PPPEXT Working Group Paul Funk
Internet-Draft Funk Software, Inc.
Category: Standards Track Simon Blake-Wilson
<draft-ietf-pppext-eap-ttls-05.txt> Basic Commerce &
Industries, Inc.
July 2004
EAP Tunneled TLS Authentication Protocol
(EAP-TTLS)
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2001-2004). All Rights Reserved.
Abstract
EAP-TTLS is an EAP protocol that extends EAP-TLS. In EAP-TLS, a TLS
handshake is used to mutually authenticate a client and server. EAP-
TTLS extends this authentication negotiation by using the secure
connection established by the TLS handshake to exchange additional
information between client and server. In EAP-TTLS, the TLS
handshake may be mutual; or it may be one-way, in which only the
server is authenticated to the client. The secure connection
established by the handshake may then be used to allow the server to
authenticate the client using existing, widely-deployed
authentication infrastructures such as RADIUS. The authentication of
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the client may itself be EAP, or it may be another authentication
protocol such as PAP, CHAP, MS-CHAP or MS-CHAP-V2.
Thus, EAP-TTLS allows legacy password-based authentication protocols
to be used against existing authentication databases, while
protecting the security of these legacy protocols against
eavesdropping, man-in-the-middle and other cryptographic attacks.
EAP-TTLS also allows client and server to establish keying material
for use in the data connection between the client and access point.
The keying material is established implicitly between client and
server based on the TLS handshake.
This document describes two versions of EAP-TTLS - version 0 and
version 1. Most of the document concerns EAP-TTLS v0, a form of the
protocol that has been implemented by multiple vendors. Section 11
defines EAP-TTLS v1, an enhanced version of the protocol that
utilizes the TLS extensions mechanism to allow authentications to
occur within, rather than after, the TLS handshake. The TLS
extension that is defined is believed to useful in its own right,
and may be used in other contexts in addition to EAP-TTLS v1.
Table of Contents
1. Introduction......................................................3
2. Motivation........................................................4
3. Terminology.......................................................6
4. Architectural Model...............................................8
4.1 Carrier Protocols.............................................9
4.2 Security Relationships.......................................10
4.3 Messaging....................................................10
4.4 Resulting Security...........................................11
5. Protocol Layering Model..........................................11
6. EAP-TTLS version 0 Overview......................................12
6.1 Phase 1: Handshake...........................................13
6.2 Phase 2: Tunnel..............................................14
6.3 Piggybacking.................................................14
6.4 Session Resumption...........................................15
6.4.1 TTLS Server Guidelines for Session Resumption............16
7. Generating Keying Material.......................................16
8. EAP-TTLS Encoding................................................17
8.1 EAP-TTLS Start Packet........................................17
8.2 EAP-TTLS Packets with No Data................................18
9. Encapsulation of AVPs within the TLS Record Layer................18
9.1 AVP Format...................................................19
9.2 AVP Sequences................................................20
9.3 Guidelines for Maximum Compatibility with AAA Servers........20
10. Tunneled Authentication..........................................20
10.1 Implicit challenge...........................................21
10.2 Tunneled Authentication Protocols............................21
10.2.1 EAP ......................................................22
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10.2.2 CHAP .....................................................23
10.2.3 MS-CHAP..................................................23
10.2.4 MS-CHAP-V2...............................................24
10.2.5 PAP ......................................................25
10.3 Performing Multiple Authentications..........................26
11. EAP-TTLS Version 1...............................................27
11.1 EAP-TTLS v1 Introduction.....................................27
11.2 Intentions Beyond EAP-TTLS...................................28
11.3 The InnerApplication Extension to TLS........................28
11.3.1 TLS/IA Overview..........................................29
11.3.2 Message Exchange.........................................31
11.3.3 Master Key Permutation...................................31
11.3.4 Session Resumption.......................................33
11.3.5 Error Termination........................................33
11.3.6 Application Session Key Material.........................33
11.3.7 Computing Verification Data..............................34
11.3.8 Attribute-Value Pairs (AVPs).............................36
11.3.9 TLS/IA Messages..........................................36
11.3.10 The InnerApplication Extension...........................37
11.3.11 The PhaseFinished Handshake Message......................37
11.3.12 The ApplicationPayload Handshake Message.................37
11.3.13 The InnerApplicationFailure Alert........................38
11.4 Binding of TLS/IA to EAP-TTLS v1.............................38
11.4.1 Flags Octet..............................................38
11.4.2 Version Negotiation......................................39
11.4.3 Acknowledgement Packets..................................39
11.4.4 Generating Keying Material...............................40
12. Discussion of Certificates and PKI...............................40
13. Message Sequences................................................42
13.1 Successful authentication via tunneled CHAP..................42
13.2 Successful authentication via tunneled EAP/MD5-Challenge.....44
13.3 Successful session resumption................................46
14. Security Considerations..........................................47
15. Changes since previous drafts....................................49
16. References.......................................................50
17. Authors' Addresses...............................................51
18. Full Copyright Statement.........................................51
1. Introduction
Extensible Authentication Protocol (EAP) [2] defines a standard
message exchange that allows a server to authenticate a client based
on an authentication protocol agreed upon by both parties. EAP may
be extended with additional authentication protocols by registering
such protocols with IANA or by defining vendor specific protocols.
Transport Layer Security (TLS) [3] is an authentication protocol
that provides for client authentication of a server or mutual
authentication of client and server, as well as secure ciphersuite
negotiation and key exchange between the parties. TLS has been
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defined as an authentication protocol for use within EAP (EAP-TLS)
[1].
Other authentication protocols are also widely deployed. These are
typically password-based protocols, and there is a large installed
base of support for these protocols in the form of credential
databases that may be accessed by RADIUS, Diameter or other AAA
servers. These include non-EAP protocols such as PAP, CHAP, MS-CHAP
and MS-CHAP-V2, as well as EAP protocols such as MD5-Challenge.
EAP-TTLS is an EAP protocol that extends EAP-TLS. In EAP-TLS, a TLS
handshake is used to mutually authenticate a client and server. EAP-
TTLS extends this authentication negotiation by using the secure
connection established by the TLS handshake to exchange additional
information between client and server. In EAP-TTLS, the TLS
handshake may be mutual; or it may be one-way, in which only the
server is authenticated to the client. The secure connection
established by the handshake may then be used to allow the server to
authenticate the client using existing, widely-deployed
authentication infrastructures such as RADIUS. The authentication of
the client may itself be EAP, or it may be another authentication
protocol such as PAP, CHAP, MS-CHAP or MS-CHAP-V2.
Thus, EAP-TTLS allows legacy password-based authentication protocols
to be used against existing authentication databases, while
protecting the security of these legacy protocols against
eavesdropping, man-in-the-middle and other cryptographic attacks.
EAP-TTLS also allows client and server to establish keying material
for use in the data connection between the client and access point.
The keying material is established implicitly between client and
server based on the TLS handshake.
In EAP-TTLS, client and server communicate using attribute-value
pairs encrypted within TLS. This generality allows arbitrary
functions beyond authentication and key exchange to be added to the
EAP negotiation, in a manner compatible with the AAA infrastructure.
2. Motivation
Most password-based protocols in use today rely on a hash of the
password with a random challenge. Thus, the server issues a
challenge, the client hashes that challenge with the password and
forwards a response to the server, and the server validates that
response against the user's password retrieved from its database.
This general approach describes CHAP, MS-CHAP, MS-CHAP-V2, EAP/MD5-
Challenge and EAP/One-Time Password.
An issue with such an approach is that an eavesdropper that observes
both challenge and response may be able to mount a dictionary
attack, in which random passwords are tested against the known
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challenge to attempt to find one which results in the known
response. Because passwords typically have low entropy, such attacks
can in practice easily discover many passwords.
While this vulnerability has long been understood, it has not been
of great concern in environments where eavesdropping attacks are
unlikely in practice. For example, users with wired or dial-up
connections to their service providers have not been concerned that
such connections may be monitored. Users have also been willing to
entrust their passwords to their service providers, or at least to
allow their service providers to view challenges and hashed
responses which are then forwarded to their home authentication
servers using, for example, proxy RADIUS, without fear that the
service provider will mount dictionary attacks on the observed
credentials. Because a user typically has a relationship with a
single service provider, such trust is entirely manageable.
With the advent of wireless connectivity, however, the situation
changes dramatically:
- Wireless connections are considerably more susceptible to
eavesdropping and man-in-the-middle attacks. These attacks may
enable dictionary attacks against low-entropy passwords. In
addition, they may enable channel hijacking, in which an attacker
gains fraudulent access by seizing control of the communications
channel after authentication is complete.
- Existing authentication protocols often begin by exchanging the
clientÆs username in the clear. In the context of eavesdropping
on the wireless channel, this can compromise the clientÆs
anonymity and locational privacy.
- Often in wireless networks, the access point does not reside in
the administrative domain of the service provider with which the
user has a relationship. For example, the access point may reside
in an airport, coffee shop, or hotel in order to provide public
access via 802.11. Even if password authentications are protected
in the wireless leg, they may still be susceptible to
eavesdropping within the untrusted wired network of the access
point.
- In the traditional wired world, the user typically intentionally
connects with a particular service provider by dialing an
associated phone number; that service provider may be required to
route an authentication to the user's home domain. In a wireless
network, however, the user does not get to choose an access
domain, and must connect with whichever access point is nearby;
providing for the routing of the authentication from an arbitrary
access point to the user's home domain may pose a challenge.
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Thus, the authentication requirements for a wireless environment
that EAP-TTLS attempts to address can be summarized as follows:
- Legacy password protocols must be supported, to allow easy
deployment against existing authentication databases.
- Password-based information must not be observable in the
communications channel between the client node and a trusted
service provider, to protect the user against dictionary attacks.
- The user's identity must not be observable in the communications
channel between the client node and a trusted service provider,
to protect the user's locational privacy against surveillance,
undesired acquisition of marketing information, and the like.
- The authentication process must result in the distribution of
shared keying information to the client and access point to
permit encryption and validation of the wireless data connection
subsequent to authentication, to secure it against eavesdroppers
and prevent channel hijacking.
- The authentication mechanism must support roaming among small
access domains with which the user has no relationship and which
will have limited capabilities for routing authentication
requests.
3. Terminology
AAA
Authentication, Authorization and Accounting - functions that are
generally required to control access to a network and support
billing and auditing.
AAA protocol
A network protocol used to communicate with AAA servers; examples
include RADIUS and Diameter.
AAA server
A server which performs one or more AAA functions: authenticating
a user prior to granting network service, providing authorization
(policy) information governing the type of network service the
user is to be granted, and accumulating accounting information
about actual usage.
AAA/H
A AAA server in the user's home domain, where authentication and
authorization for that user are administered.
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access point
A network device providing users with a point of entry into the
network, and which may enforce access control and policy based on
information returned by a AAA server. For the purposes of this
document, "access point" and "NAS" are architecturally
equivalent. "Access point" is used throughout because it is
suggestive of devices used for wireless access; "NAS" is used
when more traditional forms of access, such as dial-up, are
discussed.
access domain
The domain, including access points and other devices, that
provides users with an initial point of entry into the network;
for example, a wireless hot spot.
client
A host or device that connects to a network through an access
point.
domain
A network and associated devices that are under the
administrative control of an entity such as a service provider or
the user's home organization.
link layer protocol
A protocol used to carry data between hosts that are connected
within a single network segment; examples include PPP and
Ethernet.
NAI
A Network Access Identifier [7], normally consisting of the name
of the user and, optionally, the user's home realm.
NAS
A network device providing users with a point of entry into the
network, and which may enforce access control and policy based on
information returned by a AAA server. For the purposes of this
document, "access point" and "NAS" are architecturally
equivalent. "Access point" is used throughout because it is
suggestive of devices used for wireless access; "NAS" is used
when more traditional forms of access, such as dial-up, are
discussed.
proxy
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A server that is able to route AAA transactions to the
appropriate AAA server, possibly in another domain, typically
based on the realm portion of an NAI.
realm
The optional part of an NAI indicating the domain to which a AAA
transaction is to be routed, normally the user's home domain.
service provider
An organization with which a user has a business relationship,
that provides network or other services. The service provider may
provide the access equipment with which the user connects, may
perform authentication or other AAA functions, may proxy AAA
transactions to the user's home domain, etc.
TTLS server
A AAA server which implements EAP-TTLS. This server may also be
capable of performing user authentication, or it may proxy the
user authentication to a AAA/H.
user
The person operating the client device. Though the line is often
blurred, "user" is intended to refer to the human being who is
possessed of an identity (username), password or other
authenticating information, and "client" is intended to refer to
the device which makes use of this information to negotiate
network access. There may also be clients with no human
operators; in this case the term "user" is a convenient
abstraction.
4. Architectural Model
The network architectural model for EAP-TTLS usage and the type of
security it provides is shown below.
+----------+ +----------+ +----------+ +----------+
| | | | | | | |
| client |<---->| access |<---->| TTLS AAA |<---->| AAA/H |
| | | point | | server | | server |
| | | | | | | |
+----------+ +----------+ +----------+ +----------+
<---- secure password authentication tunnel --->
<---- secure data tunnel ---->
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The entities depicted above are logical entities and may or may not
correspond to separate network components. For example, the TTLS
server and AAA/H server might be a single entity; the access point
and TTLS server might be a single entity; or, indeed, the functions
of the access point, TTLS server and AAA/H server might be combined
into a single physical device. The above diagram illustrates the
division of labor among entities in a general manner and shows how a
distributed system might be constructed; however, actual systems
might be realized more simply.
Note also that one or more AAA proxy servers might be deployed
between access point and TTLS server, or between TTLS server and
AAA/H server. Such proxies typically perform aggregation or are
required for realm-based message routing. However, such servers play
no direct role in EAP-TTLS and are therefore not shown.
4.1 Carrier Protocols
The entities shown above communicate with each other using carrier
protocols capable of encapsulating EAP. The client and access point
communicate using a link layer carrier protocol such as PPP or
EAPOL. The access point, TTLS server and AAA/H server communicate
using a AAA carrier protocol such as RADIUS or Diameter.
EAP, and therefore EAP-TTLS, must be initiated via the link layer
protocol. In PPP or EAPOL, for example, EAP is initiated when the
access point sends an EAP-Request/Identity packet to the client.
The keying material used to encrypt and authenticate the data
connection between the client and access point is developed
implicitly between the client and TTLS server as a result of EAP-
TTLS negotiation. This keying material must be communicated to the
access point by the TTLS server using the AAA carrier protocol.
The client and access point must also agree on an
encryption/validation algorithm to be used based on the keying
material. In some systems, both these devices may be preconfigured
with this information, and distribution of the keying material alone
is sufficient. Or, the link layer protocol may provide a mechanism
for client and access point to negotiate an algorithm.
In the most general case, however, it may be necessary for both
client and access point to communicate their algorithm preferences
to the TTLS server, and for the TTLS server to select one and
communicate its choice to both parties. This information would be
transported between access point and TTLS server via the AAA
protocol, and between client and TTLS server via EAP-TTLS in
encrypted form.
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4.2 Security Relationships
The client and access point have no pre-existing security
relationship.
The access point, TTLS server and AAA/H server are each assumed to
have a pre-existing security association with the adjacent entity
with which it communicates. With RADIUS, for example, this is
achieved using shared secrets. It is essential for such security
relationships to permit secure key distribution.
The client and AAA/H server have a security relationship based on
the user's credentials such as a password.
The client and TTLS server may have a one-way security relationship
based on the TTLS server's possession of a private key guaranteed by
a CA certificate which the user trusts, or may have a mutual
security relationship based on certificates for both parties.
4.3 Messaging
The client and access point initiate an EAP conversation to
negotiate the client's access to the network. Typically, the access
point issues an EAP-Request/Identity to the client, which responds
with an EAP-Response/Identity. Note that the client does not include
the user's actual identity in this EAP-Response/Identity packet; the
user's identity will not be transmitted until an encrypted channel
has been established.
The access point now acts as a passthrough device, allowing the TTLS
server to negotiate EAP-TTLS with the client directly.
During the first phase of the negotiation, the TLS handshake
protocol is used to authenticate the TTLS server to the client and,
optionally, to authenticate the client to the TTLS server, based on
public/private key certificates. As a result of the handshake,
client and TTLS server now have shared keying material and an agreed
upon TLS record layer cipher suite with which to secure subsequent
EAP-TTLS communication.
During the second phase of negotiation, client and TTLS server use
the secure TLS record layer channel established by the TLS handshake
as a tunnel to exchange information encapsulated in attribute-value
pairs, to perform additional functions such as client authentication
and key distribution for the subsequent data connection.
If a tunneled client authentication is performed, the TTLS server
de-tunnels and forwards the authentication information to the AAA/H.
If the AAA/H performs a challenge, the TTLS server tunnels the
challenge information to the client. The AAA/H server may be a
legacy device and needs to know nothing about EAP-TTLS; it only
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needs to be able to authenticate the client based on commonly used
authentication protocols.
Keying material for the subsequent data connection between client
and access point may be generated based on secret information
developed during the TLS handshake between client and TTLS server.
At the conclusion of a successful authentication, the TTLS server
may transmit this keying material to the access point, encrypted
based on the existing security associations between those devices
(e.g., RADIUS).
The client and access point now share keying material which they can
use to encrypt data traffic between them.
4.4 Resulting Security
As the diagram above indicates, EAP-TTLS allows user identity and
password information to be securely transmitted between client and
TTLS server, and performs key distribution to allow network data
subsequent to authentication to be securely transmitted between
client and access point.
5. Protocol Layering Model
EAP-TTLS packets are encapsulated within EAP, and EAP in turn
requires a carrier protocol to transport it. EAP-TTLS packets
themselves encapsulate TLS, which is then used to encapsulate user
authentication information. Thus, EAP-TTLS messaging can be
described using a layered model, where each layer is encapsulated by
the layer beneath it. The following diagram clarifies the
relationship between protocols:
+--------------------------------------------------------+
| User Authentication Protocol (PAP, CHAP, MS-CHAP, etc.)|
+--------------------------------------------------------+
| TLS |
+--------------------------------------------------------+
| EAP-TTLS |
+--------------------------------------------------------+
| EAP |
+--------------------------------------------------------+
| Carrier Protocol (PPP, EAPOL, RADIUS, Diameter, etc.) |
+--------------------------------------------------------+
When the user authentication protocol is itself EAP, the layering is
as follows:
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+--------------------------------------------------------+
| User EAP Authentication Protocol (MD-Challenge, etc.) |
+--------------------------------------------------------+
| EAP |
+--------------------------------------------------------+
| TLS |
+--------------------------------------------------------+
| EAP-TTLS |
+--------------------------------------------------------+
| EAP |
+--------------------------------------------------------+
| Carrier Protocol (PPP, EAPOL, RADIUS, Diameter, etc.) |
+--------------------------------------------------------+
Methods for encapsulating EAP within carrier protocols are already
defined. For example, PPP [5] or EAPOL [4] may be used to transport
EAP between client and access point; RADIUS [6] or Diameter [8] are
used to transport EAP between access point and TTLS server.
6. EAP-TTLS version 0 Overview
[Authors' note: This section as well as sections 7, 8, 9 and 10,
describe version 0 of the EAP-TTLS protocol. Section 11 describes
version 1 of EAP-TTLS. Much of the material describing version 0
also applies to version 1; the version 1 documentation will refer to
the version 0 material as required. The intention is to provide a
separate draft for each of the two versions in the near future.]
A EAP-TTLS negotiation comprises two phases: the TLS handshake phase
and the TLS tunnel phase.
During phase 1, TLS is used to authenticate the TTLS server to the
client and, optionally, the client to the TTLS server. Phase 1
results in the activation of a cipher suite, allowing phase 2 to
proceed securely using the TLS record layer. (Note that the type and
degree of security in phase 2 depends on the cipher suite negotiated
during phase 1; if the null cipher suite is negotiated, there will
be no security!)
During phase 2, the TLS record layer is used to tunnel information
between client and TTLS server to perform any of a number of
functions. These might include user authentication, negotiation of
data communication security capabilities, key distribution,
communication of accounting information, etc.. Information between
client and TTLS server is exchanged via attribute-value pairs (AVPs)
compatible with RADIUS and Diameter; thus, any type of function that
can be implemented via such AVPs may easily be performed.
EAP-TTLS specifies how user authentication may be performed during
phase 2. The user authentication may itself be EAP, or it may be a
legacy protocol such as PAP, CHAP, MS-CHAP or MS-CHAP-V2. Phase 2
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user authentication may not always be necessary, since the user may
already have been authenticated via the mutual authentication option
of the TLS handshake protocol.
EAP-TTLS is also intended for use in key distribution, and specifies
how keying material for the data connection between client and
access point is generated. The keying material is developed
implicitly between client and TTLS server based on the results of
the TLS handshake; the TTLS server will communicate the keying
material to the access point over the carrier protocol However,
EAP-TTLS does not specify particular key distribution AVPs and their
use, since the needs of various systems will be different. Instead,
a general model for key distribution is suggested. Organizations may
define their own AVPs for this use, possibly using vendor-specific
AVPs, either in conformance with the suggested model or otherwise.
6.1 Phase 1: Handshake
In phase 1, the TLS handshake protocol is used to authenticate the
TTLS server to the client and, optionally, to authenticate the
client to the TTLS server.
Phase 1 is initiated when the client sends an EAP-Response/Identity
packet to the TTLS server. This packet specifically should not
include the name of the user; however, it may include the name of
the realm of a trusted provider to which EAP-TTLS packets should be
forwarded; for example, "@myisp.com".
The TTLS server responds to the EAP-Response/Identity packet with a
EAP-TTLS/Start packet, which is an EAP-Request with Type = EAP-TTLS,
the S (Start) bit set, and no data. This indicates to the client
that it should begin TLS handshake by sending a ClientHello message.
EAP packets continue to be exchanged between client and TTLS server
to complete the TLS handshake, as described in [1]. Phase 1 is
completed when the client and TTLS server exchange ChangeCipherSpec
and Finished messages. At this point, additional information may be
securely tunneled.
As part of the TLS handshake protocol, the TTLS server will send its
certificate along with a chain of certificates leading to the
certificate of a trusted CA. The client will need to be configured
with the certificate of the trusted CA in order to perform the
authentication.
If certificate-based authentication of the client is desired, the
client must have been issued a certificate and must have the private
key associated with that certificate
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6.2 Phase 2: Tunnel
In phase 2, the TLS Record Layer is used to securely tunnel
information between client and TTLS server. This information is
encapsulated in sequences of attribute-value pairs (AVPS), whose use
and format are described in later sections.
Any type of information may be exchanged during phase 2, according
to the requirements of the system. (It is expected that applications
utilizing EAP-TTLS will specify what information must be exchanged
and therefore which AVPs must be supported.)
The client begins the phase 2 exchange by encoding information in a
sequence of AVPs, passing this sequence to the TLS record layer for
encryption, and sending the resulting data to the TTLS server.
The TTLS server recovers the AVPs in clear text from the TLS record
layer. If the AVP sequence includes authentication information, it
forwards this information to the AAA/H server using the AAA carrier
protocol. Note that the EAP-TTLS and AAA/H servers may be one and
the same, in which case it simply processes the information locally.
The TTLS server may respond with its own sequence of AVPs. The TTLS
server passes the AVP sequence to the TLS record layer for
encryption and sends the resulting data to the client. For example,
the TTLS server may send key distribution information, or it may
forward an authentication challenge received from the AAA/H.
This process continues until the TTLS server has enough information
to issue either an EAP-Success or EAP-Failure. Thus, if the AAA/H
rejects the client based on forwarded authentication information,
the TTLS server would issue an EAP-Failure. If the AAA/H accepts the
client, the TTLS server would issue an EAP-Success.
The TTLS server distributes data connection keying information and
other authorization information to the access point in the same AAA
carrier protocol message that carries the EAP-Success.
6.3 Piggybacking
While it is convenient to describe EAP-TTLS messaging in terms of
two phases, it is sometimes required that a single EAP-TTLS packet
to contain both phase 1 and phase 2 TLS messages.
Such "piggybacking" occurs when the party that completes the
handshake also has AVPs to send. For example, when negotiating a
resumed TLS session, the TTLS server sends its ChangeCipherSpec and
Finished messages first, then the client sends its own
ChangeCipherSpec and Finished messages to conclude the handshake. If
the client has authentication or other AVPs to send to the TTLS
server, it must tunnel those AVPs within the same EAP-TTLS packet
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immediately following its Finished message. If the client fails to
do this, the TTLS server will incorrectly assume that the client has
no AVPs to send, and the outcome of the negotiation could be
affected.
6.4 Session Resumption
When a client and TTLS server that have previously negotiated a EAP-
TTLS session begin a new EAP-TTLS negotiation, the client and TTLS
server may agree to resume the previous session. This significantly
reduces the time required to establish the new session. This could
occur when the client connects to a new access point, or when an
access point requires reauthentication of a connected client.
Session resumption is accomplished using the standard TLS mechanism.
The client signals its desire to resume a session by including the
session ID of the session it wishes to resume in the ClientHello
message; the TTLS server signals its willingness to resume that
session by echoing that session ID in its ServerHello message.
If the TTLS server elects not to resume the session, it simply does
not echo the session ID and a new session will be negotiated. This
could occur if the TTLS server is configured not to resume sessions,
if it has not retained the requested session's state, or if the
session is considered stale. A TTLS server may consider the session
stale based on its own configuration, or based on session-limiting
information received from the AAA/H (e.g., the RADIUS Session-
Timeout attribute).
Tunneled authentication is specifically not performed for resumed
sessions; the presumption is that the knowledge of the master secret
as evidenced by the ability to resume the session is authentication
enough. This allows session resumption to occur without any
messaging between the TTLS server and the AAA/H. If periodic
reauthentication to the AAA/H is desired, the AAA/H must indicate
this to the TTLS server when the original session is established,
for example, using the RADIUS Session-Timeout attribute.
The client must, however, send other required AVPs, in particular
key distribution AVPs, that are not associated with tunneled
authentication in its first EAP-TTLS packet to the server that is
capable of containing phase 2 TLS messages. The TTLS server does not
retain client AVPs or key distribution preferences as part of
session state, and the client is expected to resend those AVPs in
each negotiation.
Thus phase 2 of a resumed session proceeds just as would a new
session, minus tunneled authentication AVPs. For example, the client
would send its key distribution preferences, and the TTLS server
would respond with its key distribution selection.
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While the TTLS server does not retain client AVPs from session to
session, it must retain authorization information returned by the
AAA/H for use in resumed sessions. A resumed session must operate
under the same authorizations as the original session, and the TTLS
server must be prepared to send the appropriate information back to
the access point. Authorization information might include the
maximum time for the session, the maximum allowed bandwidth, packet
filter information and the like. The TTLS server is responsible for
modifying time values, such as Session-Timeout, appropriately for
each resumed session.
A TTLS server must not permit a session to be resumed if that
session did not result in a successful authentication of the user
during phase 2. The consequence of incorrectly implementing this
aspect of session resumption would be catastrophic; any attacker
could easily gain network access by first initiating a session that
succeeds in the TLS handshake but fails during phase 2
authentication, and then resuming that session.
[Implementation note: Toolkits that implement TLS often cache
resumable TLS sessions automatically. Implementers must take care to
override such automatic behavior, and prevent sessions from being
cached for possible resumption until the user has been positively
authenticated during phase 2.]
6.4.1 TTLS Server Guidelines for Session Resumption
When a domain comprises multiple TTLS servers, a client's attempt to
resume a session may fail because each EAP-TTLS negotiation may be
routed to a different TTLS server.
One strategy to ensure that subsequent EAP-TTLS negotiations are
routed to the original TTLS server is for each TTLS server to encode
its own identifying information, for example, IP address, in the
session IDs that it generates. This would allow any TTLS server
receiving a session resumption request to forward the request to the
TTLS server that established the original session.
7. Generating Keying Material
When record layer security is instantiated at the end of a TLS
handshake, a pseudo-random function (PRF) is used to expand the
negotiated master secret, server random value and client random
value into a sequence of octets that is used as keying material for
the record layer. The length of this sequence depends on the
negotiated cipher suite, and contains the following components:
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client_write_MAC_secret
server_write_MAC_secret
client_write_key
server_write_key
client_write_IV (optional)
server_write_IV (optional)
The ASCII-encoded constant string "key expansion" is used as input
to the pseudo-random function to generate this sequence.
EAP-TTLS leverages this technique to create keying material for use
in the data connection between client and access point. Exactly the
same PRF is used to generate as much keying material as required,
with the constant string set to "ttls keying material", as follows:
EAP-TTLS_keying_material = PRF(SecurityParameters.master_secret,
"ttls keying material",
SecurityParameters.client_random +
SecurityParameters.server_random);
The master secret, client random and server random used to generate
the data connection keying material must be those established during
the TLS handshake. Both client and TTLS server generate this keying
material, and they are guaranteed to be the same if the handshake
succeeded. The TTLS server distributes this keying material to the
access point via the AAA carrier protocol.
[Note that the order of client_random and server_random for EAP-TTLS
is reversed from that of the TLS protocol [3]. This ordering follows
the key derivation method of EAP-TLS [1]. Altering the order of
randoms avoids namespace collisions between constant strings defined
for EAP-TTLS and those defined for the TLS protocol.]
8. EAP-TTLS Encoding
EAP-TTLS is a protocol within EAP. Its assigned EAP number is 21.
Except as described in the subsections below, EAP-TTLS's encoding of
TLS messages within EAP is identical to EAP-TLS's encoding of TLS
messages within EAP. See [1] for details.
8.1 EAP-TTLS Start Packet
The EAP-TTLS Start packet (with S-bit set) may, in a future
specification, be allowed to contain data (the EAP-TLS Start packet
does not).
Thus, the data contents of an EAP-TTLS Start packet are reserved for
future standardization; in the meantime, servers must not include
any data in an EAP-TTLS Start packet, and clients must ignore such
data but must not reject a Start packet that contains data.
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8.2 EAP-TTLS Packets with No Data
One point of clarification has to do with an EAP-TTLS packet (other
than a Start packet) that contains no data.
EAP-TLS defines the use of such a packet as a fragment ACK. When
either party must fragment an EAP-TLS packet, the other party
responds with a fragment ACK to allow the original party to send the
next fragment.
EAP-TTLS uses the fragment ACK in the same way. There are also other
instances where a EAP-TTLS packet with no data might be sent:
- When the final EAP packet of the EAP-TTLS negotiation is sent by
the TTLS server, the client must respond with a EAP-TTLS packet
with no data, to allow the TTLS server to issue its final EAP-
Success or EAP-Failure packet.
- It is possible for a EAP-TTLS packet with no data to be sent in
the middle of a negotiation. Such a packet is simply interpreted
as packet with no AVPs. For example, during session resumption,
the client sends its Finished message first, then the TTLS server
replies with its Finished message. The TTLS server cannot
piggyback key distribution AVPs within the Record Layer in the
same EAP-TTLS packet containing its Finished message, because it
must wait for the client to indicate its key distribution
preferences. But it is possible that the client has no
preferences, and thus has no AVPs to send. The client simply
sends a EAP-TTLS packet with no data, to allow the server to
continue the negotiation by sending its key distribution
selection.
9. Encapsulation of AVPs within the TLS Record Layer
Subsequent to the TLS handshake, information is tunneled between
client and TTLS server through the use of attribute-value pairs
(AVPs) encrypted within the TLS record layer.
The AVP format chosen for EAP-TTLS is compatible with the Diameter
AVP format. This does not at all represent a requirement that
Diameter be supported by any of the devices or servers participating
in a EAP-TTLS negotiation. Use of this format is merely a
convenience. Diameter is a superset of RADIUS and includes the
RADIUS attribute namespace by definition, though it does not limit
the size of an AVP as does RADIUS; RADIUS, in turn, is a widely
deployed AAA protocol and attribute definitions exist for all
commonly used password authentication protocols, including EAP.
Thus, Diameter is not considered normative except as specified in
this document. Specifically, the AVP Codes used in EAP-TTLS are
semantically equivalent to those defined for Diameter, and, by
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extension, RADIUS. Also, the representation of the Data field of an
AVP in EAP-TTLS is identical to that of Diameter.
Use of the RADIUS/Diameter namespace allows a TTLS server to easily
translate between AVPs it uses to communicate to clients and the
protocol requirements of AAA servers that are widely deployed. Plus,
it provides a well-understood mechanism to allow vendors to extend
that namespace for their particular requirements.
9.1 AVP Format
The format of an AVP is shown below. All items are in network, or
big-endian, order; that is, they have most significant octet first.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V M r r r r r r| AVP Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-ID (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-+-+-+-+
AVP Code
The AVP Code is four octets and, combined with the Vendor-ID
field if present, identifies the attribute uniquely. The first
256 AVP numbers represent attributes defined in RADIUS. AVP
numbers 256 and above are defined in Diameter.
AVP Flags
The AVP Flags field is one octet, and provides the receiver with
information necessary to interpret the AVP.
The 'V' (Vendor-Specific) bit indicates whether the optional
Vendor-ID field is present. When set to 1, the Vendor-ID field is
present and the AVP Code is interpreted according to the
namespace defined by the vendor indicated in the Vendor-ID field.
The 'M' (Mandatory) bit indicates whether support of the AVP is
required. If this bit is set to 0, this indicates that the AVP
may be safely ignored if the receiving party does not understand
or support it. If set to 1, this indicates that the receiving
party must fail the negotiation if it does not understand the
AVP; for a TTLS server, this would imply returning EAP-Failure,
for a client, this would imply abandoning the negotiation.
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The 'r' (reserved) bits are unused and must be set to 0.
AVP Length
The AVP Length field is three octets, and indicates the length of
this AVP including the AVP Code, AVP Length, AVP Flags, Vendor-ID
(if present) and Data.
Vendor-ID
The Vendor-ID field is present if the 'V' bit is set in the AVP
Flags field. It is four octets, and contains the vendor's IANA-
assigned "SMI Network Management Private Enterprise Codes" [9]
value. Vendors defining their own AVPs must maintain a consistent
namespace for use of those AVPs within RADIUS, Diameter and EAP-
TTLS.
A Vendor-ID value of zero is equivalent to absence of the Vendor-
ID field altogether.
9.2 AVP Sequences
Data encapsulated within the TLS Record Layer must consist entirely
of a sequence of zero or more AVPs. Each AVP must begin on a 4-octet
boundary relative to the first AVP in the sequence. If an AVP is not
a multiple of 4 octets, it must be padded with 0s to the next 4-
octet boundary.
Note that the AVP Length does not include the padding.
9.3 Guidelines for Maximum Compatibility with AAA Servers
For maximum compatibility, the following guidelines for AVP usage
are suggested:
- Non-vendor-specific AVPs should be selected from the set of
attributes defined for RADIUS; that is, attributes with codes
less than 256. This provides compatibility with both RADIUS and
Diameter.
- Vendor-specific AVPs should be defined in terms of RADIUS.
Vendor-specific RADIUS attributes translate to Diameter (and,
hence, to EAP-TTLS) automatically; the reverse is not true.
RADIUS vendor-specific attributes use RADIUS attribute 26 and
include vendor ID, vendor-specific attribute code and length; see
[6] for details.
10. Tunneled Authentication
EAP-TTLS permits user authentication information to be tunneled
within the TLS record layer between client and TTLS server,
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guaranteeing the security of the authentication information against
active and passive attack between the client and TTLS server. The
TTLS server decrypts and forwards this information to the AAA/H over
the AAA carrier protocol.
Any type of password or other authentication may be tunneled. Also,
multiple tunneled authentications may be performed. Normally,
tunneled authentication is used when the client has not been issued
a certificate and the TLS handshake provides only one-way
authentication of the TTLS server to the client; however, in certain
cases it may be desired to perform certificate authentication of the
client during the TLS handshake as well as tunneled user
authentication afterwards.
10.1 Implicit challenge
Certain authentication protocols that use a challenge/response
mechanism rely on challenge material that is not generated by the
authentication server, and therefore require special handling.
In CHAP, MS-CHAP and MS-CHAP-V2, for example, the NAS issues a
challenge to the client, the client then hashes the challenge with
the password and forwards the response to the NAS. The NAS then
forwards both challenge and response to a AAA server. But because
the AAA server did not itself generate the challenge, such protocols
are susceptible to replay attack.
If the client were able to create both challenge and response,
anyone able to observe a CHAP or MS-CHAP exchange could pose as that
user, even using EAP-TTLS.
To make these protocols secure under EAP-TTLS, it is necessary to
provide a mechanism to produce a challenge that the client cannot
control or predict. This is accomplished using the same technique
described above for generating data connection keying material.
When a challenge-based authentication mechanism is used, both client
and TTLS server use the pseudo-random function to generate as many
octets as are required for the challenge, using the constant string
"ttls challenge", based on the master secret and random values
established during the handshake:
EAP-TTLS_challenge = PRF(SecurityParameters.master_secret,
"ttls challenge",
SecurityParameters.client_random +
SecurityParameters.server_random);
10.2 Tunneled Authentication Protocols
This section describes the methods for tunneling specific
authentication protocols within EAP-TTLS.
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For the purpose of explication, it is assumed that the TTLS server
and AAA/H use RADIUS as a AAA carrier protocol between them.
However, this is not a requirement, and any AAA protocol capable of
carrying the required information may be used.
10.2.1 EAP
When EAP is the tunneled authentication protocol, each tunneled EAP
packet between the client and TTLS server is encapsulated in an EAP-
Message AVP, prior to tunneling via the TLS record layer.
The client's first tunneled EAP packet within phase 2 will contain
the EAP-Response/Identity. The client places the actual username in
this packet; the privacy of the user's identity is now guaranteed by
the TLS encryption. This username must be a Network Access
Identifier (NAI) [7]; that is, it must be in the following format:
username@realm
The @realm portion is optional, and is used to allow the TTLS server
to forward the EAP packet to the appropriate AAA/H.
Note that the client has two opportunities to specify realms. The
first, in the initial EAP-Response/Identity packet, indicates the
realm of the TTLS server. The second, in the tunneled
authentication, indicates the realm of the client's home network.
Thus, the access point need only know how to route to the realm of
the TTLS server; the TTLS server is assumed to know how to route to
the client's home realm. This serial routing architecture is
anticipated to be useful in roaming environments, allowing access
points or AAA proxies behind access points to be configured only
with a small number of realms.
Upon receipt of the tunneled EAP-Response/Identity, the TTLS server
forwards it to the AAA/H in a RADIUS Access-Request.
The AAA/H may immediately respond with an Access-Reject, in which
case the TTLS server completes the negotiation by sending an EAP-
Failure to the access point. This could occur if the AAA/H does not
recognize the user's identity, or if it does not support EAP.
If the AAA/H does recognize the user's identity and does support
EAP, it responds with an Access-Challenge containing an EAP-Request,
with the Type and Type-Data fields set according to the EAP protocol
with which the AAA/H wishes to authenticate the client; for example
MD-Challenge, OTP or Generic Token Card.
The EAP authentication between client and AAA/H proceeds normally,
as described in [2], with the TTLS server acting as a passthrough
device. Each EAP-Request sent by the AAA/H in an Access-Challenge is
tunneled by the TTLS server to the client, and each EAP-Response
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tunneled by the client is decrypted and forwarded by the TTLS server
to the AAA/H in an Access-Request.
This process continues until the AAA/H issues an Access-Accept or
Access-Reject, at which point the TTLS server completes the
negotiation by sending an EAP-Success or EAP-Failure to the access
point using the AAA carrier protocol.
10.2.2 CHAP
The CHAP algorithm is described in [5]; RADIUS attribute formats are
described in [6].
Both client and TTLS server generate 17 octets of challenge
material, using the constant string "ttls challenge" as described
above. These octets are used as follows:
CHAP-Challenge [16 octets]
CHAP Identifier [1 octet]
The client tunnels User-Name, CHAP-Challenge and CHAP-Password AVPs
to the TTLS server. The CHAP-Challenge value is taken from the
challenge material. The CHAP-Password consists of CHAP Identifier,
taken from the challenge material; and CHAP response, computed
according to the CHAP algorithm.
Upon receipt of these AVPs from the client, the TTLS server must
verify that the value of the CHAP-Challenge AVP and the value of the
CHAP Identifier in the CHAP-Password AVP are equal to the values
generated as challenge material. If either item does not match
exactly, the TTLS server must reject the client. Otherwise, it
forwards the AVPs to the AAA/H in an Access-Request.
The AAA/H will respond with an Access-Accept or Access-Reject. The
TTLS server will then issue an EAP-Success or EAP-Failure to the
access point.
10.2.3 MS-CHAP
The MS-CHAP algorithm is described in [10]; RADIUS attribute formats
are described in [12].
Both client and TTLS server generate 9 octets of challenge material,
using the constant string "ttls challenge" as described above. These
octets are used as follows:
MS-CHAP-Challenge [8 octets]
Ident [1 octet]
The client tunnels User-Name, MS-CHAP-Challenge and MS-CHAP-Response
AVPs to the TTLS server. The MS-CHAP-Challenge value is taken from
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the challenge material. The MS-CHAP-Response consists of Ident,
taken from the challenge material; Flags, set according the client
preferences; and LM-Response and NT-Response, computed according to
the MS-CHAP algorithm.
Upon receipt of these AVPs from the client, the TTLS server must
verify that the value of the MS-CHAP-Challenge AVP and the value of
the Ident in the client's MS-CHAP-Response AVP are equal to the
values generated as challenge material. If either item does not
match exactly, the TTLS server must reject the client. Otherwise, it
forwards the AVPs to the AAA/H in an Access-Request.
The AAA/H will respond with an Access-Accept or Access-Reject. The
TTLS server will then issue an EAP-Success or EAP-Failure to the
access point.
10.2.4 MS-CHAP-V2
The MS-CHAP-V2 algorithm is described in [11]; RADIUS attribute
formats are described in [12].
Both client and TTLS server generate 17 octets of challenge
material, using the constant string "ttls challenge" as described
above. These octets are used as follows:
MS-CHAP-Challenge [16 octets]
Ident [1 octet]
The client tunnels User-Name, MS-CHAP-Challenge and MS-CHAP2-
Response AVPs to the TTLS server. The MS-CHAP-Challenge value is
taken from the challenge material. The MS-CHAP2-Response consists of
Ident, taken from the challenge material; Flags, set to 0; Peer-
Challenge, set to a random value; and Response, computed according
to the MS-CHAP-V2 algorithm.
Upon receipt of these AVPs from the client, the TTLS server must
verify that the value of the MS-CHAP-Challenge AVP and the value of
the Ident in the client's MS-CHAP2-Response AVP are equal to the
values generated as challenge material. If either item does not
match exactly, the TTLS server must reject the client. Otherwise, it
forwards the AVPs to the AAA/H in an Access-Request.
If the authentication is successful, the AAA/H will respond with an
Access-Accept containing the MS-CHAP2-Success attribute. This
attribute contains a 42-octet string that authenticates the AAA/H to
the client based on the Peer-Challenge. The TTLS server tunnels this
AVP to the client. Note that the authentication is not yet complete;
the client must still accept the authentication response of the
AAA/H.
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Upon receipt of the MS-CHAP2-Success AVP, the client is able to
authenticate the AAA/H. If the authentication succeeds, the client
sends an EAP-TTLS packet to the TTLS server containing no data. Upon
receipt of the empty EAP-TTLS packet from the client, the TTLS
server now issues an EAP-Success.
If the authentication fails, the AAA/H will respond with an Access-
Challenge containing the MS-CHAP2-Error attribute. This attribute
contains a new Ident and a string with addition information such as
error reason and whether a retry is allowed. If the error reason is
an expired password and a retry is allowed, the client may proceed
to change the user's password. If the error reason is not an expired
password or if the client does not wish to change the user's
password, it simply abandons the EAP-TTLS negotiation.
If the client does wish to change the password, it tunnels MS-CHAP-
NT-Enc-PW, MS-CHAP2-CPW, and MS-CHAP-Challenge AVPs to the TTLS
server. The MS-CHAP2-CPW AVP is derived from from the new Ident and
Challenge received in the MS-CHAP2-Error AVP. The MS-CHAP-Challenge
AVP simply echoes the new Challenge.
Upon receipt of these AVPs from the client, the TTLS server must
verify that the value of the MS-CHAP-Challenge AVP and the value of
the Ident in the client's MS-CHAP2-CPW AVP match the values it sent
in the MS-CHAP2-Error AVP. If either item does not match exactly,
the TTLS server must reject the client. Otherwise, it forwards the
AVPs to the AAA/H in an Access-Request.
If the authentication is successful, the AAA/H will respond with an
Access-Accept containing the MS-CHAP2-Success attribute. At this
point, the negotiation proceeds as described above; the TTLS server
tunnels the MS-CHAP2-Success to the client, the client authenticates
the AAA/H based on this AVP, it either abandons the negotation on
failure or sends an EAP-TTLS packet to the TTLS server containing no
data, the TTLS server issues an EAP-Success.
Note that additional AVPs associated with MS-CHAP-V2 may be sent by
the AAA/H; for example, MS-CHAP-Domain. The TTLS server must tunnel
such authentication-related attributes along with the MS-CHAP2-
Success.
10.2.5 PAP
The client tunnels User-Name and User-Password AVPs to the TTLS
server.
Normally, in RADIUS, User-Password is padded with nulls to a
multiple of 16 octets, then encrypted using a shared secret and
other packet information.
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An EAP-TTLS client, however, does not RADIUS-encrypt the password
since no such RADIUS variables are available; this is not a security
weakness since the password will be encrypted via TLS anyway. The
client should, however, null-pad the password to a multiple of 16
octets, to obfuscate its length.
Upon receipt of these AVPs from the client, the TTLS server forwards
them to the AAA/H in a RADIUS Access-Request. (Note that in the
Access-Request, the TTLS server must encrypt the User-Password
attribute using the shared secret between the TTLS server and
AAA/H.)
The AAA/H may immediately respond with an Access-Accept or Access-
Reject. The TTLS server then completes the negotiation by sending an
EAP-Success or EAP-Failure to the access point using the AAA carrier
protocol.
The AAA/H may also respond with an Access-Challenge. The TTLS server
then tunnels the AVPs from the AAA/H's challenge to the client. Upon
receipt of these AVPs, the client tunnels User-Name and User-
Password again, with User-Password containing new information in
response to the challenge. This process continues until the AAA/H
issues an Access-Accept or Access-Reject.
At least one of the AVPs tunneled to the client upon challenge must
be Reply-Message. Normally this is sent by the AAA/H as part of the
challenge. However, if the AAA/H has not sent a Reply-Message, the
TTLS server must issue one, with null value. This allows the client
to determine that a challenge response is required.
Note that if the AAA/H includes a Reply-Message as part of an
Access-Accept or Access-Reject, the TTLS server does not tunnel this
AVP to the client. Rather, this AVP and all other AVPs sent by the
AAA/H as part of Access-Accept or Access-Reject are sent to the
access point via the AAA carrier protocol.
10.3 Performing Multiple Authentications
In some cases, it is desirable to perform multiple user
authentications. For example, a AAA/H may want first to authenticate
the user by password, then by token card.
The AAA/H may perform any number of additional user authentications
using EAP, simply by issuing a EAP-Request with a new protocol type
once the previous authentication succeeded but prior to issuing an
EAP-Success or accepting the user via the AAA carrier protocol.
For example, an AAA/H wishing to perform MD5-Challenge followed by
Generic Token Card would first issue an EAP-Request/MD5-Challenge
and receive a response. If the response is satisfactory, it would
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then issue EAP-Request/Generic Token Card and receive a response. If
that response were also satisfactory, it would issue EAP-Success.
11. EAP-TTLS Version 1
Version 1 of EAP-TTLS improves upon the original version 0 protocol
in several ways.
- Session keys developed from inner authentications are mixed with
the master secret developed during the initial TLS handshake.
This eliminates the Man-in-the-Middle (MitM) attack against
tunneled protocols for inner authentications that generate
session keys. See [15] and [16] for information about this
attack.
- A secure final exchange of the result of inner authentication is
exchanged between client and server to conclude the EAP-TTLS
exchange. This precludes any possibility of truncation attack
that could occur when the client relies solely on an unprotected
EAP-Success message to determine that the server has completed
its authentication.
- Inner authentication occurs within the TLS handshake, rather than
after it. Thus, the TLS handshake itself includes both a standard
TLS authentication as well as tunneled inner authentication(s)
using EAP or legacy protocols, as well as any other tunneled
communications required between client and server.
11.1 EAP-TTLS v1 Introduction
Version 1 of EAP-TTLS utilizes the TLS extensions mechanism to
extend the TLS handshake to include exchange of inner AVPs prior to
completion of the TLS handshake by exchange of Finished messages.
The TLS protocol provides a handshake phase and a data phase. EAP-
TTLS v0, as well as other proposed tunneled EAP types such as EAP-
PEAP and EAP-FAST, share a common strategy of utilizing the
handshake phase to establish a tunnel and the data phase to perform
protected authentication.
In EAP-TTLS v1, the AVP exchange is folded into the TLS handshake
itself; in other words, the inner authentication precedes the
conclusion of the TLS handshake, rather following it.
An advantage of this arrangement is a certain amount of
cryptographic integration of inner authentication with standard TLS
mechanisms. For example, mixing of inner session keys to thwart MitM
attacks is easily performed in such a way that both the
authentication result and the final session key is conditioned upon
these inner session keys.
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The definition of EAP-TTLS v1 proceeds by first defining the
InnerApplication extension to TLS, and then by defining the binding
of the extended TLS to EAP via EAP-TTLS v1, which in effect serves
as a carrier protocol.
11.2 Intentions Beyond EAP-TTLS
The use of TLS for EAP is a relative newcomer. TLS has long used for
many other purposes, most notably for protecting HTTP traffic.
However, TLS used in these contexts has no mechanism for
authentication beyond the certificate mechanisms that have been
defined. Any additional authentication, say in HTTP, must use
relatively primitive mechanisms defined in the HTTP protocol. It
would be very useful for the TLS protocol to provide more general
authentication mechanisms for subsequent authentication, for example
EAP.
The InnerApplication extension allows TLS to provide inner
authentication during the handshake, rather than after it. The EAP-
TTLS version 1 protocol is in fact just a binding of this extended
TLS to EAP; that it, EAP-TTLS is a carrier protocol for the extended
TLS. TLS with the InnerApplication extension can just as easily be
bound to TCP, to enable its use in HTTP.
The applicability of TLS with the InnerApplication extension
includes setting up HTTP connections (including SSL VPN
connections), establishing IPsec connections as an alternative to
IKE, obtaining credentials for single sign-on, providing for client
integrity verification, etc. The inner AVP mechanism offers both
legacy and EAP authentication capabilities, natural compatibility
with RADIUS and Diameter servers, and the flexibility to allow
arbitrary client-server exchanges for various purposes.
The authors' intention is to separately propose the TLS
InnerApplication extension as an enhancement to TLS, and then define
EAP-TTLS version 1 as a carrier protocol, or binding, of that
extended TLS to EAP. For reasons of timing, the TLS InnerApplication
extension is defined in this draft for now.
11.3 The InnerApplication Extension to TLS
The InnerApplication extension to TLS follows the guidelines of RFC
3546. The client proposes use of this extension by including an
InnerApplication message in its ClientHello handshake message, and
the server confirms its use by including an InnerApplication message
in its ServerHello handshake message.
In this document, the term "TLS/IA" shall refer to TLS with the
InnerApplication extension.
Two new handshake messages are defined for use in TLS/IA:
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- The PhaseFinished message. This message is similar to the
standard TLS Finished message; it allows the TLS/IA handshake to
operate in phases, with message and key confirmation occurring at
the end of each phase.
- The ApplicationPayload message. This message is used to carry AVP
(Attribute-Value Pair) sequences within the TLS/IA handshake, in
support of client-server applications such as authentication.
A new alert code is also defined for use in TLS/IA:
- The InnerApplicationFailure alert. This error alert allows either
party to terminate the handshake due to a failure in an
application implemented via AVP sequences carried in
ApplicationPayload messages.
11.3.1 TLS/IA Overview
In TLS/IA, the handshake is divided into phases.
The first phase is called the "initial phase", and consists of a
standard TLS handshake with PhaseFinished substituted for Finished
as the concluding message.
There are one or more subsequent phases, called "application
phases". The last application phase is called the "final phase"; any
application phase prior to the final phase is called an
"intermediate phase".
Each application phase consists of ApplicationPayload messages
exchanged by client and server to implement applications such as
authentication, plus concluding messages for cryptographic
confirmation.
Thus, the entire handshake consists of a initial phase, zero or more
intermediate phases, and a final phase. Intermediate phases are only
necessary if interim confirmation of key material generated during
an application phase is desired.
In each application phase, the client sends the first
ApplicationPayload message. ApplicationPayload messages are then
traded one at a time between client and server, until the server
concludes the phase by sending a ChangeCipherSpec and PhaseFinished
sequence to conclude an intermediate phase, or a ChangeCipherSpec
and Finished sequence to conclude the final phase. The client then
responds with its own ChangeCipherSpec and PhaseFinished sequence,
or ChangeCipherSpec and Finished sequence.
The server determines which type of concluding message is used,
either PhaseFinished or Finished, and the client MUST echo the same
type of concluding message. Each PhaseFinished or Finished message
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provides cryptographic confirmation of the integrity of all
handshake messages and keys generated from the start of the
handshake through the current phase.
Each ApplicationPayload message contains opaque data interpreted as
an AVP (Attribute-Value Pair) sequence. Each AVP in the sequence
contains a typed data element. The exchanged AVPs allow client and
server to implement "applications" within a secure tunnel. An
application may be any procedure that someone may usefully define. A
typical application might be authentication; for example, the server
may authenticate the client based on password credentials using EAP.
Other possible applications include distribution of keys, validating
client integrity, setting up IPsec parameters, setting up SSL VPNs,
and so on.
In TLS/IA, the TLS master secret undergoes multiple permutations
until a final master secret is computed at the end of the entire
handshake. Each phase of the handshake results in a new master
secret; the master secret for each phase is confirmed by the
PhaseFinished or Finished message exchange that concludes that
phase.
The initial master secret is computed during the initial phase of
the handshake, using the usual TLS algorithm, namely, that a
premaster secret is established and the TLS PRF function is used to
compute the initial master secret. This initial master secret is
confirmed via the first exchange of ChangeCipherSpec and
PhaseFinished messages.
Each subsequent master secret for an application phase is computed
using a PRF based on the current master secret, then mixing into the
result any session key material generated during authentications
during that phase. Each party computes a new master secret prior to
the conclusion of each application phase, and uses that new master
secret is to compute fresh keying material (that is, a TLS
"key_block", consisting of client and server MAC secrets, write keys
and IVs). The new master secret and keying material become part of
the pending read and write connection states. Following standard TLS
procedures, these connection states become current states upon
sending or receiving ChangeCipherSpec, and are confirmed via the
PhaseFinished or Finished message.
The final master secret, computed during the final handshake phase
and confirmed by an exchange of ChangeCipherSpec and Finished
messages, becomes the actual TLS master secret that defines the
session. This final master secret is the surviving master secret,
and each prior master secrets SHOULD be discarded when a new
connection state is instantiated. The final master secret is used
for session resumption, as well as for any session key derivation
that protocols defined over TLS may require.
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11.3.2 Message Exchange
Each intermediate handshake phase consists of ApplicationPayload
messages sent alternately by client and server, and a concluding
exchange of {ChangeCipherSpec, PhaseFinished} messages. The first
ApplicationPayload message in the each intermediate phase is sent by
the client; the first {ChangeCipherSpec, PhaseFinished} message
sequence is sent by the server. Thus the client begins the exchange
with an ApplicationPayload message and the server determines when to
conclude it by sending {ChangeCipherSpec, PhaseFinished}. When it
receives the server's {ChangeCipherSpec, PhaseFinished} messages,
the client sends its own {ChangeCipherSpec, PhaseFinished} messages.
The client then sends an ApplicationPayload message to begin the
next handshake phase.
The final handshake proceeds in the same manner as the intermediate
handshake, except that the Finished message is used rather than the
PhaseFinished message, and the client does not send an
ApplicationPayload message for the next phase because there is no
next phase.
At the start of each application handshake phase, the server MUST
wait for the client's opening ApplicationPayload message before it
sends its own ApplicationPayload message to the client. The client
MAY NOT initiate conclusion of an application handshake phase by
sending the first {ChangeCipherSpec, PhaseFinished} or
{ChangeCipherSpec, Finished message} sequence; it MUST allow the
server to initiate the conclusion of the phase.
11.3.3 Master Key Permutation
Each permutation of the master secret from one phase to the next
begins with the calculation of a preliminary 48 octet vector based
on the current master secret:
preliminary_vector = PRF(master_secret,
"InnerApplication preliminary vector",
server_random + client_random) [0..48];
Session key material generated by applications during the current
application phase are mixed into the preliminary vector by
arithmetically adding each session key to the preliminary vector to
compute the new master secret. The preliminary vector is treated as
a 48-octet integer in big-endian order; that is, the first octet is
of the highest significance. Each session key is also treated as a
big-endian integer of whatever size it happens to be. Arithmetic
carry past the most significant octet is discarded; that is, the
addition is performed modulo 2 ^ 384.
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Thus, the logical procedure for computing the next master secret
(which may also be a convenient implementation procedure) is as
follows:
1 At the start of each application handshake phase, use the current
master secret to compute the preliminary vector for the next
master secret.
2 Each time session key material is generated from an
authentication or other exchange, arithmetically add that session
key material to the preliminary vector.
3 At the conclusion of the application handshake phase, copy the
current contents of the preliminary vector (which now includes
addition of all session key material) into the new master secret,
prior to computing verify_data.
The purpose of using a PRF to compute a preliminary vector is to
ensure that, even in the absence of session keys, the master secret
is cryptographically distinct in each phase of the handshake.
The purpose of adding session keys into the preliminary vector is to
ensure that the same client entity that negotiated the original
master secret also negotiated the inner authentication(s). In the
absence of such mixing of keys generated from the standard TLS
handshake with keys generated from inner authentication, it is
possible for a hostile agent to mount a man-in-the-middle attack,
acting as server to an unsuspecting client to induce it to perform
an authentication with it, which it can then pass through the TLS
tunnel to allow it to pose as that client.
An application phase may include no authentications that produce a
session key, may include one such authentication, or may include
several. Arithmetic addition was chosen as the mixing method because
it is commutative, that is, it does not depend on the order of
operations. This allows multiple authentications to proceed
concurrently if desired, without having to synchronize the order of
master secret updates between client and server.
Addition was chosen rather than XOR in order to avoid what is
probably a highly unlikely problem; namely, that two separate
authentications produce the same session key, which, if XORed, would
mutually cancel. This might occur, for example, if two instances of
an authentication method were to be applied against different forms
of a user identity that turn out in a some cases to devolve to the
same identity.
Finally, it was decided that a more complex mixing mechanism for
session key material, such as hashing, besides not being
commutative, would not provide any additional security, due to the
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effectively random character of the preliminary vector and the
powerful PRF function which is applied to create derivative keys.
11.3.4 Session Resumption
A TLS/IA handshake may be resumed using standard mechanisms defined
in RFC 2246. In TLS/IA, session resumption is simply an alternative
form of the initial handshake phase, after which subsequent
application phases proceed.
When the initial handshake phase is resumed, client and server may
not deem it necessary to perform the same type of AVP exchange that
they might after a full handshake. In fact, the resumption itself
might provide all the security needed and no AVPs need be exchanged
at all.
If the client determines that it has no need for AVP negotiation, it
sends an ApplicationPayload message with no data as its first
application phase message. If the server concurs, it may conclude
the handshake with ChangeCipherSpec and Finished immediately upon
receiving the empty ApplicationPayload message.
Alternatively, either party may initiate AVP exchange if inner
applications must execute upon session resumption. For example,
authentication exchanges might be omitted but key distribution for
some purpose might still occur.
[Author's note: A future draft may provide a mechanism to avoid the
extra round trip incurred when neither party has a requirement to
send AVPs after session resumption.]
11.3.5 Error Termination
The TLS/IA handshake may be terminated by either party sending a
fatal alert, following standard TLS procedures.
11.3.6 Application Session Key Material
Many authentication mechanisms generate session keying material as a
by-product of authentication. Such keying material is normally
intended for use in a subsequent data connection for encryption and
validation. For example, EAP-TLS, MS-CHAP-V2 and its alter ego EAP-
MS-CHAP-V2 each generate keying material.
When encapsulated within TLS/IA, such keying material MUST NOT be
used to set up data connections; the TLS/IA master secret is a
better basis for this use.
However, such keying material generated during an application phase
MUST be used to permute the TLS/IA master secret between on phase
and the next. The purpose of this is to preclude man-in-the-middle
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attacks, in which an unsuspecting client is induced to perform an
authentication outside a tunnel with an attacker posing as a server;
the attacker can then introduce the authentication protocol into a
tunnel such as provided by TLS/IA, fooling an authentic server into
believing that the attacker is the authentic user.
By mixing keying material generating during application phase
authentication into the master secret, such attacks are thwarted,
since only a single client identity could both authenticate
successfully and have derived the session keying material.
Note that the keying material generated during authentication must
be cryptographically related to the authentication and not derivable
from data exchanged during authentication in order for the keying
material to be useful in thwarting such attacks.
The RECOMMENDED amount of keying material to mix into the master
secret is 32 octets. Up to 48 octets MAY be used.
Each authentication protocol may define how the keying material it
generates is mapped to an octet sequence of some length for the
purpose of TLS/IA mixing. However, for protocols which do not
specify this (including the multitude of protocols that pre-date
TLS/IA) the following rules are defined. The first rule that applies
SHALL be the method for determining keying material:
- If the authentication protocol maps its keying material to the
RADIUS attributes MS-MPPE-Receive-Key and MS-MPPE-Send-Key, then
the keying material for those attributes are concatenated (with
MS-MPPE-Receive-Key first), the concatenated sequence is
truncated to 32 octets if longer, and the result is used as
keying material. (Note that this rule applies to MS-CHAP-V2 and
EAP-MS-CHAP-V2.)
- If the authentication protocol uses a pseudo-random function to
generate keying material, that function is used to generate 32
octets for use as keying material.
11.3.7 Computing Verification Data
In standard TLS, the "verify_data" vector of the Finished message is
computed as follows:
PRF(master_secret, finished_label, MD5(handshake_messages) +
SHA-1(handshake_messages)) [0..11];
This allows both parties to confirm the master secret as well as the
integrity of all handshake messages that have been exchanged.
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In TLS/IA, verify_data for the initial handshake phase is computed
in exactly the same manner, though verify_data is encapsulated in a
PhaseFinished, rather than Finished, message.
In the subsequent application phases, a slight variation to this
formula is used. For each hash, the handshake messages of the
current phase are appended to the hash of the handshake messages of
the previous phase. Thus, for each application phase, the MD5 hash
input to the PRF is a hash of the MD5 hash computed for the previous
phase concatenated with the handshake messages of the current phase;
the SHA-1 hash is computed in the same way, but using the SHA-1 hash
computed for the previous phase.
Also, the master secret used in the PRF computation in each
application phase is the new master secret generated at the
conclusion of that phase.
For clarity, this is best expressed in formal notation.
Let phases be numbered from 0, where phase 0 is the initial phase.
Let:
Secret[n] be the master secret determined at the conclusion of
phase n.
Messages[n] be the handshake messages in phase n.
MD5[n] be the MD5 hash of handshake message material in phase n.
SHA-1[n] be the SHA-1 hash of handshake message material in phase
n.
PRF[n] be the verify_data generated via PRF in phase n.
Hash computations for phase 0 are as follows:
MD5[0] = MD5(Messages[0])
SHA-1[0] = SHA-1(Messages[0])
PRF[0] = PRF(master_secret, finished_label, MD5[0] + SHA-1[0])
[0..11]
Hash computations for phase i, where i > 0 (i.e. application phases)
are as follows:
MD5[i] = MD5(MD5[i-1] + Messages[i])
SHA-1[i] = SHA-1(SHA-1[i-1] + Messages[i])
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The PRF computation to generate verify_data for any phase i
(including i = 0) is as follows:
PRF[i] = PRF(Secret[i], finished_label, MD5[i] + SHA-1[i])
[0..11]
Note that for phase 0, the PRF computation is identical to the
standard TLS computation. Variations to the algorithm occur only in
application phases, in the use of new master secrets and the
inclusion of hashes of previous handshake messages as input to the
hashing algorithms.
Note that the only handshake messages that appear in an application
phase are InnerApplication messages and Finished or Phase Finished
messages. During an application phase, the handshake messages input
to the hashing algorithm by the server will include all
InnerApplication messages exchanged during that phase; the handshake
messages input to the hashing algorithm by the client will include
all InnerApplication messages exchanged during that phase plus the
server's PhaseFinished or Finished message.
11.3.8 Attribute-Value Pairs (AVPs)
AVPs used in InnerApplication messages are exactly as defined in
Section 9 of this document; that is, they are Diameter-style AVPs
and use the RADIUS-Diameter namespace.
Rules for performing authentications using these AVPs are exactly as
defined in Section 10 of this document. This includes rules for
creating implicit challenges, and rules for use of inner EAP
authentications as well as legacy protocols such as PAP, CHAP and
MS-CHAP-V1/V2. Note that all implicit challenges are based on the
then-current master secret.
11.3.9 TLS/IA Messages
All specifications of TLS/IA messages follow the usage defined in
RFC 2246.
TLS/IA defines a new TLS extension - "InnerApplication"; two new
handshake messages - "PhaseFinished" and "ApplicationPayload"; and a
new alert code - "InnerApplicationFailure".
The InnerApplication extension type is 9347 (hex).
In order to avoid potential type-assignment problems, the new
handshake message types and alert code are dynamically defined
within the InnerApplication extension message. Client and server
independently specify the values they will send. Thus, the client
assigns its own message type and alert code values for use in its
own transmissions, and includes these values in its InnerApplication
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message within ClientHello. Similarly, the server assigns its own
message type and alert code values for use in its own transmissions,
and includes these values in its InnerApplication message within
ServerHello. Each party must note the message type and alert code
values assigned by the other party and interpret messages from the
other party accordingly. Both client and server assign message types
and alert code so as not to conflict with values that that it might
otherwise send. There is no requirement that client and server
assign identical values for these items.
11.3.10 The InnerApplication Extension
Use of the InnerApplication extension follows RFC 3546. The client
proposes use of this extension by including the InnerApplication
extension in the client_hello_extension_list vector of the extended
ClientHello. If the extension is included in the ClientHello, the
server MAY accept the proposal by including the InnerApplication
extension in the server_hello_extension_list of the extended
ServerHello. If use of this extension is either not proposed by the
client or not confirmed by the server, the variations to the TLS
handshake described here MUST NOT be used.
The "extension_data" field of the Extension structure for the
InnerApplication extension SHALL contain "InnerApplication" where:
struct {
uint8 PhaseFinishedType;
uint8 ApplicationPayloadType;
uint8 InnerApplicationFailureAlertCode;
} InnerApplication;
11.3.11 The PhaseFinished Handshake Message
The PhaseFinished message concludes the initial handshake phase and
each intermediate handshake phase. It MUST be immediately preceded
by a ChangeCipherSpec message. It is defined as follows:
struct {
opaque verify_data[12];
} PhaseFinished;
11.3.12 The ApplicationPayload Handshake Message
The ApplicationPayload message carries an AVP sequence during an
application handshake phase. It is defined as follows:
struct {
opaque avps[Handshake.length];
} ApplicationPayload;
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where Handshake.length is the 24-bit length field in the
encapsulating Handshake message.
Note that the "avps" element has its length defined in square
bracket rather than angle bracket notation, implying a fixed rather
than variable length vector. This avoids the having the length of
the AVP sequence specified redundantly both in the encapsulating
Handshake message and as a length prefix in the avps element itself.
11.3.13 The InnerApplicationFailure Alert
An InnerApplicationFailure error alert may be sent by either party
during an application phase. This indicates that the sending party
considers the negotiation to have failed due to an application
carried in the AVP sequences, for example, a failed authentication.
The AlertLevel for an InnerApplicationFailure alert MUST be set to
"fatal".
Note that other alerts are possible during an application phase; for
example, decrypt_error. The InnerApplicationFailure alert relates
specifically to the failure of an application implemented via AVP
sequences; for example, failure of an EAP or other authentication
method, or information passed within the AVP sequence that is found
unsatisfactory.
11.4 Binding of TLS/IA to EAP-TTLS v1
EAP-TTLS v1 encapsulates a TLS handshake with the InnerApplication
extension (TLS/IA). EAP-TTLS v1 acts as a carrier protocol for
TLS/IA, and uses cryptographic information developed during the
TLS/IA exchange to create session keys for encrypting subsequent
data transmission between client and access point.
The format for encapsulated TLS/IA messages in EAP-TTLS v1 is
identical to the formats described for EAP-TTLS v0 in Section 8,
unless otherwise specified
11.4.1 Flags Octet
Use of version 1 of EAP-TTLS is negotiated through a new 3-bit
"Version" field in the Flags octet of the EAP-TTLS request/response
header. The Flags octet is the first octet of each EAP-TTLS message,
following immediately after the EAP type. The Version field uses
bits of the Flags octet that were formerly reserved and required to
be 0.
The new bit field definitions for the Flags octet are as follows:
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| L M S R R | Version |
+---+---+---+---+---+---+---+---+
where:
L = Length included
M = More fragments
S = Start
R = Reserved
Version = EAP-TTLS version number
For EAP-TTLS v1, Version is set to 1; that is, the bit sequence 001.
Interpretation of L, M and S are as in EAP-TTLS v0.
11.4.2 Version Negotiation
The version of EAP-TTLS is negotiated in the first exchange between
server and client. The server sets the highest version number of
EAP-TTLS that it supports in the Version field of its Start message
(in the case of EAP-TTLS v1, this is 1). In its first EAP message in
response, the client sets the Version field to the highest version
number that it supports that is no higher than the version number
offered by the server. If the client version is not acceptable to
the server, it sends an EAP-Failure to terminate the EAP session.
Otherwise, the version sent by the client is the version of EAP-TTLS
that MUST be used, and both server and client set the version field
to that version number in all subsequent EAP messages.
11.4.3 Acknowledgement Packets
An Acknowledgement packet is an EAP-TTLS v1 packet with no
additional data beyond the Flags octet, and with the L, M and S bits
of the Flags octet set to 0. (Note, however, that the Version field
MUST still be set to the appropriate version number.)
An Acknowledgement packet is sent for the following purposes:
- Fragment acknowledgement
- Error alert acknowledgement
Note that in EAP-TTLS v0 there are other cases in which a packet
with no data must be sent by the client for the simple reason that
the client has no AVPs to send. This situation does not arise in
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EAP-TTLS v1. If no AVPs are to be sent, there will nevertheless be
an ApplicationPayload message containing no data, which the client
must send.
- Fragment Acknowledgement
Each EAP-TTLS v1 message contains a sequence of TLS/IA messages
that represent a single leg of a half-duplex conversation. The
EAP carrier protocol (e.g., PPP, EAPOL, RADIUS) may impose
constraints on the length of of an EAP message. Therefore it may
be necessary to fragment an EAP-TTLS v1 message across multiple
EAP messages.
Each fragment except for the last MUST have the M bit set, to
indicate that more data is to follow; the final fragment MUST NOT
have the M bit set. The party that receives a message with the M
bit set MUST respond with an Acknowledgement packet.
- Error Alert Acknowledgement
Either party may at any time send a TLS error alert to fail the
TLS/IA handshake.
If the client sends an error alert to the server, no further EAP-
TTLS messages are exchanged, and the server sends an EAP-Failure
to terminate the conversation.
If the server sends an error alert to the client, the client MUST
respond with an Acknowledgement packet to allow the conversation
to continue. Upon receipt of the Acknowledgement packet, the
server sends an EAP-Failure to terminate the conversation.
11.4.4 Generating Keying Material
EAP-TTLS v1 uses the same mechanism as EAP-TTLS v0 to generate
keying material (session keys) for use in the data connection
between client and access point.
Note that it is the final master secret of the TLS/IA exchange that
is used to generate keying material for use in the subsequent data
connection.
12. Discussion of Certificates and PKI
Public-key cryptography, certificates, and the associated PKI are
used in EAP-TTLS to authenticate the EAP-TTLS server to the client,
and optionally the client to the EAP-TTLS server. Previous
experience with the deployment of PKI in applications has shown that
its implementation requires care. This section provides a brief
discussion of the issues implementers will face when deploying PKI
for EAP-TTLS.
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The traditional use of TLS for securing e-commerce transactions over
the Internet is perhaps the best-known deployment of PKI, and it
serves to illustrate several of the issues relevant here. In the
case of e-commerce:
- The environment is many-to-many - many consumers do business with
many merchants. Typically there is no relationship in advance
between a consumer and a merchant.
- Users are "notoriously bad" about following security guidelines.
When presented with a dialogue saying "the name in the
certificate is different from the name you requested", most users
will simply continue with the transaction.
- Support for revocation is limited. It is important to understand
that the environments in which EAP-TTLS are likely to be deployed
will typically be very different from e-commerce.
In particular, many deployments will be comparable to deploying
wireless LAN within an enterprise. In this case, the communications
topology is essentially many-to-one or many-to-few - many employees
talking to a few EAP-TTLS servers - and all clients are essentially
governed by their employer rather than autonomous.
This means:
- It may be unnecessary to rely on a public CA. Instead the
enterprise could choose to run its own CA (either insourced or
outsourced).
- The enterprise could choose to enforce stringent policies on
certificate validation and processing - for example simply
insisting connections are dropped if the correct name does not
appear in the server certificate. Such policies could be enforced
via extensions in the root certificate of the enterprise CA.
However it also means:
- EAP-TTLS servers may receive considerably less attention than the
web servers of large e-commerce sites. As a result, compromise of
EAP-TTLS servers may be more common, and therefore deployment and
use of revocation solutions may be more relevant.
One open question in the area of PKI on which the authors would like
to promote discussion is the following:
- Should EAP-TTLS enforce rules on name matching regarding the EAP-
TTLS server? For example, EAP-TTLS could mandate that
radius.xyz.realm or diameter.xyz.realm be used as the name in the
EAP-TTLS server's certificate, and that the client must match
this name with the realm it sent in the initial EAP-
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Response/Identity.
13. Message Sequences
[Author's note: The message sequences in these sections apply to
version 0 of the EAP-TTLS protocol. Messages sequences for version 1
have not yet been completed.]
This section presents EAP-TTLS message sequences for various
negotiation scenarios. These examples do not attempt to exhaustively
depict all possible scenarios.
It is assumed that RADIUS is the AAA carrier protocol both between
access point and TTLS server, and between TTLS server and AAA/H.
EAP packets that are passed unmodified between client and TTLS
server by the access point are indicated as "passthrough". AVPs that
are securely tunneled within the TLS record layer are enclosed in
curly braces ({}). Items that are optional are suffixed with
question mark (?). Items that may appear multiple times are suffixed
with plus sign (+).
13.1 Successful authentication via tunneled CHAP
In this example, the client performs one-way TLS authentication of
the TTLS server nad CHAP is used as a tunneled user authentication
mechanism.
client access point TTLS server AAA/H
------ ------------ ----------- -----
EAP-Request/Identity
<--------------------
EAP-Response/Identity
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS-Start
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
ClientHello
-------------------->
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RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS:
ServerHello
Certificate
ServerKeyExchange
ServerHelloDone
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
ClientKeyExchange
ChangeCipherSpec
Finished
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS:
ChangeCipherSpec
Finished
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
{User-Name}
{CHAP-Challenge}
{CHAP-Password}
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Request:
User-Name
CHAP-Challenge
CHAP-Password
-------------------->
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RADIUS Access-Accept
<--------------------
RADIUS Access-Accept:
EAP-Success
<--------------------
EAP-Success passthrough
<--------------------
13.2 Successful authentication via tunneled EAP/MD5-Challenge
In this example, the client performs one-way TLS authentication of
the TTLS server and EAP/MD5-Challenge is used as a tunneled user
authentication mechanism.
client access point TTLS server AAA/H
------ ------------ ----------- -----
EAP-Request/Identity
<--------------------
EAP-Response/Identity
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS-Start
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
ClientHello
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS:
ServerHello
Certificate
ServerKeyExchange
ServerHelloDone
<--------------------
Paul Funk expires January 2005 [Page 44]
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EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
ClientKeyExchange
ChangeCipherSpec
Finished
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS:
ChangeCipherSpec
Finished
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
{EAP-Response/Identity}
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Request:
EAP-Response/Identity
-------------------->
RADIUS Access-Challenge
EAP-Request/
MD5-Challenge
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS:
{EAP-Request/MD5-Challenge}
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
{EAP-Response/MD5-Challenge}
-------------------->
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RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge
EAP-Response/
MD5-Challenge
-------------------->
RADIUS Access-Accept
<--------------------
RADIUS Access-Accept:
EAP-Success
<--------------------
EAP-Success passthrough
<--------------------
13.3 Successful session resumption
In this example, the client and server resume a previous TLS
session. The ID of the session to be resumed is sent as part of the
ClientHello, and the server agrees to resume this session by sending
the same session ID as part of ServerHello.
client access point TTLS server AAA/H
------ ------------ ----------- -----
EAP-Request/Identity
<--------------------
EAP-Response/Identity
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS-Start
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
ClientHello
-------------------->
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RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS:
ServerHello
ChangeCipherSpec
Finished
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
ChangeCipherSpec
Finished
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Accept:
EAP-Success
<--------------------
EAP-Success passthrough
<--------------------
14. Security Considerations
This draft is entirely about security and the security
considerations associated with the mechanisms employed in this
document should be considered by implementers.
The following additional issues are relevant:
- Anonymity and privacy. Unlike other EAP methods, EAP-TTLS does
not communicate a username in the clear in the initial EAP-
Response/Identity. This feature is designed to support anonymity
and location privacy from attackers eavesdropping the network
path between the client and the TTLS server. However implementers
should be aware that other factors - both within EAP-TTLS and
elsewhere - may compromise a user's identity. For example, if a
user authenticates with a certificate during phase 1 of EAP-TTLS,
the subject name in the certificate may reveal the user's
identity. Outside of EAP-TTLS, the client's fixed MAC address, or
in the case of wireless connections, the client's radio
signature, may also reveal information. Additionally,
implementers should be aware that a user's identity is not hidden
Paul Funk expires January 2005 [Page 47]
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from the EAP-TTLS server and may be included in the clear in AAA
messages between the access point, the EAP-TTLS server, and the
AAA/H server.
- Trust in the EAP-TTLS server. EAP-TTLS is designed to allow the
use of legacy authentication methods to be extended to mediums
like wireless in which eavesdropping the link between the client
and the access point is easy. However implementers should be
aware of the possibility of attacks by rogue EAP-TTLS servers -
for example in the event that the phase 2 authentication method
within EAP-TTLS is susceptible to dictionary attacks. These
threats can be mitigated through the use of authentication
methods like one-time passwords which are not susceptible to
dictionary attacks, or by ensuring that clients connect only to
trusted EAP-TTLS servers.
- EAP-TTLS server certificate compromise. The use of EAP-TTLS
server certificates within EAP-TTLS makes EAP-TTLS susceptible to
attack in the event that an EAP-TTLS server's certificate is
compromised. EAP-TTLS servers should therefore take care to
protect their private key. In addition, certificate revocation
methods may be used to mitigate against the possibility of key
compromise. [13] describes a way to integrate one such method -
OCSP [14] - into the TLS handshake - use of this approach may be
appropriate within EAP-TTLS.
- Negotiation of link encryption. EAP-TTLS includes a method to
negotiate data cipher suites. It also allows data cipher suites
to be negotiated by other means - for example by having client
and access point exchange their preferences using the link layer
protocol. However the use of the EAP-TTLS negotiation is strongly
recommended because it provides a secured negotiation. In
contrast, simple unsecured preference exchange over the link
layer is susceptible to a man-in-the-middle attack that forces
the parties to use the weakest, rather than the strongest,
mutually acceptable data cipher suite. The potential of this
problem is well-illustrated by wireless LAN where for
interoperability purposes many entities will have to continue to
support WEP encryption for some time. In the event that the data
link protocol already includes a negotiation exchange, it is
recommended that the EAP-TTLS exchange still be used, with the
link layer exchange simply confirming the data cipher suite
selected using EAP-TTLS.
- Listing of data cipher preferences. EAP-TTLS negotiates data
cipher suites by having the EAP-TTLS server select the first
cipher suite appearing on the client list that also appears on
the access point list. In order to maximize security, it is
therefore recommended that the client order its list according to
security - most secure acceptable cipher suite first, least
secure acceptable cipher suite last.
Paul Funk expires January 2005 [Page 48]
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- Forward secrecy. With forward secrecy, revelation of a secret
does not compromise session keys previously negotiated based on
that secret. Thus, when the TLS key exchange algorithm provides
forward secrecy, if a TTLS server certificate's private key is
eventually stolen or cracked, tunneled user password information
will remain secure as long as that certificate is no longer in
use. Diffie-Hellman key exchange is an example of an algorithm
that provides forward secrecy. A forward secrecy algorithm should
be considered if attacks against recorded authentication or data
sessions are considered to pose a significant threat.
15. Changes since previous drafts
Other than minor editorial changes, the following changes have been
made to this draft:
Since version 04:
- An enhanced version of EAP-TTLS, called version 1, has been
defined in section 11.
Since version 03:
- Removed section on keying information.
Since version 02:
- Added password change for MS-CHAP-V2.
Since version 01:
- In section 11, the TTLS server's response with data cipher suites
has been made conditional on receiving data cipher suite
preferences from both client and access point. Also, implicit
acceptance of the client's preferred data cipher suite has been
eliminated in favor of explicitly returning the data cipher suite
selection.
Since version 00:
- A Table of Contents has been added.
- In section 3, a definition of "access domain" has been added.
- In section 6.4, the requirement has been added that TLS session
resumption must not be allowed for any negotiation that succeeds
in phase 1 TLS handshake but does not successfully complete phase
2 authentication.
- In sections 7 and 10.1, reversed the order of randoms used in
PRF, to follow EAP-TLS practice and avoid namespace collisions
Paul Funk expires January 2005 [Page 49]
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with TLS.
- In section 8, specified the assigned EAP-TTLS number.
- Added section 8.1, reserving for future standardization the
ability to add data to an EAP-TTLS Start packet.
16. References
[1] Aboba, B., and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[2] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "PPP Extensible Authentication Protocol (EAP)", RFC
3784, June 2004.
[3] Dierks, T., and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, November 1998.
[4] Institute for Electrical and Electronics Engineers, "IEEE
802.1X, Standard for Port Based Network Access Control", 2001.
[5] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[6] Rigney, C., Rubens, A., Simpson, W., and S. Willens, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[7] Aboba, B., and M. Beadles, "The Network Access Identifier",
RFC 2486, January 1999.
[8] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, July 2001.
[9] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1700,
October 1994.
[10] Zorn, G., and S. Cobb, "Microsoft PPP CHAP Extensions", RFC
2433, October 1998.
[11] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC
2759, January 2000.
[12] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC
2548, March 1999.
[13] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
T. Wright, "Transport Layer Security (TLS) Extensions", RFC
3546, June 2003.
Paul Funk expires January 2005 [Page 50]
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[14] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "Internet X.509 Public Key Infrastructure: Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[15] Asokan, N., Niemi, V., and Nyberg, K., "Man-in-the-Middle in
Tunneled Authentication",
http://www.saunalahti.fi/~asokan/research/mitm.html, Nokia
Research Center, Finland, October 24 2002.
[16] Puthenkulam, J., "The Compound Authentication Binding
Problem", draft-puthenkulam-eap-binding-04.txt, October 2003.
17. Authors' Addresses
Questions about this memo can be directed to:
Paul Funk
Funk Software, Inc.
222 Third Street
Cambridge, MA 02142
USA
Phone: +1 617 497-6339
E-mail: paul@funk.com
Simon Blake-Wilson
Basic Commerce & Industries, Inc.
304 Harper Drive, Suite 203
Moorestown, NJ 08057
Phone: +1 856 778-1660
E-mail: sblakewilson@bcisse.com
18. Full Copyright Statement
Copyright (C) The Internet Society (2001-2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
Paul Funk expires January 2005 [Page 51]
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The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Paul Funk expires January 2005 [Page 52]
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