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Versions: 00

Internet Draft                                     Jesse Walker
Expiration: December 2002                          Intel Corporation
File: draft-walker-aaa-key-distribution-00.txt     Russ Housley
                                                   RSA Labs
                                                   Nancy Cam-Winget




                    AAA Key Distribution
                Last Updated: April 15, 2002


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of [RFC2026].  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
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   Copyright (C) The Internet Society (2002). All Rights Reserved.


Abstract

   This memo describes problems with the current AAA NASREQ key
   distribution mechanisms, and proposes enhancements to the NASREQ key
   distribution model to address these problems.

   Please send comments on this document to the aaa-wg@merit.edu mailing
   list.


1.  Introduction

   The IETF AAA Working Group is developing solutions for
   Authentication, Authorization and Accounting as applied to network
   access.  The AAA Working Group has defined protocols addressing the
   network access needs specified by the NASREQ, MOBILE IP, and ROAMOPS


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   Working Groups as well as TIA 45.6.  The solution specified by the
   AAA Working Group is also thought to address the needs of IEEE
   802.11 wireless networks.  The solution is intended to augment and
   eventually replace RADIUS.

   One area under the AAA Working Group's purview is the subject of
   session key distribution.  For the purposes here, a session key is a
   cryptographic key that is used either directly or indirectly to
   protect traffic exchanged over a link between a Network Access Server
   (NAS) and one of its clients.  [NASREQ] describes the key exchange
   mechanism.  This document analyzes the NASREQ key distribution
   mechanism, identifies a number of security weaknesses, and proposes
   some enhancements that rectify the identified weaknesses.


1.1.  Requirements Terminology

   Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
   "MAY" that appear in this document are to be interpreted as described
   in [RFC2119].


2.  Description of NASREQ Key Distribution

   [NASREQ] defines session key distribution from an AAA server to a
   NAS.  The model (or models) it uses are never explicitly specified,
   but it is possible to infer the model from the documentation
   available.

   The purpose of NASREQ key distribution is to securely establish a
   session key between the NAS and the NAS client.  The AAA server may
   distribute more than one key to the NAS, each to provide different
   functions.  For instance, it can distribute both encryption and data
   authenticity keys, or send and receive keys, or master keys that can
   be used to derive other keys.

   [NASREQ] allows the AAA server to distribute a key to the NAS client
   using EAP, but does not specify how this is accomplished.  EAP fails
   to specify mechanisms.  As a result, all mechanisms assume that the
   AAA server and the NAS client already share a key that may be used
   directly to protect the link between the NAS and the NAS client, and
   so it is unnecessary to distribute any key to the NAS client.  Since
   no specified instances of EAP key distribution to the NAS client
   exist, the implicit assumption has to be that such mechanisms are
   unimportant and will not be deployed as part of the AAA architecture.
   That is, the lack of any defined EAP key distribution mechanism, and
   the further lack of any work on such a mechanism, implies that the
   AAA Working Group implicitly assumes it is only necessary to
   distribute the key to the NAS.  This de facto NASREQ key distribution
   architecture is the object of our critique.

   To effect key distribution, at some (unspecified) point in the
   authentication process, the AAA server decides to distribute a key to
   the NAS.  This is accomplished by inserting a NAS-Session-Key AVP in
   an Answer packet sent by the AAA server to the NAS.  Each distributed
   key is included in its own NAS-Session-Key AVP.



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   The NAS-Session-Key AVP has the following structure:

      NAS-Session-Key ::= < AVP Header: 408 >
                          { NAS-Key-Direction }
                          { NAS-Key-Type }
                          { NAS-Key }
                          { NAS-Key-Data }
                          [ NAS-Key-Binding ]
                          [ NAS-Key-Lifetime ]
                          [ NAS-IV ]

   Here

     o  NAS-Key-Direction tells whether the key is bi-directional, used
        for upstream communication, or for downstream communication;

     o  NAS-Key-Type tells whether the key is a cipher key, an
        integrity key, or some type of master key, used for further key
        derivation;

     o  NAS-Key is never defined;

     o  NAS-Key-Data is the distributed key itself;

     o  NAS-Key-Binding indicates the cryptographic primitive to use
        with the key, and it is optional;

     o  NAS-Key-Lifetime gives the number of seconds remaining before
        the key expires, and it is optional; and

     o  NAS-IV may be used as an initialization vector, and it is
        optional.

   To protect the key distribution, [NASREQ] permits three different
   mechanisms:

   1. IPsec can be used to protect the entire Answer packet conveying
      the NAS-Session-Key AVP.  In this case, ESP would be used to
      provide confidentiality, integrity, and data origin
      authentication.

   2. TLS can be used to protect the entire Answer packet, or TLS can
      be used to protect a portion of the packet.

   3. CMS can be used to envelope any included NAS-Session-Key AVPs.

   [NASREQ] specifies no mandatory-to-implement protections for the key
   distribution.

3.  Analysis of NASREQ Key Distribution

   This section argues there are five fundamental problems with the
   model assumed by the NASREQ key distribution mechanism:

   1. It is incompatible with static keys.




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   2. It has an inadequate, non-existent key naming scheme, which
      opens the mechanism to numerous abuses.

   3. It provides inadequate protection for distributed keys.

   4. It introduces a novel key distribution architecture with poorly
      understood security properties, while key distribution is an
      area bedeviled by subtle bugs.

   5. It is not feasible to design the key distribution exchange
      between the AAA server and the NAS in isolation of any
      associated handshaking between the NAS and the NAS client.


3.1.  Architecture incompatible with static keys

   In the de facto key distribution model described above, the key
   distributed by the AAA server to the NAS must not ever be used again,
   with either the same or with any other NAS.  To permit the
   distributed key to be reused with the same NAS, both the NAS and the
   NAS client would have to maintain a record of the consumed sequence
   spaces, nonce spaces, and replay windows used with the key, to
   prevent inadvertent compromise on reuse, something that is not in
   general feasible or desirable.  To permit the key to be used by a
   different NAS, a mechanism to prevent the original NAS from using the
   key to masquerade as the NAS client would be needed.  Some people may
   argue that this latter problem is not a genuine concern because the
   NAS is trusted.  However, the consequences of NAS compromise must be
   considered.  The NAS is not immune from compromise; within the past
   year alone, for example, the Code Red virus and SNMP buffer overrun
   alert have applied to nearly every NAS, and almost all NASes required
   patches as a consequence.  Therefore, NASREQ key distribution cannot
   be used with static keys; all distributed keys must be fresh, never-
   used-before keys.

   Another approach exists that can safely employ static keys.  Under
   this approach the static key remains a secret shared only between the
   AAA server and the NAS client; it is never shared with the NAS.  The
   AAA server employs this static key to distribute a key to the NAS
   client at the same time it distributes a key to the NAS.  The cost of
   this increased flexibility is that the AAA server generates a random
   key and distributes it both to the NAS client and to the NAS, not
   just the NAS, as in the present de facto architecture.

   Proponents of the NASREQ approach might argue that the use of
   protocols like TTLS or PEAP will alleviate this problem.  They reason
   that TTLS derives a fresh key at initial contact, and TLS-resume can
   be used to derive another fresh key at the time of reattachment.
   This is an attractive line of thinking, but suffers from two
   problems.

   The first problem is that organizations deploying TTLS or PEAP still
   have to obtain an X.509 certificate for their AAA servers and deploy
   a trust anchor that allows the X.509 certificate to be validated on




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   the NAS client.  The trust anchor only contains public data, but
   integrity must be maintained.  If the trust anchor can be altered,
   then the NAS clients cannot properly authenticate the AAA server, and
   the NAS clients are subject to rogue NASes.  This trust anchor
   provisioning can be costly.  It is probably an unacceptable cost for
   many simple deployments, especially ad hoc wireless networks.

   The second problem with this reasoning is that it leads to a more
   complex exchange between the NAS client and the AAA server than is
   actually necessary.  An approach distributing the derived key only to
   the NAS requires at least a three packet exchange between the NAS
   client and the AAA server (EAP-TLS actually requires a four packet
   exchange for TLS-resume) to generate a fresh key, while an approach
   distributing a fresh randomly generated key to both the NAS and the
   NAS client requires only a two packet exchange.  This means that at
   the time of a reattachment, when an existing key may be used, the de
   facto architecture implementation is 50% more complex than necessary.
   A 50% reduction in complexity is a giant win for security analysis,
   and, when amortized over all reattachments, it provides a significant
   performance enhancement for the case that is most time critical.

   This is not to say that TTLS and PEAP do not provide any value.  The
   argument is rather they do not solve the problem being raised here.

   Presumably, the NASREQ vision can be completed by enhancing EAP to
   distribute an analog of the NAS-Session-Key AVP, delivering a session
   key to the NAS client.  Mandating this usage going forward would
   improve flexibility by restoring the use of static keys, and would
   simplify the overall system design.


3.2.  Inadequate key naming

   Since NASREQ did not tackle the fundamental issue of key freshness,
   it also fails to address the issue of binding keys to particular
   sessions between particular entity pairs.  The protocol fails to
   explicitly name the distributed keys.  This is a much more serious
   problem than the failure to work with static keys, because the
   history of key distribution protocols shows that failing to properly
   identify keys presents a major opportunity for compromise of the
   distributed keys.  It is a major vulnerability exploited to launch
   attacks.

   To prevent these kinds of weaknesses, it is necessary to specify both
   the NAS client and the NAS identities in the key distribution, to
   allow their peers to detect when either cheats or uses the key with
   unintended parties.  It is further necessary to explicitly bind the
   key to a particular session between the NAS and the NAS client, to
   detect key reuse problems.

   The sort of key naming required represents an assertion by the AAA
   server that the parties may not use the key with any other party, nor
   with a different session.  Thus, the only reasonable identification
   has to include the AAA identities of the intended pair of peers and
   some session identifier.  This is not needless overhead.




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   The NAS-Session-Key AVP can be easily enhanced to provide this
   additional information.


3.3.  Inadequate protections for NAS-Session-Key AVPs

   The NAS-Session-Key AVP does not require any explicit mandatory
   protection.  Instead, the NASREQ specification permits
   implementations to rely upon TLS or IPsec to protect the AVP.  The
   problem with this approach is a practical implementation necessity:
   AAA server implementations are typically software only, running under
   general-purpose operating systems; many NAS devices are likewise
   based on public domain UNIX implementation.  In either environment it
   is usually easy for an attacker to insert a Trojan horse that
   intercepts the data stream between modules, e.g., between the TCP/IP
   stack and the socket layer.  Such a Trojan horse can read and replace
   distributed keys if the cryptographic protections are applied by a
   separate module.  This defect in the design may be remedied by
   mandating that the NAS-Session-Key AVP be CMS encapsulated and design
   due diligence applied to keep the cryptographic operations from
   crossing module boundaries.

   The issue is deeper than just the layer at which the cryptographic
   protections are applied.  To minimize the immunity of a key
   distribution protocol to attack, it is necessary to explicitly bind
   together the information in a way that the recipient of the
   distributed key can validate, and this binding typically crosses
   message boundaries and often must even relate to messages from all
   three parties in the protocol; section 3.5 below touches further on
   this theme.  These are application protocol issues, and it is simply
   not reasonable to expect that bilateral mechanisms at other layers
   can effectively solve these problems.  [NASREQ] as it stands today
   does not exhibit any evidence that this issue has even been
   contemplated.  For instance, the relationship between information in
   AAA Request messages and Answer messages is never spelled out, being
   left entirely as a method-specific detail.  For some aspects of key
   distribution to work properly, this relationship has to be defined.
   Key distribution is not merely a data transport operation; it is also
   a mechanism for building transitive trust; it is simply infeasible to
   specify a secure key distribution without binding data across several
   messages.

   As an example of this complaint, even CMS wrapping the NAS-Session-
   Key AVP does not explicitly protect the key distribution from replay.
   Thus, although the NAS itself can assume a CMS-wrapped key is genuine
   and issued from the AAA server, the NAS cannot determine that the AVP
   it receives has not been issued already for some prior session.
   Instead, the NAS must assume that the AAA server is playing by the
   rules and issuing only fresh keys, and that there is no man-in-the-
   middle replacing AVPs before or after they are unwrapped by a lower-
   layer security mechanism.  Many session establishment handshakes do
   not define adequate key confirmation handshakes, so the NAS could end
   up sending new data encrypted under already-used keys and IVs before
   the problem can be detected, potentially compromising previously sent
   data.  What is needed to defeat this kind of attack is to require the



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   AAA server to incorporate a challenge from the NAS (and/or the NAS
   client) into the NAS-Session-Key AVP prior to wrapping. (Inserting a
   timestamp into the AVP is another option, but maintaining
   synchronized time in many of the environments served by an AAA server
   introduces its own problems.)  Relying on TLS or IPsec does not solve
   the problem, because the replay protection afforded by such low-level
   mechanisms is not adequately bound to the AVP to prevent a Trojan
   horse from substituting an old AVP for the new one without detection.

   While it was poorly implemented, the RADIUS authenticator played a
   useful role.  It allowed the NAS to detect replay.  By removing the
   authenticator when going forward to DIAMETER, AAA created a
   structurally weaker protocol than RADIUS.  This omission is an
   opportunity, however, because it would be better to include an
   authenticator field in the NAS-Session-Key AVP than as part of the
   larger Answer packet, so it may be more tightly bound to the
   distributed key.

   Finally, the AVP wrapping algorithm is not specified with sufficient
   granularity.  Key distribution protocols make very specific
   assumption about what is encrypted, what is authenticated but not
   encrypted, and what must be sent without any protection.  [NASREQ]
   appears to apply the same protection to the entire AVP.  This again
   argues in favor of a mandatory application-specific security
   protocol.


3.4.  Novel solution to a problem bedeviled by subtle failures

   The cryptographic community has not studied the de facto
   architecture.  Key distribution as defined in [NASREQ] is a three
   party protocol, with the AAA server, the NAS, and the NAS client all
   parties with an interest in the exchange.  Cryptographers have
   defined protocols that are known to protect the interests of all
   three parties in such an exchange, but the NASREQ key distribution
   does not resemble any of these well-studied protocols.  This is
   particularly troubling, because three-party key distribution of this
   sort appears to be one of the hardest problems cryptographers have
   attempted to address.  Almost all of the initial proposals to this
   problem have been flawed.  There are examples where minor flaws have
   remained undiscovered for 20 years after the protocol was published.
   This repeated history of failure puts a premium on the most
   conservative possible design, restricting it to well scrutinized
   protocols.

   It may be possible to address many of the problems discussed here
   without adopting a classical, well-studied three-party protocol.  It
   may be, for instance, feasible to clean up the NASREQ architecture to
   securely distribute keys in an environment where TTLS or PEAP is
   mandated.  However, many years of analysis and scrutiny may be
   necessary to develop the confidence that this kind of approach is
   secure from practical attack.  It is safer to deploy a protocol known
   to work correctly than to gamble that the novel design won't be
   broken after the NASREQ application is widely deployed in the
   Internet.



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3.5.  Key Distribution Protocols not Properly Bound

   The IETF and the AAA WG have followed the time-honored custom of
   decomposing problems into separate pieces.  In the case of key
   distribution, the decomposition is into the NASREQ/DIAMETER exchange
   between the AAA server and the NAS on one hand, and the AAA server
   and NAS client on the other, the latter being under the purview of
   the EAP WG.  Unfortunately, this decomposition is not correct.  It is
   incorrect because it is difficult to create separate key distribution
   sub-protocols that, when reassembled into a single system, guarantee
   the security needs of all three parties involved.  A valid key
   distribution protocol requires that the sub-protocols interact in
   subtle and non-trivial ways and thus the key distribution
   specification has to span the domains of at least two Working Groups.

   As an example of this, [NASREQ] permits the distribution of several
   keys for the same function, e.g., several Master session keys.  On
   the other hand, while allowing this flexibility, it provides no means
   of indicating the sequence in which these keys are used, and when to
   change from one key to another.  The point is that, to be useful, the
   usage of the distributed key must be synchronized on the NAS and the
   NAS client, and the protocol does not provide the information
   necessary for synchronization.

   As a second example, in environments such as IEEE 802.11, an EAP-
   Success message is inappropriate to signal the open link between the
   NAS and the NAS client for general traffic.  Rather, a key
   confirmation handshake is required; it is inappropriate to open the
   link to data traffic until the peer has signaled that its session
   keys are in place.  It is not feasible to design all the details of
   the key confirmation handshake without also binding the handshake to
   the details of the key distribution.  The reason is that key
   confirmation requires additional information to be exchanged that
   would not be configured when there is no trusted third party.

   An objection has been raised that an approach based on a classical
   three-party protocol might be more complex than the present course of
   the AAA Working Group.  This objection is wrong on two accounts.
   First, it is a necessary complexity, because composition of
   separately designed protocols does not necessarily lead to a secure
   overall protocol.  Second, increasing the local complexity by binding
   together protocols which are intrinsically linked reduces the
   coupling of session key establishment from the remainder of the
   system, and hence also reduces global complexity.


4.      NASREQ Key Distribution Requirements

   A key distribution protocol should provide a secure means for
   affecting use of the session key.  In addition to the requirements
   already spelled out for NASREQ key distribution, the following
   requirements are also needed for such a key distribution mechanism:







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   1.   Must ensure the session key is fresh;

   2.   Must name the key;

   3.   Must bind the key to the intended session between the NAS and
        NAS client; and

   4.   Must enforce protection of the session key.


4.1.  Session key freshness

   Key distribution mechanisms must ensure that the session key
   distributed is statistically unique.  That is, a session key must
   never be used again in either subsequent sessions or reused with
   another NAS.  The protocol must allow both the NAS and the NAS client
   some means of verifying the freshness of the key distribution.


4.2.  Key naming

   Each key must be properly identified to a given session corresponding
   to a NAS and NAS client.  The protocol and key naming scheme together
   must allow the participants to detect the unauthorized use of a
   distributed key.  The protocol must allow each party to verify that
   the session peer is the other intended recipient of the distributed
   key.


4.3.  Key binding

   A key must be properly bound to a particular NAS and NAS client
   session.  The key binding is critical to allow both the NAS and NAS
   client to properly synchronize to a session key.  Since the key is
   ultimately used to establish communications between the NAS and the
   NAS client, the protocol must be explicit on when the distributed key
   becomes active as well as allowing the NAS and NAS client to validate
   and confirm receipt of the key.


4.4.  Protection of the session key

   The protocol must provide assurances to all three parties (the AAA
   server, the NAS, and the NAS client) that no other parties have
   access to the distributed session key, assuming none of the three
   publishes it either intentionally or inadvertently.


5.      Proposed NASREQ Key Distribution Architecture and Enhancements

   < To be supplied by 3 May 2002 >




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6.  Security Considerations

   This document concerns the security of [NASREQ].  The authors hope
   that the analysis presented here will be embraced by the AAA Working
   Group, resulting in a more secure protocol.


7.  Acknowledgements


8.  References

   [PROB]   Calhoun, P., Aboba, B., Guttman, E., Mitton, D., Nelson,
            D., Schoenwaelder, J., Wolff, B., Zhang, X., "AAA Problem
            Statements", work in progress, draft-ietf-aaa-issues-05.txt,
            January 2002.

   [TRANS]  Aboba, B., Wood, J., "Authentication, Authorization, and
            Accounting (AAA) Transport Profile", work in progress,
            draft-ietf-aaa-transport-05.txt, November 2001.

   [DIAM]   Calhoun, P., Akhtar, H., Arkko, J., Guttman, E., Rubens,
            A., Zorn, G., "Diameter Base Protocol", work in progress,                         draft-ietf-aaa-diameter-08.txt, November 2001

   [NASREQ] Calhoun, P., Bulley, W., Rubens, A.C., Haag, J., Zorn., G.
            "Diameter NASREQ Application", work in progress,
            draft-ietf-aaa-diameter-nasreq-08.txt, November 2001.

   [CMS]    Calhoun, P., Farrell, S., Bulley, W., "The Diameter CMS
            Security Application", work in progress,
            draft-ietf-aaa-diameter-cms-03.txt, November 2001.

   [TTLS]   Funk, P., Blake-Wilson, S., "EAP Tunneled TLS
            Authentication Protocol", work in progress,
            draft-ietf-pppext-eap-ttls-01.txt, February 2002

   [PEAP]   Anderson, H., Josefsson, S., Zorn, G., Simon, D., Palekar,
            A., "Protected EAP Protocol (PEAP)", work in progress,
            draft-josefsson-pppext-eap-tls-eap-02.txt, February 2002


9.  Author Addresses

   Jesse Walker
   Intel Corporation
   2111 N.E. 25th Avenue
   Hillsboro, OR  97214
   USA
   jesse.walker@intel.com

   Russell Housley
   RSA Laboratories
   918 Spring Knoll Drive
   Herndon, VA  20170
   USA
   rhousley@rsasecurity.com

   Nancy Cam-Winget
   Mountain View, CA  94040
   USA
   nance@winget.net

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10.  Full Copyright Statement

   Copyright (C) The Internet Society (2002). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  In addition,
   the ASN.1 modules presented in Appendices A and B may be used in
   whole or in part without inclusion of the copyright notice.
   However, this document itself may not be modified in any way, such
   as by removing the copyright notice or references to the Internet
   Society or other Internet organizations, except as needed for the
   purpose of developing Internet standards in which case the




   procedures for copyrights defined in the Internet Standards process
   shall be followed, or as required to translate it into languages
   other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns. This
   document and the information contained herein is provided on an "AS
   IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
   FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
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   WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



























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