RFC 5281
Extensible Authentication Protocol Tunneled Transport Layer Security Authenticated Protocol Version 0 (EAP-TTLSv0)
Network Working Group P. Funk Request for Comments: 5281 Unaffiliated Category: Informational S. Blake-Wilson SafeNet 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.Abstract
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.
Table of Contents
1. Introduction ....................................................4 2. Motivation ......................................................5 3. Requirements Language ...........................................7 4. Terminology .....................................................7 5. Architectural Model .............................................9 5.1. Carrier Protocols .........................................10 5.2. Security Relationships ....................................10 5.3. Messaging .................................................11 5.4. Resulting Security ........................................12 6. Protocol Layering Model ........................................12 7. EAP-TTLS Overview ..............................................13 7.1. Phase 1: Handshake ........................................14 7.2. Phase 2: Tunnel ...........................................14 7.3. EAP Identity Information ..................................15 7.4. Piggybacking ..............................................15 7.5. Session Resumption ........................................16 7.6. Determining Whether to Enter Phase 2 ......................17 7.7. TLS Version ...............................................18 7.8. Use of TLS PRF ............................................18 8. Generating Keying Material .....................................19 9. EAP-TTLS Protocol ..............................................20 9.1. Packet Format .............................................20 9.2. EAP-TTLS Start Packet .....................................21 9.2.1. Version Negotiation ................................21 9.2.2. Fragmentation ......................................22 9.2.3. Acknowledgement Packets ............................22 10. Encapsulation of AVPs within the TLS Record Layer .............23 10.1. AVP Format ...............................................23 10.2. AVP Sequences ............................................25 10.3. Guidelines for Maximum Compatibility with AAA Servers ....25 11. Tunneled Authentication .......................................26 11.1. Implicit Challenge .......................................26 11.2. Tunneled Authentication Protocols ........................27 11.2.1. EAP ...............................................27 11.2.2. CHAP ..............................................29 11.2.3. MS-CHAP ...........................................30 11.2.4. MS-CHAP-V2 ........................................30 11.2.5. PAP ...............................................32 11.3. Performing Multiple Authentications ......................33 11.4. Mandatory Tunneled Authentication Support ................34 11.5. Additional Suggested Tunneled Authentication Support .....34 12. Keying Framework ..............................................35 12.1. Session-Id ...............................................35 12.2. Peer-Id ..................................................35 12.3. Server-Id ................................................35 13. AVP Summary ...................................................35
14. Security Considerations .......................................36 14.1. Security Claims ..........................................36 14.1.1. Authentication Mechanism ..........................36 14.1.2. Ciphersuite Negotiation ...........................37 14.1.3. Mutual Authentication .............................37 14.1.4. Integrity Protection ..............................37 14.1.5. Replay Protection .................................37 14.1.6. Confidentiality ...................................37 14.1.7. Key Derivation ....................................37 14.1.8. Key Strength ......................................37 14.1.9. Dictionary Attack Protection ......................38 14.1.10. Fast Reconnect ...................................38 14.1.11. Cryptographic Binding ............................38 14.1.12. Session Independence .............................38 14.1.13. Fragmentation ....................................38 14.1.14. Channel Binding ..................................38 14.2. Client Anonymity .........................................38 14.3. Server Trust .............................................39 14.4. Certificate Validation ...................................39 14.5. Certificate Compromise ...................................40 14.6. Forward Secrecy ..........................................40 14.7. Negotiating-Down Attacks .................................40 15. Message Sequences .............................................41 15.1. Successful Authentication via Tunneled CHAP ..............41 15.2. Successful Authentication via Tunneled EAP/MD5-Challenge ........................................43 15.3. Successful Session Resumption ............................46 16. IANA Considerations ...........................................47 17. Acknowledgements ..............................................48 18. References ....................................................48 18.1. Normative References .....................................48 18.2. Informative References ...................................49
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. 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.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
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
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
"Network Access Server (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
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.
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 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 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 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 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 authentication protocols. 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., RADIUS). 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 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: +-----------------------------------------------------------+ | 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. 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 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.) 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.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. 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 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
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 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.
(page 19 continued on part 2)
