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Versions: (draft-salowey-emu-eaptunnel-req) 00 01 02 03 04 05 06 07 08 09 RFC 6678

EMU Working Group                                              K. Hoeper
Internet-Draft                                            Motorola, Inc.
Intended status: Informational                                  S. Hanna
Expires: April 10, 2010                                 Juniper Networks
                                                                 H. Zhou
                                                         J. Salowey, Ed.
                                                     Cisco Systems, Inc.
                                                         October 7, 2009


               Requirements for a Tunnel Based EAP Method
                  draft-ietf-emu-eaptunnel-req-04.txt

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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Copyright Notice




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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This memo defines the requirements for a tunnel-based Extensible
   Authentication Protocol (EAP) Method.  This method will use Transport
   Layer Security (TLS) to establish a secure tunnel.  The tunnel will
   provide support for password authentication, EAP authentication and
   the transport of additional data for other purposes.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5

   2.  Conventions Used In This Document  . . . . . . . . . . . . . .  5

   3.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Password Authentication  . . . . . . . . . . . . . . . . .  6
     3.2.  Protection of Weak EAP Methods . . . . . . . . . . . . . .  6
     3.3.  Chained EAP Methods  . . . . . . . . . . . . . . . . . . .  7
     3.4.  Identity Protection  . . . . . . . . . . . . . . . . . . .  7
     3.5.  Anonymous Service Access . . . . . . . . . . . . . . . . .  7
     3.6.  Network Endpoint Assessment  . . . . . . . . . . . . . . .  8
     3.7.  Client Authentication During Tunnel Establishment  . . . .  8
     3.8.  Extensibility  . . . . . . . . . . . . . . . . . . . . . .  8

   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  General Requirements . . . . . . . . . . . . . . . . . . .  9
       4.1.1.  RFC Compliance . . . . . . . . . . . . . . . . . . . .  9
       4.1.2.  Draw from Existing Work  . . . . . . . . . . . . . . .  9
     4.2.  Tunnel Requirements  . . . . . . . . . . . . . . . . . . . 10
       4.2.1.  TLS Requirements . . . . . . . . . . . . . . . . . . . 10
         4.2.1.1.  Cipher Suites  . . . . . . . . . . . . . . . . . . 10
           4.2.1.1.1.  Cipher Suite Negotiation . . . . . . . . . . . 10
           4.2.1.1.2.  Tunnel Data Protection Algorithms  . . . . . . 11
           4.2.1.1.3.  Tunnel Authentication and Key Establishment  . 11
         4.2.1.2.  Tunnel Replay Protection . . . . . . . . . . . . . 11
         4.2.1.3.  TLS Extensions . . . . . . . . . . . . . . . . . . 11
         4.2.1.4.  Peer Identity Privacy  . . . . . . . . . . . . . . 12
         4.2.1.5.  Session Resumption . . . . . . . . . . . . . . . . 12
       4.2.2.  Fragmentation  . . . . . . . . . . . . . . . . . . . . 12
       4.2.3.  Protection of Data External to Tunnel  . . . . . . . . 12
     4.3.  Tunnel Payload Requirements  . . . . . . . . . . . . . . . 12
       4.3.1.  Extensible Attribute Types . . . . . . . . . . . . . . 13
       4.3.2.  Request/Challenge Response Operation . . . . . . . . . 13
       4.3.3.  Mandatory and Optional Attributes  . . . . . . . . . . 13
       4.3.4.  Vendor Specific Support  . . . . . . . . . . . . . . . 13
       4.3.5.  Result Indication  . . . . . . . . . . . . . . . . . . 13
       4.3.6.  Internationalization of Display Strings  . . . . . . . 14
     4.4.  EAP Channel Binding Requirements . . . . . . . . . . . . . 14
     4.5.  Requirements Associated with Carrying Username and
           Passwords  . . . . . . . . . . . . . . . . . . . . . . . . 14
       4.5.1.  Security . . . . . . . . . . . . . . . . . . . . . . . 14
         4.5.1.1.  Confidentiality and Integrity  . . . . . . . . . . 14
         4.5.1.2.  Authentication of Server . . . . . . . . . . . . . 14
         4.5.1.3.  Server Certificate Revocation Checking . . . . . . 15
       4.5.2.  Internationalization . . . . . . . . . . . . . . . . . 15
       4.5.3.  Meta-data  . . . . . . . . . . . . . . . . . . . . . . 15



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       4.5.4.  Password Change  . . . . . . . . . . . . . . . . . . . 15
     4.6.  Requirements Associated with Carrying EAP Methods  . . . . 16
       4.6.1.  Method Negotiation . . . . . . . . . . . . . . . . . . 16
       4.6.2.  Chained Methods  . . . . . . . . . . . . . . . . . . . 16
       4.6.3.  Cryptographic Binding with the TLS Tunnel  . . . . . . 16
       4.6.4.  Peer Initiated . . . . . . . . . . . . . . . . . . . . 17
       4.6.5.  Method Meta-data . . . . . . . . . . . . . . . . . . . 17

   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18

   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
     6.1.  Cipher Suite Selection . . . . . . . . . . . . . . . . . . 18
     6.2.  Tunneled Authentication  . . . . . . . . . . . . . . . . . 19
     6.3.  Data External to Tunnel  . . . . . . . . . . . . . . . . . 19
     6.4.  Separation of TLS Tunnel and Inner Authentication
           Termination  . . . . . . . . . . . . . . . . . . . . . . . 19

   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 21

   Appendix A.  Changes from -01  . . . . . . . . . . . . . . . . . . 22

   Appendix B.  Changes from -02  . . . . . . . . . . . . . . . . . . 23

   Appendix C.  changes from -03  . . . . . . . . . . . . . . . . . . 23

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23























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1.  Introduction

   Running EAP methods within a TLS protected tunnel has been deployed
   in several different solutions.  EAP methods supporting this include
   PEAP [PEAP], TTLS [RFC5281] and EAP-FAST [RFC4851].  In general this
   has worked well so there is consensus to continue to use TLS as the
   basis for a tunnel method.  There have been various reasons for
   employing a protected tunnel for EAP processes.  They include
   protecting weak authentication exchanges, such as username and
   password.  In addition a protected tunnel can provide means to
   provide peer identity protection and EAP method chaining.  Finally,
   systems have found it useful to transport additional types of data
   within the protected tunnel.

   This document describes the requirements for an EAP tunnel method as
   well as for a password protocol supporting legacy password
   verification within the tunnel method.


2.  Conventions Used In This Document

   Use of each capitalized word within a sentence or phrase carries the
   following meaning during the EMU WG's method selection process:

      MUST - indicates an absolute requirement

      MUST NOT - indicates something absolutely prohibited

      SHOULD - indicates a strong recommendation of a desired result

      SHOULD NOT - indicates a strong recommendation against a result

      MAY - indicates a willingness to allow an optional outcome

   Lower case uses of "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and
   "MAY" carry their normal meaning and are not subject to these
   definitions.


3.  Use Cases

   To motivate and explain the requirements in this document, a
   representative set of use cases for the EAP tunnel method are
   supplied here.  The candidate tunnel method is expected to support
   all of the use cases marked as MUST.






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3.1.  Password Authentication

   Many legacy systems only support user authentication with passwords.
   Some of these systems require transport of the actual username and
   password to the authentication server.  This is true for systems
   where the authentication server does not have access to the cleartext
   password or a consistent transform of the cleartext password.
   Example of such systems are one time password (OTP) systems and other
   systems where the username and password are submitted to an external
   party for validation.  The tunnel method MUST support transporting
   username and password to the authentication server.  However, it MUST
   NOT expose the username and password to parties in the communication
   path between the peer and the EAP Server and it MUST provide
   protection against man-in-the-middle and dictionary attacks.  The
   combination of the tunnel authentication and password authentication
   MUST enable mutual authentication.

   Since EAP authentication occurs before network access is granted the
   tunnel method SHOULD enable an inner exchange to provide support for
   minimal password management tasks including password change, "new PIN
   mode", and "next token mode" required by some systems.

3.2.  Protection of Weak EAP Methods

   Some existing EAP methods have vulnerabilities that could be
   eliminated or reduced by running them inside a protected tunnel.  For
   example, a method such as EAP-MD5 does not provide mutual
   authentication or protection from dictionary attacks.  Without extra
   protection, tunnel-based EAP methods are vulnerable to a special type
   of tunnel man-in-the-middle attack [TUNNEL-MITM].  This attack is
   referred to as "tunnel MitM attack" in the remainder of this
   document.  The additional protection needed to thwart tunnel MitM
   attacks depends on the inner method executed within the tunnel.  In
   particular, when weak methods are used, security policies enforcing
   that such methods can only be executed inside a tunnel but never
   outside one are required to mitigate the attack.  On the other hand,
   a technical solution (so-called cryptographic bindings) can be used
   whenever the inner method is not susceptible to attacks outside a
   tunnel and derives keying material.  Only the latter mitigation
   technique can be made an actual requirement for tunnel-based EAP
   methods (see Section 4.6.3), while security policies are outside the
   scope of this requirement draft.  Please refer to the NIST
   Recommendation for EAP Methods Used in Wireless Network Access
   Authentication [NIST SP 800-120] for a discussion on security
   policies and complete solutions for thwarting tunnel MitM attacks.

   The tunnel method MUST support protection of weak EAP methods.
   Cryptographic protection from tunnel MitM attacks MUST be provided



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   for all key generating methods.  In combination with an appropriate
   security policy this will thwart MitM attacks against inner methods.

3.3.  Chained EAP Methods

   Several circumstances are best addressed by using chained EAP
   methods.  For example, it may be desirable to authenticate the user
   and also authenticate the device that he or she is using.  However,
   chained EAP methods from different conversations can be re-directed
   into the same conversation by an attacker giving the authenticator
   the impression that both conversations terminate at the same end-
   point.  Cryptographic binding can be used to bind the results of key
   generating methods together or to an encompassing tunnel.

   The tunnel method MUST support chained EAP methods while including
   strong protection against attacks on method chaining.

3.4.  Identity Protection

   When performing an EAP authentication, the peer may want to protect
   its identity, only disclosing its identity to a trusted backend
   authentication server.  This helps to maintain the privacy of the
   peer's identity.

   The tunnel method MUST support identity protection, ensuring that
   peer identity is not disclosed to the authenticator and any other
   intermediaries before the server that terminates the tunnel method.
   Peer identity protection provided by the tunnel method applies to
   tunnel method and inner method specific identities.  Note that the
   peer may need to expose the realm portion of the EAP outer identity
   in the NAI [RFC4282] in a roaming scenario in order to reach the
   appropriate authentication server.

3.5.  Anonymous Service Access

   When network service is provided, it is sometimes desirable for any
   user to be able to gain access to the network to enable access to
   limited services emergency communication or troubleshooting
   information.  To avoid eavesdropping, it's best to negotiate link
   layer security as with any other authentication.

   Therefore, the tunnel method SHOULD allow anonymous peers or server-
   only authentication, but still derive keys that can be used for link
   layer security.  The tunnel method MAY also allow for the bypass of
   server authentication processing on the client.  Forgoing
   authentication increases the chance of man-in-the-middle and other
   types of attacks that can compromise the derived keys used for link
   layer security.  In addition, passwords and other sensitive



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   information must not be disclosed to an unauthenticated or
   unauthorized server.

3.6.  Network Endpoint Assessment

   The Network Endpoint Assessment (NEA) protocols and reference model
   described in [RFC5209] provide a standard way to check the health
   ("posture") of a device at or after the time it connects to a
   network.  If the device does not comply with the network's
   requirements, it can be denied access to the network or granted
   limited access to remediate itself.  EAP is a convenient place for
   conducting an NEA exchange.

   The tunnel method SHOULD support carrying NEA protocols such as PB-
   TNC [I-D.ietf-nea-pb-tnc].  Depending on the specifics of the tunnel
   method, these protocols may be required to be carried in an EAP
   method.

3.7.  Client Authentication During Tunnel Establishment

   In some cases the peer will have credentials usable to authenticate
   during tunnel establishment.  These credentials may only partially
   authenticate the identity of the peer and additional authentication
   may be required inside the tunnel.  If the identity of the peer is
   fully authenticated during tunnel establishment then the tunnel may
   be used to communicate additional data.  The tunnel method MUST be
   capable of providing client side authentication during tunnel
   establishment.

3.8.  Extensibility

   The tunnel method MUST provide extensibility so that additional data
   related to authentication, authorization and network access can be
   carried inside the tunnel in the future.  This removes the need to
   develop new tunneling methods for specific purposes.

   One example of a application for extensibility is credential
   provisioning.  When a peer has authenticated with EAP, this is a
   convenient time to distribute credentials to that peer that may be
   used for later authentication exchanges.  For example, the
   authentication server can provide a private key or shared key to the
   peer that can be used by the peer to perform rapid re-authentication
   or roaming.  In addition there have been proposals to perform
   enrollment within EAP, such as [I-D.mahy-eap-enrollment].  Another
   use for extensibility is support for authentication frameworks other
   than EAP.





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4.  Requirements

4.1.  General Requirements

4.1.1.  RFC Compliance

   The tunnel method MUST include a Security Claims section with all
   security claims specified in Section 7.2 in RFC 3748 [RFC3748].  In
   addition, it MUST meet the requirement in Sections 2.1 and 7.4 of RFC
   3748 that tunnel methods MUST support protection against man-in-the-
   middle attacks.  Furthermore, the tunnel method MUST support identity
   protection as specified in Section 7.3 in RFC 3748.

   The tunnel method MUST be unconditionally compliant with RFC 4017
   [RFC4017] (using the definition of "unconditionally compliant"
   contained in section 1.1 of RFC 4017).  This means that the method
   MUST satisfy all the MUST, MUST NOT, SHOULD, and SHOULD NOT
   requirements in RFC 4017.

   The tunnel method MUST meet all the MUST and SHOULD requirements
   relevant to EAP methods contained in the EAP Key Management Framework
   [RFC5247] or its successor.  This includes the generation of the MSK,
   EMSK, Peer-Id, Server-Id and Session-Id.  This will enable the tunnel
   method to properly fit into the EAP key management framework,
   maintaining all of the security properties and guarantees of that
   framework.

   The tunnel method MUST NOT be tied to any single cryptographic
   algorithm.  Instead, it MUST support run-time negotiation to select
   among an extensible set of cryptographic algorithms.  This includes
   algorithms used with certificates presented during tunnel
   establishment.  This "cryptographic algorithm agility" provides
   several advantages.  Most important, when a weakness in an algorithm
   is discovered or increased processing power overtakes an algorithm,
   users can easily transition to a new algorithm.  Also, users can
   choose the algorithm that best meets their needs.

   The tunnel method MUST meet the SHOULD and MUST requirements
   pertinent to EAP method contained in Section 3 of RFC 4962 [RFC4962].
   This includes: cryptographic algorithm independence; strong, fresh
   session keys; replay detection; keying material confidentiality and
   integrity; and confirmation of cipher suite selection.

4.1.2.  Draw from Existing Work

   Several existing tunnel methods are already in widespread usage: EAP-
   FAST [RFC4851], EAP-TTLS [RFC5281], and PEAP.  Considerable
   experience has been gained from various deployments with these



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   methods.  This experience SHOULD be considered when evaluating tunnel
   methods.  If one of these existing tunnel methods can meet the
   requirements contained in this specification then that method SHOULD
   be preferred over a new method.

   Even if minor modifications or extensions to an existing tunnel
   method are needed, this method SHOULD be preferred over a completely
   new method so that the advantage of accumulated deployment experience
   and security analysis can be gained.

4.2.  Tunnel Requirements

   The following section discusses requirements for TLS Tunnel
   Establishment.

4.2.1.  TLS Requirements

   The tunnel based method MUST support TLS version 1.2 [RFC5246] and
   may support earlier versions to enable the possibility of backwards
   compatibility.  The following section discusses requirements for TLS
   Tunnel Establishment.

4.2.1.1.  Cipher Suites

4.2.1.1.1.  Cipher Suite Negotiation

   Cipher suite negotiations always suffer from downgrading attacks when
   they are not secured by any kind of integrity protection.  A common
   practice is a post integrity check in which, as soon as available,
   the established keys (here the tunnel key) are used to derive
   integrity keys.  These integrity keys are then used by peer and
   authentication server to verify whether the cipher suite negotiation
   has been maliciously altered by another party.

   Integrity checks prevent downgrading attacks only if the derived
   integrity keys and the employed integrity algorithms cannot be broken
   in real-time.  See Section 6.1 or [KHLC07] for more information on
   this.  Hence, the tunnel method MUST provide integrity protected
   cipher suite negotiation with secure integrity algorithms and
   integrity keys.

   All versions of TLS meet these requirements as long as the cipher
   suites used provide strong authentication, key establishment and data
   integrity protection.







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4.2.1.1.2.  Tunnel Data Protection Algorithms

   In order to prevent attacks on the cryptographic algorithms employed
   by inner authentication methods, a tunnel protocol's protection needs
   to provide a basic level of algorithm strength.  The tunnel method
   MUST provide at least one mandatory to implement cipher suite that
   provides the equivalent security of 128-bit AES for encryption and
   message authentication.  See Part 1 of the NIST Recommendation for
   Key Management [NIST SP 800-57] for a discussion of the relative
   strengths of common algorithms.

4.2.1.1.3.  Tunnel Authentication and Key Establishment

   A tunnel method MUST provide unidirectional authentication from
   authentication server to EAP peer and mutual authentication between
   authentication server and EAP peer.  The tunnel method MUST provide
   at least one mandatory to implement cipher suite that provides
   certificate-based authentication of the server and provides optional
   certificate-based authentication of the client.  Other types of
   authentication MAY be supported.

   At least one mandatory to implement cipher suite MUST be approved by
   NIST DRAFT Recommendation for Key Management, Part 3 [NIST SP
   800-57p3], i.e., the ciphersuite MUST be listed in Table 4-1, 4-2 or
   4-3 in that document.

   The mandatory to implement cipher suites MUST NOT include "export
   strength" cipher suites, cipher suites providing mutually anonymous
   authentication or static Diffie-Hellman cipher suites.

   Other ciphersuites MAY be selected following the security
   requirements for tunnel protocols in NIST DRAFT Recommendation for
   EAP Methods Used in Wireless Network Access Authentication [NIST SP
   800-120].

4.2.1.2.  Tunnel Replay Protection

   In order to prevent replay attacks on a tunnel protocol, the message
   authentication MUST be generated using a time-variant input such as
   timestamps, sequence numbers, nonces, or a combination of these so
   that any re-use of the authentication data can be detected as
   invalid.  TLS makes use of an 8 byte sequence number to protect
   against replay.

4.2.1.3.  TLS Extensions

   In order to meet the requirements in this document TLS extensions MAY
   be used.  For example, TLS extensions may be useful in providing



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   certificate revocation information via the TLS OCSP extension (thus
   meeting the requirement in Section 4.5.1.3).

4.2.1.4.  Peer Identity Privacy

   A tunnel protocol MUST support peer privacy.  This requires that the
   username and other attributes associated with the peer are not
   transmitted in the clear or to an unauthenticated, unauthorized
   party.  Peer identity protection provided by the tunnel method
   applies to tunnel methods and inner method specific identities.  If
   applicable, the peer certificate is sent confidentially (i.e.
   encrypted).

4.2.1.5.  Session Resumption

   The tunnel method MUST support TLS session resumption as defined in
   [RFC5246].  The tunnel method MAY support other methods of session
   resumption such as those defined in [RFC5077].

4.2.2.  Fragmentation

   Tunnel establishment sometimes requires the exchange of information
   that exceeds what can be carried in a single EAP message.  In
   addition information carried within the tunnel may also exceed this
   limit.  Therefore a tunnel method MUST support fragmentation and
   reassembly.

4.2.3.  Protection of Data External to Tunnel

   An attacker in the middle can modify clear text values such as
   protocol version and type code information communicated outside the
   TLS tunnel.  The tunnel method MUST provide implicit or explicit
   protection of the protocol version and type code.  If modification of
   other information external to the tunnel can cause exploitable
   vulnerabilities, the tunnel method MUST provide protection against
   modification of this additional data.

4.3.  Tunnel Payload Requirements

   This section describes the payload requirements inside the tunnel.
   These requirements frequently express features that a candidate
   protocol must be capable of offering so that a deployer can decide
   whether to make use of that feature.  This section does not state
   requirements about what features of each protocol must be used during
   a deployment.






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4.3.1.  Extensible Attribute Types

   The payload MUST be extensible.  Some standard payload attribute
   types will be defined to meet known requirements listed below, such
   as password authentication, inner EAP method, vendor specific
   attributes, and result indication.  Additional payload attributes MAY
   be defined in the future to support additional features and data
   types.

4.3.2.  Request/Challenge Response Operation

   The payload MUST support request and response type of half-duplex
   operation typical of EAP.  Multiple attributes may be sent in a
   single payload.  The payload MAY support carrying on multiple
   authentications in a single payload packet.

4.3.3.  Mandatory and Optional Attributes

   The payload MUST support marking of mandatory and optional
   attributes, as well as an attribute used for rejecting mandatory
   attributes.  Mandatory attributes are attributes sent by the
   requester that the responder is expected to understand and MUST
   respond to.  If the responder does not understand or support one of
   the mandatory attributes in the request, it MUST ignore the rest of
   the attributes and send a NAK attribute to decline the request.  The
   NAK attribute MUST support inclusion of which mandatory attribute is
   not supported.  The optional attributes are attributes that are not
   mandatory to support and respond to.  If the responder does not
   understand or support the optional attributes, it can ignore these
   attributes.

4.3.4.  Vendor Specific Support

   The payload MUST support communication of an extensible set of
   vendor-specific attributes.  These attributes will be segmented into
   uniquely identified vendor specific name spaces.  They can be used
   for experiments or vendor specific features.

4.3.5.  Result Indication

   The payload MUST support result indication and its acknowledgement,
   so both the EAP peer and server will end up with a synchronized
   state.  The result indication is needed after each chained inner
   authentication method and at the end of the authentication, so
   separate result indication for intermediate and final result MUST be
   supported.





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4.3.6.  Internationalization of Display Strings

   The payload MAY provide a standard attribute format that supports
   international strings.  This attribute format MUST support encoding
   strings in UTF-8 [RFC3629] format.  Any strings sent by the server
   intended for display to the user MUST be sent in UTF-8 format and
   SHOULD be able to be marked with language information and adapted to
   the user's language preference as indicated by RFC 4646 [RFC4646].
   Note that in some cases, such as when transmitting error codes, it is
   acceptable to exchange numeric codes that can be translated by the
   client to support the particular local language.  These numeric codes
   are not subject internationalization during transmission.

4.4.  EAP Channel Binding Requirements

   The tunnel method MUST be capable of meeting EAP channel binding
   requirements described in [I-D.clancy-emu-chbind].

4.5.  Requirements Associated with Carrying Username and Passwords

   This section describes the requirements associated with tunneled
   password authentication.  The password authentication mentioned here
   refers to user or machine authentication using a legacy password
   database or verifier, such as LDAP, OTP, etc.  These typically
   require the password in its original text form in order to
   authenticate the peer, hence they require the peer to send the clear
   text user name and password to the EAP server.

4.5.1.  Security

   Many internal EAP methods have the peer send its password in the
   clear To the EAP server.  Other methods (e.g. challenge-response
   methods) are vulnerable to attacks if an eavesdropper can intercept
   the traffic.  For any such methods, the security measures in the
   following sections MUST be met.

4.5.1.1.  Confidentiality and Integrity

   The clear text password exchange MUST be integrity and
   confidentiality protected.  As long as the password exchange occurs
   inside an authenticated and encrypted tunnel, this requirement is
   met.

4.5.1.2.  Authentication of Server

   The EAP server MUST be authenticated before the peer can send the
   clear text password to the server.




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4.5.1.3.  Server Certificate Revocation Checking

   In some cases, the EAP peer needs to present its password to the
   server before it has network access to check the revocation status of
   the server's credentials.  Therefore, the tunnel method MUST support
   mechanisms to check the revocation status of a credential.  The
   tunnel method SHOULD make use of Online Certificate Status Protocol
   (OCSP) [RFC2560] or Server-based Certificate Validation Protocol
   (SCVP) [RFC5055] to obtain the revocation status of the EAP server
   certificate.

4.5.2.  Internationalization

   The password authentication exchange MUST support user names and
   passwords in international languages.  It MUST support encoding of
   user name and password strings in UTF-8 [RFC3629] format.  The method
   MUST specify how username and password normalizations and/or
   comparisons is performed in reference to SASLPrep [RFC4013] or Net-
   UTF-8 [RFC5198].

   Any strings sent by the server intended for display to the user MUST
   be sent in UTF-8 format and SHOULD be able to be marked with language
   information and adapted to the user's language preference as
   indicated by RFC 4646 [RFC4646].  Note that in some cases, such as
   when transmitting error codes, it is acceptable to exchange numeric
   codes that can be translated by the client to support the particular
   local language.  These numeric codes are not subject
   internationalization during transmission.

4.5.3.  Meta-data

   The password authentication exchange SHOULD support additional
   associated meta-data which can be used to indicate whether the
   authentication is for a user or a machine.  This allows the EAP
   server and peer to request and negotiate authentication specifically
   for a user or machine.  This is useful in the case of multiple inner
   authentications where the user and machine both need to be
   authenticated.

4.5.4.  Password Change

   The password authentication exchange MUST support password change.
   The exchange SHOULD be extensible to support other "housekeeping"
   functions, such as the management of PINs or other data, required by
   some systems.






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4.6.  Requirements Associated with Carrying EAP Methods

   The tunnel method MUST be able to carry inner EAP methods without
   modifying them.  EAP methods MUST NOT be redefined inside the tunnel.

4.6.1.  Method Negotiation

   The tunnel method MUST support the protected negotiation of the inner
   EAP method.  It MUST NOT allow the inner EAP method negotiation to be
   downgraded or manipulated by intermediaries.

4.6.2.  Chained Methods

   The tunnel method SHOULD support the chaining of multiple EAP
   methods.  The tunnel method MUST allow for the communication of
   intermediate result and verification of compound binding between
   executed inner methods when chained methods are employed.

4.6.3.  Cryptographic Binding with the TLS Tunnel

   The tunnel method MUST provide a mechanism to bind the tunnel
   protocol and the inner EAP method.  This property is referred to as
   cryptographic binding.  Such bindings are an important tool for
   mitigating the tunnel MitM attacks on tunnel methods [TUNNEL-MITM].
   Cryptographic bindings enable the complete prevention of tunnel MitM
   attacks without the need of additional security policies as long as
   the inner method derives keys and is not vulnerable to attacks
   outside a protected tunnel [KHLC07].  Even though weak or non-key
   deriving inner methods may be permitted, and thus security policies
   preventing tunnel MitM attacks are still necessary, the tunnel method
   MUST provide cryptographic bindings, because only this allows
   migrating to more secure, policy-independent implementations.

   Cryptographic bindings are typically achieved by securely mixing the
   established keying material (say tunnel key TK) from the tunnel
   protocol with the established keying material (say method key MK)
   from the inner authentication method(s) in order to derive fresh
   keying material.  If chained EAP methods are executed in the tunnel,
   all derived inner keys are combined with the tunnel key to create a
   new compound tunnel key (CTK).  In particular, CTK is used to derive
   the EAP MSK, EMSK and other transient keys (TEK), such as transient
   encryption keys and integrity protection keys.  The key hierarchy for
   tunnel methods executions that derive compound keys for the purpose
   of cryptographic binding is depicted in Figure 1.

   In the case of the sequential executions of n inner methods, a
   chained compound key CTK_i MUST be computed upon the completion of
   each inner method i such that it contains the compound key of all



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   previous inner methods, i.e.  CTK_i=f(CTK_i-1, MK_i) with 0 < i <= n
   and CTK_0=TK, where f() is a good key derivation function, such as
   one that complies with NIST Recommendation for Key Derivation Using
   Pseudorandom Functions [NIST SP 800-108].  CTK_n SHOULD serve as the
   key to derive further keys.  Figure 1 depicts the key hierarchy in
   the case of a single inner method.  Transient keys derived from the
   compound key CTK are used in a cryptographic protocol to verify the
   integrity of the tunnel and the inner authentication method.

                               -----------
                               | TK | MK |
                               -----------
                                  |   |
                                  v   v
                                --------
                                | CTK  |
                                --------
                                    |
                                    v
                             ----------------
                             |      |       |
                             v      v       v
                         -------  ------  -------
                         | TEK | | MSK | | EMSK |
                         ------- ------- --------

                          Figure 1: Compound Keys

   Furthermore, all compound keys CTK_i and all keys derived from it
   SHOULD be derived in accordance to the guidelines for key derivations
   and key hierarchies as specified in Section 4.2.1.1.3.  In
   particular, all derived keys MUST have a lifetime assigned that does
   not exceed the lifetime of any key higher in the key hierarchy, and
   MUST prevent domino effects where a compromise in one part of the
   system leads to compromises in other parts of the system.

4.6.4.  Peer Initiated

   The tunnel method SHOULD allow for the peer to initiate an inner EAP
   authentication in order to meet its policy requirements for
   authenticating the server.

4.6.5.  Method Meta-data

   The tunnel method SHOULD allow for the communication of additional
   data associated with an EAP method.  This can be used to indicate
   whether the authentication is for a user or a machine.  This allows
   the EAP server and peer to request and negotiate authentication



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   specifically for a user or machine.  This is useful in the case of
   multiple inner EAP authentications where the user and machine both
   need to be authenticated.


5.  IANA Considerations

   This document has no IANA considerations.


6.  Security Considerations

   A tunnel method is often deployed to provide mutual authentication
   between EAP Peer and EAP Server and to generate strong key material
   for use in protecting lower layer protocols.  In addition the tunnel
   is used to protect the communication of additional data, including
   peer identity between the EAP Peer and EAP Server from disclosure to
   or modification by an attacker.  These sections cover considerations
   that affect the ability for a method to achieve these goals.

6.1.  Cipher Suite Selection

   TLS supports a wide variety of cipher suites providing a variety of
   security properties.  The selection of strong cipher suites is
   critical to the security of the tunnel method.  Selection of a cipher
   suite with weak or no authentication, such as an anonymous Diffie-
   Hellman based cipher suite will greatly increase the risk of system
   compromise.  Since a tunnel method uses the TLS tunnel to transport
   data, the selection of a ciphersuite with weak data encryption and
   integrity algorithms will also increase the vulnerability of the
   method to attacks.

   A tunnel protocol is prone to downgrading attacks if the tunnel
   protocol supports any key establishment algorithm that can be broken
   on-line.  In a successful downgrading attack, an adversary breaks the
   selected "weak" key establishment algorithm and optionally the "weak"
   authentication algorithm without being detected.  Here, "weak" refers
   to a key establishment algorithm that can be broken in real-time, and
   an authentication scheme that can be broken off-line, respectively.
   See [KHLC07] for more details.  The requirements in this document
   disapprove the use of key establishment algorithms that can be broken
   on-line.

   Mutually anonymous tunnel protocols are prone to man-in-the-middle
   attacks described in [KHLC07].  During such an attack, an adversary
   establishes a tunnel with each the peer and the authentication
   server, while peer and server believe that they established a tunnel
   with each other.  Once both tunnels have been established, the



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   adversary can eavesdrop on all communications within the tunnels,
   i.e. the execution of the inner authentication method(s).
   Consequently, the adversary can eavesdrop on the identifiers that are
   exchanged as part of the EAP method and thus, the privacy of peer
   and/or authentication server is compromised along with any other data
   transmitted within the tunnels.  This document requires server
   authentication to avoid the risks associated with anonymous cipher
   suites.

6.2.  Tunneled Authentication

   In many cases a tunnel method provides mutual authentication by
   authenticating the server during tunnel establishment and
   authenticating the peer within the tunnel using an EAP method.  As
   described in [TUNNEL-MITM], this mode of operation can allow tunnel
   man-in-the-middle attackers to authenticate to the server as the peer
   by tunneling the inner EAP protocol messages to and from a peer
   executing the method outside a tunnel or with an untrustworthy
   server.  Cryptographic binding between the established keying
   material from the inner authentication method(s) and the tunnel
   protocol verifies that the endpoints of the tunnel and the inner
   authentication method(s) are the same.  This can thwart the attack if
   the inner method derived keys of sufficient strength that they cannot
   be broken in real-time.

   In cases where the inner authentication method does not generate any
   or only weak key material, security policies must be enforced such
   that the peer cannot execute the inner method with the same
   credentials outside a protective tunnel or with an untrustworthy
   server.

6.3.  Data External to Tunnel

   The tunnel method will use data that is outside the TLS tunnel such
   as the EAP type code or version numbers.  If an attacker can
   compromise the protocol by modifying these values the tunnel method
   MUST protect this data from modification.  In some cases external
   data may not need additional protection because it is implicitly
   verified during the protocol operation.

6.4.  Separation of TLS Tunnel and Inner Authentication Termination

   Terminating the inner method at a different location than the outer
   tunnel needs careful consideration.  The inner method data may be
   vulnerable to modification and eavesdropping between the server that
   terminates the tunnel and the server that terminates the inner
   method.  For example if a clear text password is used then it may be
   sent to the inner method server in a RADIUS password attribute which



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   uses weak encryption that may not be suitable protection for many
   environments.

   In some cases terminating the tunnel at a different location may make
   it difficult for a peer to authenticate the server and trust it for
   further communication.  For example, if the TLS tunnel is terminated
   by a different organization the peer needs to be able to authenticate
   and authorize the tunnel server to handle secret credentials that it
   shares with the home server that terminates the inner method.  This
   may not meet the security policy of many environments."


7.  References

7.1.  Normative References

   [I-D.clancy-emu-chbind]
              Clancy, C. and K. Hoeper, "Channel Binding Support for EAP
              Methods", draft-clancy-emu-chbind-04 (work in progress),
              November 2008.

   [RFC2560]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
              Adams, "X.509 Internet Public Key Infrastructure Online
              Certificate Status Protocol - OCSP", RFC 2560, June 1999.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management",
              BCP 132, RFC 4962, July 2007.

   [RFC5055]  Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
              Polk, "Server-Based Certificate Validation Protocol
              (SCVP)", RFC 5055, December 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible



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              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.

7.2.  Informative References

   [I-D.ietf-nea-pb-tnc]
              Sahita, R., Hanna, S., and K. Narayan, "PB-TNC: A Posture
              Broker Protocol (PB) Compatible with TNC",
              draft-ietf-nea-pb-tnc-05 (work in progress),
              September 2009.

   [I-D.mahy-eap-enrollment]
              Mahy, R., "An Extensible Authentication Protocol (EAP)
              Enrollment Method", draft-mahy-eap-enrollment-01 (work in
              progress), March 2006.

   [KHLC07]   Hoeper, K. and L. Chen, "Where EAP Security Claims Fail",
              ICST QShine , August 2007.

   [NIST SP 800-108]
              Chen, L., "Recommendation for Key Derivation Using
              Pseudorandom Functions", Draft NIST Special
              Publication 800-108, April 2008.

   [NIST SP 800-120]
              Hoeper, K. and L. Chen, "Recommendation for EAP Methods
              Used in Wireless Network Access Authentication", NIST
              Special Publication 800-120, September 2009.

   [NIST SP 800-57]
              Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
              "Recommendation for Key Management - Part 1: General
              (Revised)", NIST Special Publication 800-57, part 1,
              March 2007.

   [NIST SP 800-57p3]
              Barker, E., Burr, W., Jones, A., Polk, W., , S., and M.
              Smid, "Recommendation for Key Management, Part 3
              Application-Specific Key Management Guidance", Draft NIST
              Special Publication 800-57,part 3, October 2008.

   [PEAP]     Microsoft Corporation, "[MS-PEAP]: Protected Extensible
              Authentication Protocol (PEAP) Specification",
              August 2009.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.




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   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

   [RFC4646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", RFC 4646, September 2006.

   [RFC4851]  Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
              Flexible Authentication via Secure Tunneling Extensible
              Authentication Protocol Method (EAP-FAST)", RFC 4851,
              May 2007.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5209]  Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
              Tardo, "Network Endpoint Assessment (NEA): Overview and
              Requirements", RFC 5209, June 2008.

   [RFC5281]  Funk, P. and S. Blake-Wilson, "Extensible Authentication
              Protocol Tunneled Transport Layer Security Authenticated
              Protocol Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.

   [TUNNEL-MITM]
              Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
              in Tunnelled Authentication Protocols", Cryptology ePrint
              Archive:  Report 2002/163, November 2002.


Appendix A.  Changes from -01
   o  Added combined mutual authentication in section 3.1
   o  Changed reference from SP 800-52 to SP 800-57,part 3
   o  In section 6.2 changed terminology to tunnel MitM and security
      policy enforcement
   o  Reworded text in section 3.2 to clarify MITM protection
   o  Added more specific text about derivation of the CTK
   o  Removed resource constrained section
   o  Added support for Non EAP authentication as a use for
      extensibility
   o  Added text to emergency services section to emphasize that
      sensitive information should not be sent if the server is
      unauthenticated.
   o  Reworded TLS requirements





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   o  Reworded external data protection requirements
   o  Added text to section 4.6 that states method must not be re-
      defined inside the tunnel.
   o  Editorial fixes


Appendix B.  Changes from -02
   o  Editorial Fixes
   o  Clarified client authentication during tunnel establishment
   o  Changed text so that the tunnel method MUST meet all MUST and
      SHOULD requirements relevant to EAP methods in RFCs 4962 and 5247


Appendix C.  changes from -03
   o  Resolution of open issues:
      http://trac.tools.ietf.org/wg/emu/trac/report/9
   o  Revised section 2 to match other similar RFC(Issue 6)
   o  Cleaned up section 3.2 (issue 8)
   o  Clarified identity protection scope in section 3.4 and
      4.2.1.4(issue 9)
   o  Changed Emergency Services to anonymous authentication(section
      3.5)(issue 10)
   o  Clarified section 4.1.1 (issue 15)
   o  Cleaned up TLS requirements in section 4.2.1(issue 11)
   o  Replaced text in 4.2.1.1.3 with suitable reference
   o  Improved wording in 4.2.3 and 6.3 (issue 13)
   o  Update internationalization requirements in 4.3.6 and 4.5.2
      (Issues 25,18)
   o  Updated text in 4.5.1 (issue 16)
   o  Changed meta-data to SHOULD in 4.5.3 and 4.6.5(Issue 20)
   o  Changed chained methods to SHOULD in 4.6.2(issue 19)
   o  Added security consideration for inner method termination(issue
      24)
   o  Updated references
   o  Editorial changes(issues 7,22,17)


Authors' Addresses

   Katrin Hoeper
   Motorola, Inc.
   1301 E Algonquin Rd
   Schaumburg, IL  60196
   USA

   Email: khoeper@motorola.com





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   Stephen Hanna
   Juniper Networks
   3 Beverly Road
   Bedford, MA  01730
   USA

   Email: shanna@juniper.net


   Hao Zhou
   Cisco Systems, Inc.
   4125 Highlander Parkway
   Richfield, OH  44286
   USA

   Email: hzhou@cisco.com


   Joseph Salowey (editor)
   Cisco Systems, Inc.
   2901 3rd. Ave
   Seattle, WA  98121
   USA

   Email: jsalowey@cisco.com


























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