EAP Mutual Cryptographic Binding
draft-ietf-emu-crypto-bind-01

Network Working Group                                         S. Hartman
Internet-Draft                                              M. Wasserman
Intended status: Informational                         Painless Security
Expires: July 9, 2013                                           D. Zhang
                                                                  Huawei
                                                         January 5, 2013

                    EAP Mutual Cryptographic Binding
                   draft-ietf-emu-crypto-bind-01.txt

Abstract

   As the Extensible Authentication Protocol (EAP) evolves, EAP peers
   rely increasingly on information received from the EAP server.  EAP
   extensions such as channel binding or network posture information are
   often carried in tunnel methods; peers are likely to rely on this
   information.  EAP has provided a facility that protects tunnel
   methods against man-in-the-middle attacks.  However, cryptographic
   binding focuses on protecting the server rather than the peer.  This
   memo explores attacks possible when the peer is not protected from
   man-in-the-middle attacks and recommends mutual cryptographic
   binding, a new form of cryptographic binding that protects both peer
   and server along with other mitigations.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 9, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal

Hartman, et al.           Expires July 9, 2013                  [Page 1]
Internet-Draft            Mutual Crypto Binding             January 2013

   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  An Example Problem . . . . . . . . . . . . . . . . . . . . . .  5
   3.  The Server insertion Attack  . . . . . . . . . . . . . . . . .  7
     3.1.  Conditions for the Attack  . . . . . . . . . . . . . . . .  7
     3.2.  Mitigation Strategies  . . . . . . . . . . . . . . . . . .  8
       3.2.1.  Server Authentication  . . . . . . . . . . . . . . . .  8
       3.2.2.  Server Policy  . . . . . . . . . . . . . . . . . . . .  9
       3.2.3.  Existing Cryptographic Binding . . . . . . . . . . . . 12
       3.2.4.  IntroducingEMSK-based Cryptographic Binding  . . . . . 13
       3.2.5.  Mix Key into Long-Term Credentials . . . . . . . . . . 14
     3.3.  Intended Intermediates . . . . . . . . . . . . . . . . . . 14
   4.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Mutual Cryptographic Binding . . . . . . . . . . . . . . . 16
     4.2.  State Tracking . . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  Certificate Naming . . . . . . . . . . . . . . . . . . . . 16
     4.4.  Inner Mixing . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  Survey of Tunnel Methods . . . . . . . . . . . . . . . . . . . 18
   6.  Survey of Inner Methods  . . . . . . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24

Hartman, et al.           Expires July 9, 2013                  [Page 2]
Internet-Draft            Mutual Crypto Binding             January 2013

1.  Introduction

   The Extensible Authentication Protocol [RFC3748] provides
   authentication between a peer (a party accessing some service) and a
   authentication server.  Traditionally, peers have not relied
   significantly on information received from EAP servers.  However
   facilities such as EAP Channel Binding [I-D.ietf-emu-chbind] provide
   the peer with confirmation of information about the resource it is
   accessing.  Other facilities such as EAP Posture Transport
   [I-D.ietf-nea-pt-eap] permit a peer and EAP server to discuss the
   security properties of accessed networks.  Both of these facilities
   provide peers with information they need to rely on and provide
   attackers who are able to impersonate an EAP server to a peer with
   new opportunities for attack.

   Instead of adding these new facilities to all EAP methods, work has
   focused on adding support to tunnel methods
   [I-D.ietf-emu-eaptunnel-req].  There are numerous tunnel methods
   including [RFC4851], [RFC5281], and work on building a standards
   track tunnel method [I-D.ietf-emu-eap-tunnel-method].  These tunnel
   methods are extensible.  By adding an extension to support a facility
   such as channel binding to a tunnel method, it can be used with any
   inner method carried in the tunnel.

   Tunnel methods need to be careful about man-in-the-middle attacks.
   Interested people could refer to section 3.2 and 4.6.3 in
   [I-D.ietf-emu-eaptunnel-req] and [TUNNEL-MITM] for a detailed
   description of these attacks.  An example of the attack can happen
   when a peer is willing to perform authentication inside and outside a
   tunnel.  In this case, an attacker can impersonate the EAP server and
   offer the inner method to the peer.  However, on the other side, the
   attacker acts as a man-in-the-middle and opens a tunnel to the real
   EAP server.  Figure 1 illustrates this attack.  At the end of the
   attack, the EAP server believes it is talking to the peer.  At the
   inner method level, this is true.  At the outer method level,
   however, the server is talking to the attacker.

Hartman, et al.           Expires July 9, 2013                  [Page 3]
Internet-Draft            Mutual Crypto Binding             January 2013

       Peer                Attacker         Service         AAA Server
        |                     |                 |                |
        |                     |                 |                |
        |Peer Initiates connection to a Service |                |
        |-------------------------------------->|                |
        |                     |                 |                |
        |                     |        Tunnel Establishment      |
        |                     |<-------------------------------->|
        |                     |                 |                |
        |                     |..................................|
        |                     |              Tunnel              |
        |    Non-Tunneled     |                 |                |
        |       method        |  Tunneled authentication method  |
        |<===================>|<================================>|
        |                     |                 |                |
        |                     |..................................|
        |                     |                 |                |
        |                     |    Attacker     |<--- MSK keys --|
        |                     | Connected as    |                |
        |                     |      Peer       |                |
        |                     |<--------------->|                |

   A classic tunnel attack where the attacker inserts an extra tunnel
   between the attacker and EAP server.

                      Figure 1: Classic Tunnel Attack

   There are several mitigation strategies for this classic attack.
   First, security policies can be set up so that the same method is not
   offered by a server both inside and outside a tunnel.  A technical
   solution is available if the inner method is sufficiently strong:
   cryptographic binding is a security property of a tunnel method under
   which the EAP server confirms that the inner and outer parties are
   the same.  One common way to do this is to ask the outer party (the
   other end of the tunnel) to prove knowledge of the Master Session Key
   (MSK) of the inner method.  As defined in [RFC3748], cryptographic
   binding is proposed to help each peer prove that the inner and outer
   exchanges are with the same party.  However, it is typically failed
   in making this proof.  Instead, it is typically limited to proving to
   the server that the inner and outer peer are the same.  In the
   following sections, security risks caused by such limitation are
   introduced.

Hartman, et al.           Expires July 9, 2013                  [Page 4]
Internet-Draft            Mutual Crypto Binding             January 2013

2.  An Example Problem

   The GSS-EAP mechanism [I-D.ietf-abfab-gss-eap] provides application
   authentication using EAP.  A peer could reasonably trust some
   applications significantly more than others.  If the peer sends
   confidential information to some applications, an attacker may gain
   significant value from convincing the peer that the attacker is the
   trusted application.  Channel bindings are used to tell the peer
   which application service is being connected to.  Prior to channel
   bindings, peers could not distinguish one Network Access Service
   (NAS) from another, so attacks where one NAS impersonated another
   were out-of-scope.  However channel bindings add this capability and
   thus expands the threat model of EAP.  The GSS-EAP mechanism requires
   distinguishing one service from another.

   A relatively untrusted service, say a print server, has been
   compromised.  A user is attempting to connect to a trusted service
   such as a financial application.  Both the print server and the
   financial application use an Authentication, Authorization and
   Accounting protocol (AAA) to transport EAP authentication back to the
   user's EAP server.  The print server mounts a man-in-the-middle
   attack on the user's connection to the financial application and
   claims to be the application.

   The print server offers a tunnel method towards the peer.  The print
   server extracts the inner method from the tunnel and sends it on
   towards the AAA server.  Channel binding happens at the tunnel method
   though.  So, the print server is happy to confirm that it is the
   financial application.  After the inner method completes, the EAP
   server sends the MSK to the print server over the AAA protocol.  If
   only the MSK is needed for cryptographic binding then the print
   server can successfully perform cryptographic binding and may be able
   to impersonate the financial application to the peer.

Hartman, et al.           Expires July 9, 2013                  [Page 5]
Internet-Draft            Mutual Crypto Binding             January 2013

       Peer                Attacker         Service         AAA Server
        |                     |                 |                |
        |                     |                 |                |
        |Peer Initiates connection to a Service |                |
        |---------------------+----X----------->|                |
        |   (Intercepted by an attacker)        |                |
        |                     |                 |                |
        |                     |                 |                |
        | Tunnel Establishment|                 |                |
        |<------------------->|                 |                |
        |.....................|                 |                |
        |       Tunnel        |                 |                |
        |                     |                                  |
        |      Tunneled       |             Non-Tunneled         |
        |       Method        |        Authentication Method     |
        |<===================>|<================================>|
        |                     |(Same as Inner Method from Tunnel)|
        |.....................|                 |                |
        |                     |                 |                |
        |        Peer         |                 |                |
        |    Connected to     |<----------------------MSK keys --|
        |      Attacker       |                 |                |
        |<------------------->|                 |                |
        |                     |                 |                |

   A modified tunnel attack in which an extra server rather than extra
   client is inserted.

        Figure 2: Channel Binding Requires More than Crypto Binding

   This attack is not specific to GSS-EAP.  The channel bindings
   specification [I-D.ietf-emu-chbind] describes a number of situations
   where channel bindings are important for network access.  In these
   situations one NAS could impersonate another by using a similar
   attack.

Hartman, et al.           Expires July 9, 2013                  [Page 6]
Internet-Draft            Mutual Crypto Binding             January 2013

3.  The Server insertion Attack

   The previous section described an example of the server insertion
   attack.  In this attack, one party adds a layer of tunneling such
   that from the perspective of the EAP peer, there are more methods
   than from the perspective of the EAP server.  This attack is most
   beneficial when the party inserting the extra tunnel is a legitimate
   NAS, so mitigations need to be able to prevent a legitimate NAS from
   inappropriately adding a layer of tunneling.  Some deployments
   utilize an intentional intermediary that adds an extra level of EAP
   tunneling between the peer and the EAP server; see Section 3.3 for a
   discussion.

3.1.  Conditions for the Attack

   For an inserted server attack to have value, the attacker needs to
   gain an advantage from its attack.  An advantage to the attacker
   could come from:

   o  The attacker can send information to a peer that the peer would
      trust from the EAP server but not the attacker.  Examples of this
      include channel binding responses.

   o  The peer sending information to the attacker that was intended for
      the EAP server.  For example, the inner user identity may disclose
      privacy-sensitive information.  The channel binding request may
      disclose what service the peer wishes to connect to.

   o  The attacker may influence session parameters.  For example, if
      the attacker can influence the MSK, then the attacker may be able
      to read or influence session traffic and mount an attack on the
      confidentiality or integrity of the resulting session.

   o  An attacker may impact availability of the session.  In practice
      though, an attacker that can mount a server insertion attack is
      likely to be able to impact availability in other ways.

   For this attack to be possible, the following conditions need to
   hold:

   1.  The attacker needs to be able to establish a tunnel method with
       the peer over which the peer will authenticate.

   2.  The attacker needs to be able to respond to any inner
       authentication.  For example an attacker who is a legitimate NAS
       can forward the inner authentication over AAA towards the EAP
       server.  Note that the inner authentication may not be EAP.

Hartman, et al.           Expires July 9, 2013                  [Page 7]
Internet-Draft            Mutual Crypto Binding             January 2013

   3.  Typically, the attacker needs to be able to complete the tunnel
       method after inner authentication.  This may not be necessary if
       the attacker is gaining advantage from information sent by the
       peer over the tunnel.

   4.  In some cases the attacker may need to complete a Secure
       Association Protocol (SAP) or otherwise demonstrate knowledge of
       the MSK after the tunnel method successfully completes.

   Attackers who are legitimate NASes are the primary focus of this
   memo.  Previous work has provided mitigation against attackers who
   are not a NAS; these mitigations are briefly discussed.

3.2.  Mitigation Strategies

   In this section, the strategies of mitigating the security risks
   discussed in Section 2 are introduced.

3.2.1.  Server Authentication

   If a peer confirms the identity of the party that the tunnel method
   is established with, the peer prevents the first condition (the
   attacker establishing a tunnel method).  Many tunnel methods rely on
   TLS [RFC5281] [I-D.ietf-emu-eap-tunnel-method].  The specifications
   for these methods tend to encourage or mandate certificate checking.
   If the TLS certificate is validated back to a trust anchor and the
   identity of the tunnel method server confirmed, then the first attack
   condition cannot be met.

   Many challenges make server authentication difficult.  For instance,
   there is not an obvious naming mechanism by which to identify a
   tunnel method server and its associated functionality.  It is also
   not obvious where in the tunnel server certificate the name should be
   found.  One particularly problematic practice is to use a certificate
   that names the host on which the tunnel server runs.  Given such a
   name it is very difficult for a peer to understand whether that
   server is intended to be a tunnel method server for the realm.

   Moreover, it's not clear what trust anchors to use for tunnel
   servers.  Using commercial Certificate Authorities (CAs) is probably
   undesirable because tunnel servers often operate in a closed
   community and are often provisioned with certificates issued by that
   community.  Using commercial CAs can be particularly problematic with
   peers that support hostnames in certificates.  Then anyone who can
   obtain a certificate for any host in the domain being contacted can
   impersonate a tunnel server.

   These difficulties lead to poor deployment of good certificate

Hartman, et al.           Expires July 9, 2013                  [Page 8]
Internet-Draft            Mutual Crypto Binding             January 2013

   validation.  Many peers make it easy to disable certificate
   validation.  Other peers validate back to trust anchors but do not
   check names of certificates.  What name types are supported and what
   configuration is easy to perform depends significantly on the peer in
   question.

   Specifications also make the problem worse.  For example [RFC5281]
   indicates that the only impact of failing to perform certificate
   validation is that the inner method can be attacked.  Administrators
   and implementors believing this claim may believe that protection
   from passive attacks is sufficient.

   In addition, some deployments such as provisioning or strong inner
   methods are designed to work without certificate validation.Thsi
   requirement is discussed in Section 3.9 of the tunnel requirements
   [I-D.ietf-emu-eaptunnel-req] .

3.2.2.  Server Policy

   A server policy can potentially prevent the second condition
   (attacker being able to respond to inner authentication) from being
   possible.  If the server only performs a particular inner
   authentication within a tunnel, then the attacker cannot gain a
   response to the inner authentication without their being such a
   tunnel.  The attacker may be able to add a second layer of tunnels;
   see Figure 3.  The inner tunnel may limit the attacker's
   capabilities; for example if channel binding is performed over tunnel
   t2 in the figure, then an attacker cannot observe or influence it.

Hartman, et al.           Expires July 9, 2013                  [Page 9]
Internet-Draft            Mutual Crypto Binding             January 2013

       Peer                Attacker         Service         AAA Server
        |                     |                 |                |
        |                     |                 |                |
        |Peer Initiates connection to a Service |                |
        |---------------------+----X----------->|                |
        |   (Intercepted by an attacker)        |                |
        |                     |                 |                |
        |                     |                 |                |
        | Tunnel Establishment|                 |                |
        |<------------------->|                 |                |
        |.....................|                 |                |
        |       Tunnel t1     |                 |                |
        |                     |                 |                |
        |.......................................... .............|
        |                        Tunnel t2                       |
        |                                                        |
        |                                                        |
        |                       Inner Method                     |
        |<======================================================>|
        |                                                        |
        |.......................................... .............|
        |                     |                 |                |
        |.....................|                 |                |
        |                     |                 |                |
        |        Peer         |                 |                |
        |    Connected to     |<----------------------MSK keys --|
        |      Attacker       |                 |                |
        |<------------------->|                 |                |
        |                     |                 |                |

   A tunnel t1 from the peer to the attacker contains a tunnel t2 from
   the peer to the home EAP server.  Inside t2 is an inner
   authentication.

               Figure 3: Multiple Layered           Tunnels

   A peer policy can be combined with this server policy to help prevent
   conditions 1 (attacker can establish a tunnel the peer will use) and
   2 (attacker can respond to inner authentication).  If the peer
   requires exactly one tunnel of a particular type and the EAP server
   only performs inner authentication over a tunnel of this type, then
   the attacker cannot establish tunnel t1 in the figure above.

   An attacker may be able to mount a more traditional man-in-the-middle
   attack in this instance; see Figure 4.  This policy on the peer and
   EAP server combined with a tunnel method that supports cryptographic
   binding will allow the EAP server to detect the attacker.  This means

Hartman, et al.           Expires July 9, 2013                 [Page 10]
Internet-Draft            Mutual Crypto Binding             January 2013

   the attacker cannot act as a legitimate NAS and in particular does
   not obtain the MSK.  So, if the tunnel between the attacker and peer
   also requires cryptographic binding and if the cryptographic binding
   requires both the EAP server and peer to prove knowledge of the inner
   MSK, then the authentication will fail.  If cryptographic binding is
   not performed, then this attack may succeed.
            Please view in a fixed-width font such as Courier.

        Peer                Attacker         Service         AAA Server
         |                     |                 |                |
         |                     |                 |                |
         |Peer Initiates connection to a Service |                |
         |---------------------+----X----------->|                |
         |   (Intercepted by an attacker)        |                |
         |                     |                 |                |
         |                     |                 |                |
         | Tunnel Establishment|       Tunnel Establishment       |
         |<------------------->|<-------------------------------->|
         |.....................|.................... .............|
         |       Tunnel 1      |             Tunnel 2             |
         |                     |                                  |
         |      Tunneled       |                                  |
         |       Method        |        Tunneled Method           |
         |<===================>|<================================>|
         |                     |                                  |
         |.....................|..................................|
         |                     |                 |                |
         |        Peer         |                 |                |
         |    Connected to     |                 |                |
         |      Attacker       |                 |                |
         |<------------------->|                 |                |
         |                     |                 |                |

   A tunnel t1 extends from the peer to the attacker. a tunnel t2
   extends from the attacker to the home EAP server.  An inner EAP
   authentication is forwarded unmodified by the attacker from t1 to t2.
   The attacker can observe this inner authentication.

            Figure 4: A Traditional    Man-in-the-Middle Attack

   Cryptographic binding is only a valuable component of a defense if
   the inner authentication is a key-deriving EAP method.  Most tunnel
   methods also support non-EAP inner authentication such as Microsoft
   Chap version 2 [RFC2759].  This may undermine cryptographic binding
   in a number of ways.  An attacker may be able to convert an EAP
   method into a compatible non-EAP form of the same credential to
   suppress cryptographic binding.  In addition, an inner authentication

Hartman, et al.           Expires July 9, 2013                 [Page 11]
Internet-Draft            Mutual Crypto Binding             January 2013

   may be available through an entirely different means.  For example, a
   Lightweight Directory Access Protocol [RFC4510] or other directory
   server may provide an attacker a way to get challenges and provide
   responses for an authentication mechanism entirely outside of the
   AAA/EAP context.  An attacker with this capability may be able to get
   around server policy requiring an inner authentication be used only
   in a given type of tunnel.
   An attacker can convert an inner authentication using an EAP method
   to a inner authentication that does not use EAP in some cases.  This
   may avoid cryptographic binding.

                    Converting EAP Inner Authentication

   An attacker may contact another authentication resource to gain a
   challenge useful for an inner authentication.

                  Non-EAP Sources of Inner Authentication

   To Recap, the following policy conditions appear sufficient to
   prevent a server insertion attack:

   1.  Peer and EAP server require a particular inner EAP method used
       within a particular tunnel method

   2.  The inner EAP method's authentication is only available within
       the tunnel and through no other means including non-EAP means

   3.  The inner EAP method produces a key

   4.  The tunnel method uses cryptographic binding and the peer
       requires the other end of the tunnel to prove knowledge of the
       inner MSK.

3.2.3.  Existing Cryptographic Binding

   The most advanced examples of cryptographic binding today work at two
   levels.  First, the server and peer prove to each other knowledge of
   the inner MSK.  Then, the inner MSK is combined into some outer key
   material to form the tunnel's keys.  This is sufficient to detect an
   inserted server or peer provided that the attacker does not learn the
   inner MSK.  This seems sufficient to defend against attackers who
   cannot act as a legitimate NAS.

   The definition of cryptographic binding in RFC 3748 does not require
   these steps.  To meet that definition it would be sufficient for a
   peer to prove knowledge of the inner key to the EAP server.  This
   would open some additional attacks.  For example by indicating
   success an attacker might be able to mask a cryptographic binding

Hartman, et al.           Expires July 9, 2013                 [Page 12]
Internet-Draft            Mutual Crypto Binding             January 2013

   failure.  Especially if only the tunnel key material is used for the
   final keys, the peer is unlikely to be able to detect the failure.

   As discussed in the previous section, cryptographic binding is only
   effective when the inner method is EAP.

3.2.4.  IntroducingEMSK-based Cryptographic Binding

   Cryptographic binding can be strengthened when the inner EAP method
   supports an Extended Master Session Key (EMSK).  The EMSK is never
   disclosed to any party other than the EAP server or peer, so even a
   legitimate NAS cannot learn the EMSK.  So, if the same techniques
   currently applied to the inner MSK are applied to the inner EMSK,
   then condition 3 (completing tunnel authentication) will not hold
   because the attacker cannot complete this new form of cryptographic
   binding.  This does not prevent the attacker from learning
   confidential information such as a channel binding request sent over
   the tunnel prior to cryptographic binding.

   Obviously as with all forms of cryptographic binding, cryptographic
   binding only works for key-deriving inner EAP methods.  Also, some
   deployments (see Section 3.3 insert intermediates between the peer
   and the EAP server.  EMSK-based cryptographic binding is incompatible
   with these deployments because the intermediate cannot learn the
   EMSK.

   Formally, EMSK-based cryptographic binding is a security claim for
   EAP tunnel methods that holds when:

   1.  The peer proves to the server that the peer participating in any
       inner method is the same as the peer for the tunnel method.

   2.  The server proves to the peer that the server for any inner
       method is the same as the server for the tunnel method.

   3.  The MSK and EMSK for the tunnel depend on the MSK and EMSK of
       inner methods.

   4.  The peer MUST be able to force the authentication to fail if the
       peer is unable to confirm the identity of the server.

   5.  Proofs offered need to be secure even against attackers who know
       the inner method MSK.

   If EMSK-based cryptographic binding is not an optional facility it
   provides a strong defense against server insertion attacks and other
   tunnel MITM attacks for inner methods that provide an EMSK.  The
   strength of the defense is dependent on the strength of the inner

Hartman, et al.           Expires July 9, 2013                 [Page 13]
Internet-Draft            Mutual Crypto Binding             January 2013

   method.  EMSK-Based cryptographic binding MAY be provided as an
   optional facility.  The value of EMSK-based cryptographic binding is
   reduced somewhat if it is an optional feature.  It permits
   configurations where a peer uses other means to authenticate the
   server if the peer has sufficient information configured to validate
   the certificate and identity of an EAP server while using EMSK-based
   cryptographic binding for deployments where that is possible.

   If EMSK-based cryptographic binding is an optional facility, the
   negotiation of whether to use it MUST be protected by the inner MSK
   or EMSK.  Typically the MSK will be used as the primary advantage of
   making EMSK-based cryptographic binding an optional facility is to
   permit intermediates who know only the MSK to decline to use EMSK-
   based cryptographic binding.  The peer MUST have an opportunity to
   fail the authentication after the server declines to use EMSK-based
   cryptographic binding.

3.2.5.  Mix Key into Long-Term Credentials

   Another defense against tunnel MITM attacks potentially including
   server insertion attacks is to use a different credential for
   tunneled methods from other authentications.  This may prevent the
   second condition (attacker being able to respond to inner
   authentication) from taking place.  For example, if key material from
   the tunnel is mixed into a shared secret or password that is the
   basis of the inner authentication, then the second condition will not
   hold unless the attacker already knows this shared secret.  The
   advantage of this approach is that it seems to be the only way to
   strengthen non-EAP inner authentications within a tunnel.

   There are several disadvantages.  Choosing a function to mix the
   tunnel key material into the inner authentication will be very
   dependent on the inner authentication.  In addition, this appears to
   involve a layering violation.  However, exploring the possibility of
   providing a solution like this seems important because it can
   function for inner authentications where no other approach will work.

3.3.  Intended Intermediates

   Some deployments introduce a tunnel server separate from the EAP
   server; see [RFC5281] for an example of this style of deployment.
   The only difference between such an intermediate and an attacker is
   that the intermediate provides some function valuable to the peer or
   EAP server and that the intermediate is trusted by the peer.  If
   peers are configured with the necessary information to validate
   certificates of these intermediates and to confirm their identity,
   then tunnel MITM and inserted server attacks can be defended against.
   The intermediates need to be trusted with regard to channel binding

Hartman, et al.           Expires July 9, 2013                 [Page 14]
Internet-Draft            Mutual Crypto Binding             January 2013

   and other services that the peer depends on.

   Support for trusted intermediates is not a requirement according to
   the tunnel method requirements.

   It seems reasonable to treat trusted intermediates as a special case
   if they are supported and to focus on the security of the case where
   there are not intermediates in the tunnel as the common case.

Hartman, et al.           Expires July 9, 2013                 [Page 15]
Internet-Draft            Mutual Crypto Binding             January 2013

4.  Recommendations

4.1.  Mutual Cryptographic Binding

   The EAP Tunnel Method [I-D.ietf-emu-eap-tunnel-method] should gain
   support for EMSK-based cryptographic binding.

   As channel binding support is added to existing EAP methods, EMSK-
   based cryptographic binding or some other form of cryptographic
   binding that protects against server insertion should also be added
   to these methods.  Mutual cryptographic binding may also be valuable
   when other services are added to EAP methods that may require a peer
   trust an EAP server.

4.2.  State Tracking

   Today, mutual authentication in EAP is thought of as a security claim
   about a method.  However, in practice it's an attribute of a
   particular exchange.  Mutual authentication can be obtained via
   checking certificates, through mutual cryptographic binding, or in
   very controlled cases through carefully crafted peer and server
   policy combined with existing cryptographic binding.  Using services
   like channel binding that involve the peer trusting the EAP server
   should require mutual authentication be present in the session.

   to accomplish this, implementations including channel binding or
   other peer services MUST track whether mutual authentication has
   happened.  They SHOULD default to not permitting these peer services
   unless mutual authentication has happened.  They SHOULD support a
   configuration where the peer fails to authenticate unless mutual
   authentication takes place.  Discussion of whether this configuration
   should be recommended as a default is required.

   The EAP Tunnel Method should permit peers to force authentication
   failure if they are unable to perform mutual authentication.  The
   protocol should permit this to be deferred until after mutual
   cryptographic binding is considered.

   Services such as channel binding should be deferred until after
   cryptographic binding/mutual cryptographic binding.

4.3.  Certificate Naming

   Work is required to promote interoperable deployment of server
   certificate validation by peers.  A standard way to name EAP servers
   is required.  Recommendations for what name forms peers should
   implement is required.

Hartman, et al.           Expires July 9, 2013                 [Page 16]
Internet-Draft            Mutual Crypto Binding             January 2013

4.4.  Inner Mixing

   More consideration of the proposal to mix some key material into
   inner authentications is desired.  As stated today, the proposal is
   under-defined and fairly invasive.  Are there versions of this
   proposal that would be valuable?  Is there a way to view it as
   something more abstract so that it does not involve tunnel and inner
   method specific combinatorial explosion?

Hartman, et al.           Expires July 9, 2013                 [Page 17]
Internet-Draft            Mutual Crypto Binding             January 2013

5.  Survey of Tunnel Methods

Hartman, et al.           Expires July 9, 2013                 [Page 18]
Internet-Draft            Mutual Crypto Binding             January 2013

6.  Survey of Inner Methods

Hartman, et al.           Expires July 9, 2013                 [Page 19]
Internet-Draft            Mutual Crypto Binding             January 2013

7.  Security Considerations

Hartman, et al.           Expires July 9, 2013                 [Page 20]
Internet-Draft            Mutual Crypto Binding             January 2013

8.  Acknowledgements

   The authors would like to thank Alan DeKok for helping to explore
   these attacks.  Alan focused the discussion on the importance of
   inner authentications that are not EAP and proposed mixing in key
   material as a way to resolve these authentications.

   Jari Arkko provided a review of the attack and valuable context on
   past efforts in developing cryptographic binding.

   Sam Hartman's and margaret Wasserman's work on this memo is funded by
   Huawei.

Hartman, et al.           Expires July 9, 2013                 [Page 21]
Internet-Draft            Mutual Crypto Binding             January 2013

9.  References

9.1.  Normative References

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

9.2.  Informative References

   [I-D.ietf-abfab-gss-eap]
              Hartman, S. and J. Howlett, "A GSS-API Mechanism for the
              Extensible Authentication Protocol",
              draft-ietf-abfab-gss-eap-09 (work in progress),
              August 2012.

   [I-D.ietf-emu-chbind]
              Hartman, S., Clancy, T., and K. Hoeper, "Channel Binding
              Support for EAP Methods", draft-ietf-emu-chbind-16 (work
              in progress), May 2012.

   [I-D.ietf-emu-eap-tunnel-method]
              Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
              "Tunnel EAP Method (TEAP) Version 1",
              draft-ietf-emu-eap-tunnel-method-04 (work in progress),
              October 2012.

   [I-D.ietf-emu-eaptunnel-req]
              Zhou, H., Salowey, J., Hoeper, K., and S. Hanna,
              "Requirements for a Tunnel Based EAP Method",
              draft-ietf-emu-eaptunnel-req-09 (work in progress),
              December 2010.

   [I-D.ietf-nea-pt-eap]
              Cam-Winget, N. and P. Sangster, "PT-EAP: Posture Transport
              (PT) Protocol For EAP Tunnel Methods",
              draft-ietf-nea-pt-eap-06 (work in progress),
              December 2012.

   [RFC2759]  Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
              RFC 2759, January 2000.

   [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510,
              June 2006.

   [RFC4851]  Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
              Flexible Authentication via Secure Tunneling Extensible

Hartman, et al.           Expires July 9, 2013                 [Page 22]
Internet-Draft            Mutual Crypto Binding             January 2013

              Authentication Protocol Method (EAP-FAST)", RFC 4851,
              May 2007.

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

   [TUNNEL-MITM]
              "".

Hartman, et al.           Expires July 9, 2013                 [Page 23]
Internet-Draft            Mutual Crypto Binding             January 2013

Authors' Addresses

   Sam Hartman
   Painless Security

   Email: hartmans-ietf@mit.edu

   Margaret Wasserman
   Painless Security

   Email: mrw@painless-security.com
   URI:   http://www.painless-security.com/

   Dacheng Zhang
   Huawei

   Email: zhangdacheng@huawei.com

Hartman, et al.           Expires July 9, 2013                 [Page 24]