EMU Working Group                                              K. Hoeper
Internet-Draft                                            Motorola, Inc.
Intended status: Informational                                  S. Hanna
Expires: August 31, 2009 January 7, 2010                                Juniper Networks
                                                                 H. Zhou
                                                         J. Salowey, Ed.
                                                     Cisco Systems, Inc.
                                                       February 27,
                                                            July 6, 2009

               Requirements for a Tunnel Based EAP Method

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   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.

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.  Emergency Services Authentication  . . . . . . . . . . . .  7
     3.6.  Network Endpoint Assessment  . . . . . . . . . . . . . . .  8
     3.7.  Client Authentication During Tunnel Establishment  . . . .  8
     3.8.  Extensibility  . . . . . . . . . . . . . . . . . . . . . .  8

   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  General Requirements . . . . . . . . . . . . . . . . . . .  8  9
       4.1.1.  RFC Compliance . . . . . . . . . . . . . . . . . . . .  8  9
       4.1.2.  Draw from Existing Work  . . . . . . . . . . . . . . .  9
     4.2.  Tunnel Requirements  . . . . . . . . . . . . . . . . . . . 10
       4.2.1.  TLS Requirements . . . . . . . . . . . . . . . . . . . 10  Cipher Suites  . . . . . . . . . . . . . . . . . . 10
   Cipher Suite Negotiation . . . . . . . . . . . 10
   Tunnel Data Protection Algorithms  . . . . . . 10
   Tunnel Authentication and Key Establishment  . 11  Tunnel Replay Protection . . . . . . . . . . . . . 12  TLS Extensions . . . . . . . . . . . . . . . . . . 13  Peer Identity Privacy  . . . . . . . . . . . . . . 13  Session Resumption . . . . . . . . . . . . . . . . 13
       4.2.2.  Fragmentation  . . . . . . . . . . . . . . . . . . . . 13
       4.2.3.  Protection of Data External to Tunnel  . . . . . . . . 13
     4.3.  Tunnel Payload Requirements  . . . . . . . . . . . . . . . 13
       4.3.1.  Extensible Attribute Types . . . . . . . . . . . . . . 14
       4.3.2.  Request/Challenge Response Operation . . . . . . . . . 14
       4.3.3.  Mandatory and Optional Attributes  . . . . . . . . . . 14
       4.3.4.  Vendor Specific Support  . . . . . . . . . . . . . . . 14
       4.3.5.  Result Indication  . . . . . . . . . . . . . . . . . . 14
       4.3.6.  Internationalization of Display Strings  . . . . . . . 15
     4.4.  EAP Channel Binding Requirements . . . . . . . . . . . . . 15
     4.5.  Requirements Associated with Carrying Username and
           Passwords  . . . . . . . . . . . . . . . . . . . . . . . . 15
       4.5.1.  Security . . . . . . . . . . . . . . . . . . . . . . . 15  Confidentiality and Integrity  . . . . . . . . . . 15  Authentication of Server . . . . . . . . . . . . . 15  Server Certificate Revocation Checking . . . . . . 15
       4.5.2.  Internationalization . . . . . . . . . . . . . . . . . 16
       4.5.3.  Meta-data  . . . . . . . . . . . . . . . . . . . . . . 16
       4.5.4.  Password Change  . . . . . . . . . . . . . . . . . . . 16
     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  . . . . . . 17
       4.6.4.  Peer Initiated . . . . . . . . . . . . . . . . . . . . 18
       4.6.5.  Method Meta-data . . . . . . . . . . . . . . . . . . . 18

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

   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     6.1.  Cipher Suite Selection . . . . . . . . . . . . . . . . . . 19
     6.2.  Tunneled Authentication  . . . . . . . . . . . . . . . . . 20
     6.3.  Data External to Tunnel  . . . . . . . . . . . . . . . . . 20

   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

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

1.  Introduction

   Running EAP methods within a TLS protected tunnel has been deployed
   in several different solutions.  EAP methods supporting this include
   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

   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

   Because this specification is an informational specification (not
   able to directly use [RFC2119]), the following capitalized words are
   used to express requirements language used in this specification.
   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

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.

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 this use case.
   However, it MUST NOT expose the username and password to untrusted 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 attacks documented in 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,
   including cryptographic protection from tunnel MitM attacks.  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 the 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.
   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.  Emergency Services Authentication

   When wireless VOIP service is provided, some regulations require any
   user to be able to gain access to the network to make an emergency
   telephone call.  To avoid eavesdropping on this call, 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
   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

3.7.  Client Authentication During Tunnel Establishment

   In some cases where client 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 can
   may be performed as part required inside the tunnel.  If the identity of the peer is
   fully authenticated during tunnel establishment it is efficient for then the tunnel method may
   be used to allow
   this. communicate additional data.  The tunnel method MUST be
   capable of providing client side authentication during tunnel

3.8.  Extensibility

   The tunnel method MUST provide extensibility so that additional types
   of 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.

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, all the tunnel methods 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 EAP method MUST and SHOULD requirements
   relevant to EAP methods contained in the EAP Key Management Framework
   [RFC5247] or its successor.  The tunnel method MUST include MSK and
   EMSK generation.  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
   "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; confirm cipher suite selection; and uniquely named keys.

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
   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

4.2.1.  TLS Requirements

   The tunnel based method MUST support TLS version 1.2 [RFC5246] and
   SHOULD support TLS version 1.0 [RFC2246] and version 1.1 [RFC4346] to
   enable the possibility of backwards compatibility with existing
   deployments.  The following section discusses requirements for TLS
   Tunnel Establishment.  Cipher Suites  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.  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.  Tunnel Authentication and Key Establishment

   A tunnel method MUST provide unidirectional authentication from
   authentication server to EAP peer or 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
   certificate-based authentication of the server and provides optional
   certificate based
   certificate-based authentication of the client.  Other types of
   authentication MAY be supported.

   At least one mandatory to implement cipher suite MUST meet the
   following requirements for secure key establishment along with the
   previous requirements for authentication and data protection

   o  One-way key derivation, i.e., a compromised key leads to the
      compromise of all descendant keys but does not affect the security
      of any precedent key in the same branch of the key hierarchy.

   o  Cryptographically separated keys, i.e., a compromised key in one
      branch of the key hierarchy does not affect the security of keys
      in other branches.

   o  Cryptographically separated entities, i.e., keys held by different
      entities are cryptographically separate.  As a result, the
      compromise of a single peer does not compromise keying material
      held by any other peer within the system, including session keys
      and long-term keys.

   o  Identity binding, i.e., each derived key is bound to the EAP peer
      and authentication server by including their identifiers as input
      to the key derivation.

   o  Context binding, i.e., each derived key is bound to its context by
      including appropriate key labels in the input of the key

   o  Key lifetime, i.e., each key has a lifetime assigned that does not
      exceed the lifetime of any key higher in the key hierarchy.

   o  Mutual implicit key authentication, i.e., the keying material
      derived upon a successful key establishment execution is only
      known to the EAP peer and authentication server and is kept

   o  Key freshness, i.e.  EAP peer and EAP server are assured that the
      derived keys are fresh and the re-use of expired key material is
      prevented.  The freshness property is typically achieved by using
      one or more of the following techniques: nonces, sequence numbers,

   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.  Part 3 of the
   publication Recommendation for Key Management [NIST SP 800-57p3] can be
   consulted for a list of acceptable TLS v1.0, v1.1 and v 1.2 cipher
   suites and NIST
   publication Recommendation for Key Derivation Using Pseudorandom
   Functions [NIST SP 800-108] for additional information on secure key

   In addition a tunnel method SHOULD provide cipher suites to meet the
   following additional recommendations for good key establishment

   o  Key control , i.e., EAP peer and authentication server each
      contribute to the key computation of the tunnel key.  This
      property prevents that a single protocol participant controls the
      value of an established key.  In that way, protocol participants
      can ensure that generated keys are fresh and have good random

   o  Key confirmation, i.e., one protocol participant is assured that
      another participant actually possesses a particular secret key.
      In the case of mutual key confirmation both the EAP peer and the
      authentication server are assured that they possess the same key.
      Key confirmation is commonly achieved by using one of the derived
      keys to generate a message authentication code.  Mutual key
      confirmation combined with mutual implicit key authentication
      leads to mutual explicit key authentication.

   o  Forward secrecy (FS), i.e., if a long-term secret key is
      compromised, it does not compromise keys that have been
      established in previous EAP executions.  This property is
      typically achieved by executing an ephemeral Diffie-Hellman key
      establishment.  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.  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
   certificate revocation information via the TLS OCSP extension (thus
   meeting the requirement in Section  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.  If applicable, the peer certificate is sent confidentially
   (i.e. encrypted).  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

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.  If modification of this information can cause
   vulnerabilities, the tunnel method MUST provide protection against
   modification of this 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.

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

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

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

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.

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

   Due to the fact that the EAP peer needs to send clear text password
   to the EAP server to authenticate against the legacy user
   information, the security measures in the following sections MUST be
   met.  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.  Authentication of Server

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

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.  Any
   strings sent by the server during the password exchange and 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.

4.5.3.  Meta-data

   The password authentication exchange MUST 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

4.5.4.  Password Change

   The password authentication exchange MUST support password change, as
   well as other multiple round trips exchanges like new pin mode and
   next token mode for OTP verifiers. other "housekeeping" functions required by some

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 MUST 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 described in [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
   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
                         -------  ------  -------
                         | 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  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 MUST 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 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

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

   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
   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

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

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.

7.  References

7.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

7.2.  Informative References

              Sahita, R., Hanna, S., and K. Narayan, "PB-TNC: A Posture
              Broker Protocol (PB) Compatible with TNC",
              draft-ietf-nea-pb-tnc-04 (work in progress),
              November 2008. April 2009.

              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", Draft
              NIST Special Publication 800-120, December 2008.

   [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.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

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

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [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.

   [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.

              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
   o  Added text to emergency services section to emphasize that
      sensitive information should not be sent if the server is
   o  Reworded TLS requirements
   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

Authors' Addresses

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

   Email: khoeper@motorola.com

   Stephen Hanna
   Juniper Networks
   3 Beverly Road
   Bedford, MA  01730

   Email: shanna@juniper.net
   Hao Zhou
   Cisco Systems, Inc.
   4125 Highlander Parkway
   Richfield, OH  44286

   Email: hzhou@cisco.com

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

   Email: jsalowey@cisco.com