Internet DRAFT - draft-walker-aaa-key-distribution

draft-walker-aaa-key-distribution








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 
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   The limited permissions granted above are perpetual and will not be 
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   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



























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