Extensible Authentication Protocol Tunneled Transport Layer Security Authenticated Protocol Version 0 (EAP-TTLSv0)
draft-funk-eap-ttls-v0-05
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 5281.
|
|
|---|---|---|---|
| Authors | Paul Funk , Simon Blake-Wilson | ||
| Last updated | 2020-01-21 (Latest revision 2008-04-30) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 5281 (Informational) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Jari Arkko | ||
| Send notices to | emu-chairs@ietf.org, ldondeti@qualcomm.com |
draft-funk-eap-ttls-v0-05
Network Working Group Paul Funk
Internet-Draft Unaffiliated
Intended status: Informational Simon Blake-Wilson
<draft-funk-eap-ttls-v0-05.txt> SafeNet
Expires: October 2008 April 2008
EAP Tunneled TLS Authentication Protocol Version 0
(EAP-TTLSv0)
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The IETF Trust (2008). All Rights Reserved.
Abstract
EAP-TTLS is an EAP method that provides additional functionality
beyond what is available in EAP-TLS [RFC5216]. 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
Paul Funk expires October 2008 [Page 1]
Internet-Draft April 2008
authenticate the client using existing, widely-deployed
authentication mechanisms. 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 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 EAP-TTLSv0; that is, the original version 0
of the EAP-TTLS protocol, which has been widely deployed.
Table of Contents
1. Introduction .....................................................4
2. Motivation .......................................................5
3. Requirements Language ............................................6
4. Terminology ......................................................6
5. Architectural Model ..............................................9
5.1 Carrier Protocols .............................................9
5.2 Security Relationships .......................................10
5.3 Messaging ....................................................10
5.4 Resulting Security ...........................................11
6. Protocol Layering Model .........................................11
7. EAP-TTLS Overview ...............................................12
7.1 Phase 1: Handshake ...........................................13
7.2 Phase 2: Tunnel ..............................................13
7.3 EAP Identity Information .....................................14
7.4 Piggybacking .................................................15
7.5 Session Resumption ...........................................15
7.6 Determining Whether to Enter Phase 2 .........................16
7.7 TLS Version ..................................................17
7.8 Use of TLS PRF ...............................................17
8. Generating Keying Material ......................................18
9. EAP-TTLS Protocol ...............................................19
9.1 Packet Format ................................................19
9.2 EAP-TTLS Start Packet ........................................20
9.2.1 Version Negotiation ......................................20
9.2.2 Fragmentation ............................................21
9.2.3 Acknowledgement Packets ..................................21
10. Encapsulation of AVPs within the TLS Record Layer ...............21
10.1 AVP Format ...................................................22
10.2 AVP Sequences ................................................24
10.3 Guidelines for Maximum Compatibility with AAA Servers ........24
11. Tunneled Authentication .........................................24
Paul Funk expires October 2008 [Page 2]
Internet-Draft April 2008
11.1 Implicit challenge ...........................................24
11.2 Tunneled Authentication Protocols ............................25
11.2.1 EAP ......................................................26
11.2.2 CHAP .....................................................27
11.2.3 MS-CHAP ..................................................28
11.2.4 MS-CHAP-V2 ...............................................29
11.2.5 PAP ......................................................30
11.3 Performing Multiple Authentications ..........................31
11.4 Mandatory Tunneled Authentication Support ....................32
11.5 Additional Suggested Tunneled Authentication Support .........32
12. Keying Framework ................................................33
12.1 Session-Id ...................................................33
12.2 Peer-Id ......................................................33
12.3 Server-Id ....................................................33
13. AVP Summary .....................................................33
14. Security Considerations .........................................34
14.1 Security Claims ..............................................34
14.1.1 Authentication mechanism .................................34
14.1.2 Ciphersuite negotiation ..................................35
14.1.3 Mutual authentication ....................................35
14.1.4 Integrity protection .....................................35
14.1.5 Replay protection ........................................35
14.1.6 Confidentiality ..........................................35
14.1.7 Key derivation ...........................................35
14.1.8 Key strength .............................................35
14.1.9 Dictionary attack protection .............................35
14.1.10 Fast reconnect ...........................................35
14.1.11 Cryptographic binding ....................................36
14.1.12 Session independence .....................................36
14.1.13 Fragmentation ............................................36
14.1.14 Channel binding ..........................................36
14.2 Client Anonymity .............................................36
14.3 Server Trust .................................................37
14.4 Certificate Validation .......................................37
14.5 Certificate Compromise .......................................37
14.6 Forward secrecy. .............................................38
14.7 Negotiating-Down Attacks .....................................38
15. Message Sequences ...............................................38
15.1 Successful authentication via tunneled CHAP ..................39
15.2 Successful authentication via tunneled EAP/MD5-Challenge .....40
15.3 Successful session resumption ................................43
16. IANA Considerations .............................................44
17. Acknowledgements ................................................44
18. References ......................................................45
18.1 Normative References .........................................45
18.2 Informative References .......................................46
19. Authors' Addresses ..............................................47
20. Intellectual Property Statement .................................47
21. Disclaimer of Validity ..........................................47
22. Copyright Statement .............................................48
23. Acknowledgement .................................................48
Paul Funk expires October 2008 [Page 3]
Internet-Draft April 2008
1. Introduction
Extensible Authentication Protocol (EAP) [RFC3748] defines a
standard message exchange that allows a server to authenticate a
client using an authentication method agreed upon by both parties.
EAP may be extended with additional authentication methods by
registering such methods with IANA or by defining vendor specific
methods.
Transport Layer Security (TLS) [RFC4346] 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 defined as an authentication protocol for use within EAP
(EAP-TLS) [RFC5216].
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 [RFC2865], Diameter
[RFC3588] or other AAA servers. These include non-EAP protocols such
as PAP [RFC1661], CHAP [RFC1661], MS-CHAP [RFC2433] or MS-CHAP-V2
[RFC2759], as well as EAP protocols such as MD5-Challenge [RFC3748].
EAP-TTLS is an EAP method that provides functionality beyond what is
available in 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. 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 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
Paul Funk expires October 2008 [Page 4]
Internet-Draft April 2008
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
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
client's username in the clear. In the context of eavesdropping
on the wireless channel, this can compromise the client's
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
Paul Funk expires October 2008 [Page 5]
Internet-Draft April 2008
in an airport, coffee shop, or hotel in order to provide public
access via 802.11 [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.
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 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 access
domains with which the user has no relationship and which will
have limited capabilities for routing authentication requests.
3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
4. Terminology
AAA
Paul Funk expires October 2008 [Page 6]
Internet-Draft April 2008
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.
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. Since the access point
terminates the server side of the EAP conversation, for the
purposes of this document it is therefore equivalent to the
"authenticator", as used in the EAP specification [RFC3748].
Since the access point acts as a client to a AAA server, for the
purposes of this document it is therefore also equivalent to the
"NAS", as used in AAA specifications such as [RFC2865].
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. Since it terminates the client side of the EAP
conversation, for the purposes of this document, it is therefore
equivalent to the "peer", as used in the EAP specification
[RFC3748].
domain
Paul Funk expires October 2008 [Page 7]
Internet-Draft April 2008
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
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 [RFC4282], normally consisting of the
name of the user and, optionally, the user's home realm.
proxy
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.
Paul Funk expires October 2008 [Page 8]
Internet-Draft April 2008
5. 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 ---->
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.
5.1 Carrier Protocols
The entities shown above communicate with each other using carrier
protocols capable of encapsulating EAP. The client and access point
communicate typically 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 carrier
protocol between client and access point. 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.
Paul Funk expires October 2008 [Page 9]
Internet-Draft April 2008
5.2 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.
5.3 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 need not include
the user's actual identity in this EAP-Response/Identity packet
other than for routing purposes (e.g. realm information; see section
7.3 and [RFC3748] section 5.1); the user's actual identity need 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 authentication (one-
way or mutual), validation of client integrity and configuration,
provisioning of information required for data connectivity, etc.
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
Paul Funk expires October 2008 [Page 10]
Internet-Draft April 2008
challenge information to the client. The AAA/H server may be a
legacy device and needs to know nothing about EAP-TTLS; it only
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 (MSK/EMSK; see section 8) is 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.
5.4 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 generates keying material to allow network data
subsequent to authentication to be securely transmitted between
client and access point.
6. 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
attribute-value pairs (AVPs) which may carry user authentication or
other 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:
+-----------------------------------------------------------+
| AVPs, including authentication (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:
Paul Funk expires October 2008 [Page 11]
Internet-Draft April 2008
+-----------------------------------------------------------+
| EAP Method (MD-Challenge, etc.) |
+-----------------------------------------------------------+
| AVPs, including 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 [RFC1661] or EAPOL [802.1X] may be used to
transport EAP between client and access point; RADIUS [RFC2865] or
Diameter [RFC3588] are used to transport EAP between access point
and TTLS server.
7. EAP-TTLS Overview
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, client integrity
validation, 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
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.
Paul Funk expires October 2008 [Page 12]
Internet-Draft April 2008
Functions other than authentication MAY also be performed during
phase 2. This document does not define any such functions; however,
any organization or standards body is free to specify how additional
functions may be performed through the use of appropriate AVPs.
EAP-TTLS 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.
7.1 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.
The TTLS server initiates the EAP-TTLS method with an EAP-TTLS/Start
packet, which is an EAP-Request with Type = EAP-TTLS and the S
(Start) bit set. 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 [RFC5216]. 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.
7.2 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.)
Paul Funk expires October 2008 [Page 13]
Internet-Draft April 2008
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 forward an authentication challenge received
from the AAA/H.
This process continues until the AAA/H either accepts or rejects the
client, resulting in the TTLS server completing the EAP-TTLS
negotiation and indicating success or failure to the encapsulating
EAP protocol (which normally results in a final EAP-Success or EAP-
Failure being sent to the client).
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 final EAP-Success or other
success indication.
7.3 EAP Identity Information
The identity of the user is provided during phase 2, where it is
protected by the TLS tunnel. However, prior to beginning the EAP-
TTLS authentication, the client will typically issue an EAP-
Response/Identity packet as part of the EAP protocol, containing a
username in clear text. To preserve user anonymity against
eavesdropping, this packet specifically SHOULD NOT include the
actual name of the user; instead, it SHOULD use a blank or
placeholder such as "anonymous". However, this privacy constraint is
not intended to apply to any information within the EAP-
Response/Identity that is required for routing; thus, the EAP-
Response/Identity packet MAY include the name of the realm of a
trusted provider to which EAP-TTLS packets should be forwarded; for
example, "anonymous@myisp.com".
Note that at the time the initial EAP-Response/Identity packet is
sent the EAP method is yet to be negotiated. If, in addition to EAP-
TTLS, the client is willing to negotiate use of EAP methods that do
not support user anonymity, then the client MAY include the name of
the user in the EAP-Response/Identity to meet the requirements of
the other candidate EAP methods.
Paul Funk expires October 2008 [Page 14]
Internet-Draft April 2008
7.4 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
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
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.
7.5 Session Resumption
When a client and TTLS server that have previously negotiated an
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, causing a new session to 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.
Paul Funk expires October 2008 [Page 15]
Internet-Draft April 2008
The client MAY send other AVPs in its first phase 2 message of a
session resumption, to initiate non-authentication functions. If it
does not, the TTLS server, at its option, MAY send AVPs to the
client to initiate non-authentication functions, or MAY simply
complete the EAP-TTLS negotiation and indicate success or failure to
the encapsulating EAP protocol.
The TTLS server 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.]
7.6 Determining Whether to Enter Phase 2
Entering phase 2 is optional, and may be initiated by either client
or TTLS server. If no further authentication or other information
exchange is required upon completion of phase 1, it is possible to
successfully complete the EAP-TTLS negotiation without ever entering
phase 2 or tunneling any AVPs.
Scenarios in which phase 2 is never entered include:
- Successful session resumption, with no additional information
exchange required,
- Authentication of the client via client certificate during phase
1, with no additional authentication or information exchange
required.
The client always has the first opportunity to initiate phase 2 upon
completion of phase 1. If the client has no AVPs to send, it either
sends an Acknowledgement (see section 9.2.3) if the TTLS server
Paul Funk expires October 2008 [Page 16]
Internet-Draft April 2008
sends the final phase 1 message, or simply does not piggyback a
phase 2 message when it issues the final phase 1 message (as will
occur during session resumption).
If the client does not initiate phase 2, the TTLS server, at its
option, may either complete the EAP-TTLS negotiation without
entering phase 2 or initiate phase 2 by tunneling AVPs to the
client.
For example, suppose a successful session resumption occurs in phase
1. The following sequences are possible:
- Neither client nor TTLS server has additional information to
exchange. The client completes phase 1 without piggybacking phase
2 AVPs, and the TTLS server indicates success to the
encapsulating EAP protocol without entering phase 2.
- The client has no additional information to exchange, but the
TTLS server does. The client completes phase 1 without
piggybacking phase 2 AVPs, but the TTLS server extends the EAP-
TTLS negotiation into phase 2 by tunneling AVPs in its next EAP-
TTLS message.
- The client has additional information to exchange, and piggybacks
phase 2 AVPs with its final phase 1 message, thus extending the
negotiation into phase 2.
7.7 TLS Version
TLS version 1.0 [RFC2246], 1.1 [RFC4346], or any subsequent version
MAY be used within EAP-TTLS. TLS provides for its own version
negotiation mechanism.
For maximum interoperability, EAP-TTLS implementations SHOULD
support TLS version 1.0.
7.8 Use of TLS PRF
EAP-TTLSv0 utilizes a pseudo-random function (PRF) to generate
keying material (section 8) and to generate implicit challenge
material for certain authentication methods (section 11.1). The PRF
used in these computations is the TLS PRF used in the TLS handshake
negotiation that initiates the EAP-TTLS exchange.
TLS versions 1.0 [RFC2246] and 1.1 [RFC4346] define the same PRF
function, and any EAP-TTLSv0 implementation based on these versions
of TLS must use the PRF defined therein. It is expected that future
versions of or extensions to the TLS protocol will permit
alternative PRF functions to be negotiated. If an alternative PRF
function is specified for the underlying TLS version or has been
negotiated during the TLS handshake negotiation, then that
Paul Funk expires October 2008 [Page 17]
Internet-Draft April 2008
alternative PRF function must be used in EAP-TTLSv0 computations
instead of the TLS 1.0/1.1 PRF.
The TLS PRF function used in this specification is denoted as
follows:
PRF-nn(secret, label, seed)
where:
nn is the number of generated octets
secret is a secret key
label is a string (without null-terminator)
seed is a binary sequence.
The TLS 1.0/1.1 PRF has invariant output regardless of how many
octets are generated. However, it is possible that alternative PRF
functions will include the size of the output sequence as input to
the PRF function; this means generating 32 octets and generating 64
octets from the same input parameters will no longer result in the
first 32 octets being identical. For this reason, the PRF is always
specified with an "nn", indicating the number of generated octets.
8. Generating Keying Material
Upon successful conclusion of an EAP-TTLS negotiation, 128 octets of
keying material is generated and exported for use in securing the
data connection between client and access point. The first 64 octets
of the keying material constitutes the MSK, the second 64 octets
constitutes the EMSK.
The keying material is generated using the TLS PRF function
[RFC4346], with inputs consisting of the TLS master secret, the
ASCII-encoded constant string "ttls keying material", the TLS client
random, and the TLS server random. The constant string is not null-
terminated.
Keying Material = PRF-128(SecurityParameters.master_secret,
"ttls keying material",
SecurityParameters.client_random +
SecurityParameters.server_random)
MSK = Keying Material [0..63]
EMSK = Keying Material [64..127]
Note that the order of client_random and server_random for EAP-TTLS
is reversed from that of the TLS protocol [RFC4346]. This ordering
Paul Funk expires October 2008 [Page 18]
Internet-Draft April 2008
follows the key derivation method of EAP-TLS [RFC5216]. Altering the
order of randoms avoids namespace collisions between constant
strings defined for EAP-TTLS and those defined for the TLS protocol.
The TTLS server distributes this keying material to the access point
via the AAA carrier protocol. When RADIUS is the AAA carrier
protocol, the MPPE-Recv-Key and MPPE-Send-Key attributes [RFC2548]
may be used to distribute the first 32 octets and second 32 octets
of the MSK, respectively.
9. EAP-TTLS Protocol
9.1 Packet Format
The EAP-TTLS packet format is shown below. The fields are
transmitted left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags | Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Length | Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 for request, 2 for response.
Identifier
The Identifier field is one octet and aids in matching responses
with requests. The Identifier field MUST be changed for each
request packet and MUST be echoed in each response packet.
Length
The Length field is two octets and indicates the number of octets
in the entire EAP packet, from the Code field through the Data
field.
Type
21 (EAP-TTLS)
Flags
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| L | M | S | R | R | V |
+---+---+---+---+---+---+---+---+
L = Length included
M = More fragments
Paul Funk expires October 2008 [Page 19]
Internet-Draft April 2008
S = Start
R = Reserved
V = Version (000 for EAP-TTLSv0)
The L bit is set to indicate the presence of the four octet TLS
Message Length field. The M bit indicates that more fragments are
to come. The S bit indicates a Start message. The V field is set
to the version of EAP-TTLS, and is set to 000 for EAP-TTLSv0.
Message Length
The Message Length field is four octets, and is present only if
the L bit is set. This field provides the total length of the raw
data message sequence prior to fragmentation.
Data
For all packets other than a Start packet, the Data field
consists of the raw TLS message sequence or fragment thereof. For
a Start packet, the Data field may optionally contain an AVP
sequence.
9.2 EAP-TTLS Start Packet
The S bit MUST be set on the first packet sent by the server to
initiate the EAP-TTLS protocol. It MUST NOT be set on any other
packet.
This packet MAY contain additional information in the form of AVPs,
which may provide useful hints to the client; for example, the
server identity may be useful to the client to allow it to pick the
correct TLS session ID for session resumption. 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.
9.2.1 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 V field of its Start message (in
the case of EAP-TTLSv0, this is 0). In its first EAP message in
response, the client sets the V 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 MUST set the V field
to that version number in all subsequent EAP messages.
Paul Funk expires October 2008 [Page 20]
Internet-Draft April 2008
9.2.2 Fragmentation
Each EAP-TTLS message contains a single leg of a half-duplex
conversation. The EAP carrier protocol (e.g., PPP, EAPOL, RADIUS)
may impose constraints on the length of an EAP message. Therefore it
may be necessary to fragment an EAP-TTLS 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.
If there are multiple fragments, the first fragment MUST have the L
bit set and include the length of the entire raw message prior to
fragmentation. Fragments other than the first MUST NOT have the L
bit set. Unfragmented messages MAY have the L bit set and include
the length of the message (though this information is redundant).
Upon receipt of a packet with M bit set, the receiver MUST transmit
an Acknowledgement packet. The receiver is responsible for
reassembly of fragmented packets.
9.2.3 Acknowledgement Packets
An Acknowledgement packet is an EAP-TTLS 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 V field MUST still be
set to the appropriate version number.)
An Acknowledgement packet is sent for the following purposes:
- Fragment Acknowledgement
A Fragment Acknowledgement is sent in response to an EAP packet
with M bit set.
- When the final EAP packet of the EAP-TTLS negotiation is sent by
the TTLS server, the client must respond with an Acknowledgement
packet, to allow the TTLS server to proceed with the EAP protocol
upon completion of EAP-TTLS (typically by sending or causing to
be sent a final EAP-Success or EAP-Failure to the client).
10. Encapsulation of AVPs within the TLS Record Layer
Subsequent to the TLS handshake, information may be 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
Paul Funk expires October 2008 [Page 21]
Internet-Draft April 2008
in an 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 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.
It is expected that the AVP Codes used in EAP-TTLS will carry
roughly the same meaning in EAP-TTLS as they do in Diameter and, by
extension, RADIUS. However, although EAP-TTLS uses the same AVP
Codes and syntax as Diameter, the semantics may differ, and most
Diameter AVPs do not have any well-defined semantics in EAP-TTLS. A
separate "EAP-TTLS AVP Usage" registry lists the AVPs that can be
used within EAP-TTLS and their semantics in this context (see
Section 16 for details). A TTLS server copying AVPs between an EAP-
TTLS exchange and a Diameter or RADIUS exchange with a backend MUST
NOT make assumptions about AVPs whose usage in either EAP-TTLS or
the backend protocol it does not understand and therefore MUST NOT
copy an AVP between an EAP-TTLS exchange and a Diameter or RADIUS
exchange unless the semantics of the AVP are understood and defined
in both contexts.
10.1 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
Paul Funk expires October 2008 [Page 22]
Internet-Draft April 2008
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 {RFC2865].
AVP numbers 256 and above are defined in Diameter [RFC3588].
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.
The 'r' (reserved) bits are unused and MUST be set to 0 by the
sender and MUST be ignored by the receiver.
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"
[RFC3232] 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.
Note that the 'M' (Mandatory) bit provides a means for extending the
functionality of EAP-TTLS while preserving backward compatibility
when desired. By setting the 'M' bit of the appropriate AVP(s) to 0
or 1, the party initiating the function indicates that support of
the function by the other party is either optional or required.
Paul Funk expires October 2008 [Page 23]
Internet-Draft April 2008
10.2 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.
10.3 Guidelines for Maximum Compatibility with AAA Servers
For maximum compatibility with AAA servers, the following guidelines
for AVP usage are suggested:
- Non-vendor-specific AVPs intended for use with AAA servers 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 intended for use with AAA servers 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 [RFC2865] for details.
11. Tunneled Authentication
EAP-TTLS permits user authentication information to be tunneled
within the TLS record layer between client and TTLS server, ensuring
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.
11.1 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.
Paul Funk expires October 2008 [Page 24]
Internet-Draft April 2008
In CHAP, MS-CHAP and MS-CHAP-V2, for example, the access point
issues a challenge to the client, the client then hashes the
challenge with the password and forwards the response to the access
point. The access point 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-nn(SecurityParameters.master_secret,
"ttls challenge",
SecurityParameters.client_random +
SecurityParameters.server_random);
The number of octets to be generated (nn) depends on the
authentication method, and is indicated below for each
authentication method requiring implicit challenge generation.
11.2 Tunneled Authentication Protocols
This section describes the methods for tunneling specific
authentication protocols within EAP-TTLS.
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.
The client determines which authentication protocol will be used via
the initial AVPs it sends to the server, as described in the
following sections.
Note that certain of the authentication protocols described below
utilize vendor-specific AVPs originally defined for RADIUS. RADIUS
and Diameter differ in the encoding of vendor-specific AVPs: RADIUS
uses the Vendor-Specific attribute (code 26), while Diameter uses
setting of the V-bit to indicate the presence of Vendor-ID. The
RADIUS form of the vendor-specific attribute is always convertible
Paul Funk expires October 2008 [Page 25]
Internet-Draft April 2008
to a Diameter AVP with V-bit set. All vendor-specific AVPs described
below MUST be encoded using the preferred Diameter V-bit mechanism;
that is, the AVP code of 26 MUST NOT be used to encode vendor-
specific AVPs within EAP-TTLS.
11.2.1 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.
Note that because Diameter AVPs are not limited to 253 octets of
data, as are RADIUS attributes, the RADIUS mechanism of
concatenating multiple EAP-Message attributes to represent a longer-
than-253-octet EAP packet is not appropriate in EAP-TTLS. Thus, a
tunneled EAP packet within a single EAP-TTLS message MUST be
contained in a single EAP-Message AVP.
The client initiates EAP by tunneling EAP-Response/Identity to the
TTLS server. Depending on the requirements specified for the inner
method, the client MAY now place the actual username in this packet;
the privacy of the user's identity is now guaranteed by the TLS
encryption. This username is typically a Network Access Identifier
(NAI) [RFC4282]; that is, it is typically 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, untunneled EAP-Response/Identity packet prior
to starting EAP-TTLS, indicates the realm of the TTLS server. The
second, occurring as part of the EAP exchange within the EAP-TTLS
tunnel, 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. (Refer to section 7.3 for additional information
distinguishing the untunneled and tunneled versions of the EAP-
Response/Identity packets.)
Note that TTLS processing of the initial identity exchange is
different from plain EAP. The state machine of TTLS is different.
However, it is expected that the server side is capable of dealing
with client initiation, because even normal EAP protocol runs are
client-initiated over AAA. On the client side there are various
implementation techniques to deal with the differences. Even a TTLS-
unaware EAP protocol run could be used, if TTLS makes it appear as
Paul Funk expires October 2008 [Page 26]
Internet-Draft April 2008
if EAP-Request/Identity message was actually received. This is
similar to what authenticators do when operating between a client
and a AAA server.
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
MD5-Challenge, OTP or Generic Token Card.
The EAP authentication between client and AAA/H proceeds normally,
as described in [RFC3748], 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 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.
Note that EAP-TTLS does not impose special rules on EAP Notification
packets; such packets MAY be used within a tunneled EAP exchange
according to the rules specified in [RFC3748].
EAP-TTLS provides a reliable transport for the tunneled EAP
exchange. However, [RFC3748] assumes an unreliable transport for EAP
messages (see section 3.1), and provides for silent discard of any
EAP packet that violates the protocol or fails a method-specific
integrity check, on the assumption that such a packet is likely a
counterfeit sent by an attacker. But since the tunnel provides a
reliable transport for the inner EAP authentication, errors that
would result in silent discard according to [RFC3748] presumably
represent implementation errors when they occur within the tunnel,
and SHOULD be treated as such in preference to being silently
discarded. Indeed, silently discarding an EAP message within the
tunnel effectively puts a halt to the progress of the exchange, and
will result in long timeouts in cases that ought to result in
immediate failures.
11.2.2 CHAP
The CHAP algorithm is described in [RFC1661]; RADIUS attribute
formats are described in [RFC2865].
Paul Funk expires October 2008 [Page 27]
Internet-Draft April 2008
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 initiates CHAP by tunneling 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.
11.2.3 MS-CHAP
The MS-CHAP algorithm is described in [RFC2433]; RADIUS attribute
formats are described in [RFC2548].
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 initiates MS-CHAP by tunneling User-Name, MS-CHAP-
Challenge and MS-CHAP-Response AVPs to the TTLS server. The MS-CHAP-
Challenge value is taken from 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.
Paul Funk expires October 2008 [Page 28]
Internet-Draft April 2008
11.2.4 MS-CHAP-V2
The MS-CHAP-V2 algorithm is described in [RFC2759]; RADIUS attribute
formats are described in [RFC2548].
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 initiates MS-CHAP-V2 by tunneling 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.
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 (that
is, with a zero-length Data field). Upon receipt of the empty EAP-
TTLS packet from the client, the TTLS server considers the MS-CHAP-
V2 authentication to have succeeded.
If the authentication fails, the AAA/H will respond with an Access-
Challenge containing the MS-CHAP-Error attribute. This attribute
contains a new Ident and a string with addition information such as
error reason and whether a retry is allowed. The TTLS server tunnels
this AVP to the client. 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.
Paul Funk expires October 2008 [Page 29]
Internet-Draft April 2008
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 the new Ident and
Challenge received in the MS-CHAP-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-CHAP-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 negotiation on
failure or sends an EAP-TTLS packet to the TTLS server containing no
data (that is, with a zero-length Data field), causing the TTLS
server to consider the MS-CHAP-V2 authentication to have succeeded.
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.
11.2.5 PAP
The client initiates PAP by tunneling 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.
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
Paul Funk expires October 2008 [Page 30]
Internet-Draft April 2008
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.
11.3 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 EAP type once
the previous authentication completes. Note that each new EAP method
is subject to negotiation; that is, the client may respond to the
EAP request for a new EAP type with an EAP-Nak, as described in
[RFC3748].
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
then issue EAP-Request/Generic Token Card and receive a response. If
that response were also satisfactory, it would accept the user.
The entire inner EAP exchange comprising multiple authentications is
considered a single EAP sequence, in that each subsequent request
MUST contain distinct a EAP Identifier from the previous, even as
one authentication completes and another begins.
The peer identity indicated in the original EAP-Response/Identity
that initiated the EAP sequence is intended to apply to each of the
sequential authentications, and in the absence of an application
profile standard specifying otherwise, additional EAP-Identity
exchanges SHOULD NOT occur.
Paul Funk expires October 2008 [Page 31]
Internet-Draft April 2008
The conditions for overall success or failure when multiple
authentications are used are a matter of policy on client and
server; thus, either party may require that all inner
authentications succeed, or that at least one inner authentication
succeed, as a condition for success of the overall authentication.
Each EAP method is intended to run to completion. Should the TTLS
server abandon a method and start a new one, client behavior is not
defined in this document and is a matter of client policy.
Note that it is not always feasible to use the same EAP method twice
in a row, since it may not be possible to determine when the first
authentication completes and the new authentication begins if the
EAP type does not change. Certain EAP methods, such as EAP-TLS, use
a Start bit to distinguish the first request, thus allowing each new
authentication using that type to be distinguished from the
previous. Other methods, such as EAP-MS-CHAP-V2, terminate in a
well-defined manner, allowing a second authentication of the same
type to commence unambiguously. While use of the same EAP method for
multiple authentications is relatively unlikely, implementers should
be aware of the issues and avoid cases that would result in
ambiguity.
Multiple authentications using non-EAP methods or a mixture of EAP
and non-EAP methods is not defined in this document, nor is it known
whether such an approach has been implemented.
11.4 Mandatory Tunneled Authentication Support
To ensure interoperability, in the absence of an application profile
standard specifying otherwise, an implementation compliant with this
specification MUST implement EAP as a tunneled authentication method
and MUST implement MD5-Challenge as an EAP type, though such an
implementation MAY allow the use of EAP, any EAP type, or any other
tunneled authentication method to be enabled or disabled by
administrative action on either client or TTLS server.
In addition, in the absence of an application profile standard
specifying otherwise, an implementation compliant with this
specification MUST allow an administrator to configure the use of
tunneled authentication without the M (Mandatory) bit set on any
AVP.
11.5 Additional Suggested Tunneled Authentication Support
The following information is provided as non-normative guidance
based on the experience of the authors and reviewers of this
specification with existing implementations of EAP-TTLSv0.
Paul Funk expires October 2008 [Page 32]
Internet-Draft April 2008
The following authentication methods are commonly used, and servers
wishing for broad interoperability across multiple media should
consider implementing them:
- PAP (both for password and token authentication)
- MS-CHAP-V2
- EAP-MS-CHAP-V2
- EAP-GTC
12. Keying Framework
In compliance with [KEYFRAME], Session-Id, Peer-Id and Server-Id are
here defined.
12.1 Session-Id
The Session-Id uniquely identifies an authentication exchange
between the client and TTLS server. It is defined as follows:
Session-Id = 0x015 || client.random || server.random
12.2 Peer-Id
The Peer-Id represents the identity to be used for access control
and accounting purposes. When the client presents a certificate as
part of the TLS handshake, the Peer-Id is determined based on
information in the certificate, as specified in section 5.2 of
[RFC5216]. Otherwise, the Peer-Id is null.
12.3 Server-Id
The Server-Id identifies the TTLS server. When the TTLS server
presents a certificate as part of the TLS handshake, the Server-Id
is determined based on information in the certificate, as specified
in section 5.2 of [RFC5216]. Otherwise, the Server-Id is null.
13. AVP Summary
The following table lists each AVP defined in this document, whether
the AVP may appear in a packet from server to client ("Request")
and/or in a packet from client to server ("Response"), and whether
the AVP MUST be implemented ("MI").
Name Request Response MI
---------------------------------------------------
User-Name X
User-Password X
CHAP-Password X
Paul Funk expires October 2008 [Page 33]
Internet-Draft April 2008
Reply-Message X
CHAP-Challenge X
EAP-Message X X X
MS-CHAP-Response X
MS-CHAP-Error X
MS-CHAP-NT-Enc-PW X
MS-CHAP-Domain X
MS-CHAP-Challenge X
MS-CHAP2-Response X
MS-CHAP2-Success X
MS-CHAP2-CPW X
14. Security Considerations
14.1 Security Claims
Pursuant to RFC3748, security claims for EAP-TTLSv0 are as follows:
Authentication mechanism: TLS plus arbitrary additional protected
authentication(s)
Ciphersuite negotiation: Yes
Mutual authentication: Yes, in recommended implementation
Integrity protection: Yes
Replay protection: Yes
Confidentiality: Yes
Key derivation: Yes
Key strength: Up to 384 bits
Dictionary attack prot.: Yes
Fast reconnect: Yes
Cryptographic binding: No
Session independence: Yes
Fragmentation: Yes
Channel binding: No
14.1.1 Authentication mechanism
EAP-TTLSv0 utilizes negotiated underlying authentication protocols,
both in the phase 1 TLS handshake and the phase 2 tunneled
authentication. In a typical deployment, at a minimum the TTLS
server authenticates to the client in phase 1, and the client
authenticates to the AAA/H server in phase 2. Phase 1 authentication
of the TTLS server to the client is typically by certificate; the
client may optionally authenticate to the TTLS server by certificate
as well. Phase 2 authentication of the client to the AAA/H server is
typically by password or security token via an EAP or supported non-
EAP authentication mechanism; this authentication mechanism may
provide authentication of AAA/H server to the client as well (mutual
authentication).
Paul Funk expires October 2008 [Page 34]
Internet-Draft April 2008
14.1.2 Ciphersuite negotiation
Ciphersuite negotiation is inherited from TLS.
14.1.3 Mutual authentication
In the recommended minimum configuration, the TTLS server is
authenticated to the client in phase 1, and the client and AAA/H
server mutually authenticate in phase 2.
14.1.4 Integrity protection
Integrity protection is inherited from TLS.
14.1.5 Replay protection
Replay protection is inherited from TLS.
14.1.6 Confidentiality
Confidentiality is inherited from TLS. Note, however, that EAP-
TTLSv0 contains no provision for encryption of success or failure
EAP packets.
14.1.7 Key derivation
Both MSK and EMSK are derived. Key derivation PRF is inherited from
TLS, and cryptographic agility of this mechanism depends on the
cryptographic agility of the TLS PRF.
14.1.8 Key strength
Key strength is limited by the size of the TLS master secret, which
for versions 1.0 and 1.1 is 48 octets (384 bits). Effective key
strength may be less, depending on the attack resistance of the
negotiated DH group, certificate RSA/DSA group, etc. BCP 86
[RFC3766] Section 5 offers advice on the required RSA or DH module
and DSA subgroup size in bits, for a given level of attack
resistance in bits. For example, a 2048-bit RSA key is recommended
to provide 128-bit equivalent key strength. The National Institute
for Standards and Technology (NIST) also offers advice on
appropriate key sizes in [SP800-57].
14.1.9 Dictionary attack protection
Phase 2 password authentication is protected against eavesdropping
and therefore against offline dictionary attack by TLS encryption.
14.1.10 Fast reconnect
Fast reconnect is provided by TLS session resumption.
Paul Funk expires October 2008 [Page 35]
Internet-Draft April 2008
14.1.11 Cryptographic binding
[MITM] describes a vulnerability that is characteristic of tunneled
authentication protocols, in which an attacker authenticates as a
client via a tunneled protocol by posing as an authenticator to a
legitimate client using a non-tunneled protocol. When the same proof
of credentials can be used in both authentications, the attacker
merely shuttles the credential proof between them. EAP-TTLSv0 is
vulnerable to such an attack. Care should be taken to avoid using
authentication protocols and associated credentials both as inner
TTLSv0 methods and as untunneled methods.
Extensions to EAP-TTLSv0 or a future version of EAP-TTLS should be
defined to perform a cryptographic binding of keying material
generated by inner authentication methods and the keying material
generated by the TLS handshake. This avoids the Man-in-the-Middle
problem when used with key-generating inner methods. Such an
extension mechanism has been proposed [TTLS-EXT].
14.1.12 Session independence
TLS guarantees the session independence of its master secret, from
which the EAP-TTLSv0 MSK/EMSK is derived.
14.1.13 Fragmentation
Provision is made for fragmentation of lengthy EAP packets.
14.1.14 Channel binding
Support for channel binding may be added as a future extension,
using appropriate AVPs.
14.2 Client Anonymity
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
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.
Paul Funk expires October 2008 [Page 36]
Internet-Draft April 2008
Note that if a client authenticating with a certificate wishes to
shield its certificate, and hence its identity, from eavesdroppers,
it may use the technique described in the "Privacy" section of
[RFC5216], in which the client sends an empty certificate list, the
TTLS server issues a ServerHello upon completion of the TLS
handshake to begin a second, encrypted handshake, during which the
client will send its certificate list. Note that for this feature to
work the client must know in advance that the TTLS server supports
it.
14.3 Server Trust
Trust of the server by the client is established via a server
certificate conveyed during the TLS handshake. The client should
have a means of determining which server identities are authorized
to act as a TTLS server and may be trusted, and should refuse to
authenticate with servers it does not trust. The consequence of
pursuing authentication with a hostile server is exposure of the
inner authentication to attack; e.g. offline dictionary attack
against the client password.
14.4 Certificate Validation
When either client or server presents a certificate as part of the
TLS handshake, it should include the entire certificate chain minus
the root to facilitate certificate validation by the other party.
When either client or server receives a certificate as part of the
TLS handshake, it should validate the certification path to a
trusted root. If intermediate certificates are not provided by the
sender, the receiver may use cached or pre-configured copies if
available, or may retrieve them from the Internet if feasible.
Clients and servers should implement policies related to the
Extended Key Usage (EKU) extension [RFC3280] of certificates it
receives, to ensure that the other party's certificate usage
conforms to the certificate's purpose. Typically, a client EKU, when
present, would be expected to include id-kp-clientAuth; a server
EKU, when present, would be expected to include id-kp-serverAuth.
Note that absence of the EKU extension or a value of
anyExtendedKeyUsage implies absence of constraint on the
certificate's purpose.
14.5 Certificate Compromise
Certificates should be checked for revocation to reduce exposure to
imposture using compromised certificates.
Checking a server certificate against the most recent revocation
list during authentication is not always possible for a client, as
it may not have network access until completion of the
Paul Funk expires October 2008 [Page 37]
Internet-Draft April 2008
authentication. This problem can be alleviated through the use of
OCSP [RFC2560] during the TLS handshake, as described in [RFC4366].
14.6 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.
14.7 Negotiating-Down Attacks
EAP-TTLS negotiates its own protocol version prior to, and therefore
outside the security established by the TLS tunnel. In principle,
therefore, it is subject to a negotiating-down attack, in which an
intermediary modifies messages in transit to cause a lower version
of the protocol to be agreed upon, each party assuming that the
other does not support as high a version as it actually does.
The version of the EAP-TTLS protocol described in this document is
0, and is therefore not subject to such an attack. However, any new
version of the protocol using a higher number than 0 should define a
mechanism to ensure against such an attack. One such mechanism might
be the TTLS server's reiteration of the protocol version that it
proposed in an AVP within the tunnel, such AVP to be inserted with
M-bit clear even when version 0 is agreed upon.
15. Message Sequences
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 (+).
Paul Funk expires October 2008 [Page 38]
Internet-Draft April 2008
15.1 Successful authentication via tunneled CHAP
In this example, the client performs one-way TLS authentication of
the TTLS server. 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
-------------------->
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
-------------------->
Paul Funk expires October 2008 [Page 39]
Internet-Draft April 2008
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
-------------------->
RADIUS Access-Accept
<--------------------
RADIUS Access-Accept:
EAP-Success
<--------------------
EAP-Success
<--------------------
15.2 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
<--------------------
Paul Funk expires October 2008 [Page 40]
Internet-Draft April 2008
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
<--------------------
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
<--------------------
Paul Funk expires October 2008 [Page 41]
Internet-Draft April 2008
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}
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge
EAP-Response/
MD5-Challenge
-------------------->
RADIUS Access-Accept
<--------------------
RADIUS Access-Accept:
EAP-Success
<--------------------
EAP-Success
<--------------------
Paul Funk expires October 2008 [Page 42]
Internet-Draft April 2008
15.3 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
-------------------->
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Challenge:
EAP-Request/TTLS:
ServerHello
ChangeCipherSpec
Finished
<--------------------
EAP-Request passthrough
<--------------------
EAP-Response/TTLS:
ChangeCipherSpec
Finished
-------------------->
Paul Funk expires October 2008 [Page 43]
Internet-Draft April 2008
RADIUS Access-Request:
EAP-Response passthrough
-------------------->
RADIUS Access-Accept:
EAP-Success
<--------------------
EAP-Success
<--------------------
16. IANA Considerations
IANA has assigned the number 21 (decimal) as the method type of the
EAP-TTLS protocol. Mechanisms for defining new RADIUS and Diameter
AVPs and AVP values are outlined in [RFC2865] and [RFC3588],
respectively. No additional IANA registrations are specifically
contemplated in this document.
Section 11 of this document specifies how certain authentication
mechanisms may be performed within the secure tunnel established by
EAP-TTLS. New mechanisms and other functions MAY also be performed
within this tunnel. Where such extensions use AVPs that are not
vendor-specific, their semantics must be specified in new RFCs; that
is, there are TTLS-specific processing rules related to the use of
each individual AVP, even though such AVPs have already been defined
for RADIUS or DIAMETER.
This specification requires the creation of a new registry -- EAP-
TTLS AVP Usage -- to be managed by IANA, listing each non-vendor-
specific RADIUS/Diameter AVP that has been defined for use within
EAP-TTLS, along with a reference to the RFC or other document which
specifies its semantics. The initial list of AVPs shall be those
listed in section 13 of this document. The purpose of this registry
is to avoid potential ambiguity resulting from the same AVP being
utilized in different functional contexts. This registry does not
assign numbers to AVPs, as the AVP numbers are assigned out of the
RADIUS and Diameter namespaces as outlined in [RFC2865] and
[RFC3588]. Only top-level AVPs -- that is, AVPs not encapsulated
within Grouped AVPs -- will be registered. AVPs should be added to
this registry based on IETF Consensus as defined in [RFC2434].
17. Acknowledgements
Thanks to Bernard Aboba, Jari Arkko, Lakshminath Dondeti, Stephen
Hanna, Ryan Hurst, Avi Lior and Gabriel Montenegro for careful
reviews and useful comments.
Paul Funk expires October 2008 [Page 44]
Internet-Draft April 2008
18. References
18.1 Normative References
[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol
(PPP)", STD 51, RFC 1661, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T., and C. Allen, "The TLS Protocol Version
1.0", RFC 2246, November 1998.
[RFC2433] Zorn, G., and S. Cobb, "Microsoft PPP CHAP Extensions",
RFC 2433, October 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS
Attributes", RFC 2548, March 1999.
[RFC2759] Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
RFC 2759, January 2000.
[RFC2865] Rigney, C., Rubens, A., Simpson, W., and S. Willens,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced
by an On-line Database", RFC 3232, January 2002.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and
J. Arkko, "Diameter Base Protocol", RFC 3588, September
2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "PPP Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC4282] Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC4346] Dierks, T., and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", RFC 4346, April
2006.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP TLS
Authentication Protocol", RFC 5216, March 2008.
Paul Funk expires October 2008 [Page 45]
Internet-Draft April 2008
[KEYFRAME] Aboba, B., Simon, D. and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management
Framework", Internet Draft (work in progress), draft-
ietf-eap-keying-22.txt, November 2007.
18.2 Informative References
[802.1X] Institute of Electrical and Electronics Engineers,
"Local and Metropolitan Area Networks: Port-Based
Network Access Control", IEEE Standard 802.1X-2004,
December 2004.
[802.11] Institute of Electrical and Electronics Engineers,
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific Requirements Part
11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications", IEEE Standard
802.11, 2007.
[TTLS-EXT] Hanna, S, and P. Funk, "Key Agility Extensions for EAP-
TTLSv0", Internet Draft (work in progress), draft-
hanna-eap-ttls-agility-00.txt, September 24, 2007.
[RFC2560] 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.
[RFC3280] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3766] Orman. H. and P. Hoffman, "Determining Strengths for
Public Keys Used for Exchanging Symmetric Keys", RFC
3766, April 2004.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
J., and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[MITM] 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.
[SP800-57] National Institute of Standards and Technology,
"Recommendation for Key Management", Special
Publication 800-57, May 2006.
Paul Funk expires October 2008 [Page 46]
Internet-Draft April 2008
19. Authors' Addresses
Questions about this memo can be directed to:
Paul Funk
43 Linnaean St.
Cambridge, MA 02138
E-mail: PaulFunk@alum.mit.edu
Simon Blake-Wilson
SafeNet
Amstelveenseweg 88-90
1054XV, Amsterdam
The Netherlands
E-mail: sblakewilson@nl.safenet-inc.com
20. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
21. Disclaimer of Validity
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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 October 2008 [Page 47]
Internet-Draft April 2008
22. Copyright Statement
Copyright (C) The IETF Trust (2008). This document is subject to
the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
23. Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Paul Funk expires October 2008 [Page 48]