EAP WG H. Tschofenig
Internet-Draft D. Kroeselberg
Expires: August 10, 2006 A. Pashalidis
Siemens
Y. Ohba
Toshiba
F. Bersani
France Telecom
February 6, 2006
EAP IKEv2 Method
draft-tschofenig-eap-ikev2-09.txt
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Abstract
This document specifies EAP-IKEv2, an EAP authentication method that
is based on the Internet Key Exchange (IKEv2) protocol. EAP-IKEv2
provides mutual authentication and session key establishment between
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an EAP peer and an EAP server. It supports authentication techniques
that are based on passwords, high-entropy shared keys, and public key
certificates. These techniques can be combined in a number of ways.
EAP-IKEv2 further provides support for cryptographic ciphersuite
negotiation, hash function agility, identity confidentiality (in
certain modes of operation), fragmentation, and a "fast reconnect"
mode.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 8
4. Fast Reconnect . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Key Derivation . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Error handling . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Specification of Protocol Fields . . . . . . . . . . . . . . . 17
7.1. The Flags, Message Length, and Integrity Checksum Data
fields . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.2. EAP-IKEv2 header . . . . . . . . . . . . . . . . . . . . . 19
7.3. Security Association Payload . . . . . . . . . . . . . . . 19
7.4. Key Exchange Payload . . . . . . . . . . . . . . . . . . . 20
7.5. Nonce Payload . . . . . . . . . . . . . . . . . . . . . . 20
7.6. Identification Payload . . . . . . . . . . . . . . . . . . 20
7.7. Certificate Payload . . . . . . . . . . . . . . . . . . . 20
7.8. Certificate Request Payload . . . . . . . . . . . . . . . 20
7.9. Encrypted Payload . . . . . . . . . . . . . . . . . . . . 20
7.10. Authentication Payload . . . . . . . . . . . . . . . . . . 20
7.11. Notify Payload . . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8.1. Protected Ciphersuite Negotiation . . . . . . . . . . . . 22
8.2. Mutual Authentication . . . . . . . . . . . . . . . . . . 22
8.3. Integrity Protection . . . . . . . . . . . . . . . . . . . 22
8.4. Replay Protection . . . . . . . . . . . . . . . . . . . . 23
8.5. Confidentiality . . . . . . . . . . . . . . . . . . . . . 23
8.6. Key Strength . . . . . . . . . . . . . . . . . . . . . . . 23
8.7. Dictionary Attack Resistance . . . . . . . . . . . . . . . 24
8.8. Fast Reconnect . . . . . . . . . . . . . . . . . . . . . . 24
8.9. Cryptographic Binding . . . . . . . . . . . . . . . . . . 24
8.10. Session Independence . . . . . . . . . . . . . . . . . . . 24
8.11. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 25
8.12. Channel Binding . . . . . . . . . . . . . . . . . . . . . 25
9. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
11.1. Normative References . . . . . . . . . . . . . . . . . . . 28
11.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. EAP-IKEv2 Protocol Runs with Failed Authentication . 29
A.1. Full EAP-IKEv2 . . . . . . . . . . . . . . . . . . . . . . 29
A.2. EAP-IKEv2 Fast Reconnect . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
Intellectual Property and Copyright Statements . . . . . . . . . . 33
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1. Introduction
This document specifies EAP-IKEv2, an EAP authentication method that
is based on the Internet Key Exchange Protocol version 2 (IKEv2) [1].
It 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.
o 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.
o Passwords: these are low-entropy bit strings that are known to
both the server and the peer.
o 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) in each direction, e.g. that the EAP server
authenticates itself based on a public/private key pair and the EAP
client based on a symmetric key. In particular, the following
combinations are expected to be used in practice. These are referred
to as "use cases" in the remainder of this document.
1. EAP server: asym. key pair, EAP peer: asym. key pair
2. EAP server: asym. key pair, EAP peer: symmetric key
3. EAP server: asym. key pair, EAP peer: password
4. EAP server: symmetric key, EAP peer: symmetric key
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.
o The next section provides an overview of some of the terms and
abbreviations used in this document.
o Section 3 provides an overview of the full EAP-IKEv2 exchange and
thereby specifies the protocol message composition.
o Section 4 specifies the message composition for the EAP-IKEv2
"fast reconnect" mode of operation.
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o Section 5 specifies how exportable session keys are derived.
o Section 6 specifies how possible errors that may occur during
protocol execution are handled.
o Section 7 specifies the format of the EAP-IKEv2 data fields is
specified, where Section 7.1 also describes how fragmentation is
handled in EAP-IKEv2.
o Section 8 provides a list of claimed security properties.
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2. Terminology
This document makes use of terms defined in [2] and [1]. 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].
A list of abbreviations that are used in this document follows.
o AUTH: Denotes a data field containing either a MAC or a signature.
This field is in embedded into an Authentication payload, defined
in Section 7.10.
o CERT: Public key certificate or similar structure.
o CERTREQ: Certificate Request.
o EMSK: Extended Master Session Key, defined in [2].
o HDR: EAP-IKEv2 header, defined in Section 7.2.
o I: Initiator, the party that sends the first message of an EAP-
IKEv2 protocol run. This is always the EAP server.
o MAC: Message Authentication Code. The result of a cryptographic
operation that involves a symmetric key.
o MSK: Master Session Key, defined in [2].
o prf: Pseudorandom function: a cryptographic function whose output
is assumed to be indistinguishable from that of a truly random
function.
o R: Responder, the party that sends the second message of an EAP-
IKEv2 protocol run. This is always the EAP peer.
o 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.
o 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.
o SK: Session Key. In this document, the notation SK{x} denotes that
x is embedded within an Encrypted payload, i.e. that x is
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encrypted and integrity-protected using EAP-IKEv2 internal keys.
These keys are different in each direction.
o SK_xx: EAP-IKEv2 internal key, defined in section 2.14 of [1].
o SKEYSEED: Keying material, defined in section 2.14 of [1].
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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] 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], AUTH})
6. R->I: EAP-Res (HDR, SK{IDr, [CERT], AUTH})
7. R<-I: EAP-Success
Figure 1: EAP-IKEv2 full, successful protocol run
Figure 1 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]. 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
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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]. 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]. 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]. 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]. 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. 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].
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), 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.
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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 paylod 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.
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4. Fast Reconnect
This section specifies a "fast reconnect" mode of operation for EAP-
IKEv2. This mode can only be used by an EAP server/EAP peer pair
that has already been mutually authenticated in a previous EAP-IKEv2
protocol run.
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.
The EAP-IKEv2 fast reconnect exchange is similar, albeit not
identical, to the IKE-SA rekeying procedure as specified in section
2.18 of [1]. 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(Id)
3. R<-I: EAP-Req(HDR, SK{SA, Ni, [KEi]})
4. R->I: EAP-Res(HDR, SK{SA, Nr, [KEr]})
5. R<-I: EAP-Success
Figure 2: EAP-IKEv2 successful fast reconnect protocol run
Figure 2 shows the message composition 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 and are optional; they are not
required if the EAP server is known to the peer and vice versa.
Messages 3 and 4 establish a fresh EAP-IKEv2 security context. In
message 3, the initiator MAY select a new (non-zero) SPI value in the
IKE_SA Initiator's SPI field of the EAP-IKEv2 header. The value of
the IKE_SA Responder's SPI field MUST be the one from the previous
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. 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
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protocol run.
Upon reception of message 3, the responder (EAP peer) decrypts and
verifies the Encrypted payload. If successful (as assumed in
Figure 2), it constructs message 4 in a fashion similar to the
construction of message 3. Note that the IKE_SA Responder's SPI
field in the EAP-IKEv2 header of message 4 MUST contain the same
value as in message 3. The responder MAY choose a new (non-zero)
value for the IKE_SA Initiator's SPI in message 4. Upon reception of
message 4, the initiator (EAP server) decrypts and verifies the
Encrypted payload. If successful, 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 MAY 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]. 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.
Note: In EAP-IKEv2, the EAP server initiates the fast reconnect mode
and thereby causes fresh session keys to be established. If the
client wishes to initiate this "fast rekeying", it needs to indicate
this to the network by an appropriate out-of-band means (e.g. at the
link-layer).
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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 these keys are exported by the EAP-IKEv2 process and be
used in accordance with the EAP keying framework [5].
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]. The generation is 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]. The same prf is used in the same
fashion in order to generate the MSK and the EMSK.
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 from
messages 3 and 4 shown in Figure 1 (in the context of a full EAP-
IKEv2 protocol run) or Figure 2 (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. If the output size
of the prf is less than 128 octets, then the prf is used iteratively,
as specified in section 2.13 of [1], until 128 octets are generated.
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
only be exported when an EAP-IKEv2 protocol run completes
successfully.
Finally, note that the EAP-IKEv2 method does not produce an
initialisation vector.
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6. 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 7.11).
If 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 of the full and "fast reconnect" EAP-IKEv2 protocol
run deviates from that described in Section 3 and Section 4. The
message flows in the presence of authentication failures are
specified in Appendix A.
If, in message 3 of a full or "fast reconnect" EAP-IKEv2 protocol run
(see Figure 1 and Figure 2), 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 to the initiator. This effectively
becomes message 4 in the protocol run.
4. R->I: EAP-Res (HDR, N(INVALID_KE_PAYLOAD))
Figure 3: Error handling in case of unsupported D-H value
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 accepted DH Group in big
endian order, as specified in section 3.10.1 of [1].
If, during a full EAP-IKEv2 protocol run (see Figure 1), the
initiator receives a message conforming to Figure 3 instead of the
usual message 4, then the protocol continues with a new message 3
that the initiator sends to the peer. In this new message 3 the
initiator SHOULD use a Diffie-Hellman value that is drawn from the
group that is indicated in the Notify payload of message 4 in
Figure 3. If local policy does not allow this, or if the initiator
does not support the indicated group, then it MUST resend the
original message 3 (or a "usual" message 3 with fresh values), up to
a predetermined number of times. If the error persists (i.e. if the
initiator keeps receiving a message conforming to Figure 3) beyond
that, then the initiator MUST give up with an EAP-Failure message.
If, during a fast reconnect EAP-IKEv2 protocol run (see Figure 2),
the initiator receives a message conforming to Figure 3 instead of
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the usual message 4, then 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 same
group as the one from which the Diffie-Hellman value in message 3 of
the initial full EAP-IKEv2 protocol run with this peer was drawn. If
the error persists (i.e. if the initiator receives another message
conforming to Figure 3), then the initiator MUST give up with an EAP-
Failure message.
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.
o The packet contains an Encrypted payload that, when decrypted with
the appropriate key, yields an invalid decryption.
o The packet contains an Encrypted payload with a Checksum field
that does not verify with the appropriate key.
o The packet contains an Integrity Checksum Data field (see
Figure 4) that is incorrect.
o The packet does not contain a compulsory field.
o 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).
o 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).
o The packet causes a defragmentation error.
o The format of the packet is invalid.
If an incoming packet causes both an authentication failure and a
channel binding error (and no other errors), then the packet MUST be
treated as if it only caused the authentication failure.
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
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the packet MUST be silently discarded.
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7. Specification of Protocol Fields
In this section, the format of the EAP-IKEv2 data fields and
applicable processing rules are specified. Figure 4 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]. The EAP
Type for this EAP method is TBD.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: General packet format of EAP-IKEv2
The Flags field is always present and is used for fragmentation
support, as described in Section 7.1. 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 7.1. The field denoted as
"HDR + payloads" in Figure 4 contains the EAP-IKEv2 header (see
Section 7.2), 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].
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 7.1.
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], 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 are also highlighted in the remainder of
this section.
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7.1. 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.
Fragmentation support in EAP-IKEv2 is provided by the Flags and
Message Length fields shown in Figure 4. 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
Figure 5: Flags field
Only the first three bits (0-2) are used; all remaining bits MUST be
set to zero. 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. 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 contains the length of only the current fragment. If the L
bit is not set, 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, 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
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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 7.9). 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.
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.
7.2. 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].
In the first EAP-IKEv2 message that is sent by the initiator (message
3 in Figure 1), 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], 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. In messages 3 and 4
in Figure 2 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]. Note that this is not the same field as the
Flags field shown in Figure 4.
7.3. 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], in particular section 2.7 and 3.3.
In EAP-IKEv2 the SA payload MUST contain a single Proposal
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Substructure where the Protocol ID value is 1 (IKE).
7.4. 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].
7.5. 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].
7.6. 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].
7.7. Certificate Payload
The Certificate payload, denoted CERT, is constructed and formatted
according to the rules specified in section 3.6 of [1]. 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.
7.8. Certificate Request Payload
The Certificate payload, denoted CERTREQ, is constructed and
formatted according to the rules specified in section 3.7 of [1].
7.9. Encrypted Payload
The Encrypted payload, denoted SK{...}, is constructed and formatted
according to the rules specified in section 3.14 of [1].
7.10. Authentication Payload
The Authentication payload, denoted AUTH, is constructed and
formatted according to the rules specified in sections 2.15 and 3.8
of [1].
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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]. 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.
7.11. Notify Payload
The Notify payload, denoted N(...), is constructed and formatted
according to the rules specified in section 3.10 of [1]. The
Protocol ID field of this payload MUST be set to 1 (IKE_SA).
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8. Security Considerations
As mentioned in Section 3, 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 8.7).
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
adhere to the same rules with regard to mandatory-to-implement
cryptographic algorithms as IKEv2. These rules are specified in [4].
The remainder of this section describes EAP-IKEv2 in terms of
specific security terminology as required by [2]. The discussion
makes reference to the use cases defined in Section 1 above.
8.1. 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.
8.2. Mutual Authentication
EAP-IKEv2 supports mutual authentication.
8.3. 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), and one as part of an
Encrypted payload (see Section 7.9).
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8.4. 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.
8.5. 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) 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.
8.6. 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
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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.
8.7. Dictionary Attack Resistance
EAP-IKEv2 can be used in a variety of use cases, as explained in
Section 1. 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).
8.8. Fast Reconnect
EAP-IKEv2 supports a "fast reconnect" mode of operation, as described
in Section 4.
8.9. Cryptographic Binding
EAP-IKEv2 is not a tunnel EAP method. Thus, cryptographic binding
does not apply to EAP-IKEv2.
8.10. 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] applies.
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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.
8.11. Fragmentation
EAP-IKEv2 provides support for fragmentation, as described in
Section 7.1.
8.12. Channel Binding
Channel binding support in EAP-IKEv2 is TBD.
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9. IAB Considerations
IANA should allocate a value for the EAP method type indicating EAP-
IKEv2.
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10. Acknowledgements
The authors would like to thank Bernard Aboba, Jari Arkko, Guenther
Horn, Thomas Otto, Paulo Pagliusi and John Vollbrecht for their
insightful comments and suggestions.
In addition, the authors would like to thank the members of the PANA
design team, in particular D. Forsberg and A. Yegin, for their
comments and input to the initial version of the draft.
Finally, 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. .
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11. References
11.1. Normative References
[1] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306,
December 2005.
[2] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP) (RFC
3748)", Request For Comments 3748, June 2004.
[3] Bradner, S., "Internet Key Exchange (IKEv2) Protocol", RFC 2119,
March 1997.
[4] Schiller, J., "Cryptographic Algorithms for Use in the Internet
Key Exchange Version 2 (IKEv2)", RFC 4307, December 2005.
11.2. Informative References
[5] Aboba, B., Simon, D., Arkko, J., Eronen, P., and H. Levkowetz,
"Extensible Authentication Protocol (EAP) Key Management
Framework", Internet Draft (work in progress), January 2006.
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Appendix A. EAP-IKEv2 Protocol Runs with Failed Authentication
This appendix illustrates how authentication failures are handled
within EAP-IKEv2.
A.1. Full EAP-IKEv2
Figure 6 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
Figure 6: EAP-IKEv2 with failed server authentication
The difference to the full successful exchange described in Section 3
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 message 7,
an EAP-Failure message MUST be returned by the EAP server.
Figure 7 shows the message flow in case the EAP server fails to
authenticate the EAP peer.
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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
Figure 7: EAP-IKEv2 with failed peer authentication
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. The EAP
server then answers with an EAP-Failure.
A.2. EAP-IKEv2 Fast Reconnect
Figure 8 shows the message flow in case the EAP peer fails to
authenticate the EAP server. in a fast reconnect protocol run.
1. R<-I: EAP-Request/Identity
2. R->I: EAP-Response/Identity(Id)
3. R<-I: EAP-Req (HDR, SK {SA, Ni, [KEi]})
4. R->I: EAP-Res (HDR, SK {N(AUTHENTICATION_FAILED)})
5. R <-- I: EAP-Failure
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Figure 8: EAP-IKEv2 fast reconnect with failed server authentication
The message format of message 3 is identical to that of message 6 in
Figure 6. The message processing is analogous to the case of failed
server authentication in a full EAP-IKEv2 protocol run.
Figure 9 shows the message flow in case the EAP server fails to
authenticate the EAP peer. in a fast reconnect protocol run
1. R<-I: EAP-Request/Identity
2. R->I: EAP-Response/Identity(Id)
3. R<-I: EAP-Req (HDR, SK {SA, Ni, [KEi]})
4. R->I: EAP-Res (HDR, SK {SA, Nr, [KEr]})
5. R<-I: EAP-Req (HDR, SK {N(AUTHENTICATION_FAILED)})
6. R->I: EAP-Res (HDR, SK {})
7. R<-I: EAP-Failure
Figure 9: EAP-IKEv2 fast reconnect with failed peer authentication
This case is analogous to the case of peer authentication failure in
a full EAP-IKEv2 protocol run. The EAP peer MUST send an empty EAP-
IKEv2 informational message (empty Encrypted payload, message 6) in
reply to the error indication message of the EAP server (message 5).
The EAP server answers with an EAP-Failure.
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Authors' Addresses
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
Dirk Kroeselberg
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Dirk.Kroeselberg@siemens.com
Andreas Pashalidis
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Andreas.Pashalidis@siemens.com
Yoshihiro Ohba
Toshiba America Research, Inc.
1 Telcordia Drive
Piscataway, NJ 08854
USA
Email: yohba@tari.toshiba.com
Florent Bersani
France Telecom R&D
38, rue du General Leclerc
Issy-Les-Moulineaux, Cedex 92794
France
Email: florent.bersani1@francetelecom.com
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