EAP Tunnel: Requirements for a Tunnel-Based Extensible Authentication Protocol (EAP) Method

Internet Engineering Task Force (IETF)                         K. Hoeper
Request for Comments: 6678                      Motorola Solutions, Inc.
Category: Informational                                         S. Hanna
ISSN: 2070-1721                                         Juniper Networks
                                                                 H. Zhou
                                                         J. Salowey, Ed.
                                                     Cisco Systems, Inc.
                                                               July 2012


                           Requirements for a
      Tunnel-Based Extensible Authentication Protocol (EAP) Method

Abstract

   This memo defines the requirements for a tunnel-based Extensible
   Authentication Protocol (EAP) Method.  This tunnel 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.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6678.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Password Authentication  . . . . . . . . . . . . . . . . .  5
     3.2.  Protection of Weak EAP Methods . . . . . . . . . . . . . .  5
     3.3.  Chained EAP Methods  . . . . . . . . . . . . . . . . . . .  6
     3.4.  Identity Protection  . . . . . . . . . . . . . . . . . . .  6
     3.5.  Anonymous Service Access . . . . . . . . . . . . . . . . .  7
     3.6.  Network Endpoint Assessment  . . . . . . . . . . . . . . .  7
     3.7.  Client Authentication during Tunnel Establishment  . . . .  7
     3.8.  Extensibility  . . . . . . . . . . . . . . . . . . . . . .  8
     3.9.  Certificate-Less Authentication and Generic EAP Method
           Extension  . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  General Requirements . . . . . . . . . . . . . . . . . . .  9
       4.1.1.  RFC Compliance . . . . . . . . . . . . . . . . . . . .  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  . . . . . . 10
           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  . . . . . . . . . . . . . . 11
         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 . . . . . . . . . . . . . . 12
       4.3.2.  Request/Challenge Response Operation . . . . . . . . . 13
       4.3.3.  Indicating Criticality of Attributes . . . . . . . . . 13
       4.3.4.  Vendor-Specific Support  . . . . . . . . . . . . . . . 13
       4.3.5.  Result Indication  . . . . . . . . . . . . . . . . . . 13
       4.3.6.  Internationalization of Display Strings  . . . . . . . 13
     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 . . . . . . 14
       4.5.2.  Internationalization . . . . . . . . . . . . . . . . . 15
       4.5.3.  Metadata . . . . . . . . . . . . . . . . . . . . . . . 15
       4.5.4.  Password Change  . . . . . . . . . . . . . . . . . . . 15
     4.6.  Requirements Associated with Carrying EAP Methods  . . . . 15
       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 EAP Authentication  . . . . . . . . . . 17
       4.6.5.  Method Metadata  . . . . . . . . . . . . . . . . . . . 17
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
     5.1.  Cipher Suite Selection . . . . . . . . . . . . . . . . . . 18
     5.2.  Tunneled Authentication  . . . . . . . . . . . . . . . . . 19
     5.3.  Data External to Tunnel  . . . . . . . . . . . . . . . . . 19
     5.4.  Separation of TLS Tunnel and Inner Authentication
           Termination  . . . . . . . . . . . . . . . . . . . . . . . 19
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 21

1.  Introduction

   An Extensible Authentication Protocol (EAP) tunnel method is an EAP
   method that establishes a secure tunnel and executes other EAP
   methods under the protection of that secure tunnel.  An EAP tunnel
   method can be used in any lower-layer protocol that supports EAP
   authentication.  There are several existing EAP tunnel methods that
   use Transport Layer Security (TLS) to establish the secure tunnel.
   EAP methods supporting this include Protected EAP [PEAP], Tunneled
   Transport Layer Security EAP (TTLS) [RFC5281] and EAP Flexible
   Authentication via Secure Tunneling (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 a 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 EAP Method Update (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

   Lowercase 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.  It is mandatory for a candidate tunnel method to
   support all of the use cases that are marked below 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.
   Examples of such systems are some 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 cleartext username and password to the EAP server.  It
   MUST NOT reveal information about the username and password to
   parties in the communication path between the peer and the EAP
   server.  The advantage any attacker gains against the tunnel method
   when employing a username and password for authentication MUST be
   through interaction and not computation.  The tunnel MUST support
   protection from man-in-the-middle 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, an EAP-MD5 method does not provide mutual authentication or
   protection from dictionary attacks.  Without extra protection, EAP
   tunnel methods are vulnerable to a special type of tunnel man-in-the-
   middle (MitM) 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.  When weak methods are
   used, these attacks can be mitigated via security policies that
   require the method to be used only within a tunnel.  On the other
   hand, a technical solution (so-called cryptographic bindings) can be
   used whenever the inner method derives key material and is not
   susceptible to attacks outside a tunnel.  Only the latter mitigation
   technique can be made an actual requirement for EAP tunnel methods
   (see Section 4.6.3), while security policies are outside the scope of
   this requirement document.  Please refer to the NIST "Recommendation
   for EAP Methods Used in Wireless Network Access Authentication"
   [NIST-SP-800-120] and [LCN-2010] 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
   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 being used.  However, chained EAP
   methods from different conversations can be redirected into the same
   conversation by an attacker giving the authenticator the impression
   that both conversations terminate at the same endpoint.
   Cryptographic binding can be used to bind the results of chained key-
   generating methods together or to an encompassing tunnel.

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

3.4.  Identity Protection

   When performing an EAP authentication, the peer may want to protect
   its identity and only disclose it to a trusted EAP server.  This
   helps to maintain peer privacy.

   The tunnel method MUST support identity protection, therefore the
   identity of the peer used for authentication purposes MUST NOT be
   obtainable by any entity other than the EAP server terminating the
   tunnel method.  Peer identity protection provided by the tunnel
   method applies to the identities that are specific to the tunnel
   method and inner method being used.  In a roaming scenario, note that
   the peer may need to expose the realm portion of the EAP outer
   identity in the Network Access Identifier (NAI) [RFC4282] in order to
   reach the appropriate authentication server.

3.5.  Anonymous Service Access

   When network service is provided, it is sometimes desirable for a
   user to gain network access in order to access limited services for
   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, while still deriving 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.

   Foregoing user or server 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.  Therefore, passwords and
   other sensitive information MUST NOT be disclosed to an
   unauthenticated server, or to a server that is not authorized to
   authenticate the user.

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 a
   Posture Broker protocol compatible with Trusted Network Connect
   (PB-TNC) [RFC5793].  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 that allow it 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.  For example, a
   communication device may be authenticated during tunnel
   establishment; in addition, user authentication may be required to
   satisfy authentication policy.  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.

   An 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
   [EAP-ENROLL].  Another use for extensibility is support for alternate
   authentication frameworks within the tunnel.

3.9.  Certificate-Less Authentication and Generic EAP Method Extension

   In some cases, the peer will not have a way to verify a server
   certificate and, in some cases, a server might not have a certificate
   to verify.  Therefore, it is desirable to support certificate-less
   authentication.  An application for this is credential provisioning
   where the peer and server authenticate each other with a shared
   password and credentials for subsequent authentication (e.g., a key
   pair and certificate, or a shared key) can be passed inside the
   tunnel.  Another application is to extend existing EAP methods with
   new features such as EAP channel bindings.

   Great care must be taken when using tunnels with no server
   authentication for the protection of an inner method.  For example,
   the client may lack the appropriate trust roots to fully authenticate
   the server, but may still establish the tunnel to execute an inner
   EAP method to perform mutual authentication and key derivation.  In
   these cases, the inner EAP method MUST provide resistance to
   dictionary attack and a cryptographic binding between the inner
   method and the tunnel method MUST be established.  Furthermore, the
   cipher suite used to establish the tunnel MUST derive the master key
   using contributions from both client and server, as in ephemeral
   Diffie-Hellman cipher suites.

   The tunnel method MAY allow for certificate-less authentication.

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 of 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 any successor.  This includes the generation of the
   Master Session Key (MSK), Extended Master Session Key (EMSK),
   Peer-Id, Server-Id, and Session-Id.  These requirements 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, such as
   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.

4.2.  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 greater than SSL 2.0 in order to enable
   the possibility of backwards compatibility.

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-negotiation 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 the
   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 5.1 or [HC07] for more information on
   this.  Hence, the tunnel method MUST provide integrity-protected
   cipher suite negotiation with secure integrity algorithms and
   integrity keys.

   TLS provides protected cipher suite negotiation as long as all the
   cipher suites supported provide authentication, key establishment,
   and data integrity protection as discussed in Section 5.1.

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
   the NIST "Draft Recommendation for Key Management", Part 3
   [NIST-SP-800-57p3], i.e., the cipher suite MUST be listed in Table
   4-1, 4-2, or 4-3 in that document.

   The mandatory-to-implement cipher suites MUST only include cipher
   suites that use strong cryptographic algorithms.  They MUST NOT
   include cipher suites providing mutually anonymous authentication or
   static Diffie-Hellman cipher suites.

   Other cipher suites MAY be selected following the security
   requirements for tunnel protocols in the NIST "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 reuse of the authentication data can be detected as invalid.
   TLS provides sufficient replay protection to meet this requirement as
   long as weak cipher suites discussed in Section 5.1 are avoided.

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
   certificate revocation information via the TLS Online Certificate
   Status Protocol (OCSP) extension [RFC6066] (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 establishment of the tunnel and protection of 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

   A man-in-the-middle attacker can modify cleartext 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.

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

4.3.3.  Indicating Criticality of Attributes

   It is expected that new attributes will be defined to be carried
   within the tunnel method.  In some cases, it is necessary for the
   sender to know if the receiver did not understand the attribute.  To
   support this, there MUST be a way for the sender to mark attributes
   such that the receiver will indicate if an attribute is not
   understood.

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 namespaces.  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 indications for intermediate and final results MUST
   be supported.

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 5646 [RFC5646].
   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 to internationalization during transmission.

4.4.  EAP Channel Binding Requirements

   The tunnel method MUST be capable of meeting EAP channel binding
   requirements described in [RFC6677].  As discussed in [RFC5056], EAP
   channel bindings differ from channel bindings discussed in other
   contexts.  Cryptographic binding between the TLS tunnel and the inner
   method discussed in Section 4.6.3 relates directly to the non-EAP
   channel binding concepts discussed in RFC 5056.

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 the Lightweight Directory Access
   Protocol (LDAP) [RFC4511], 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 cleartext username 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 cleartext 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 sends the
   cleartext password to the server.

4.5.1.3.  Server Certificate Revocation Checking

   When certificate authentication is used during tunnel establishment,
   the EAP peer may need 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 usernames and
   passwords in international languages.  It MUST support encoding of
   username and password strings in UTF-8 [RFC3629] format.  The method
   MUST specify how username and password normalizations and/or
   comparisons are performed in reference to SASLprep [RFC4013],
   Net-UTF-8 [RFC5198], or their replacements.

   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 5646 [RFC5646].  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 to
   internationalization during transmission.

4.5.3.  Metadata

   The password authentication exchange SHOULD support additional
   associated metadata that 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.

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
   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 results and for the 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 [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 [LCN-2010].  Even though weak or non-key-deriving
   inner methods may be permitted.  Thus, security policies preventing
   tunnel MitM attacks are still necessary, and 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 (shown as "TEK" below),
   such as transient encryption keys and integrity protection keys.  The
   key hierarchy for tunnel method 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 key derivation function, such as one
   that complies with the 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 follow the recommendations for key derivations and key
   hierarchies as specified in [NIST-SP-800-108].  In particular, all
   derived keys MUST have a lifetime assigned that does not exceed the
   lifetime of any key higher in the key hierarchy.  The derivation MUST
   prevent a compromise in one part of the system from leading to
   compromises in other parts of the system that relay on keys at the
   same or higher level in the hierarchy.

4.6.4.  Peer-Initiated EAP Authentication

   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 Metadata

   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
   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.  Security Considerations

   A tunnel method is often deployed to provide mutual authentication
   between EAP Peer and EAP Server and to generate 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.

5.1.  Cipher Suite Selection

   TLS supports a wide range of cipher suites providing a variety of
   security properties.  The selection of 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 cipher suite 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 [HC07] 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 [HC07].  During such an attack, an adversary
   establishes one tunnel with the peer and one with the authentication
   server, while the 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
   suites.

5.2.  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 that
   is 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 are of sufficient strength that they
   cannot be broken in real-time.

   In cases where the inner authentication method does not generate any
   key material 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.

5.3.  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.

5.4.  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 cleartext password is used, then it may be
   sent to the inner method server in a RADIUS password attribute, which
   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 the peer shares with the home server that terminates
   the inner method.  This may not meet the security policy of many
   environments.

6.  References

6.1.  Normative References

   [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.

   [RFC6677]     Hartman, S., Ed., Clancy, T., and K. Hoeper, "Channel
                 Binding Support for Extensible Authentication Protocol
                 (EAP) Methods", RFC 6677, July 2012.

6.2.  Informative References

   [EAP-ENROLL]  Mahy, R., "An Extensible Authentication Protocol (EAP)
                 Enrollment Method", Work in Progress, March 2006.

   [HC07]        Hoeper, K. and L. Chen, "Where EAP Security Claims
                 Fail", Institute for Computer Sciences, Social
                 Informatics and Telecommunications Engineering (ICST),
                 The Fourth International Conference on Heterogeneous
                 Networking for Quality, Reliability, Security and
                 Robustness (QShine 2007), August 2007.

   [LCN-2010]    Hoeper, K. and L. Chen, "An Inconvenient Truth about
                 Tunneled Authentications", Proceedings of 35th Annual
                 IEEE Conference on Local Computer Networks (LCN 2010),
                 September 2009.

   [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., Rose, 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, <http:// download.microsoft.com/download/9/5/E/
                 95EF66AF-9026-4BB0-A41D-A4F81802D92C/%5BMS-
                 PEAP%5D.pdf>.

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

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

   [RFC4511]     Sermersheim, J., "Lightweight Directory Access Protocol
                 (LDAP): The Protocol", RFC 4511, June 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.

   [RFC5056]     Williams, N., "On the Use of Channel Bindings to Secure
                 Channels", RFC 5056, November 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.

   [RFC5646]     Phillips, A. and M. Davis, "Tags for Identifying
                 Languages", BCP 47, RFC 5646, September 2009.

   [RFC5793]     Sahita, R., Hanna, S., Hurst, R., and K. Narayan,
                 "PB-TNC: A Posture Broker (PB) Protocol Compatible with
                 Trusted Network Connect (TNC)", RFC 5793, March 2010.

   [RFC6066]     Eastlake, D., "Transport Layer Security (TLS)
                 Extensions: Extension Definitions", RFC 6066, January
                 2011.

   [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.

Authors' Addresses

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

   EMail: khoeper@motorolasolutions.com


   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