


Network Working Group                                         Y. Sheffer
Internet-Draft                                               Check Point
Intended status: Informational                              July 6, 2008
Expires: January 7, 2009


         Using EAP-GTC for Simple User Authentication in IKEv2
                     draft-sheffer-ikev2-gtc-00.txt

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Abstract

   Despite many years of effort, simple username-password authentication
   is still prevalent.  In many cases a password is the only credential
   available to the end user.  IKEv2 uses EAP as a sub-protocol for user
   authentication.  This provides a well-specified and extensible
   architecture.  To this day EAP does not provide a simple password-
   based authentication method.  The only existing password
   authentication methods either require the peer to know the password
   in advance (EAP-MD5), or are needlessly complex when used within
   IKEv2 (e.g.  PEAP).  This document codifies the common practice of
   using EAP-GTC for this type of authentication, with the goal of
   achieving maximum interoperability.  The various security issues are
   extensively analyzed.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Alternatives to EAP-GTC in IKEv2  . . . . . . . . . . . . . . . 4
     3.1.  Non-password credentials  . . . . . . . . . . . . . . . . . 4
     3.2.  Using the IKE preshared secret  . . . . . . . . . . . . . . 4
     3.3.  EAP-MD5 , EAP-MSCHAPv2 and mutual authentication
           schemes . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Using EAP-GTC in IKE: Details . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
     6.1.  Key generation and MITM protection  . . . . . . . . . . . . 6
     6.2.  Protection of credentials between the IKE gateway and
           the AAA server  . . . . . . . . . . . . . . . . . . . . . . 6
     6.3.  Server authentication . . . . . . . . . . . . . . . . . . . 6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . . . 8
     A.1.  -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9



























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1.  Introduction

   "Oh dear!  It's possible that we have added EAP to IKE to support a
   case that EAP can't support." -- C. Kaufman.

   Despite many years of effort, simple username-password authentication
   is still prevalent.  In many cases a password is the only credential
   available to the end user.

   IKEv2 [RFC4306] uses the Extensible Authentication Protocol (EAP) as
   a sub-protocol for user authentication.  This provides a well-
   specified and extensible architecture and enables useful capabilities
   like SIM authentication.  Unfortunately, for a number of reasons EAP
   still does not provide a simple password-based authentication method.
   The only existing password authentication methods either require the
   peer to know the password in advance (EAP-MD5), or are needlessly
   complex when used within IKEv2 (e.g.  PEAP).

   Technically, the IKE preshared secret authentication mode can be used
   for password authentication.  In fact even the IKEv2 RFC winks at
   this practice.  But this use jeopardizes the protocol's security and
   should clearly be avoided (more details below).

   EAP is used in IKEv2 at a stage when the remote access gateway has
   already been authenticated.  At this point the user has a high enough
   level of trust to send his or her password to the gateway.  Such an
   exchange is enabled by the EAP Generic Token Card (GTC) method, which
   is a simple text transport between the two EAP peers.  To quote
   [RFC3748]:

      The EAP GTC method is intended for use with the Token Cards
      supporting challenge/response authentication and MUST NOT be used
      to provide support for cleartext passwords in the absence of a
      protected tunnel with server authentication.

   IKEv2 does indeed provide "a protected tunnel with server
   authentication".  The current document updates [RFC3748] by making an
   exception and allowing the use of GTC to carry secret credentials, in
   this specific situation.  Section 6 further elaborates on the
   security properties of this solution.

   Other protocols provide a similar protected tunnel, for example TLS-
   EAP, described in [I-D.nir-tls-eap].  These protocols however are out
   of scope for this document.







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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  Alternatives to EAP-GTC in IKEv2

   This section presents a few of the alternatives to EAP-GTC, and
   explains why they are either insecure or impractical given today's
   common identity management infrastructure.

3.1.  Non-password credentials

   Certificate-based authentication, especially when combined with
   hardware protection (e.g. a hardware token), can be deployed in a
   more secure manner than the form of password authentication which we
   discuss.  However, due to a host of issues to do with cost,
   inconvenience and reliability this solution has not gained wide
   market acceptance over the last 10 years.

3.2.  Using the IKE preshared secret

   Sec. 2.15 of RFC 4306 points out that the generation of the IKE
   preshared secret from a weak password is insecure.  Such use is
   vulnerable to off line password guessing by an active attacker.  All
   the attacker needs to do is respond correctly to the first IKE_INIT
   message, and then record the third IKE message.  This is then
   followed by a dictionary attack to obtain the password.

3.3.  EAP-MD5 , EAP-MSCHAPv2 and mutual authentication schemes

   Challenge-response schemes, like EAP-MD5 and EAP-MSCHAPv2, have a
   clear security advantage over sending the plaintext password to the
   gateway.  Password-based mutual authentication schemes like SRP have
   a further advantage in that the gateway's authentication is much
   stronger than when using certificates alone, since the AAA server
   proves its knowledge of a per-client credential, and the gateway
   proves that it has been authorized by the AAA server for that
   particular client.

   Unfortunately all of these methods also suffer from a major drawback:
   the gateway must have a priori access to the plaintext password.
   While many RADIUS servers may indeed have such access, other very
   common deployments do not provide it.  One typical example is when
   the gateway directly accesses an LDAP directory (or a Microsoft
   Active Directory) to authenticate the user.  The usual way to do that



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   is by issuing an LDAP Bind operation into the directory, using the
   just-received plaintext password.  Often in this case it is the IKE
   gateway that terminates the EAP protocol, and it needs a way to
   obtain the raw password.

   An additional issue with mutual authentication schemes is their heavy
   IP encumbrance, which has resulted in a scarcity of standards using
   them and a low rate of market adoption.


4.  Using EAP-GTC in IKE: Details

   EAP-GTC is specified in [RFC3748], Sec. 5.6.  This section is non-
   normative, and is merely an interpretation of this specification in
   the context of IKEv2.

   Simple authentication requires a non secret identity ("user name")
   and a secret credential ("password").  Both of these are arbitrary
   Unicode strings, although implementations may impose length
   constraints.

   In the case of EAP-GTC, the user name is conveyed in the IKE IDi
   payload.  According to [RFC4718], Sec. 3.4, the user name can be
   encoded in one of two ways: as a simple user name, in which case the
   ID_KEY_ID identification type is used; or as a combination user name
   plus realm, in which case the format is a NAI [RFC4282] and the
   identification type is ID_RFC822_ADDR.  In either case, the user name
   is a Unicode string encoded as UTF-8.  Using the EAP Identity payload
   is redundant, and if it is used, it should be identical to the IDi
   payload.

   EAP-GTC consists of a simple 2-message exchange.  The contents of the
   Type-Data field in the Request should not be interpreted in any way,
   and should be displayed to the user.  This field contains a Unicode
   string, encoded as UTF-8.

   The password is sent in the EAP Response.  The Type-Data field of the
   Response is also a Unicode string encoded as UTF-8.  Note that none
   of the IDi payload, the EAP Request or the EAP Response is null-
   terminated.

   If either or both the user name and the password are non-ASCII, they
   should be normalized by the IKE client before the IKE/EAP message is
   constructed.  The normalization method is SASLprep, [RFC4013].







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5.  IANA Considerations

   This document does not require any action by IANA.


6.  Security Considerations

6.1.  Key generation and MITM protection

   Modern EAP methods generate a key shared between the two protocol
   peers.  GTC does not (and cannot) generate such a key.  RFC 4306
   mandates that:

      EAP methods that do not establish a shared key SHOULD NOT be used,
      as they are subject to a number of man-in-the-middle attacks
      [EAPMITM] if these EAP methods are used in other protocols that do
      not use a server-authenticated tunnel.

   However GTC must never be used in such a situation, since the client
   would be sending its credentials openly to an unauthenticated server.
   When using GTC with IKEv2, the implementation (or local
   administrators) MUST ensure that the same credentials are never used
   in such a manner.

6.2.  Protection of credentials between the IKE gateway and the AAA
      server

   In the proposed solution, the raw credentials are sent from the IKE
   gateway to a AAA server, typically a RADIUS server.  These
   credentials and the associated messaging MUST be strongly protected.
   Some of the existing options include:
   o  An IPsec tunnel between the gateway and the AAA server.
   o  RADIUS over TCP with TLS, [I-D.winter-radsec].
   o  RADIUS over UDP with DTLS, [I-D.dekok-radext-dtls] (expired).
   The legacy RADIUS security mechanism (Sec. 5.2 of [RFC2865]) is
   considered weak and SHOULD NOT be used when better alternatives are
   available.

6.3.  Server authentication

   The client may only send its cleartext credentials after it has
   positively authenticated the server.  This authentication is
   specified, albeit rather vaguely, in [RFC4306] and is out of scope of
   the current document.  Unauthenticated (BTNS) derivatives of IKE MUST
   NOT be used with EAP-GTC.






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7.  Acknowledgments

   I would like to thank Yoav Nir and Charlie Kaufman for their helpful
   comments.


8.  References

8.1.  Normative References

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

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

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

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

8.2.  Informative References

   [EAPMITM]  Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
              in Tunneled Authentication Protocols", November 2002,
              <http://eprint.iacr.org/2002/163>.

   [I-D.dekok-radext-dtls]
              DeKok, A., "DTLS as a Transport Layer for RADIUS",
              draft-dekok-radext-dtls-00 (work in progress),
              February 2007.

   [I-D.nir-tls-eap]
              Nir, Y., Tschofenig, H., and P. Gutmann, "TLS using EAP
              Authentication", draft-nir-tls-eap-03 (work in progress),
              April 2008.

   [I-D.winter-radsec]
              Winter, S., McCauley, M., and S. Venaas, "RadSec Version 2
              - A Secure and Reliable Transport for the RADIUS
              Protocol", draft-winter-radsec-01 (work in progress),
              February 2008.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.



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

   [RFC4718]  Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
              Implementation Guidelines", RFC 4718, October 2006.


Appendix A.  Change Log

A.1.  -00

   Initial version.


Author's Address

   Yaron Sheffer
   Check Point Software Technologies Ltd.
   5 Hasolelim St.
   Tel Aviv  67897
   Israel

   Email: yaronf@checkpoint.com




























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