EAP Working Group L. Blunk
Internet-Draft Merit Network, Inc
Obsoletes: 2284 (if approved) J. Vollbrecht
Expires: November 14, 2003 Vollbrecht Consulting LLC
B. Aboba
Microsoft
J. Carlson
Sun
H. Levkowetz, Ed.
ipUnplugged
May 16, 2003
Extensible Authentication Protocol (EAP)
<draft-ietf-eap-rfc2284bis-03.txt>
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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
methods. EAP typically runs directly over data link layers such as
PPP or IEEE 802, without requiring IP. EAP provides its own support
for duplicate elimination and retransmission, but is reliant on lower
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layer ordering guarantees. Fragmentation is not supported within EAP
itself; however, individual EAP methods may support this.
This document obsoletes RFC 2284. A summary of the changes between
this document and RFC 2284 is available in Appendix B.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Specification of Requirements . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . 4
1.3 Security claims terminology for EAP methods . . . . . 5
2. Extensible Authentication Protocol (EAP) . . . . . . . . . . 9
2.1 Support for sequences . . . . . . . . . . . . . . . . 10
2.2 EAP multiplexing model . . . . . . . . . . . . . . . . 11
3. Lower layer behavior . . . . . . . . . . . . . . . . . . . . 14
3.1 Lower layer requirements . . . . . . . . . . . . . . . 14
3.2 EAP usage within PPP . . . . . . . . . . . . . . . . . 16
3.2.1 PPP Configuration Option Format . . . . . . . . 16
3.3 EAP usage within IEEE 802 . . . . . . . . . . . . . . 17
3.4 Lower layer indications . . . . . . . . . . . . . . . 17
4. EAP Packet format . . . . . . . . . . . . . . . . . . . . . 17
4.1 Request and Response . . . . . . . . . . . . . . . . . 18
4.2 Success and Failure . . . . . . . . . . . . . . . . . 21
5. Initial EAP Request/Response Types . . . . . . . . . . . . . 23
5.1 Identity . . . . . . . . . . . . . . . . . . . . . . . 23
5.2 Notification . . . . . . . . . . . . . . . . . . . . . 24
5.3 Nak . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3.1 Legacy Nak . . . . . . . . . . . . . . . . . . . 25
5.3.2 Expanded Nak . . . . . . . . . . . . . . . . . . 26
5.4 MD5-Challenge . . . . . . . . . . . . . . . . . . . . 28
5.5 One-Time Password (OTP) . . . . . . . . . . . . . . . 30
5.6 Generic Token Card (GTC) . . . . . . . . . . . . . . . 31
5.7 Expanded Types . . . . . . . . . . . . . . . . . . . . 32
5.8 Experimental . . . . . . . . . . . . . . . . . . . . . 33
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 34
6.1 Packet Codes . . . . . . . . . . . . . . . . . . . . . 35
6.2 Method Types . . . . . . . . . . . . . . . . . . . . . 35
7. Security Considerations . . . . . . . . . . . . . . . . . . 35
7.1 Threat model . . . . . . . . . . . . . . . . . . . . . 36
7.2 Security claims . . . . . . . . . . . . . . . . . . . 37
7.3 Identity protection . . . . . . . . . . . . . . . . . 38
7.4 Man-in-the-middle attacks . . . . . . . . . . . . . . 38
7.5 Packet modification attacks . . . . . . . . . . . . . 39
7.6 Dictionary attacks . . . . . . . . . . . . . . . . . . 40
7.7 Connection to an untrusted network . . . . . . . . . . 40
7.8 Negotiation attacks . . . . . . . . . . . . . . . . . 41
7.9 Implementation idiosyncrasies . . . . . . . . . . . . 41
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7.10 Key derivation . . . . . . . . . . . . . . . . . . . . 41
7.11 Weak ciphersuites . . . . . . . . . . . . . . . . . . 42
7.12 Link layer . . . . . . . . . . . . . . . . . . . . . . 43
7.13 Separation of authenticator and backend authentication
server . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 45
Normative References . . . . . . . . . . . . . . . . . . . . 45
Informative References . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
A. Method Specific Behavior . . . . . . . . . . . . . . . . . . 49
A.1 Message Integrity Checks . . . . . . . . . . . . . . . 49
B. Changes from RFC 2284 . . . . . . . . . . . . . . . . . . . 50
C. Open issues . . . . . . . . . . . . . . . . . . . . . . . . 51
Intellectual Property and Copyright Statements . . . . . . . 53
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1. Introduction
This document defines the Extensible Authentication Protocol (EAP),
an authentication framework which supports multiple authentication
methods. EAP typically runs directly over data link layers such as
PPP or IEEE 802, without requiring IP. EAP provides its own support
for duplicate elimination and retransmission, but is reliant on lower
layer ordering guarantees. 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]. EAP encapsulation on
IEEE 802 wired media is described in [IEEE-802.1X], and encapsulation
on IEEE wireless LANs in [IEEE-802.11i].
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 method 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 peers.
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:
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authenticator
The end of the EAP link initiating EAP authentication. The
term Authenticator is used in [IEEE-802.1X], and
authenticator has the same meaning in this document.
peer
The end of the EAP Link that responds to the authenticator.
In [IEEE-802.1X], 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. When used,
this server typically executes EAP methods for the
authenticator. This terminology is also used in
[IEEE-802.1X].
Displayable Message
This is interpreted to be a human readable string of
characters. The message encoding MUST follow the UTF-8
transformation format [RFC2279].
EAP server
The entity that terminates the EAP authentication method
with the peer. In the case where no backend authentication
server is used the EAP server is part of the authenticator.
In the case where the authenticator operates in
pass-through mode, the EAP server is located on 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.
1.3 Security claims terminology for EAP methods
These terms are used to described the security properties of EAP
methods:
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
independent one-way methods, running in opposite directions
do not provide mutual authentication as defined here.
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Integrity protection
This refers to providing data origin authentication and
protection against unauthorized modification of information
for EAP packets (including EAP Requests and Responses).
When making this claim, a method specification MUST
describe the EAP packets and fields within the EAP packet
that are protected.
Replay protection
This refers to protection against replay of an EAP method
or its messages, including 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 (see Section 7.3).
Key derivation
This refers to the ability of the EAP method to derive
exportable keying material such as the Master Session Key
(MSK), and Extended Master Session Key (EMSK). The MSK is
used only for further key derivation, not directly for
protection of the EAP conversation or subsequent data. Use
of the EMSK is reserved.
Key strength
If the effective key strength is N bits, the best currently
known methods to recover the key (with non-negligible
probability) require an effort comparable to 2^N operations
of a typical block cipher.
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
This property applies only for the use of multiple methods
in a combination, such as in authentication sequences or
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tunnels. It refers to the ability of the peer to
demonstrate to the authenticator that it has acted as the
peer for each authentication method within the
conversation. Similarly, the authenticator demonstrates to
the peer that it has acted as the authenticator for each
authentication method within the conversation. If this is
not possible, then there may be a vulnerability 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
authenticated, integrity and replay protected on a
per-packet basis.
Message Integrity Check (MIC)
A keyed hash function used for authentication and integrity
protection of data. This is usually called a Message
Authentication Code (MAC), but IEEE 802 specifications (and
this document) use the acronym MIC to avoid confusion with
Medium Access Control.
Cryptographic binding
The demonstration of the EAP peer to the EAP server that a
single entity has acted as the EAP peer for all methods
executed within a sequence or tunnel. Binding MAY also
imply that the EAP server demonstrates to the peer that a
single entity has acted as the EAP server for all methods
executed within a sequence or tunnel. If executed
correctly, binding serves to mitigate man-in-the-middle
vulnerabilities.
Cryptographic separation
Two keys (x and y) are "cryptographically separate" if an
adversary that knows all messages exchanged in the protocol
cannot compute x from y or y from x without "breaking" some
cryptographic assumption. In particular, this definition
allows that the adversary has the knowledge of all nonces
sent in cleartext as well as all predictable counter values
used in the protocol. Breaking a cryptographic assumption
would typically require inverting a one- way function or
predicting the outcome of a cryptographic pseudo- random
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number generator without knowledge of the secret state. In
other words, if the keys are cryptographically separate,
there is no shortcut to compute x from y or y from x, but
the work an adversary must do to perform this computation
is equivalent to performing exhaustive search for the
secret state value."
EAP Master key (MK)
A key derived between the EAP client and server during the
EAP authentication process that is purely local to the EAP
method. The MK MUST NOT be exported from the EAP method or
be made available to a third party. Since derivation of
the MK is a residue of the successful completion of the EAP
authentication exchange, proof of MK possession may be used
to shorten future EAP exchanges between the same EAP client
and server, a technique known as "fast resume".
Master Session Key (MSK)
Keying material that is derived between the EAP client and
server. The MSK is used in the derivation of Transient
Session Keys (TSKs) for the ciphersuite negotiated between
the EAP peer and authenticator. Where a backend
authentication server is present, acting as an EAP server,
it will typically transport the MSK to the authenticator.
The MSK differs from the MK in that it not assumed to
remain local to the EAP method, and is known by all parties
in the EAP exchange: the peer, authenticator and the
authentication server (if present). The MSK MAY be derived
from the MK via a one-way function, or it may be an
independent quantity. However possession of the MSK MUST
NOT provide any information useful in determining the MK.
Extended Master Session Key (EMSK)
Additional keying material derived between the EAP client
and server that is exported by the EAP method. However,
unlike the MSK, the EMSK is known only to the EAP peer and
EAP server and MUST NOT be provided to a third party. The
EMSK therefore MUST NOT be transported by the backend
authentication server to the authenticator. The EMSK is
reserved for future uses that are not defined yet. For
example, it could be used to derive additional keying
material for purposes such as fast handoff,
man-in-the-middle vulnerability protection, etc.
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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. EAP is a 'lock step'
protocol, so that other than the initial Request, a new Request
cannot be sent prior to receiving a valid Response. The
authenticator MUST NOT send a Success or Failure packet as a
result of a timeout. After a suitable number of timeouts have
elapsed, the authenticator SHOULD end the EAP conversation.
[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
authentication may take place in the reverse direction. Both peers
may act as authenticators and authenticatees at the same time. For a
discussion of security issues in peer-to-peer operation, see Section
7.7.
Advantages:
o The EAP protocol can support multiple authentication mechanisms
without having to pre-negotiate a particular one.
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o Network Access Server (NAS) devices (e.g., a 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 peers while at the same time acting as a
pass-through for non-local peers and authentication methods it
does not implement locally.
o Separation of the authenticator from the backend authentication
server simplifies credentials management and policy decision
making.
Disadvantages:
o 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] in order to use EAP.
o 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 the peer and
authenticator MUST utilize only one authentication method (Type 4 or
greater) within an EAP conversation, after which the authenticator
MUST send a Success or Failure packet.
Once a peer has sent a Response of the same Type as the initial
Request, an authenticator MUST NOT send a Request of a different Type
prior to completion of the final round of a given method (with the
exception of a Notification-Request) and MUST NOT send a Request for
an additional method of any Type after completion of the initial
authentication method; a peer receiving such Requests MUST treat them
as invalid, and silently discard them. As a result, Identity Requery
is not supported.
A peer MUST NOT send a Nak (legacy or expanded) in reply to a
Request, after an initial non-Nak Response has been sent. Since
spoofed EAP Request packets may be sent by an attacker, an
authenticator receiving an unexpected Nak SHOULD silently discard it
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and log the event.
Multiple authentication methods within an EAP conversation are not
supported due to their vulnerability to man-in-the-middle attacks
(see Section 7.4) and incompatibility with existing implementations.
Where a single EAP authentication method is utilized, but other
methods are run within it (a "tunneled" method) the prohibition
against multiple authentication methods does not apply. Such
"tunneled" methods appear as a single authentication method to EAP.
Backward compatibility can be provided, since a peer not supporting a
"tunneled" method can reply to the initial EAP-Request with a Nak
(legacy or expanded). To address security vulnerabilities,
"tunneled" methods MUST support protection against man-in-the-middle
attacks.
2.2 EAP multiplexing model
Conceptually, EAP implementations consist of the following
components:
[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]; IEEE 802.11 wireless LANs [IEEE-802.11];
UDP (L2TP [RFC2661] and ISAKMP [PIC]); and TCP [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 duplicate detection and
retransmission, 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.
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+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
| EAP method| EAP method| | EAP method| EAP method|
| Type = X | Type = Y | | Type = X | Type = Y |
| V | | | ^ | |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
| ! | | ! |
| EAP ! Layer | | EAP ! Layer |
| ! | | ! |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
| ! | | ! |
| Lower ! Layer | | Lower ! Layer |
| ! | | ! |
+-+-+-+-!-+-+-+-+-+-+-+-+ +-+-+-+-!-+-+-+-+-+-+-+-+
! !
! Peer ! Authenticator
+------------>-------------+
Figure 1: EAP Multiplexing Model
Within EAP, the Type field functions much like a port number in UDP
or TCP. 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.
Since EAP authentication methods may wish to access the Identity, the
Identity Request and Response can be assumed to be accessible to
authentication methods (Types 4 or greater) in addition to the
Identity method. The Identity Type is discussed in Section 5.1.
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.
Nak (Type 3) or Expanded Nak (Type 254) are utilized for the purposes
of method negotiation. Peers respond to an initial EAP Request for
an unacceptable Type with a Nak Response (Type 3) or Expanded Nak
Response (Type 254). It cannot be assumed that the contents of the
Nak Response(s) are available to another method. The Nak Type(s) are
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(s) and
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Notification Request/Response messages MUST NOT be used to carry data
destined for delivery to other EAP methods.
Where an authenticator operates as a pass-through, it forwards
packets back and forth between the peer and a backend authentication
server, based on the EAP layer header fields (Code, Identifier,
Length). This includes performing validity checks on the Code,
Identifer and Length fields, as described in Section 4.1.
Since pass-through authenticators rely on a backend authenticator
server to implement methods, the EAP method layer header fields
(Type, Type-Data) are not examined as part of the forwarding
decision. The forwarding model is illustrated in Figure 2. Compliant
pass-through authenticator implementations MUST by default be capable
of forwarding packets from any EAP method. The use of the RADIUS
protocol for encapsulation of EAP in pass-through operation is
described in [RFC2869bis].
Peer Pass-through Authenticator Authentication
Server
+-+-+-+-+-+-+ +-+-+-+-+-+-+
| | | |
|EAP method | |EAP method |
| Layer | | Layer |
| V | | ^ |
+-+-+-!-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-!-+-+-+
| ! | | | | | ! |
|EAP !Layer| | EAP Layer | EAP Layer | |EAP !Layer|
| ! | | +-----+-----+ | | ! |
| ! | | ! | ! | | ! |
+-+-+-!-+-+-+ +-+-+-!-+-+-+-+-+-!-+-+-+ +-+-+-!-+-+-+
| ! | | ! | ! | | ! |
|Lower!Layer| |Lower!Layer| AAA ! /IP | | AAA ! /IP |
| ! | | ! | ! | | ! |
+-+-+-!-+-+-+ +-+-+-!-+-+-+-+-+-!-+-+-+ +-+-+-!-+-+-+
! ! ! !
! ! ! !
+-------->-----+ +------->------+
Figure 2: Pass-through Authenticator
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
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an encapsulated EAP packet.
3. Lower layer behavior
3.1 Lower layer requirements
EAP makes the following assumptions about lower layers:
[1] Unreliable transport. 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 it
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.
Note that EAP Success and Failure packets are not retransmitted.
Without a reliable lower layer, and a non-negligible error rate,
these packets can be lost, resulting in timeouts. It is therefore
desirable for implementations to improve their resilience to loss
of EAP Success or Failure packets, as described in Section 4.2.
[2] Lower layer error detection. While EAP does not assume that the
lower layer is reliable, it does rely on lower layer error
detection (e.g., CRC, Checksum, MIC, etc.). EAP methods may not
include a MIC, or if they do, it may not be computed over all the
fields in the EAP packet, such as the Code, Identifier, Length or
Type fields. As a result, without lower layer error detection,
undetected errors could creep into the EAP layer or EAP method
layer header fields, resulting in authentication failures.
For example, EAP TLS [RFC2716], which computes its MIC over the
Type-Data field only, regards MIC validation failures as a fatal
error. Without lower layer error detection, this method and
others like it will not perform reliably.
[3] Lower layer security. EAP assumes that lower layers either
provide physical security (e.g., wired PPP or IEEE 802 links) or
support per-packet authentication, integrity and replay
protection. EAP SHOULD NOT be used on physically insecure links
(e.g., wireless or the Internet) where subsequent data is not
protected by per-packet authentication, integrity and replay
protection.
After EAP authentication is complete, the peer will typically
transmit data to the network via the authenticator. In order to
provide assurance that the peer transmitting data is the same
entity that successfully completed EAP authentication, the lower
layer needs to bind per-packet integrity, authentication and
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replay protection to the original EAP authentication, using keys
derived during EAP authentication. Alternatively, the lower
layer needs 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, ciphersuite negotiation and key activation is
controlled by the lower layer. In PPP, ciphersuites are
negotiated within ECP so that it is not possible to use keys
derived from EAP authentication until the completion of ECP.
Therefore an initial EAP exchange cannot protected by a PPP
ciphersuite, although EAP re-authentication can be protected.
In IEEE 802 media, key activation also typically occurs after
completion of EAP authentication. Therefore an initial EAP
exchange typically cannot be protected by lower layer
ciphersuite, although an EAP re-authentication or
pre-authentication exchange can be protected.
[4] MTU known a-priori. 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 a-priori. EAP does not support path MTU
discovery.
[5] 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.
[6] Ordering guarantees. EAP does not require the Identifier to be
monotonically increasing, and so is reliant on lower layer
ordering guarantees for correct operation. 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."
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Lower layer transports for EAP MUST preserve ordering between a
source and destination, at a given priority level (the ordering
guarantee provided by [IEEE-802]).
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
"backend" 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
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 EAP 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type
3
Length
4
Authentication Protocol
C227 (Hex) for Extensible Authentication Protocol (EAP)
3.3 EAP usage within IEEE 802
The encapsulation of EAP over IEEE 802 is defined in [IEEE-802.1X].
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 Lower layer indications
The reliability and security of lower layer indications is dependent
on the lower layer. Since EAP is media independent, the presence or
absence of lower layer security is not taken into account in the
processing of EAP messages.
Lower layer failure indications provided to EAP by the lower layer
MUST be processed and will cause an EAP exchange in progress to be
aborted. However, lower layer success indications MUST NOT affect
EAP message processing; an EAP implementation cannot conclude that
authentication has succeeded based on those indications. This ensures
that an attacker spoofing lower layer indications can at best succeed
in a denial of service attack.
A discussion of some reliability and security issues with lower layer
indications in PPP, IEEE 802 wired networks and IEEE 802.11 wireless
LANs can be found in the Security Considerations, Section 7.12.
4. EAP Packet format
A summary of the EAP 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
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) is sent by the
authenticator to the peer. 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.
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Retransmitted Requests MUST be sent with the same Identifier value
in order to distinguish them from new Requests. The contents of
the data field are 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
currently outstanding Request. An authenticator receiving a
Response whose Identifier value does not match that of the
currently outstanding Request MUST silently discard the Response.
The Type field of a Response MUST either match that of the
Request, or correspond to a legacy or Expanded Nak (see Section
5.3). An EAP server receiving a Response not meeting this
requirement MUST silently discard it.
Implementation Note: 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 peer is responsible
for detecting and handling duplicate Request messages before
processing them in any way, including passing them on to an
outside party. The authenticator is also responsible for
discarding Response messages with a non-matching Identifier
value before acting on them in any way, including passing them
on to the backend authentication server for verification.
Since the authenticator can retransmit before receiving a
Response from the peer, the authenticator can receive multiple
Responses, each with a matching Identifier. Until a new Request
is received by the authenticator, the Identifier value is not
updated, so that the authenticator forwards Responses to the
backend authentication server, one at a time.
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 [PIC]), the authenticator retransmission timer
SHOULD be set to an infinite value, so that retransmissions do
not occur at the EAP layer. Note that in this case the peer
may still maintain a timeout value so as to avoid waiting
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indefinitely for a Request.
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
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.
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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.
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 are
also Nak Response Types for indicating that a Request Type is
unacceptable to the peer (see Section 5.3). An initial
specification of Types follows in Section 5 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 after
completion of an EAP authentication method (Type 4 or greater), to
indicate that the peer has authenticated successfully to the
authenticator. 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 after unsuccessful completion of the EAP method in
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progress, 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.
EAP Success or Failure packets MUST NOT be sent by an EAP server
prior to completion of the final round of a given method. A peer EAP
implementation receiving a Success or Failure packet prior to
completion of the method in progress MUST silently discard it. By
default, an EAP peer MUST silently discard a "canned" EAP Success
message (an EAP Success message sent immediately upon connection).
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.
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. If acknowledged result indications are
desired, these MAY be implemented within individual EAP methods.
Since only a single EAP authentication method is supported within
an EAP conversation, a peer that successfully authenticates the
authenticator MAY, in the event that an EAP Success is not
received, conclude that the EAP Success packet was lost and enable
the link.
A summary of the Success and Failure 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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Length
4
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
considered special case Types.
The remaining Types define authentication exchanges. Nak (Type 3) or
Expanded Nak (Type 254) are valid only for Response packets, they
MUST NOT be sent in a Request.
All EAP implementations MUST support Types 1-4, which are defined in
this document, and SHOULD support Type 254. Implementations MAY
support other Types defined here or in future RFCs.
1 Identity
2 Notification
3 Nak (Response only)
4 MD5-Challenge
5 One Time Password (OTP)
6 Generic Token Card (GTC)
254 Expanded Types
255 Experimental use
EAP methods MAY support authentication based on shared secrets. If
the shared secret is a passphrase entered by the user,
implementations MAY support entering passphrases with non-ASCII
characters. In this case, the input should be processed using an
appropriate stringprep [RFC3454] profile, and encoded in octets using
UTF-8 encoding [RFC2279]. A possible registered stringprep profile is
specified in [SASLPREP].
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 is an expectation of
interaction with a user. A Response of Type 1 (Identity) SHOULD
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be sent in Response to a Request with a Type of 1 (Identity).
Since Identity Requests and Responses are not protected, from a
privacy perspective, it may be preferable for protected
method-specific Identity exchanges to be used instead. Where the
peer is configured to only accept authentication methods
supporting protected identity exchanges, the peer MAY provide an
abbreviated Identity Response (such as omitting the peer-name
portion of the NAI [RFC2486]). For further discussion of identity
protection, see Section 7.3.
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).
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 when there is
no outstanding Request, prior to completion of an EAP
authentication method. The peer MUST respond to a Notification
Request with a Notification Response unless the EAP authentication
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method specification prohibits the use of Notification message.
In any case, a Nak Response MUST NOT be sent in response to a
Notification Request.
An EAP method MAY indicate within its specification that
Notification messages must not be sent during that method. In this
case, the peer MUST silently discard Notification Requests from
the point where an initial Request for that Type is answered with
a Response of the same Type.
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).
5.3 Nak
5.3.1 Legacy Nak
Description
The legacy 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.
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Since the legacy Nak Type is valid only 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 legacy 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 a peer receives a Request for an unacceptable authentication
Type (4-253,255), or a peer lacking support for Expanded Types
receives a Request for Type 254, a Nak Response (Type 3) MUST be
sent. The Type-Data field of the Nak Response (Type 3) 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. A peer supporting Expanded Types that
receives a Request for an unacceptable authentication Type (4-253,
255) MAY include the value 254 in the Nak Response (Type 3) in
order to indicate the desire for an Expanded authentication Type.
If the authenticator can accommodate this preference, it will
respond with an Expanded Type Request (Type 254).
5.3.2 Expanded Nak
Description
The Expanded Nak Type is valid only in Response messages. It MUST
be sent only in reply to a Request of Type 254 (Expanded Type)
where the authentication Type is unacceptable. The Expanded Nak
Type uses the Expanded Type format itself, and the Response
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contains one or more authentication Types desired by the peer, all
in Expanded Type format. Type zero (0) is used to indicate that
the sender has no viable alternatives. The general format of the
Expanded Type is described in Section 5.7.
Since the Expanded Nak Type is valid only 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 an Expanded Nak Response
MUST match the Identifier field of the Request packet that it is
sent in response to.
Length
>=40
Type
254
Vendor-Id
0 (IETF)
Vendor-Type
3 (Nak)
Vendor-Data
The Expanded Nak Type is only sent when the Request contains an
Expanded Type (254) as defined in Section 5.7. The Vendor-Data
field of the Nak Response MUST contain one or more authentication
Types (4 or greater), all in expanded format, 8 octets per Type,
or the value zero (0), also in Expanded Type format, to indicate
no proposed alternative. The desired authentication Types may
include a mixture of Vendor-Specific and IETF Types. For example,
an Expanded Nak Response indicating a preference for OTP (Type 5),
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and an MIT (Vendor-Id=20) Expanded Type of 6 would appear as
follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 (Nak) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 (OTP) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 20 (MIT) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An Expanded Nak Response indicating a no desired alternative would
appear as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 (Nak) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=254 | 0 (IETF) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 (No alternative) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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), Nak (Type 3) or Expanded Nak (Type 254). The
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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.
Note: [RFC1994] treats the shared secret as an octet string, and
does not specify how it is entered into the system (or if it is
handled by the user at all). EAP MD5-Challenge implementations MAY
support entering passphrases with non-ASCII characters. See
Section 5 for instructions how the input should be processed and
encoded into octets.
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
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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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: 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: N/A
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 an OTP challenge in the format described in
[RFC2289]. A Response MUST be sent in reply to the Request. The
Response MUST be of Type 5 (OTP), Nak (Type 3) or Expanded Nak
(Type 254). 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
the Length field of the Request/Reply packet.
Note: [RFC2289] does not specify how the secret pass-phrase is
entered by the user, or how the pass-phrase is converted into
octets. EAP OTP implementations MAY support entering passphrases
with non-ASCII characters. See Section 5 for instructions how the
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input should be processed and encoded into octets.
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: N/A
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), Nak (Type 3) or
Expanded Nak (Type 254). 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
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authentication. The length of the data is determined by the
Length field of the Response packet.
EAP GTC implementations MAY support entering a response with
non-ASCII characters. See Section 5 for instructions how the
input should be processed and encoded into octets.
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: N/A
Acknowledged S/F: No
5.7 Expanded Types
Description
Since many of the existing uses of EAP are vendor-specific, the
Expanded method Type is available to allow vendors to support
their own Expanded Types not suitable for general usage.
The Expanded 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 Expanded 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 Expanded
Type where Vendor-Id is 0. Peers not equipped to interpret the
Expanded Type MUST send a Nak as described in Section 5.3.1, and
negotiate a more suitable authentication method.
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A summary of the Expanded Type 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
254 for Expanded Type
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 the 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.
5.8 Experimental
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Description
The Experimental Type has no fixed format or content. It is
intended for use when experimenting with new EAP Types. This Type
is intended for experimental and testing purposes. No guarantee
is made for interoperability between peers using this Type, as
outlined in [IANA-EXP].
Type
255
Type-Data
Undefined
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
SHOULD NOT be made for purposes unrelated to authentication.
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".
For registration requests where a Designated Expert should be
consulted, the responsible IESG area director should appoint the
Designated Expert. The intention is that any allocation will be
accompanied by a published RFC. But in order to allow for the
allocation of values prior to the RFC being approved for publication,
the Designated Expert can approve allocations once it seems clear
that an RFC will be published. The Designated expert will post a
request to the EAP WG mailing list (or a successor designated by the
Area Director) for comment and review, including an Internet-Draft.
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, as well
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as informing IANA. 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.
6.1 Packet Codes
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.
6.2 Method Types
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 1-41 have been allocated, with 20 available for re-use.
Method Types 42-191 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 Expanded Type. Where the
Vendor-Id field is non-zero, the Expanded Type is used for 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
IETF 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] in
[IEEE-802.1X]. 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.
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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 attacker may try to discover user identities by snooping
authentication traffic.
[2] An attacker may try to modify or spoof EAP packets.
[3] An attacker may launch denial of service attacks by spoofing
lower layer indications or Success/Failure packets; by replaying
EAP packets; or by generating packets with overlapping
Identifiers.
[4] An attacker may attempt to recover the pass-phrase by mounting an
offline dictionary attack.
[5] An attacker may attempt to convince the peer to connect to an
untrusted network, by mounting a man-in-the-middle attack.
[6] An attacker may attempt to disrupt the EAP negotiation in order
cause a weak authentication method to be selected.
[7] An attacker may attempt to recover keys 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.
[9] An attacker may attempt to perform downgrading attacks on lower
layer ciphersuite negotiation in order to ensure that a weaker
ciphersuite is used subsequently to EAP authentication.
Where EAP is used over wired networks, an attacker typically requires
access to the physical infrastructure in order to carry out these
attacks. However, where EAP is used over wireless networks, EAP
packets may be forwarded by authenticators (e.g., pre-authentication)
so that the attacker need not be within the coverage area of an
authenticator in order to carry out an attack on it or its peers.
Where EAP is used over the Internet, attacks may be carried out at an
even greater distance.
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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 lower layers.
[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, 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. This estimate is meant for potential
users of the method to determine if the keys produced are strong
enough for the intended application.
The effective key strength SHOULD be stated as number of bits,
defined as follows: If the effective key strength is N bits, the
best currently known methods to recover the key (with
non-negligible probability) require an effort comparable to 2^N
operations of a typical block cipher. The statement SHOULD be
accompanied by a short rationale, explaining how this number was
arrived at. This explanation SHOULD include the parameters
required to achieve the stated key strength based on current
knowledge of the algorithms.
(Note: Although it is difficult to define what "comparable
effort" and "typical block cipher" exactly mean, reasonable
approximations are sufficient here. Refer to e.g. [SILVERMAN]
for more discussion.)
The key strength depends on the methods used to derive the keys.
For instance, if keys are derived from a shared secret (such as a
password or a long-term secret), and possibly some public
information such as nonces, the effective key strength is limited
by the strength of the long-term secret (assuming that the
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derivation procedure is computationally simple). To take another
example, when using public key algorithms, the strength of the
symmetric key depends on the strength of the public keys used.
[e] Description of key hierarchy. EAP methods deriving keys MUST
either provide a reference to a key hierarchy specification, or
describe how Master Session Keys (MSKs) and Extended Master
Session Keys (EMSKs) are to be derived.
[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 use a method-specific identity exchange 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 EAP is tunneled within another protocol that omits peer
authentication, there exists a potential vulnerability to
man-in-the-middle attack.
As noted in Section 2.1, EAP does not permit sequences of
authentication methods. Were a sequence of EAP authentication
methods to be permitted, 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
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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.
This attack may be mitigated by the following measures:
[a] Requiring mutual authentication within EAP tunneling mechanisms.
[b] Requiring cryptographic binding 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 tunnels when a single, strong method is
available.
7.5 Packet modification attacks
While EAP methods may support per-packet data origin authentication,
integrity and replay protection, support is not provided within the
EAP layer.
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 wireless (802.11 or cellular) 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
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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. For details, see Appendix A.
To provide protection, EAP also may be encapsulated within a
protected channel created by protocols such as ISAKMP [RFC2408] as is
done in [PIC] or within TLS [RFC2246]. However, as noted in Section
7.4, EAP tunneling may result in a man-in-the-middle vulnerability.
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].
Where EAP runs over lower layers which are not physically secure, in
order to protect against dictionary attacks, an authentication
algorithm resistant to dictionary attack (as defined in Section 7.2)
SHOULD be used.
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 peer does not authenticate the authenticator. Where the lower
layer is not physically secure (such as where EAP runs over wireless
media or the Internet), the peer is vulnerable 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
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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 Nak Response packets, although it is
possible for a method to include coverage of Nak Responses within a
method-specific MIC.
Within or associated with each authenticator, it is not anticipated
that a particular named peer will support a choice of methods. This
would make the peer vulnerable to attacks that negotiate the least
secure method from among a set. 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.
7.9 Implementation idiosyncrasies
The interaction of EAP with lower layers 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 keys. In order to provide keying material for use in a
subsequently negotiated ciphersuite, an EAP method supporting key
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derivation MUST export a Master Session Key (MSK) of at least 64
octets, and an Extended Master Session Key (EMSK) of at least 64
octets.
The MSK and EMSK are 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. Each
ciphersuite is responsible for specifying how to derive the TSKs from
the MSK. The EAP method is also responsible for the derivation of
Transient EAP Keys (TEKs) used for protection of the EAP conversation
itself.
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 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.
Non-overlapping substrings of the MSK MUST be cryptographically
separate from each other. This is required because some existing
ciphersuites form TSKs by simply splitting the MSK to pieces of
appropriate length. Likewise, non-overlapping substrings of EMSK
MUST be cryptographically separate from each other, and from
substrings of the MSK.
The EMSK is reserved for future use and MUST remain on the EAP peer
and EAP server where it is derived; it MUST NOT be transported to, or
shared with, additional parties, or used to derive any other keys.
(This restriction will be relaxed in a future document that specifies
how the EMSK can be used.)
This specification does not provide detailed guidance on how EAP
methods are to derive the MSK, EMSK and TEKs, or how the TSKs are to
be derived from the 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.
Further details on EAP Key Derivation are provided within [KEYFRAME].
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
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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.
Additionally, if the lower layer performs ciphersuite negotiation, it
should be understood that EAP does not provide by itself integrity
protection of that negotiation. Therefore, in order to avoid
downgrading attacks which would lead to weaker ciphersuites being
used, clients implementing lower layer ciphersuite negotiation SHOULD
protect against negotiation downgrading.
This can be done by enabling users to configure which are the
acceptable ciphersuites as a matter of security policy; or, the
ciphersuite negotiation MAY be authenticated using keying material
derived from the EAP authentication and a MIC algorithm agreed upon
in advance by lower-layer peers.
7.12 Link layer
There exists a number of reliability and security issues with link
layer indications in PPP, IEEE 802 wired networks and IEEE 802.11
wireless LANs:
[a] PPP. In PPP, link layer indications such as LCP-Terminate (a
link failure indication) and NCP (a link success indication) are
not authenticated or integrity protected. They can therefore be
spoofed by an attacker with access to the physical medium.
[b] IEEE 802 wired networks. On wired networks, IEEE 802.1X messages
are sent to a non-forwardable multicast MAC address. As a
result, while the IEEE 802.1X EAPOL-Start and EAPOL-Logoff frames
are not authenticated or integrity protected, only an attacker
with access to the physical link can spoof these messages.
[c] IEEE 802.11 wireless LANs. In IEEE 802.11, link layer
indications include Disassociate and Deauthenticate frames (link
failure indications), and Association and Reassociation Response
frames (link success indications). These messages are not
authenticated or integrity protected, and although they are not
forwardable, they are spoofable by an attacker within range.
In IEEE 802.11, IEEE 802.1X data frames may be sent as Class 3
unicast data frames, and are therefore forwardable. This implies
that while EAPOL-Start and EAPOL-Logoff messages may be
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authenticated and integrity protected, they can be spoofed by an
authenticated attacker far from the target when
"pre-authentication" is enabled.
In IEEE 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. To avoid unnecessary resets, it is
advisable to damp these indications, rather than passing them
directly to the EAP. Since EAP supports retransmission, it is
robust against transient connectivity losses.
7.13 Separation of authenticator and backend authentication server
It is possible for the EAP peer and authenticator to mutually
authenticate, and derive a Master Session Key (MSK) for a ciphersuite
used to protect subsequent data traffic. This does not present an
issue on the peer, since the peer and EAP client reside on the same
machine; all that is required is for the EAP client module to derive
and pass a Transient Session Key (TSK) to the ciphersuite module.
However, in the case where the authenticator and authentication
server reside on different machines, there are several implications
for security.
[a] Authentication will occur between the peer and the authentication
server, not between the peer and the authenticator. This means
that it is not possible for the peer to validate the identity of
the authenticator that it is speaking to, using EAP alone.
[b] As discussed in [RFC2869bis], the authenticator is dependent on
the AAA protocol in order to know the outcome of an
authentication conversation, and does not look at the
encapsulated EAP packet (if one is present) to determine the
outcome. In practice this means that the AAA protocol spoken
between the authenticator and authentication server MUST support
per-packet authentication, integrity and replay protection.
[c] Where EAP is used over lower layers which are not physically
secure, subsequent to completion of the EAP conversation, a
subsequent protocol SHOULD be run between the peer and
authentication in order to mutually authenticate the peer and
authenticator; guarantee liveness of the TSKs; provide protected
ciphersuite and capabilities negotiation; and provide for
synchronized key usage.
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[d] An EAP Master Session Key (MSK) negotiated between the peer and
authentication server MAY be transmitted to the authenticator.
Therefore a mechanism needs to be provided to transmit the MSK
from the authentication server to the authenticator that needs
it. The specification of the key transport and wrapping
mechanism is outside the scope of this document.
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, Glen Zorn of Cisco
Systems, Jesse Walker of Intel, Bill Arbaugh, Nick Petroni and Bryan
Payne of the University of Maryland, Steve Bellovin of AT&T Research,
Paul Funk of Funk Software, Pasi Eronen of Nokia, Joseph Salowey of
Cisco and Paul Congdon of HP and members of the EAP working group.
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.
[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.
Blunk, et al. Expires November 14, 2003 [Page 45]
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[IEEE-802]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Overview and
Architecture", IEEE Standard 802, 1990.
[IEEE-802.1X]
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.
[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.
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[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[KRBATTACK]
Wu, T., "A Real-World Analysis of Kerberos Password
Security", Stanford University Computer Science
Department, http://theory.stanford.edu/~tjw/krbpass.html.
[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.
[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]
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
Specifications", IEEE Standard 802.3, September 1998.
[IEEE-802.11]
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.
[SILVERMAN]
Silverman, Robert D., "A Cost-Based Security Analysis of
Symmetric and Asymmetric Key Lengths", RSA Laboratories
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Bulletin 13, April 2000 (Revised November 2001), http://
www.rsasecurity.com/rsalabs/bulletins/bulletin13.html.
[RFC2869bis]
Aboba, B. and P. Calhoun, "RADIUS Support For Extensible
Authentication Protocol (EAP)",
draft-aboba-radius-rfc2869bis-21 (work in progress), May
2003.
[IANA-EXP]
Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful",
draft-narten-iana-experimental-allocations-03 (work in
progress), December 2002.
[KEYFRAME]
Aboba, B. and D. Simon, "EAP Keying Framework",
draft-aboba-pppext-key-problem-06 (work in progress),
March 2003.
[SASLPREP]
Zeilenga, K., "SASLprep: Stringprep profile for user names
and passwords", draft-ietf-sasl-saslprep-01 (work in
progress), May 2003.
[IEEE-802.11i]
Institute of Electrical and Electronics Engineers,
"Unapproved Draft Supplement to Standard for
Telecommunications and Information Exchange Between
Systems - LAN/MAN Specific Requirements - Part 11:
Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications: Specification for Enhanced
Security", IEEE Draft 802.11i (work in progress), 2003.
Authors' Addresses
Larry J. Blunk
Merit Network, Inc
4251 Plymouth Rd., Suite 2000
Ann Arbor, MI 48105-2785
USA
Phone: +1 734-647-9563
Fax: +1 734-647-3185
EMail: ljb@merit.edu
Blunk, et al. Expires November 14, 2003 [Page 48]
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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
Fax: +1 425 936 6605
EMail: bernarda@microsoft.com
James Carlson
Sun Microsystems, Inc
1 Network Drive
Burlington, MA 01803-2757
USA
Phone: +1 781 442 2084
Fax: +1 781 442 1677
EMail: james.d.carlson@sun.com
Henrik Levkowetz
ipUnplugged AB
Arenavagen 33
Stockholm S-121 28
SWEDEN
Phone: +46 8 725 9513
EMail: henrik@levkowetz.com
Appendix A. Method Specific Behavior
A.1 Message Integrity Checks
Today, EAP methods commonly define message integrity checks (MICs)
that cover more than one EAP packet. For example, EAP-TLS [RFC2716]
defines a MIC over a TLS record that could be split into multiple
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fragments; within the FINISHED message, the MIC is computed over
previous messages. Where the MIC covers more than one EAP packet, a
MIC validation failure is typically considered a fatal error.
If a per-packet MIC is employed within an EAP method, then peers,
authentication servers, and authenticators not operating in
pass-through mode MUST validate the MIC. MIC validation failures
SHOULD be logged. Whether a MIC validation failure is considered a
fatal error or not is determined by the EAP method specification.
Within EAP-TLS [RFC2716] a MIC validation failure is treated as a
fatal error, since that is what is specified in TLS [RFC2246].
However, it is also possible to develop EAP methods that support
per-packet MICs, and respond to verification failures by silently
discarding the offending packet.
In this document, descriptions of EAP message handling assume that
per-packet MIC validation, where it occurs, is effectively performed
as though it occurs before sending any responses or changing the
state of the host which received the packet.
Appendix B. Changes from RFC 2284
This section lists the major changes between [RFC2284] and this
document. Minor changes, including style, grammar, spelling and
editorial changes are not mentioned here.
o The Terminology section (Section 1.2) has been expanded, defining
more concepts and giving more exact definitions.
o In Section 2, it is explicitly specified that more than one
exchange of Request and Response packets may occur as part of the
EAP authentication exchange. How this may and may not be used is
specified in detail in Section 2.1.
o Also in Section 2, some requirements on the authenticator when
acting in pass-through mode has been made explicit.
o An EAP multiplexing model (Section 2.2) has been added, to
illustrate a typical implementation of EAP. There is no
requirement that an implementation conforms to this model, as long
as the on-the-wire behavior is consistent with it.
o As EAP is now in use with a variety of lower layers, not just PPP
for which it was first designed, Section 3 on lower layer behavior
has been added.
o In the description of the EAP Request and Response interaction
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(Section 4.1), it has been more exactly specified when packets
should be silently discarded, and also the behavior on receiving
duplicate requests. The implementation notes in this section has
been substantially expanded.
o In Section 4.2, it has been clarified that Success and Failure
packets must not contain additional data, and the implementation
note has been expanded. A subsection giving requirements on
processing of success and failure packets has been added.
o Section 5 on EAP Request/Response Types lists two new Type values:
the Expanded Type (Section 5.7), which is used to expand the Type
value number space, and the Experimental Type. In the Expanded
Type number space, the new Expanded Nak (Section 5.3.2) Type has
been added. Clarifications have been made in the description of
most of the existing Types. Security claims summaries have been
added for authentication methods.
o In Section 5.5, support for OTP Extended Responses [RFC2243] has
been added to EAP OTP.
o An IANA Considerations section (Section 6) has been added, giving
registration policies for the numbering spaces defined for EAP.
o The Security Considerations (Section 7) have been greatly
expanded, aiming at giving a much more comprehensive coverage of
possible threats and other security considerations.
o Appendix A has been added on method-specific behavior, providing
guidance on how EAP method-specific integrity checks should be
processed. Where possible, it is desirable for a method-specific
MIC to be computed over the entire EAP packet, including the EAP
layer header (Code, Identifier, Length) and EAP method layer
header (Type, Type-Data).
Appendix C. Open issues
(This section should be removed by the RFC editor before publication)
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:
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http://www.levkowetz.com/pub/ietf/drafts/eap/
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Intellectual Property Statement
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
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Blunk, et al. Expires November 14, 2003 [Page 54]
