Network Working Group P. Funk
Request for Comments: 5281 Unaffiliated
Category: Informational S. Blake-Wilson
August 2008 Extensible Authentication Protocol Tunneled Transport Layer Security
Authenticated Protocol Version 0 (EAP-TTLSv0)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
EAP-TTLS is an EAP (Extensible Authentication Protocol) method that
encapsulates a TLS (Transport Layer Security) session, consisting of
a handshake phase and a data phase. During the handshake phase, the
server is authenticated to the client (or client and server are
mutually authenticated) using standard TLS procedures, and keying
material is generated in order to create a cryptographically secure
tunnel for information exchange in the subsequent data phase. During
the data phase, the client is authenticated to the server (or client
and server are mutually authenticated) using an arbitrary
authentication mechanism encapsulated within the secure tunnel. The
encapsulated authentication mechanism 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. The data phase
may also be used for additional, arbitrary data exchange.
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
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
EAP-TTLS is an EAP method that provides functionality beyond what is
available in EAP-TLS. EAP-TTLS has been widely deployed and this
specification documents what existing implementations do. It has
some limitations and vulnerabilities, however. These are addressed
in EAP-TTLS extensions and ongoing work in the creation of
standardized tunneled EAP methods at the IETF. Users of EAP-TTLS are
strongly encouraged to consider these in their deployments.
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 authentication 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
functions beyond authentication and key exchange to be added to the
EAP negotiation, in a manner compatible with the AAA infrastructure.
The main limitation of EAP-TTLS is that its base version lacks
support for cryptographic binding between the outer and inner
authentication. Please refer to Section 14.1.11 for details and the
conditions where this vulnerability exists. It should be noted that
an extension for EAP-TTLS [TTLS-EXT] fixed this vulnerability. Users
of EAP-TTLS are strongly encouraged to adopt this extension.
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
- 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
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
- 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].
Authentication, Authorization, and Accounting - functions that are
generally required to control access to a network and support
billing and auditing.
A network protocol used to communicate with AAA servers; examples
include RADIUS and Diameter.
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.
A AAA server in the user's home domain, where authentication and
authorization for that user are administered.
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
"Network Access Server (NAS)", as used in AAA specifications such
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.
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
A network and associated devices that are under the administrative
control of an entity such as a service provider or the user's home
A protocol used to carry data between hosts that are connected
within a single network segment; examples include PPP and
A Network Access Identifier [RFC4282], normally consisting of the
name of the user and, optionally, the user's home realm.
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.
The optional part of an NAI indicating the domain to which a AAA
transaction is to be routed, normally the user's home domain.
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.
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.
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
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 (EAP over LAN). The access point, TTLS server, and AAA/H
server communicate using a AAA carrier protocol such as RADIUS or
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 the
EAP-TTLS negotiation. This keying material must be communicated to
the access point by the TTLS server using the AAA carrier protocol.
5.2. Security Relationships
The client and access point have no pre-existing security
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.
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
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 issues a challenge, the TTLS server tunnels the
challenge information to the client. The AAA/H server may be a
legacy device and needs to know nothing about EAP-TTLS; it only needs
to be able to authenticate the client based on commonly used
Keying material for the subsequent data connection between client and
access point (Master Session Key / Extended Master Session Key
(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.,
The client and access point now share keying material that 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
| 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
| 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
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
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.
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
the 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
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
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.) 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
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
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, "firstname.lastname@example.org".
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.
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
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
Tunneled authentication is specifically not performed for resumed
sessions; the presumption is that the knowledge of the master secret
(as evidenced by the ability to resume the session) is authentication
enough. This allows session resumption to occur without any
messaging between the TTLS server and the AAA/H. If periodic
reauthentication to the AAA/H is desired, the AAA/H must indicate
this to the TTLS server when the original session is established, for
example, using the RADIUS Session-Timeout attribute.
The client 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
- Authentication of the client via client certificate during phase
1, with no additional authentication or information exchange
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 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 the 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
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 alternative PRF
function must be used in EAP-TTLSv0 computations instead of the TLS
The TLS PRF function used in this specification is denoted as
PRF-nn(secret, label, seed)
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.