EAP Working Group L. Blunk
Internet-Draft Merit Network, Inc
Obsoletes: 2284 (if approved) J. Vollbrecht
Expires: July 2, 2003 Vollbrecht Consulting LLC
B. Aboba
Microsoft
J. Carlson
Sun
January 2003
Extensible Authentication Protocol (EAP)
<draft-ietf-eap-rfc2284bis-00.txt>
Status of this Memo
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all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on July 2, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines the Extensible Authentication Protocol (EAP),
an authentication framework which supports multiple authentication
mechanisms. EAP typically runs directly over the link layer without
requiring IP, but is reliant on lower layer ordering guarantees as in
PPP and IEEE 802. EAP does provide its own support for duplicate
elimination and retransmission. Fragmentation is not supported
within EAP itself; however, individual EAP methods may support this.
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While EAP was originally developed for use with PPP, it is also now
in use with IEEE 802.
This document obsoletes RFC 2284. A summary of the changes between
this document and RFC 2284 is available in the "Change Log" Appendix.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Specification of Requirements . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Extensible Authentication Protocol (EAP) . . . . . . . . . . 7
2.1 Support for sequences . . . . . . . . . . . . . . . . . . . 8
2.2 EAP multiplexing model . . . . . . . . . . . . . . . . . . . 9
3. Lower layer behavior . . . . . . . . . . . . . . . . . . . . 12
3.1 Lower layer requirements . . . . . . . . . . . . . . . . . . 12
3.2 EAP usage within PPP . . . . . . . . . . . . . . . . . . . . 13
3.3 EAP usage within IEEE 802 . . . . . . . . . . . . . . . . . 14
3.4 Link layer indications . . . . . . . . . . . . . . . . . . . 14
4. EAP Packet format . . . . . . . . . . . . . . . . . . . . . 15
4.1 Request and Response . . . . . . . . . . . . . . . . . . . . 16
4.2 Success and Failure . . . . . . . . . . . . . . . . . . . . 19
5. Initial EAP Request/Response Types . . . . . . . . . . . . . 21
5.1 Identity . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.2 Notification . . . . . . . . . . . . . . . . . . . . . . . . 23
5.3 Nak . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.4 MD5-Challenge . . . . . . . . . . . . . . . . . . . . . . . 25
5.5 One-Time Password (OTP) . . . . . . . . . . . . . . . . . . 26
5.6 Generic Token Card (GTC) . . . . . . . . . . . . . . . . . . 27
5.7 Vendor-specific . . . . . . . . . . . . . . . . . . . . . . 28
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 29
6.1 Definition of Terms . . . . . . . . . . . . . . . . . . . . 30
6.2 Recommended Registration Policies . . . . . . . . . . . . . 30
7. Security Considerations . . . . . . . . . . . . . . . . . . 31
7.1 Threat model . . . . . . . . . . . . . . . . . . . . . . . . 31
7.2 Security claims . . . . . . . . . . . . . . . . . . . . . . 32
7.3 Identity protection . . . . . . . . . . . . . . . . . . . . 33
7.4 Man-in-the-middle attacks . . . . . . . . . . . . . . . . . 33
7.5 Packet modification attacks . . . . . . . . . . . . . . . . 34
7.6 Dictionary attacks . . . . . . . . . . . . . . . . . . . . . 35
7.7 Connection to an untrusted network . . . . . . . . . . . . . 35
7.8 Negotiation attacks . . . . . . . . . . . . . . . . . . . . 35
7.9 Implementation idiosyncrasies . . . . . . . . . . . . . . . 36
7.10 Key derivation . . . . . . . . . . . . . . . . . . . . . . . 36
7.11 Weak ciphersuites . . . . . . . . . . . . . . . . . . . . . 38
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 38
Normative References . . . . . . . . . . . . . . . . . . . . 38
Informative References . . . . . . . . . . . . . . . . . . . 39
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 41
A. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 42
B. Open issues . . . . . . . . . . . . . . . . . . . . . . . . 42
Intellectual Property and Copyright Statements . . . . . . . 43
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1. Introduction
This document defines the Extensible Authentication Protocol (EAP),
an authentication framework which supports multiple authentication
mechanisms. EAP typically runs directly over the link layer without
requiring IP, but is reliant on lower layer ordering guarantees as in
PPP and IEEE 802. EAP does provide its own support for duplicate
elimination and retransmission. Fragmentation is not supported
within EAP itself; however, individual EAP methods may support this.
EAP may be used on dedicated links as well as switched circuits, and
wired as well as wireless links. To date, EAP has been implemented
with hosts and routers that connect via switched circuits or dial-up
lines using PPP [RFC1661]. It has also been implemented with switches
and access points using IEEE 802 [IEEE.802.1990]. EAP encapsulation
on IEEE 802 wired media is described in [IEEE.802-1X.2001].
One of the advantages of the EAP architecture is its flexibility.
EAP is used to select a specific authentication mechanism, typically
after the authenticator requests more information in order to
determine the specific authentication mechanism(s) to be used.
Rather than requiring the authenticator to be updated to support each
new authentication method, EAP permits the use of a backend
authentication server which may implement some or all authentication
methods, with the authenticator acting as a pass-through for some or
all methods and users.
Within this document, authenticator requirements apply regardless of
whether the authenticator is operating as a pass-through or not.
Where the requirement is meant to apply to either the authenticator
or backend authentication server, depending on where the EAP
authentication is terminated, the term "EAP server" will be used.
1.1 Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. The key
words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
are to be interpreted as described in [RFC2119].
1.2 Terminology
This document frequently uses the following terms:
authenticator
The end of the link requiring the authentication. This
terminology is also used in [IEEE.802-1X.2001], and has the
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same meaning in this document.
peer The other end of the point-to-point link (PPP),
point-to-point LAN segment (IEEE 802 wired media) or 802.11
wireless link, which is being authenticated by the
authenticator. In [IEEE.802-1X.2001], this end is known as
the Supplicant.
backend authentication server
A backend authentication server is an entity that provides
an authentication service to an authenticator. This service
verifies the claim of identity made by the peer from the
credentials provided by the peer. This terminology is also
used in [IEEE.802-1X.2001].
Displayable Message
This is interpreted to be a human readable string of
characters, and MUST NOT affect operation of the protocol.
The message encoding MUST follow the UTF-8 transformation
format [RFC2279].
EAP server
The entity that terminates the EAP authentication with the
peer. In the case where there is no backend authentication
server, this term refers to the authenticator. Where the
authenticator operates in pass-through, it refers to the
backend authentication server.
Silently Discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the event, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
Security claims terminology (see Section 7.2):
Mutual authentication
This refers to an EAP method in which, within an
interlocked exchange, the authenticator authenticates the
peer and the peer authenticates the authenticator. Two
one-way conversations, running in opposite directions do
not provide mutual authentication as defined here.
Integrity protection
This refers to per-packet authentication and integrity
protection of EAP packets, including EAP Requests and
Responses, and method-specific success and failure
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indications. When making this claim, a method specification
MUST describe the fields within the EAP packet that are
protected.
Replay protection
This refers to protection against replay of EAP messages,
including EAP Requests and Responses, and method-specific
success and failure indications.
Confidentiality
This refers to encryption of EAP messages, including EAP
Requests and Responses, and method-specific success and
failure indications. A method making this claim MUST
support identity protection.
Key derivation
This refers to the ability of the EAP method to derive a
Master Key which is not exported, as well as a ciphersuite-
independent Master Session Keys. Both the Master Key and
Master Session Keys are used only for further key
derivation, not directly for protection of the EAP
conversation or subsequent data.
Key strength
This refers to the effective entropy of the derived Master
Session Keys, independent of their physical length. For
example, a 128-bit key derived from a password might have
an effective entropy much less than 128 bits.
Dictionary attack resistance
Where password authentication is used, users are
notoriously prone to select poor passwords. A method may be
said to be dictionary attack resistant if, when there is a
weak password in the secret, the method does not allow an
attack more efficient than brute force.
Fast reconnect
The ability, in the case where a security association has
been previously established, to create a new or refreshed
security association in a smaller number of round-trips.
Man-in-the-Middle resistance
The ability for the peer to demonstrate to the
authenticator that it has acted as the peer for each method
within the conversation. Similarly, the authenticator
demonstrates to the peer that it has acted as the
authenticator for each method within the conversation. If
this is not possible, then the authentication sequence or
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tunnel may be vulnerable to a man-in-the-middle attack.
Acknowledged result indications
The ability of the authenticator to provide the peer with
an indication of whether the peer has successfully
authenticated to it, and for the peer to acknowledge
receipt, as well as providing an indication of whether the
authenticator has successfully authenticated to the peer.
Since EAP Success and Failure packets are neither
acknowledged nor integrity protected, this claim requires
implementation of a method- specific result exchange that
is integrity protected.
2. Extensible Authentication Protocol (EAP)
The EAP authentication exchange proceeds as follows:
[1] The authenticator sends a Request to authenticate the peer. The
Request has a type field to indicate what is being requested.
Examples of Request types include Identity, MD5-challenge, etc.
The MD5-challenge type corresponds closely to the CHAP
authentication protocol [RFC1994]. Typically, the authenticator
will send an initial Identity Request; however, an initial
Identity Request is not required, and MAY be bypassed. For
example, the identity may not be required where it is determined
by the port to which the peer has connected (leased lines,
dedicated switch or dial-up ports); or where the identity is
obtained in another fashion (via calling station identity or MAC
address, in the Name field of the MD5-Challenge Response, etc.).
[2] The peer sends a Response packet in reply to a valid Request. As
with the Request packet the Response packet contains a Type
field, which corresponds to the Type field of the Request.
[3] The authenticator sends an additional Request packet, and the
peer replies with a Response. The sequence of Requests and
Responses continues as long as needed.
[4] The conversation continues until the authenticator cannot
authenticate the peer (unacceptable Responses to one or more
Requests), in which case the authenticator implementation MUST
transmit an EAP Failure (Code 4). Alternatively, the
authentication conversation can continue until the authenticator
determines that successful authentication has occurred, in which
case the authenticator MUST transmit an EAP Success (Code 3).
Since EAP is a peer-to-peer protocol, an independent and simultaneous
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authentication may take place in the reverse direction. Both peers
may act as authenticators and authenticatees at the same time.
Advantages
The EAP protocol can support multiple authentication mechanisms
without having to pre-negotiate a particular one.
Devices (e.g. a NAS, switch or access point) do not have to
understand each authentication method and MAY act as a
pass-through agent for a backend authentication server. Support
for pass-through is optional. An authenticator MAY authenticate
local users while at the same time acting as a pass-through for
non-local users and authentication methods it does not implement
locally.
For sessions in which the authenticator acts as a pass-through, it
MUST determine the outcome of the authentication solely based on
the Accept/Reject indication sent by the backend authentication
server; the outcome MUST NOT be determined by the contents of an
EAP packet sent along with the Accept/Reject indication, or the
absence of such an encapsulated EAP packet.
Separation of the authenticator from the backend authentication
server simplifies credentials management and policy decision
making.
Disadvantages
For use in PPP, EAP does require the addition of a new
authentication type to PPP LCP and thus PPP implementations will
need to be modified to use it. It also strays from the previous
PPP authentication model of negotiating a specific authentication
mechanism during LCP. Similarly, switch or access point
implementations need to support [IEEE.802-1X.2001] in order to use
EAP.
Where the authenticator is separate from the backend
authentication server, this complicates the security analysis and,
if needed, key distribution.
2.1 Support for sequences
An EAP conversation MAY utilize a sequence of methods. A common
example of this is an Identity request followed by a single EAP
authentication method such as an MD5-Challenge. However, within or
associated with each EAP server, it is not anticipated that a
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particular named peer will utilize multiple authentication methods
(Type 4 or greater), either by supporting a choice of methods or by
using multiple methods in sequence. This would make the peer
vulnerable to attacks that negotiate the least secure method from
among a set (negotiation attacks, described in Section 7.8) or
man-in-the-middle attacks (described in Section 7.4). Instead, for
each named peer there SHOULD be an indication of exactly one method
used to authenticate that peer name. If a peer needs to make use of
different authentication methods under different circumstances, then
distinct identities SHOULD be employed, each of which identifies
exactly one authentication method. If additional authentication
methods are required beyond the initial one, the authenticator MAY
send a Request packet for a subsequent authentication method, or it
MAY send another Identity request. If the peer does not support
additional methods, it SHOULD respond with a Nak, indicating no
acceptable alternative, as described in Section 5.3. However, peer
implementations MAY not respond at all, in which case a timeout will
result and authentication will fail. Since the authenticator
presumably requires successful completion of the sequence in order to
grant access, authentication failure is the correct result.
Therefore, it is not necessary for the authenticator to determine
that the peer supports sequences prior to sending a Request for a
subsequent authentication method.
The above prescription also applies in the situation where an
authenticator sends a message of a different Type prior to completion
of the final round of a given method. If the peer wishes to continue
authenticating with the method in progress, it SHOULD send a Nak in
response to such a Request, indicating the Type in progress as the
alternative. Otherwise it MAY send a Response with the same Type as
the Request. Since an EAP packet with a different Type may be sent
by an attacker, an authenticator receiving a Nak including a
preference for the Type in progress SHOULD log the event, but
otherwise not take any action.
Once a peer has sent a Response of the same Type as a Request, some
existing peer implementations might expect the method to run to
completion. As a result, these implementations silently discard EAP
Requests of a Type different from the method in progress, despite the
requirement for a Response in Section 4.1. For this reason, EAP
authenticators that must interoperate with these peers are
discouraged from switching methods before the final round of a given
method has completed.
2.2 EAP multiplexing model
Conceptually, EAP implementations consist of the following
components:
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[a] Lower layer. The lower layer is responsible for transmitting and
receiving EAP frames between the peer and authenticator. EAP has
been run over a variety of lower layers including PPP; wired IEEE
802 LANs [IEEE.802-1X.2001]; IEEE 802.11 wireless LANs
[IEEE.802-11.1999]; UDP (L2TP [RFC2661] and ISAKMP
[I-D.ietf-ipsra-pic]); and TCP [I-D.ietf-ipsra-pic]. Lower layer
behavior is discussed in Section 3.
[b] EAP layer. The EAP layer receives and transmits EAP packets via
the lower layer, implements the EAP state machine, and delivers
and receives EAP messages to and from EAP methods.
[c] EAP method. EAP methods implement the authentication algorithms
and receive and transmit EAP messages via the EAP layer. Since
fragmentation support is not provided by EAP itself, this is the
responsibility of EAP methods, which are discussed in Section 5.
The EAP multiplexing model is illustrated in figure 1 below. Note
that there is no requirement that an implementation conform to this
model, as long as the on-the-wire behavior is consistent with it.
Within EAP, the Type field functions much like a port number in UDP
or TCP. With the exception of Types handled by the EAP layer, it is
assumed that the EAP layer multiplexes incoming EAP packets according
to their Type, and delivers them only to the EAP method corresponding
to that Type code, with one exception.
Since EAP methods may wish to access the Identity, the Identity
Response can be assumed to be stored within the EAP layer so as to be
available to methods of Types other than 1 (Identity). The Identity
Type is discussed in Section 5.1.
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+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
| EAP method| EAP method| | EAP method| EAP method|
| Type = X | Type = Y | | Type = X | Type = Y |
| ! | | | ^ | |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
| ! | | ! |
| EAP ! Layer | | EAP ! Layer |
| ! | | ! |
| (Nak, ! Success, | | (Nak, ! Success, |
| ! Failure, | | ! Failure, |
| ! Notification, | | ! Notification, |
| ! Identity) | | ! Identity) |
| ! | | ! |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
| ! | | ! |
| Lower ! Layer | | Lower ! Layer |
| ! | | ! |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
! !
+------------>-------------+
Figure 1: EAP Multiplexing Model
A Notification Response is only used as confirmation that the peer
received the Notification Request, not that it has processed it, or
displayed the message to the user. It cannot be assumed that the
contents of the Notification Request or Response is available to
another method. The Notification Type is discussed in Section 5.2.
The Nak method is utilized for the purposes of method negotiation.
Peers MUST respond to an EAP Request for an unacceptable Type with a
Nak Response. It cannot be assumed that the contents of the Nak
Response is available to another method. The Nak Type is discussed
in Section 5.3.
EAP packets with codes of Success or Failure do not include a Type,
and therefore are not delivered to an EAP method. Success and Failure
are discussed in Section 4.2.
Given these considerations, the Success, Failure, Nak Response and
Notification Request/Response messages MUST NOT used to carry data
destined for delivery to EAP authentication Types (4 or greater).
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3. Lower layer behavior
3.1 Lower layer requirements
EAP makes the following assumptions about lower layers:
[1] Lower layer CRC or checksum is not necessary. In EAP, the
authenticator retransmits Requests that have not yet received
Responses, so that EAP does not assume that lower layers are
reliable. Since EAP defines its own retransmission behavior,
when run over a reliable lower layer, it is possible (though
undesirable) for retransmission to occur both in the lower layer
and the EAP layer.
If lower layers exhibit a high loss rate, then retransmissions
are likely, and since EAP Success and Failure are not
retransmitted, timeouts are also likely to result. EAP methods
such as EAP TLS [RFC2716] include a message integrity check (MIC)
and regard MIC errors as fatal. Therefore if a checksum or CRC is
not provided by the lower layer, then some methods may not behave
well.
[2] Lower layer data security. After EAP authentication is complete,
the peer will typically transmit data to the network, through the
authenticator. In order to provide assurance that the peer
transmitting data is the one that successfully completed EAP
authentication, it is necessary for the lower layer to provide
per- packet integrity, authentication and replay protection that
is bound to the original EAP authentication, or for the lower
layer to be physically secure. Otherwise it is possible for
subsequent data traffic to be hijacked, or replayed.
As a result of these considerations, EAP SHOULD be used only when
lower layers provide physical security for data (e.g. wired PPP
or IEEE 802 links), or for insecure links, where per-packet
authentication, integrity and replay protection is provided.
Where keying material for the lower layer ciphersuite is itself
provided by EAP, typically the lower layer ciphersuite cannot be
enabled until late in the EAP conversation, after key derivation
has completed. Thus it may only be possible to use the lower
layer ciphersuite to protect a portion of the EAP conversation,
such as the EAP Success or Failure packet.
[3] Known MTU. The EAP layer does not support fragmentation and
reassembly. However, EAP methods SHOULD be capable of handling
fragmentation and reassembly. As a result, EAP is capable of
functioning across a range of MTU sizes, as long as the MTU is
known.
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[4] Possible duplication. Where the lower layer is reliable, it will
provide the EAP layer with a non-duplicated stream of packets.
However, while it is desirable that lower layers provide for non-
duplication, this is not a requirement. The Identifier field
provides both the peer and authenticator with the ability to
detect duplicates.
[5] Ordering guarantees. EAP does not require the Identifier to be
monotonically increasing, and so is reliant on lower layer
ordering guarantees for correct operation. Also, EAP was
originally defined to run on PPP, and [RFC1661] Section 1 has an
ordering requirement:
"The Point-to-Point Protocol is designed for simple links which
transport packets between two peers. These links provide full-
duplex simultaneous bi-directional operation, and are assumed to
deliver packets in order."
Lower lower transports for EAP MUST preserve ordering between a
source and destination, at a given priority level (the level of
ordering guarantee provided by [IEEE.802.1990]).
3.2 EAP usage within PPP
In order to establish communications over a point-to-point link, each
end of the PPP link must first send LCP packets to configure the data
link during Link Establishment phase. After the link has been
established, PPP provides for an optional Authentication phase before
proceeding to the Network-Layer Protocol phase.
By default, authentication is not mandatory. If authentication of the
link is desired, an implementation MUST specify the Authentication-
Protocol Configuration Option during Link Establishment phase.
If the identity of the peer has been established in the
Authentication phase, the server can use that identity in the
selection of options for the following network layer negotiations.
When implemented within PPP, EAP does not select a specific
authentication mechanism at PPP Link Control Phase, but rather
postpones this until the Authentication Phase. This allows the
authenticator to request more information before determining the
specific authentication mechanism. This also permits the use of a
"back-end" server which actually implements the various mechanisms
while the PPP authenticator merely passes through the authentication
exchange. The PPP Link Establishment and Authentication phases, and
the Authentication-Protocol Configuration Option, are defined in The
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Point-to-Point Protocol (PPP) [RFC1661].
3.2.1 PPP Configuration Option Format
A summary of the PPP Authentication-Protocol Configuration Option
format to negotiate the EAP Authentication Protocol is shown below.
The fields are transmitted from left to right.
Exactly one EAP packet is encapsulated in the Information field of a
PPP Data Link Layer frame where the protocol field indicates type hex
C227 (PPP EAP).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Authentication-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
3
Length
4
Authentication-Protocol
C227 (Hex) for PPP Extensible Authentication Protocol (EAP)
3.3 EAP usage within IEEE 802
The encapsulation of EAP over IEEE 802 is defined in
[IEEE.802-1X.2001]. The IEEE 802 encapsulation of EAP does not
involve PPP, and IEEE 802.1X does not include support for link or
network layer negotiations. As a result, within IEEE 802.1X it is not
possible to negotiate non-EAP authentication mechanisms, such as PAP
or CHAP [RFC1994].
3.4 Link layer indications
The reliability and security of link layer indications is dependent
on the medium. Link layer failure indications accepted by the link
layer and provided to EAP MUST be processed. However, link layer
success indications MUST NOT result in an EAP implementation
concluding that authentication has succeeded, since these indications
are typically unauthenticated.
In PPP, link layer indications are not authenticated and are
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therefore subject to spoofing, provided that the attacker can gain
access to the physical medium. This includes LCP-Terminate (a link
failure indication), NCP (a link success indication), and "link down"
(a link failure indication).
In IEEE 802 wired networks, the IEEE 802.1X EAPOL-Start and
EAPOL-Logoff frames are not authenticated or integrity protected,
whereas within 802.11, authentication and integrity protection is
possible depending on when they are sent and the ciphersuite that has
been negotiated. Therefore, depending on the circumstances,
EAPOL-Start and EAPOL-Logoff frames may or may not be authenticated
and integrity protected.
In 802.11 a "link down" indication is an unreliable indication of
link failure, since wireless signal strength can come and go and may
be influenced by radio frequency interference generated by an
attacker. In 802.11, control and management frames are not
authenticated and an attacker within range can gain access to the
physical medium. Link layer indications include Disassociate and
Deauthenticate frames (link failure indications), and Association and
Reassociation Response frames (link success indications).
4. EAP Packet format
A summary of the EAP packet format is shown below. The fields are
transmitted from left to right.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
Code
The Code field is one octet and identifies the type of EAP packet.
EAP Codes are assigned as follows:
1 Request
2 Response
3 Success
4 Failure
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Since EAP only defines Codes 1-4, EAP packets with other codes
MUST be silently discarded by both authenticators and peers.
Identifier
The Identifier field is one octet and aids in matching Responses
with Requests.
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length and Data fields.
Octets outside the range of the Length field should be treated as
Data Link Layer padding and should be ignored on reception.
Data
The Data field is zero or more octets. The format of the Data
field is determined by the Code field.
4.1 Request and Response
Description
The Request packet (Code field set to 1) MUST be sent by the
authenticator to the peer; the peer MUST NOT send Request packets
to the authenticator. Each Request has a Type field which serves
to indicate what is being requested. Additional Request packets
MUST be sent until a valid Response packet is received, or an
optional retry counter expires. In [IEEE.802-1X.2001], the retry
counter is effectively set to zero, so that retransmission never
occurs, and instead the peer times out and authentication is
restarted.
Retransmitted Requests MUST be sent with the same Identifier value
in order to distinguish them from new Requests. The contents of
the data field is dependent on the Request type. The peer MUST
send a Response packet in reply to a valid Request packet.
Responses MUST only be sent in reply to a valid Request and never
retransmitted on a timer.
The Identifier field of the Response MUST match that of the
Request; if it does not match, then the Response MUST be silently
discarded. Authenticators receiving a Response for which the
authenticator has no outstanding Request MUST silently discard the
Response.
The authenticator MUST NOT send a new Request until a valid
Response is received to an outstanding Request. Since the
authenticator can retransmit before receiving a valid Response
from the peer, the authenticator can receive duplicate Responses.
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The authenticator MUST silently discard these duplicate Responses.
If a Message Integrity Check (MIC) is employed within an EAP
method, then implementations MUST silently discard any message
that fails this check. In this document, the descriptions of EAP
message handling assume that MIC validation is effectively
performed as though it occurs before examining any of the EAP
message fields (such as 'Code').
Implementation Note: These obligations apply regardless of
whether or not pass-through is implemented. The authenticator
is responsible for retransmitting Request messages. If the
Request message is obtained from elsewhere (such as from a
backend authentication server), then the authenticator will
need to save a copy of the Request in order to accomplish this.
The authenticator is also responsible for discarding Response
messages with the wrong Identifier value before acting on them
in any way, including passing them on to the backend
authentication server for verification. Similarly, the peer is
responsible for detecting and handling duplicate Request
messages before processing them in any way, including passing
them on to an outside party.
Because the authentication process will often involve user
input, some care must be taken when deciding upon
retransmission strategies and authentication timeouts. By
default, where EAP is run over an unreliable lower layer, the
EAP retransmission timer SHOULD be computed as described in
[RFC2988]. This includes use of Karn's algorithm to filter RTT
estimates resulting from retransmissions. A maximum of 3-5
retransmissions is suggested.
When run over a reliable lower layer (e.g. EAP over ISAKMP/TCP,
as within [I-D.ietf-ipsra-pic]), the EAP retransmission timer
SHOULD be set to an infinite value, so that retransmissions do
not occur at the EAP layer.
Where the authentication process requires user input, the
measured round trip times are largely determined by user
responsiveness rather than network characteristics, so that RTO
estimation is not helpful. Instead, the retransmission timer
SHOULD be set so as to provide sufficient time for the user to
respond, with longer timeouts required in certain cases, such
as where Token Cards (see Section 5.6) are involved.
In order to provide the EAP authenticator with guidance as to
the appropriate timeout value, a hint can be communicated to
the authenticator by the backend authentication server (such as
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via the RADIUS Session-Timeout attribute).
A summary of the Request and Response packet format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Type-Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
1 for Request
2 for Response
Identifier
The Identifier field is one octet. The Identifier field MUST be
the same if a Request packet is retransmitted due to a timeout
while waiting for a Response. Any new (non-retransmission)
Requests MUST modify the Identifier field. In order to avoid
confusion between new Requests and retransmissions, the Identifier
value chosen for each new Request need only be different from the
previous Request, but need not be unique within the conversation.
One way to achieve this is to start the Identifier at an initial
value and increment it for each new Request. Initializing the
first Identifier with a random number rather than starting from
zero is recommended, since it makes sequence attacks somewhat
harder.
Since the Identifier space is unique to each session,
authenticators are not restricted to only 256 simultaneous
authentication conversations. Similarly, with re-authentication,
an EAP conversation might continue over a long period of time, and
is not limited to only 256 roundtrips.
If a peer receives a valid duplicate Request for which it has
already sent a Response, it MUST resend its original Response. If
a peer receives a duplicate Request before it has sent a Response,
but after it has determined the initial Request to be valid (i.e.
it is waiting for user input), it MUST silently discard the
duplicate Request. An EAP message may be found invalid for a
variety of reasons: failed lower layer CRC or checksum, malformed
EAP packet, EAP method MIC failure, etc.
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Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, and Type-Data
fields. Octets outside the range of the Length field should be
treated as Data Link Layer padding and should be ignored on
reception.
Type
The Type field is one octet. This field indicates the Type of
Request or Response. A single Type MUST be specified for each EAP
Request or Response. Normally, the Type field of the Response
will be the same as the Type of the Request. However, there is
also a Nak Response Type for indicating that a Request type is
unacceptable to the peer. An initial specification of Types
follows in a later section of this document.
Type-Data
The Type-Data field varies with the Type of Request and the
associated Response.
4.2 Success and Failure
The Success packet is sent by the authenticator to the peer to
acknowledge successful authentication. The authenticator MUST
transmit an EAP packet with the Code field set to 3 (Success). If
the authenticator cannot authenticate the peer (unacceptable
Responses to one or more Requests) then the implementation MUST
transmit an EAP packet with the Code field set to 4 (Failure). An
authenticator MAY wish to issue multiple Requests before sending a
Failure response in order to allow for human typing mistakes. Success
and Failure packets MUST NOT contain additional data.
Implementation Note: Because the Success and Failure packets are
not acknowledged, the authenticator cannot know whether they have
been received. As a result, these packets are not retransmitted by
the authenticator, and if they are lost, the peer will timeout. If
acknowledged success and failure indications are desired, these
MAY be implemented within individual EAP methods.
A summary of the Success and Failure packet format is shown below.
The fields are transmitted from left to right.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
3 for Success
4 for Failure
Identifier
The Identifier field is one octet and aids in matching replies to
Responses. The Identifier field MUST match the Identifier field
of the Response packet that it is sent in response to.
Length
4
4.2.1 Processing of success and failure
Within EAP, success and failure indications consist of the EAP
Success and Failure messages, as well as method-specific indications.
Within EAP, these indications may be protected or unprotected.
EAP Success and Failure packets are considered unprotected
indications which may be spoofed, since as described in Section 4.2,
they contain no message integrity check (MIC).
In order to provide additional protection against tampering, EAP
methods MAY support a MIC that covers some or all of the EAP packet,
including headers. In addition, such a MIC MAY include coverage of
previous Request and Response messages, so as to enable protection of
other packets to that do not contain MICs, such as Identity Request/
Response, Notification Request/Response and Nak Response.
EAP methods also MAY include support for method-specific acknowledged
success and failure indications. This enables the authenticator to
indicate whether the peer has successfully authenticated, as well as
for the peer to acknowledge receipt of that indication, and respond
with an indication of whether the authenticator has successfully
authenticated to the peer. If a key has previously been derived, the
result exchange MAY be protected by a Message Integrity Check (MIC),
and if so, then this success/failure indication is considered
protected.
In order to decrease vulnerability to spoofing of success and failure
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indications, the following processing rules are recommended:
[a] Processing of protected success and failure indications. Where a
method-specific protected success/failure indication has been
received, the implementation MUST validate the EAP method MIC,
with a MIC failure handled via silent discard, as specified in
Section 4.1.
[b] Receipt of EAP Success and Failure packets prior to method
completion. A peer EAP implementation receiving an EAP Success
packet prior to completion of the method in progress MUST
silently discard it. This ensures that a rogue authenticator will
not be able to bypass mutual authentication by sending an EAP
Success prior to conclusion of the EAP method conversation. A
peer EAP implementation receiving an EAP Failure packet prior to
completion of the method in progress MAY silently discard it.
When using EAP methods that provide their own (protected) error
indications, premature EAP Failure packets are unexpected, so
that this technique may be more readily employed.
[c] Authentication requirement. An EAP peer implementation that has
been configured to require authentication MUST silently discard a
"canned" EAP Success message (an EAP Success message sent
immediately upon connection).
[d] Contradictory indications. Where protected and unprotected result
indications are both available, protected indications take
precedence. For example, where an EAP method provides a
protected indication that authentication failure has occurred in
either direction, the implementation MUST silently discard
subsequent EAP Success packets. Similarly, where an EAP method
provides a protected indication that authentication has succeeded
in both directions, the EAP implementation MAY silently discard
EAP Failure packets.
[e] Processing of EAP Success and Failure in the absence of protected
indications. Subsequent to the completion of the EAP
authentication method (Types 4 and greater), and in the absence
of protected result indications, EAP Success and Failure packets
MUST be accepted and processed by the EAP implementation.
5. Initial EAP Request/Response Types
This section defines the initial set of EAP Types used in Request/
Response exchanges. More Types may be defined in follow-on
documents. The Type field is one octet and identifies the structure
of an EAP Request or Response packet. The first 3 Types are
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considered special case Types.
The remaining Types define authentication exchanges. The Nak Type is
valid only for Response packets, it MUST NOT be sent in a Request.
The Nak Type MUST only be sent in response to a Request which uses an
authentication Type code (i.e., Type of 4 or greater).
All EAP implementations MUST support Types 1-4, which are defined in
this document, and SHOULD support Type 254. Follow-on RFCs MAY define
additional EAP Types.
1 Identity
2 Notification
3 Nak (Response only)
4 MD5-Challenge
5 One Time Password (OTP)
6 Generic Token Card (GTC)
254 Vendor-specific
255 Experimental use
5.1 Identity
Description
The Identity Type is used to query the identity of the peer.
Generally, the authenticator will issue this as the initial
Request. An optional displayable message MAY be included to prompt
the peer in the case where there expectation of interaction with a
user. A Response of Type 1 (Identity) SHOULD be sent in Response
to a Request with a Type of 1 (Identity).
Since Identity Requests and Responses are not protected, from a
security perspective, it may be preferable for protected method-
specific Identity exchanges to be used instead.
Implementation Note: The peer MAY obtain the Identity via user
input. It is suggested that the authenticator retry the
Identity Request in the case of an invalid Identity or
authentication failure to allow for potential typos on the part
of the user. It is suggested that the Identity Request be
retried a minimum of 3 times before terminating the
authentication phase with a Failure reply. The Notification
Request MAY be used to indicate an invalid authentication
attempt prior to transmitting a new Identity Request
(optionally, the failure MAY be indicated within the message of
the new Identity Request itself).
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Type
1
Type-Data
This field MAY contain a displayable message in the Request,
containing UTF-8 encoded ISO 10646 characters [RFC2279]. The
Response uses this field to return the Identity. If the Identity
is unknown, this field should be zero bytes in length. The field
MUST NOT be null terminated. The length of this field is derived
from the Length field of the Request/Response packet and hence a
null is not required.
5.2 Notification
Description
The Notification Type is optionally used to convey a displayable
message from the authenticator to the peer. An authenticator MAY
send a Notification Request to the peer at any time. The peer MUST
respond to a Notification Request with a Notification Response; a
Nak Response MUST NOT be sent.
The peer SHOULD display this message to the user or log it if it
cannot be displayed. The Notification Type is intended to provide
an acknowledged notification of some imperative nature, but it is
not an error indication, and therefore does not change the state
of the peer. Examples include a password with an expiration time
that is about to expire, an OTP sequence integer which is nearing
0, an authentication failure warning, etc. In most circumstances,
Notification should not be required.
Type
2
Type-Data
The Type-Data field in the Request contains a displayable message
greater than zero octets in length, containing UTF-8 encoded ISO
10646 characters [RFC2279]. The length of the message is
determined by Length field of the Request packet. The message
MUST NOT be null terminated. A Response MUST be sent in reply to
the Request with a Type field of 2 (Notification). The Type-Data
field of the Response is zero octets in length. The Response
should be sent immediately (independent of how the message is
displayed or logged).
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5.3 Nak
Description
The Nak Type is valid only in Response messages. It is sent in
reply to a Request where the desired authentication Type is
unacceptable. Authentication Types are numbered 4 and above. The
Response contains one or more authentication Types desired by the
Peer. Type zero (0) is used to indicate that the sender has no
viable alternatives.
Since the Nak Type is only valid in Responses and has very limited
functionality, it MUST NOT be used as a general purpose error
indication, such as for communication of error messages, or
negotiation of parameters specific to a particular EAP method.
Code
2 for Response.
Identifier
The Identifier field is one octet and aids in matching Responses
with Requests. The Identifier field of a Nak Response MUST match
the Identifier field of the Request packet that it is sent in
response to.
Length
>=6
Type
3
Type-Data
Where the Request contains a Type within the original EAP Type
space (1-253, 255), or the Request contains an expanded Type as
defined in Section 5.7, but the peer does not support expanded
Types, then the Type-Data field of the Nak Response MUST contain
one or more octets indicating the desired authentication Type(s),
one octet per Type, or the value zero (0) to indicate no proposed
alternative. When the Nak Response includes as one of the Type(s)
the value 254, this indicates an expanded Type of preference
indicated by the relative order. If the authenticator can
accomodate this preference, it will respond with a an expanded
Type Request.
If a peer supporting expanded Types receives an expanded Type
Request, then the Type-Data field of the Nak Response, if sent,
MUST contain one or more authentication Types, all of which MUST
be in the format below (8 octets per Type). This includes the
encoding of zero (0) in the Vendor-Type field, to indicate no
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proposed alternative. See Section 5.7 for details on the Vendor-Id
and Vendor-Type fields. If the peer does not support expanded
Types, then the Type-Data field of the Nak Response MUST contain
one or more authentication Type(s), one octet per Type, or the
value zero (0) to indicate no proposed alternative. However, the
value 254 MUST NOT be included as one of the preferred
authentication Types.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.4 MD5-Challenge
Description
The MD5-Challenge Type is analogous to the PPP CHAP protocol
[RFC1994] (with MD5 as the specified algorithm). The Request
contains a "challenge" message to the peer. A Response MUST be
sent in reply to the Request. The Response MAY be either of Type
4 (MD5-Challenge) or Type 3 (Nak). The Nak reply indicates the
peer's desired authentication Type(s). EAP peer and EAP server
implementations MUST support the MD5-Challenge mechanism. An
authenticator that supports only pass-through MUST allow
communication with a backend authentication server that is capable
of supporting MD5-Challenge, although the EAP authenticator
implementation need not support MD5-Challenge itself. However, if
the EAP authenticator can be configured to authenticate peers
locally (e.g. not operate in pass-through), then the requirement
for support of the MD5-Challenge mechanism applies.
Note that the use of the Identifier field in the MD5-Challenge
Type is different from that described in [RFC1994]. EAP allows
for retransmission of MD5-Challenge Request packets while
[RFC1994] states that both the Identifier and Challenge fields
MUST change each time a Challenge (the CHAP equivalent of the
MD5-Challenge Request packet) is sent.
Type
4
Type-Data
The contents of the Type-Data field is summarized below. For
reference on the use of these fields see the PPP Challenge
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Handshake Authentication Protocol [RFC1994].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value-Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Security Claims ) (see Section 7.2):
Intended use: Wired networks, including PPP, PPPOE, and
IEEE 802 wired media. Use over the
Internet or with wireless media only when
protected.
Mechanism: Password or pre-shared key.
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key Derivation: No
Key strength: N/A
Dictionary attack prot: No
Key hierarchy: N/A
Fast reconnect: No
MiTM resistance: No
Acknowledged S/F: No
5.5 One-Time Password (OTP)
Description
The One-Time Password system is defined in "A One-Time Password
System" [RFC2289] and "OTP Extended Responses" [RFC2243]. The
Request contains a displayable message containing an OTP
challenge. A Response MUST be sent in reply to the Request. The
Response MUST be of Type 5 (OTP) or Type 3 (Nak). The Nak
Response indicates the peer's desired authentication Type(s).
Type
5
Type-Data
The Type-Data field contains the OTP "challenge" as a displayable
message in the Request. In the Response, this field is used for
the 6 words from the OTP dictionary [RFC2289]. The messages MUST
NOT be null terminated. The length of the field is derived from
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the Length field of the Request/Reply packet.
Security Claims (see Section 7.2):
Intended use: Wired networks, including PPP, PPPOE, and
IEEE 802 wired media. Use over the
Internet or with wireless media only when
protected.
Mechanism: One-Time Password
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key Derivation: No
Key strength: N/A
Dictionary attack prot: No
Key hierarchy: N/A
Fast reconnect: No
MiTM resistance: No
Acknowledged S/F: No
5.6 Generic Token Card (GTC)
Description
The Generic Token Card Type is defined for use with various Token
Card implementations which require user input. The Request
contains a displayable message and the Response contains the Token
Card information necessary for authentication. Typically, this
would be information read by a user from the Token card device and
entered as ASCII text. A Response MUST be sent in reply to the
Request. The Response MUST be of Type 6 (GTC) or Type 3 (Nak).
The Nak Response indicates the peer's desired authentication
Type(s).
Type
6
Type-Data
The Type-Data field in the Request contains a displayable message
greater than zero octets in length. The length of the message is
determined by the Length field of the Request packet. The message
MUST NOT be null terminated. A Response MUST be sent in reply to
the Request with a Type field of 6 (Generic Token Card). The
Response contains data from the Token Card required for
authentication. The length of the data is determined by the
Length field of the Response packet.
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Security Claims (see Section 7.2):
Intended use: Wired networks, including PPP, PPPOE, and
IEEE 802 wired media. Use over the
Internet or with wireless media only when
protected.
Mechanism: Hardware token.
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key Derivation: No
Key strength: N/A
Dictionary attack prot: No
Key hierarchy: N/A
Fast reconnect: No
MiTM resistance: No
Acknowledged S/F: No
5.7 Vendor-specific
Description
Due to EAP's popularity, the original Method Type space, which
only provides for 255 values, is being allocated at a pace, which
if continued, would result in exhaustion within a few years. Since
many of the existing uses of EAP are vendor-specific, the
Vendor-Specific Method Type is available to allow vendors to
support their own extended Types not suitable for general usage.
The Vendor-specific type is also used to expand the global Method
Type space beyond the original 255 values. A Vendor-Id of 0 maps
the original 255 possible types onto a namespace of 2^32-1
possible types, allowing for virtually unlimited expansion. (Type
0 is only used in a Nak Response, to indicate no acceptable
alternative)
An implementation that supports the Vendor-specific attribute MUST
treat EAP types that are less than 256 equivalently whether they
appear as a single octet or as the 32-bit Vendor-Type within a
Vendor-specific type where Vendor-Id is 0. Peers not equipped to
interpret the Vendor-specific Type MUST send a Nak as described in
Section 5.3, and negotiate a more suitable authentication method.
A summary of the Vendor-specific Type format is shown below. The
fields are transmitted from left to right.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
254 for Vendor-specific
Vendor-Id
The Vendor-Id is 3 octets and represents the SMI Network
Management Private Enterprise Code of the Vendor in network byte
order, as allocated by IANA. A Vendor-Id of zero is reserved for
use by the IETF in providing an expanded global EAP Type space.
Vendor-Type
The Vendor-Type field is four octets and represents the vendor-
specific Method Type.
If Vendor-Id is zero, the Vendor-Type field is an extension and
superset of the existing namespace for EAP types. The first 256
types are reserved for compatibility with single-octet EAP types
that have already been assigned or may be assigned in the future.
Thus, EAP types from 0 through 255 are semantically identical
whether they appear as single octet EAP types or as Vendor-Types
when Vendor-Id is zero.
Vendor-Data
The Vendor-Data field is defined by the vendor. Where a Vendor-Id
of zero is present, the Vendor-Data field will be used for
transporting the contents of EAP methods of Types defined by the
IETF.
6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP
protocol, in accordance with BCP 26, [RFC2434].
There are two name spaces in EAP that require registration: Packet
Codes and Method Types.
EAP is not intended as a general-purpose protocol, and allocations
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SHOULD NOT be made for purposes unrelated to authentication.
6.1 Definition of Terms
The following terms are used here with the meanings defined in BCP
26: "name space", "assigned value", "registration".
The following policies are used here with the meanings defined in BCP
26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IETF Consensus", "Standards Action".
6.2 Recommended Registration Policies
For registration requests where a Designated Expert should be
consulted, the responsible IESG area director should appoint the
Designated Expert. For Designated Expert with Specification Required,
the request is posted to the EAP WG mailing list (or, if it has been
disbanded, a successor designated by the Area Director) for comment
and review, and MUST include a pointer to a public specification.
Before a period of 30 days has passed, the Designated Expert will
either approve or deny the registration request and publish a notice
of the decision to the EAP WG mailing list or its successor. A denial
notice must be justified by an explanation and, in the cases where it
is possible, concrete suggestions on how the request can be modified
so as to become acceptable.
For registration requests requiring Expert Review, the EAP mailing
list should be consulted. If the EAP mailing list is no longer
operational, an alternative mailing list may be designated by the
responsible IESG Area Director.
Packet Codes have a range from 1 to 255, of which 1-4 have been
allocated. Because a new Packet Code has considerable impact on
interoperability, a new Packet Code requires Standards Action, and
should be allocated starting at 5.
The original EAP Method Type space has a range from 1 to 255, and is
the scarcest resource in EAP, and thus must be allocated with care.
Method Types 1-36 have been allocated, with 20 available for re-use.
Method Types 37-191 may be allocated on the advice of a Designated
Expert, with Specification Required.
Allocation of blocks of Method Types (more than one for a given
purpose) should require IETF Consensus. EAP Type Values 192-253 are
reserved and allocation requires Standards Action.
Method Type 254 is allocated for the Vendor-Specific Type. Where the
Vendor-Id field is non-zero, the Vendor-Specific Type is used for
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functions specific only to one vendor's implementation of EAP, where
no interoperability is deemed useful. When used with a Vendor-Id of
zero, Method Type 254 can also be used to provide for an expanded
Method Type space. Method Type values 256-4294967295 may be
allocated after Type values 1-191 have been allocated.
Method Type 255 is allocated for Experimental use, such as testing of
new EAP methods before a permanent Type code is allocated.
7. Security Considerations
EAP was designed for use with dialup PPP [RFC1661] and was later
adapted for use in wired IEEE 802 networks [IEEE.802.1990] in
[IEEE.802-1X.2001]. On these networks, an attacker would need to
gain physical access to the telephone or switch infrastructure in
order to mount an attack. While such attacks have been documented,
such as in [DECEPTION], they are assumed to be rare.
However, subsequently EAP has been proposed for use on wireless
networks, and over the Internet, where physical security cannot be
assumed. On such networks, the security vulnerabilities are greater,
as are the requirements for EAP security.
This section defines the threat model and security terms and
describes the security claims section required in EAP method
specifications. We then discuss threat mitigation.
7.1 Threat model
On physically insecure networks, it is possible for an attacker to
gain access to the physical medium. This enables a range of attacks,
including the following:
[1] An adversary may try to discover user identities by snooping
authentication traffic.
[2] An adversary may try to modify or spoof EAP packets.
[3] An adversary may launch denial of service attacks by spoofing
layer 2 indications or EAP layer success/failure indications,
replaying EAP packets, or generating packets with overlapping
Identifiers.
[4] An adversary might attempt to recover the pass-phrase by mounting
an offline dictionary attack.
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[5] An adversary may attempt to convince the peer to connect to an
untrusted network, by mounting a man-in-the-middle attack.
[6] An adversary may attempt to disrupt the EAP negotiation in order
to weaken the authentication.
[7] An attacker may attempt to recover the key by taking advantage of
weak key derivation techniques used within EAP methods.
[8] An attacker may attempt to take advantage of weak ciphersuites
subsequently used after the EAP conversation is complete.
Where EAP is used over wireless networks, an attacker needs to be
within the coverage area of the wireless medium in order to carry out
these attacks. However, where EAP is used over the Internet, no such
restrictions apply.
7.2 Security claims
In order to clearly articulate the security provided by an EAP
method, EAP method specifications MUST include a Security Claims
section including the following declarations:
[a] Intended use. This includes a statement of whether the method is
intended for use over a physically secure or insecure network, as
well as a statement of the applicable media.
[b] Mechanism. This is a statement of the authentication technology:
certificates, pre-shared keys, passwords, token cards, etc.
[c] Security claims. This is a statement of the claimed security
properties of the method, using terms defined in Section 1.2:
mutual authentication, integrity protection, replay protection,
confidentiality, key derivation, key strength, dictionary attack
resistance, fast reconnect, man-in-the-middle resistance,
acknowledged result indications. The Security Claims section of
an EAP method specification SHOULD provide justification for the
claims that are made. This can be accomplished by including a
proof in an Appendix, or including a reference to a proof.
[d] Key strength. If the method derives keys, then the effective key
strength MUST be estimated.
[e] Description of key hierarchy. EAP methods deriving keys MUST
either provide a reference to a key hierarchy specification, or
describe how keys used for authentication/integrity, encryption
and IVs are to be derived from the provided keying material, and
how cryptographic separation is maintained between keys used for
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different purposes.
[f] Indication of vulnerabilities. In addition to the security claims
that are made, the specification MUST indicate which of the
security claims detailed in Section 1.2 are NOT being made.
7.3 Identity protection
An Identity exchange is optional within the EAP conversation.
Therefore, it is possible to omit the Identity exchange entirely, or
to postpone it until later in the conversation once a protected
channel has been established.
However, where roaming is supported as described in [RFC2607], it may
be necessary to locate the appropriate backend authentication server
before the authentication conversation can proceed. The realm
portion of the Network Access Identifier (NAI) [RFC2486] is typically
included within the Identity-Response in order to enable the
authentication exchange to be routed to the appropriate backend
authentication server. Therefore while the peer-name portion of the
NAI may be omitted in the Identity- Response, where proxies or relays
are present, the realm portion may be required.
7.4 Man-in-the-middle attacks
Where a sequence of methods is utilized for authentication or EAP is
tunneled within another protocol that omits peer authentication,
there exists a potential vulnerability to man-in-the-middle attack.
Where a sequence of EAP methods is utilized for authentication, the
peer might not have proof that a single entity has acted as the
authenticator for all EAP methods within the sequence. For example,
an authenticator might terminate one EAP method, then forward the
next method in the sequence to another party without the peer's
knowledge or consent. Similarly, the authenticator might not have
proof that a single entity has acted as the peer for all EAP methods
within the sequence.
This enables an attack by a rogue EAP authenticator tunneling EAP to
a legitimate server. Where the tunneling protocol is used for key
establishment but does not require peer authentication, an attacker
convincing a legitimate peer to connect to it will be able to tunnel
EAP packets to a legitimate server, successfully authenticating and
obtaining the key. This allows the attacker to successfully establish
itself as a man-in-the-middle, gaining access to the network, as well
as the ability to decrypt data traffic between the legitimate peer
and server.
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This attack may be mitigated by the following measures:
[a] Requiring mutual authentication within EAP tunneling mechanisms.
[b] Requiring cryptographic binding between EAP methods executed
within a sequence or between the EAP tunneling protocol and the
tunneled EAP methods. Where cryptographic binding is supported, a
mechanism is also needed to protect against downgrade attacks
that would bypass it.
[c] Limiting the EAP methods authorized for use without protection,
based on peer and authenticator policy.
[d] Avoiding the use of sequences or tunnels when a single, strong
method is available.
7.5 Packet modification attacks
While individual EAP methods may support per-packet data origin
authentication, integrity and replay protection, EAP itself does not
provide built-in support for this.
Since the Identifier is only a single octet, it is easy to guess,
allowing an attacker to successfully inject or replay EAP packets.
An attacker may also modify EAP headers within EAP packets where the
header is unprotected. This could cause packets to be inappropriately
discarded or misinterpreted.
In the case of PPP and IEEE 802 wired links, it is assumed that such
attacks are restricted to attackers who can gain access to the
physical link. However, where EAP is run over physically insecure
lower layers such as IEEE 802.11 or the Internet (such as within
protocols supporting PPP, EAP or Ethernet Tunneling), this assumption
is no longer valid and the vulnerability to attack is greater.
To protect EAP messages sent over physically insecure lower layers,
methods providing mutual authentication and key derivation, as well
as per-packet origin authentication, integrity and replay protection
SHOULD be used. Method-specific MICs may be used to provide
protection. Since EAP messages of Types Identity, Notification, and
Nak do not include their own MIC, it may be desirable for the EAP
method MIC to cover information contained within these messages, as
well as the header of each EAP message. To provide protection, EAP
also may be encapsulated within a protected channel created by
protocols such as ISAKMP [RFC2408] as is done in [I-D.ietf-ipsra-pic]
or within TLS [RFC2246]. However, as noted in Section 7.4, EAP
tunneling may result in a man-in-the-middle vulnerability.
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7.6 Dictionary attacks
Password authentication algorithms such as EAP-MD5, MS-CHAPv1
[RFC2433] and Kerberos V [RFC1510] are known to be vulnerable to
dictionary attacks. MS-CHAPv1 vulnerabilities are documented in
[PPTPv1]; Kerberos vulnerabilities are described in [KRBATTACK],
[KRBLIM], and [KERB4WEAK].
In order to protect against dictionary attacks, an authentication
algorithm resistant to dictionary attack (as defined in Section 7.2)
may be used. This is particularly important when EAP runs over media
which are not physically secure.
If an authentication algorithm is used that is known to be vulnerable
to dictionary attack, then the conversation may be tunneled within a
protected channel, in order to provide additional protection.
However, as noted in Section 7.4, EAP tunneling may result in a
man-in-the-middle vulnerability, and therefore dictionary attack
resistant methods are preferred.
7.7 Connection to an untrusted network
With EAP methods supporting one-way authentication, such as EAP-MD5,
the authenticator's identity is not verified. Where the lower layer
is not physically secure (such as where EAP runs over wireless media
or IP), this enables the peer to connect to a rogue authenticator. As
a result, where the lower layer is not physically secure, a method
supporting mutual authentication is recommended.
In EAP there is no requirement that authentication be full duplex or
that the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.
This will, of course, depend on the specific protocols negotiated.
However, in general, completing a single unitary mutual
authentication is preferable to two one-way authentications, one in
each direction. This is because separate authentications that are
not bound cryptographically so as to demonstrate they are part of the
same session are subject to man-in-the-middle attacks, as discussed
in Section 7.4.
7.8 Negotiation attacks
In a negotiation attack, the attacker attempts to convince the peer
and authenticator to negotiate a less secure EAP method. EAP does not
provide protection for the Nak packet, although it is possible for a
method to include coverage of Nak Responses within a method-specific
MIC.
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To avoid negotiation attacks in situations where EAP runs over
physically insecure media, for each named peer there SHOULD be an
indication of exactly one method used to authenticate that peer name,
as described in Section 2.1.
7.9 Implementation idiosyncrasies
The interaction of EAP with lower layer transports such as PPP and
IEEE 802 are highly implementation dependent.
For example, upon failure of authentication, some PPP implementations
do not terminate the link, instead limiting traffic in Network-Layer
Protocols to a filtered subset, which in turn allows the peer the
opportunity to update secrets or send mail to the network
administrator indicating a problem. Similarly, while in IEEE 802.1X
an authentication failure will result in denied access to the
controlled port, limited traffic may be permitted on the uncontrolled
port.
In EAP there is no provision for retries of failed authentication.
However, in PPP the LCP state machine can renegotiate the
authentication protocol at any time, thus allowing a new attempt.
Similarly, in IEEE 802.1X the Supplicant or Authenticator can
re-authenticate at any time. It is recommended that any counters used
for authentication failure not be reset until after successful
authentication, or subsequent termination of the failed link.
7.10 Key derivation
It is possible for the peer and EAP server to mutually authenticate,
and derive a Master Key (MK). The MK is unique to the peer and EAP
server and MUST NOT be exported by the EAP method, or used directly
to protect the EAP conversation or subsequent data. As a result,
possession of the MK represents proof of a successful authentication,
and this is potentially useful in enabling features such as fast
reconnect, or fast handoff.
In order to provide keying material for use in a subsequently
negotiated ciphersuite, the EAP method exports a Master Session Key
(MSK). Like the EAP Master Key, EAP Master Session Keys are also not
used directly to protect data; however, they are of sufficient size
to enable subsequent derivation of Transient Session Keys (TSKs) for
use with the selected ciphersuite.
EAP methods provide Master Session Keys and not Transient Session
Keys so as to allow EAP methods to be ciphersuite and media
independent. Depending on the lower layer, EAP methods may run before
or after ciphersuite negotiation, so that the selected ciphersuite
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may not be known to the EAP method. By providing keying material
usable with any ciphersuite, EAP methods can used with a wide range
of ciphersuites and media. Since the peer and EAP client reside on
the same machine, TSKs can be provided to the lower layer security
module without needing to leave the machine.
In the case where the backend authentication server and authenticator
reside on different machines, there are several implications for
security:
[a] Mutual authentication may occur between the peer and the backend
authentication server, if the negotiated EAP method supports
this. However, where the authenticator and backend authentication
server are separate, the peer and authenticator do not mutually
authenticate within EAP. However, subsequent to completion of
the EAP conversation, the lower layer may support mutual
authentication between the peer and authenticator. For example,
IEEE 802.11i includes a Transient Session Key derivation protocol
known as the 4-way handshake, which guarantees liveness of the
TSKs, provides for mutual authentication between the peer and
authenticator, replay protection, and protected ciphersuite
negotiation.
[b] The MSK negotiated between the peer and backend authentication
server will need to be transmitted to the authenticator. The
specification of this transit mechanism is outside the scope of
this document.
This specification does not provide detailed guidance on how EAP
methods are to derive the MK and MSK. Key derivation is an art that
is best practiced by professionals; rather than inventing new key
derivation algorithms, reuse of existing algorithms such as those
specified in IKE [RFC2409], or TLS [RFC2246] is recommended.
However, some general guidelines can be provided:
[1] The MK is for use only by the EAP authenticator and peer and MUST
NOT be exported by the EAP method or provided to a third party.
[2] Since the MSK is exported by the EAP method, while the MK is not,
possession of the MSK MUST NOT provide information useful in
determining the MK.
[3] The MSK and TSKs MUST be fresh. Otherwise it is infeasible to
detect messages replayed from prior sessions.
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[4] TSKs MUST be cryptographically independent from each other so
that if an attacker obtains one of them, he will not have gained
information useful in determining the other ones.
[5] There MUST be a way to determine whether TSKs belong to this or
to some other session.
[6] The MSK derived by EAP methods MUST be bound to the peers as well
as to the authentication method, so as to avoid a
man-in-the-middle attack (see Section 7.4).
7.11 Weak ciphersuites
If after the initial EAP authentication, data packets are sent
without per-packet authentication, integrity and replay protection,
an attacker with access to the media can inject packets, "flip bits"
within existing packets, replay packets, or even hijack the session
completely. Without per-packet confidentiality, it is possible to
snoop data packets.
As a result, as noted in Section 3.1, where EAP is used over a
physically insecure lower layer, per-packet authentication, integrity
and replay protection SHOULD be used, and per-packet confidentiality
is also recommended.
8. Acknowledgments
This protocol derives much of its inspiration from Dave Carrel's AHA
draft as well as the PPP CHAP protocol [RFC1994]. Valuable feedback
was provided by Yoshihiro Ohba of Toshiba America Research, Jari
Arkko of Ericsson, Sachin Seth of Microsoft, and Glen Zorn of Cisco
Systems.
Normative References
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2243] Metz, C., "OTP Extended Responses", RFC 2243, November
1997.
Blunk, et al. Expires July 2, 2003 [Page 38]
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[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2289] Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time
Password System", RFC 2289, February 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[IEEE.802.1990]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Overview and
Architecture", IEEE Standard 802, 1990.
[IEEE.802-1X.2001]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X, September 2001.
Informative References
[DECEPTION]
Slatalla, M. and J. Quittner, "Masters of Deception",
HarperCollins , New York, 1995.
[RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A.
and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,
January 1999.
[RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier",
RFC 2486, January 1999.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
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[RFC2408] Maughan, D., Schneider, M. and M. Schertler, "Internet
Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC
2433, October 1998.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.
and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC
2661, August 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[KRBATTACK]
Wu, T., "A Real-World Analysis of Kerberos Password
Security".
[KRBLIM] Bellovin, S. and M. Merrit, "Limitations of the Kerberos
authentication system, Proceedings of the 1991 Winter
USENIX Conference, pp. 253-267", , 1991.
[KERB4WEAK]
Dole, B., Lodin, S. and E. Spafford, "Misplaced trust:
Kerberos 4 session keys, Proceedings of the Internet
Society Network and Distributed System Security Symposium,
pp. 60-70", , March 1997.
[I-D.ietf-ipsra-pic]
Aboba, B., Krawczyk, H. and Y. Sheffer, "PIC, A Pre-IKE
Credential Provisioning Protocol", draft-ietf-ipsra-pic-06
(work in progress), October 2002.
[PPTPv1] Schneier, B. and Mudge, "Cryptanalysis of Microsoft's
Point-to- Point Tunneling Protocol, Proceedings of the 5th
ACM Conference on Communications and Computer Security,
ACM Press", , November 1998.
[IEEE.802-3.1996]
Institute of Electrical and Electronics Engineers,
"Information technology - Telecommunications and
Information Exchange between Systems - Local and
Metropolitan Area Networks - Specific requirements - Part
3: Carrier sense multiple access with collision detection
(CSMA/CD) Access Method and Physical Layer
Blunk, et al. Expires July 2, 2003 [Page 40]
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Specifications", IEEE Standard 802.3, 1996.
[IEEE.802-11.1999]
Institute of Electrical and Electronics Engineers,
"Information Technology - Telecommunications and
Information Exchange between Systems - Local and
Metropolitan Area Network - Specific Requirements - Part
11: Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications", IEEE Standard 802.11, 1999.
Authors' Addresses
Larry J. Blunk
Merit Network, Inc
4251 Plymouth Rd., Suite 2000
Ann Arbor, MI 48105-2785
USA
Phone: +1 734-647-9563
EMail: ljb@merit.edu
John R. Vollbrecht
Vollbrecht Consulting LLC
9682 Alice Hill Drive
Dexter, MI 48130
USA
Phone:
EMail: jrv@umich.edu
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 706 6605
EMail: bernarda@microsoft.com
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James Carlson
Sun Microsystems, Inc
1 Network Drive
Burlington, MA 01803-2757
USA
Phone: +1 781 442 2084
EMail: james.d.carlson@sun.com
Appendix A. Change Log
This section lists the major changes between [RFC2284] and this
document. Minor changes, including style, grammar, spelling and
editorial changes are not mentioned here.
[To be written]
Appendix B. Open issues
Open issues relating to this specification are tracked on the
following web site:
http://www.drizzle.com/~aboba/EAP/eapissues.html
The current working documents for this draft are available at this
web site:
http://www.levkowetz.com/pub/ietf/drafts/eap/
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