEv2 responder identifier IDr*, then the server SHOULD continue the protocol with a message conforming to the format of message 5. The AUTH payload in that message SHOULD contain a value that is computationally indistinguishable from a value that it would contain if IDr* was valid and authorised. This can be accomplished, for example, by generating a random key and calculate AUTH as usual (however, this document does not mandate a specific mechanism). Only after receiving message 6, the server SHOULD respond with an authentication failure notification, i.e., a message conforming to message 6 in Figure 7 (EAP-IKEv2 with failed server authentication). The purpose of this behaviour is to prevent an adversary from probing the EAP-IKEv2 peer identifier space.

If, in the context of use cases 1, 2, or 3 and during a full EAP-IKEv2 protocol run (see Figure 1 (EAP-IKEv2 full, successful protocol run)), the initiator receives, in message 4, an SK{IDr} payload that decrypts to an EAP-IKEv2 responder identifier IDr*, then the server MUST continue the protocol as usual (note that such a payload would not be required in these use cases). The server MUST compare IDr* with the IDr received in message 6 and, in case of a mismatch, MUST respond with an authentication failure notification, i.e., a message conforming to message 6 in Figure 7 (EAP-IKEv2 with failed server authentication). If no mismatch is detected, normal processing applies.

Other errors do not trigger messages with Notification payloads to be sent, and MUST be treated as if nothing happened (i.e., the erroneous EAP-IKEv2 packet MUST be silently discarded). This includes situations where at least one of the following conditions is met, with respect to an incoming EAP-IKEv2 packet.

• The packet contains an Encrypted payload that, when decrypted with the appropriate key, yields an invalid decryption.
• The packet contains an Encrypted payload with a Checksum field that does not verify with the appropriate key.
• The packet contains an Integrity Checksum Data field (see Figure 4 (General packet format of EAP-IKEv2)) that is incorrect.
• The packet does not contain a compulsory field.
• A field in the packet contains an invalid value (e.g., an invalid combination of flags, a length field that is inconsistent with the real length of the field or packet, or the responder's choice of a cryptographic algorithm is different to NONE and any of those that were offered by the initiator).
• The packet contains an invalid combination of fields (e.g., it contains two or more Notify payloads with the same Notify Message Type value, or two or more Transform substructures with the same Transform Type and Transform ID value).
• The packet causes a defragmentation error.
• The format of the packet is invalid.
• The identity provided by the EAP peer in the EAP-Response/Identity cannot be associated with either an established security context (in case of a fast reconnect) or with a real user account (in case of a full protocol exchange) then the packet is silently discarded. With an outstanding message from the EAP server the client may either retransmit the previous request or, in case of a fast reconnect, assume that state information was deleted (e.g., due to garbage collection) at the EAP server and to fall back to a previously used FRID or to the full protocol exchange.

If an incoming packet contains an error for which behaviour is specified in this section, and an error that, in the absence of the former error, would cause the packet to be silently discarded, then the packet MUST be silently discarded.

8.  Specification of Protocol Fields

In this section, the format of the EAP-IKEv2 data fields and applicable processing rules are specified. Figure 4 (General packet format of EAP-IKEv2) shows the general packet format of EAP-IKEv2 messages, and the embedding of EAP-IKEv2 into EAP. The EAP-IKEv2 messages are embedded in the Data field of the standard EAP Request/Response packets. The Code, Identifier, Length and Type fields are described in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP) (RFC 3748),” June 2004.). The EAP Type for this EAP method is TBD by IANA.

   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      |   Flags       |       Message Length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Message Length          |       HDR + payloads          ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Integrity Checksum Data                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Flags field is always present and is used for fragmentation support, as described in Section 8.1 (The Flags, Message Length, and Integrity Checksum Data fields). The Message Length field is not always present; it's presence is determined by a certain flag in the Flags field, as described in Section 8.1 (The Flags, Message Length, and Integrity Checksum Data fields). The field denoted as "HDR + payloads" in Figure 4 (General packet format of EAP-IKEv2) contains the EAP-IKEv2 header (see Section 8.2 (EAP-IKEv2 header)), followed by a number of payloads, in accordance with the composition of EAP-IKEv2 messages, as described in the previous sections. Note that each payload begins with a generic payload header that is specified in Section 3.2 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

The Integrity Checksum Data field is not always present; its presence is determined by a certain flag in the Flags field, as described in Section 8.1 (The Flags, Message Length, and Integrity Checksum Data fields).

In the remainder of this section, the protocol fields that are used in EAP-IKEv2 are specified. This specification heavily relies on the IKEv2 specification [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.), and many fields are constructed, formatted and processed in way that is almost identical to that in IKEv2. However, certain deviations from standard IKEv2 formatting and processing exist. These deviations are highlighted in the remainder of this section.

8.1.  The Flags, Message Length, and Integrity Checksum Data fields

This section describes EAP-IKEv2 fragmentation, and specifies the encoding and processing rules for the Flags, Message Length, and Integrity Checksum Data field shown in Figure 4 (General packet format of EAP-IKEv2).

Fragmentation support in EAP-IKEv2 is provided by the Flags and Message Length fields shown in Figure 4 (General packet format of EAP-IKEv2). These are encoded and used as follows.

 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|L M I 0 0 0 0 0|
+-+-+-+-+-+-+-+-+

L = Length included
M = More fragments
I = Integrity Checksum Data included

The Flags field is defined in Figure 5 (Flags field). Only the first three bits (0-2) are used; all remaining bits MUST be set to zero and and ignored on receipt. The L flag indicates the presence of a Message Length field, and the M flag indicates whether or not the current EAP message has more fragments. In particular, if the L bit is set, then a Message Length field MUST be present in the EAP message, as shown in Figure 4 (General packet format of EAP-IKEv2). The Message Length field is four octets long and contains the length of the entire message (i.e., the length of the EAP Data field.). Note that, in contrast, the Length field shown in Figure 4 (General packet format of EAP-IKEv2) contains the length of only the current fragment. (Note that there exist two fields that are related to length: the Length field, which is a generic EAP field, and the Message Length field, which is an EAP-IKEv2-specific field.) If the L bit is not set, then the Message Length field MUST NOT be present.

The M flag MUST be set on all fragments except the last one. In the last fragment, the M flag MUST NOT be set. Reliable fragment delivery is provided by the retransmission mechanism of EAP.

The Integrity Checksum Data field contains a cryptographic checksum that covers the entire EAP message, starting with the Code field, and ending at the end of the EAP Data field. This field, shown in Figure 4 (General packet format of EAP-IKEv2), is present only if the I bit is set in the Flags field. The Integrity Checksum Data field immediately follows the EAP Data field without padding.

Whenever possible, the Integrity Checksum Data field MUST be present (and the I bit set) for each fragment, including the case where the entire EAP-IKEv2 message is carried in a single fragment. The algorithm and keys that are used to compute the Integrity Checksum Data field MUST be identical to those used to compute the Integrity Checksum Data field of the Encrypted Payload (see Section 8.9 (Encrypted Payload)). That is, the algorithm and keys that were negotiated and established during this EAP-IKEv2 protocol run are used. Note that this means that different keys are used to compute the Integrity Checksum Data field in each direction. Also note that, for messages where this algorithm and the keys are not yet established, the Integrity Checksum Data field cannot be computed and is therefore not included. This applies, for example, to messages 3 and 4 in Figure 1 (EAP-IKEv2 full, successful protocol run).

In order to minimize the exposure to denial-of-service attacks on fragmented packets, messages that are not protected with an Integrity Checksum Data field SHOULD NOT be fragmented. Note, however, that those packets are not likely to be fragmented anyway since they do not carry certificates.

8.2.  EAP-IKEv2 header

The EAP-IKEv2 header, denoted HDR in this specification, is constructed and formatted according to the rules specified in Section 3.1 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

In the first EAP-IKEv2 message that is sent by the initiator (message 3 in Figure 1 (EAP-IKEv2 full, successful protocol run)), the IKE_SA Responder's SPI field is set to zero. This is because, at this point in time, the initiator does not know what SPI value the responder will choose for this protocol run. In all other messages, both SPI fields MUST contain non-zero values that reflect the initiator and responder-chosen SPI values.

In accordance with [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.), for this version of EAP-IKEv2, the MjVer (major version) and MnVer (minor version) fields in the header MUST be 2 and 0 respectively. The value of the Exchange Type field MUST be set to 34 (IKE_SA_INIT) in messages 3 and 4, and to 35 (IKE_SA_AUTH) in messages 5 and 6 in Figure 1 (EAP-IKEv2 full, successful protocol run). In messages 3 and 4 in Figure 2 (Fast Reconnect Protocol Run) this value MUST be set to 36 (CREATE_CHILD_SA).

The Flags field of the EAP-IKEv2 header is also constructed according to Section 3.1 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). Note that this is not the same field as the Flags field shown in Figure 4 (General packet format of EAP-IKEv2).

The Message ID field is constructed as follows. Messages 3 and 4 in a full protocol run MUST carry Message ID value 0. Messages 5 and 6 in a full protocol run (see Figure 1) MUST carry Message ID value 1. Messages 3 and 4 in a fast reconnect protocol run MUST carry Message ID value 2.

8.3.  Security Association Payload

The SA payload is used for the negotiation of cryptographic algorithms between the initiator and the responder. The rules for its construction adhere to [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.), in particular sections 2.7 and 3.3.

In EAP-IKEv2, all Proposal Substructures in the SA payload MUST carry Protocol ID value 1 (IKE).

8.4.  Key Exchange Payload

The Key Exchange payload, denoted KEi if constructed by the initiator and KEr if constructed by the responder, is formatted according to the rules specified in Section 3.4 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

8.5.  Nonce Payload

The Nonce payload, denoted Ni if constructed by the initiator and Nr if constructed by the responder, is constructed and formatted according to the rules specified in Section 3.9 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

8.6.  Identification Payload

The Identification payload, denoted IDi if it contains an identifier for the initiator and IDr if it contains an identifier for the responder, is constructed and formatted according to the rules specified in Section 3.5 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

8.7.  Certificate Payload

The Certificate payload, denoted CERT, is constructed and formatted according to the rules specified in Section 3.6 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). Note that certain certificate encodings for the EAP server certificate, e.g., those that need to be resolved via another network protocol, cannot be used in some typical EAP-IKEv2 deployment scenarios. A user, for example, that authenticates himself by means of EAP-IKEv2 in order to obtain network access, cannot resolve the server certificate at the time of EAP-IKEv2 protocol execution.

8.8.  Certificate Request Payload

The Certificate payload, denoted CERTREQ, is constructed and formatted according to the rules specified in Section 3.7 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

8.9.  Encrypted Payload

The Encrypted payload, denoted SK{...}, is constructed and formatted according to the rules specified in Section 3.14 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

8.10.  Authentication Payload

The Authentication payload, denoted AUTH, is constructed and formatted according to the rules specified in Section 2.15 and Section 3.8 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

The contents of the Authentication payload depend on which party generates this field, the use case, and the algorithm that corresponds to the credential (asymmetric key, symmetric key, or password) that this party uses to authenticate itself. The Authentication payload contains either a MAC or a signature.

If the party that generates the Authentication payload authenticates itself based on a shared secret (i.e., a password or a symmetric key), then the Authentication payload MUST contain a MAC. This MAC is calculated using a key that is derived from the shared secret, according to Section 2.15 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). According to that section, the shared secret is padded with the string "Key Pad for IKEv2" as part of this key derivation. For the EAP-IKEv2 method, this rule is overridden, in that the padding string is redefined as "Key Pad for EAP-IKEv2". The latter padding string MUST be used for the derivation of the MAC key from a shared secret in the context of EAP-IKEv2. This is done in order to avoid the same MAC key to be used for both IKEv2 and EAP-IKEv2 in scenarios where the same shared secret is used for both. Note that using a shared secret (e.g., a password) in the context EAP-IKEv2 that is identical or similar to a shared secret that is used in another context (including IKEv2) is nevertheless NOT RECOMMENDED.

8.11.  Notify Payload

The Notify payload, denoted N(...), is constructed and formatted according to the rules specified in Section 3.10 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The Protocol ID field of this payload MUST be set to 1 (IKE_SA).

8.12.  Next Fast-ID Payload

The Next Fast-ID Payload is defined as follows.

                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Next Payload  !C!  RESERVED   !         Payload Length        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                     Fast-Reconnect-ID (FRID)                  ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Next Fast-ID payload, denoted NFID, does not have an equivalent in IKEv2. Nevertheless, the Next Payload, C, RESERVED, and Payload Length fields of this payload are constructed according to Section 3.2 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The payload ID is registered in Section 11 (IANA Considerations). The Fast-Reconnect-ID field contains a fast reconnect identifer that the peer can use in the next fast reconnect protocol run, as described in Section 4 (Fast Reconnect). In environments where a realm portion is required, Fast-Reconnect-ID includes both a username portion and a realm name portion. The Fast-Reconnect-ID MUST NOT include any terminating null characters. The encoding of the Fast-Reconnect-ID field MUST follow the NAI format [4] (Aboba, B., Beadles, M., Arkko, J., and P. Eronen, “The Network Access Identifier,” December 2005.).

9.  Payload Types and Extensibility

In EAP-IKEv2, each payload is identified by means of a type field which, as specified in [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.), is indicated in the "Next Payload" field of the preceding payload. However, the identifer space from which EAP-IKEv2 payload types are drawn is independent from the payload type space of IKEv2. This is because EAP-IKEv2 and IKEv2 may evolve in a different way and, as such, payload types that appear in one protocol do not necessary appear in the other. An example of this is the "Next Fast-ID" (NFID) payload which does not exist in IKEv2.

The values for the payload types defined in this document are listed in Section 11 (IANA Considerations). Payload type values 13-127 are reserved to IANA for future assignment in EAP-IKEv2. Payload type values 128-255 are for private use among mutually consenting parties.

10.  Security Considerations

As mentioned in Section 3 (Protocol Overview), in EAP-IKEv2, the EAP server always assumes the role of the initiator (I), and the EAP peer takes on the role of the responder (R) of an exchange. This is in order to ensure that, in scenarios where the peer authenticates itself based on a password (i.e., in use case 3), operations that involve this password only take place after the server has been successfully authenticated. In other words, this assignment of initiator and responder roles results in protection against offline dictionary attacks on the password that is used by the peer to authenticate itself (see Section 10.7 (Dictionary Attack Resistance)).

In order for two EAP-IKEv2 implementations to be interoperable, they must support at least one common set of cryptographic algorithms. In order to promote interoperability, EAP-IKEv2 implementations MUST support the following algorithms based on the "MUST/MUST-" recommendations given in [5] (Schiller, J., “Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2),” December 2005.):

    Diffie-Hellman Groups: 1024 MODP Group
    IKEv2 Transform Type 1 Algorithms: ENCR_3DES
    IKEv2 Transform Type 2 Algorithms: PRF_HMAC_SHA1
    IKEv2 Transform Type 3 Algorithms: AUTH_HMAC_SHA1_96

All other options of [5] (Schiller, J., “Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2),” December 2005.) MAY be implemented.

The remainder of this section describes EAP-IKEv2 in terms of specific security terminology as required by [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP) (RFC 3748),” June 2004.). The discussion makes reference to the use cases defined in Section 1 (Introduction).

10.1.  Protected Ciphersuite Negotiation

In message 3, the EAP server provides the set of ciphersuites it is willing to accept in an EAP-IKEv2 protocol run. Hence, the server is in control of the ciphersuite. An EAP peer that does not support any of the indicated ciphersuites is not able to authenticate. The local security policy of the peer MUST specify the set of ciphersuites that the peer accepts. The server MUST verify that the ciphersuite that is indicated as being chosen by the peer in message 4, belongs to the set of ciphersuites that were offered in message 3. If this verification fails, the server MUST silently discard the packet.

10.2.  Mutual Authentication

EAP-IKEv2 supports mutual authentication.

10.3.  Integrity Protection

EAP-IKEv2 provides integrity protection of EAP-IKEv2 traffic. This protection is offered after authentication is completed and is facilitated by inclusion of two Integrity Checksum Data fields: one at the end of the EAP packet (see Figure 4 (General packet format of EAP-IKEv2)), and one as part of an Encrypted payload (see Section 8.9 (Encrypted Payload)).

10.4.  Replay Protection

EAP-IKEv2 provides protection against replay attacks by a variety of means. This includes the requirement that the Authentication payload is computed as a function of, among other things, a server-provided nonce and a peer-provided nonce. These nonces are required to be practically unpredictable by an adversary. Assuming that the algorithm that is used to compute the Authentication payload does not contain cryptographic weaknesses, the probability that an Authentication payload that is valid in a particular protocol run, will also be valid in a subsequent run, is therefore negligible.

10.5.  Confidentiality

EAP-IKEv2 provides confidentiality of certain EAP-IKEv2 fields, namely those included in Encrypted payloads. With respect to identity confidentiality, the following claims are made. Note that identity confidentiality refers to the EAP-IKEv2 identity of the EAP peer.

Identity confidentiality is provided in the face of a passive adversary, i.e., an adversary that does not modify traffic as it is in transit. Whenever the optional SK{IDr} payload in message 4 of a full EAP-IKEv2 protocol (see Figure 1 (EAP-IKEv2 full, successful protocol run)) is not included, identity confidentiality is also provided in the face of an active adversary. This payload MUST NOT be included in use cases 1, 2, and 3. In use case 4 this payload MUST be included. Therefore, in use case 4, EAP-IKEv2 does not provide identity confidentiality in the face of an active adversary.

Note, however, that the EAP peer provides it's identity in message 2 in Figure 1 (EAP-IKEv2 full, successful protocol run) in cleartext. In order to provide identity confidentiality as discussed in the previous paragraphs it is necessary to obfuscated the username part of the identity (the realm part must stay intact to allow correct message routing by the AAA infrastructure). The EAP server then uses the identity information in message 4. The same mechanism is also used by other EAP methods to provide identity confidentiality, for example EAP-TTLS [8] (Funk, P. and S. Blake-Wilson, “EAP Tunneled TLS Authentication Protocol (EAP-TTLS),” July 2004.).

10.6.  Key Strength

EAP-IKEv2 supports the establishment of session keys (MSK and EMSK) of a variety of key strengths, with the theoretical maximum at 512 bits per key (since this is the size of the MSK and the EMSK). However, in practice the effective key strength is likely to be significantly lower, and depends on the authentication credentials used, the negotiated ciphersuite (including the output size of the pseudorandom function), the Diffie-Hellman group used, and on the extent to which the assumptions on which the underlying cryptographic algorithms depend really hold. Of the above mechanisms, the one that offers the lowest key strength can be regarded as a measure of the effective key strength of the resulting session keys. Note that this holds for other EAP methods, too.

Due to the large variety of possible combinations, no indication of a practical effective key strength for MSK or EMSK is given here. However, those responsible for the deployment of EAP-IKEv2 in a particular environment should consider the threats this environment may be exposed to, and configure the EAP-IKEv2 server and peer policies and authentication credentials such that the established session keys are of a sufficiently high effective key strength.

10.7.  Dictionary Attack Resistance

EAP-IKEv2 can be used in a variety of use cases, as explained in Section 1 (Introduction). In some of these uses cases, namely use case 1, 2, and 4, dictionary attacks cannot be launched since no passwords are used. In use case 3, EAP-IKEv2 provides protection against offline dictionary attacks, since operations that involve the password are executed only after the server has authenticated itself (based on a credential other than a password).

In order to reduce exposure against online dictionary attacks, in use case 3, the server SHOULD provide the capability to log failed peer authentication events, and SHOULD implement a suitable policy in case of consecutive failed peer authentication attempts within a short period of time (such as responding with an EAP-Failure instead of message 5 for a predetermined amount of time).

When passwords are used with method 4 (instead of using an key with high entropy) then dictionary attacks are possible, as described in Section 8 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.):

" When using pre-shared keys, a critical consideration is how to assure the randomness of these secrets. The strongest practice is to ensure that any pre-shared key contain as much randomness as the strongest key being negotiated. Deriving a shared secret from a password, name, or other low-entropy source is not secure. These sources are subject to dictionary and social engineering attacks, among others. "

Hence, the usage of passwords with mode 4 where the EAP peer and the EAP server rely on a shared secret that was derived from a password is insecure. It is strongly recommended to use mode 3 when passwords are used by the EAP peer.

10.8.  Fast Reconnect

EAP-IKEv2 supports a "fast reconnect" mode of operation, as described in Section 4 (Fast Reconnect).

10.9.  Cryptographic Binding

EAP-IKEv2 is not a tunnel EAP method. Thus, cryptographic binding does not apply to EAP-IKEv2.

10.10.  Session Independence

EAP-IKEv2 provides session independence in a number of ways, as follows. Firstly, knowledge of captured EAP-IKEv2 conversations (i.e., the information that a passive adversary may obtain) does not enable the adversary to compute the Master Session Key (MSK) and Extended Master Session Key (EMSK) that resulted from these conversations. This holds even in the case where the adversary later obtains access to the server and/or the peer's long-term authentication credentials that were used in these conversations. That is, EAP-IKEv2 provides support for "perfect forward secrecy". However, whether or not this support is made use of in a particular EAP-IKEv2 protocol run, depends on when the peer and the server delete the Diffie-Hellman values that they used in that run, and on whether or not they use fresh Diffie-Hellman values in each protocol run. The discussion in Section 2.12 of [1] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) applies.

Secondly, an active adversary that does not know the peer's and the server's long-term authentication credentials cannot learn the MSK and EMSK that were established in a particular protocol run of EAP-IKEv2, even if it obtains access to the MSK and EMSK that were established in other protocol runs of EAP-IKEv2. This is because the MSK and the EMSK are a function of, among other things, data items that are assumed to be generated independently at random in each protocol run.

10.11.  Fragmentation

EAP-IKEv2 provides support for fragmentation, as described in Section 8.1 (The Flags, Message Length, and Integrity Checksum Data fields).

10.12.  Channel Binding

Channel binding is not supported in EAP-IKEv2.

10.13.  Summary

EAP security claims are defined in Section 7.2.1 of [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP) (RFC 3748),” June 2004.). The security claims for EAP-IKEv2 are as follows:

            Ciphersuite negotiation:   Yes
            Mutual authentication:     Yes
            Integrity protection:      Yes
            Replay protection:         Yes
            Confidentiality:           Yes
            Key derivation:            Yes; see Section 5
            Key strength:              Variable
            Dictionary attack prot.:   Yes; see Section 10.7
            Fast reconnect:            Yes; see Section 4
            Crypt. binding:            N/A
            Session independence:      Yes; see Section 10.10
            Fragmentation:             Yes; see Section 10.11
            Channel binding:           No

11.  IANA Considerations

IANA should allocate a value for the EAP method type indicating EAP-IKEv2.

In addition, IANA is requested to create a new registry for EAP-IKEv2 payloads, and populate it with the following initial entries listed below.

The following payload type values are used by this document.

  Next Payload Type                  | Value
  -----------------------------------+----------------------------------
  No Next payload                    | TBD by IANA (suggested value: 0)
  Security Association payload       | TBD by IANA (suggested value: 1)
  Key Exchange payload               | TBD by IANA (suggested value: 2)
  Identification payload             |
      (when sent by initiator, IDi)  | TBD by IANA (suggested value: 3)
  Identification payload             |
      (when sent by responder, IDr)  | TBD by IANA (suggested value: 4)
  Certificate payload                | TBD by IANA (suggested value: 5)
  Certificate Request payload        | TBD by IANA (suggested value: 6)
  Authentication payload             | TBD by IANA (suggested value: 7)
  Nonce payload                      | TBD by IANA (suggested value: 8)
  Notification payload               | TBD by IANA (suggested value: 9)
  Vendor ID payload                  | TBD by IANA (suggested value: 10)
  Encrypted payload                  | TBD by IANA (suggested value: 11)
  Next Fast-ID payload               | TBD by IANA (suggested value: 12)
  RESERVED TO IANA                   | 13-127
  PRIVATE USE                        | 128-255

Payload type values 13-127 are reserved to IANA for future assignment in EAP-IKEv2. Payload type values 128-255 are for private use among mutually consenting parties.

The semantic of the above-listed payloads is provided in this document and refer to IKEv2 when necessary.

Following the policies outline in [9] (Aboba, B., “IANA Considerations for RADIUS (Remote Authentication Dial In User Service),” July 2003.) new payload type values with a description of their semantic will be assigned after Expert Review initiated by the Security Area Directors in consultation with the EMU working group chairs or the working group chairs of a designated successor working group. Updates can be provided based on expert approval only. A designated expert will be appointed by the Security Area Directors. Based on expert approval it is possible to delete entries from the registry or to mark entries as "deprecated".

Each registration must include the payload type value and the semantic of the payload.

12.  Contributors

The authors are grateful to Krzysztof Rzecki, Rafal Mijal, Piotr Marnik, and Pawel Matejski, who, during their implementation of EAP-IKEv2 (see http://eap-ikev2.sourceforge.net/), provided invaluable feedback and identified a number of errors in previous versions of this draft.

13.  Acknowledgements

The authors also thank Pasi Eronen for his invaluable comments as expert reviewer assigned by the EAP working group chairs Jari Arkko and Bernard Aboba. The authors would also like to thank Guenther Horn, Thomas Otto, Paulo Pagliusi and John Vollbrecht for their insightful comments and suggestions. The members of the PANA design team, in particular D. Forsberg and A. Yegin, also provided comments on the initial version of this draft. We would like to thank Hugo Krawczyk for his feedback regarding the usage of the password based authentication.

The authors are grateful to the members of the EAP keying design team for their discussion in the area of the EAP Key Management Framework.

We would like to thank Jari Arkko for his support and for his comments. Finally, we would like to thank Charlie Kaufman, Bernard Aboba, and Paul Hoffman for their comments during IETF Last Call.

14.  References

14.1. Normative References

14.2. Informative References

Appendix A.  EAP-IKEv2 Protocol Runs with Failed Authentication

This appendix illustrates how authentication failures are handled within EAP-IKEv2. Note that authentication failures only occur in full EAP-IKEv2 protocol runs.

Figure 7 (EAP-IKEv2 with failed server authentication) shows the message flow in case the EAP peer fails to authenticate the EAP server.

1. R<-I: EAP-Request/Identity

2. R->I: EAP-Response/Identity(Id)

3. R<-I: EAP-Req (HDR, SAi1, KEi, Ni)

4. R->I: EAP-Res (HDR, SAr1, KEr, Nr, [CERTREQ], [SK{IDr}])

5. R<-I: EAP-Req (HDR, SK {IDi, [CERT], [CERTREQ], [IDr], AUTH})

6. R->I: EAP-Res(HDR, SK {N(AUTHENTICATION_FAILED)})

7. R<-I: EAP-Failure

The difference to the full successful exchange described in Section 3 (Protocol Overview) is that, in message 6, the EAP peer MUST answer the EAP server with an Encrypted payload that contains a Notify payload with the Notify Message Type value set to 24 (AUTHENTICATION_FAILED). In that message, the Message ID field in the EAP-IKEv2 header (HDR), MUST carry Message ID value 2. In message 7, an EAP-Failure message MUST be returned by the EAP server.

Figure 8 (EAP-IKEv2 with failed peer authentication) shows the message flow in case the EAP server fails to authenticate the EAP peer.

1. R<-I: EAP-Request/Identity

2. R->I: EAP-Response/Identity(Id)

3. R<-I: EAP-Req (HDR, SAi1, KEi, Ni)

4. R->I: EAP-Res (HDR, SAr1, KEr, Nr, [CERTREQ], [SK{IDr}])

5. R<-I: EAP-Req (HDR, SK {IDi, [CERT], [CERTREQ], AUTH})

6. R->I: EAP-Res (HDR, SK {IDr, [CERT], AUTH})

7. R<-I: EAP-Req (HDR, SK {N(AUTHENTICATION_FAILED)})

8. R->I: EAP-Res (HDR, SK {})

9. R<-I: EAP-Failure

Compared to the full successful exchange, one additional roundtrip is required. In message 7 the EAP server MUST send an EAP request with Encrypted payload that contains a Notify payload with the Notify Message Type value set to 24 (AUTHENTICATION_FAILED), instead of sending an EAP-Success message. The EAP peer, upon receiving message 7, MUST send an empty EAP-IKEv2 (informational) message in reply to the EAP server's error indication, as shown in message 8. In both message 7 and 8, the Message ID field in the EAP-IKEv2 header (HDR), MUST carry Message ID value 2. Finally, by means of message 9, the EAP server answers with an EAP-Failure.

Authors' Addresses

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