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BEST CURRENT PRACTICE

Network Working Group                                         R. Housley
Request for Comments: 4962                                Vigil Security
BCP: 132                                                        B. Aboba
Category: Best Current Practice                                Microsoft
                                                               July 2007


   Guidance for Authentication, Authorization, and Accounting (AAA)
                             Key Management

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document provides guidance to designers of Authentication,
   Authorization, and Accounting (AAA) key management protocols.  The
   guidance is also useful to designers of systems and solutions that
   include AAA key management protocols.  Given the complexity and
   difficulty in designing secure, long-lasting key management
   algorithms and protocols by experts in the field, it is almost
   certainly inappropriate for IETF working groups without deep
   expertise in the area to be designing their own key management
   algorithms and protocols based on Authentication, Authorization, and
   Accounting (AAA) protocols.  The guidelines in this document apply to
   documents requesting publication as IETF RFCs.  Further, these
   guidelines will be useful to other standards development
   organizations (SDOs) that specify AAA key management.
















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Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Specification .................................3
      1.2. Mandatory to Implement .....................................3
      1.3. Terminology ................................................3
   2. AAA Environment Concerns ........................................5
   3. AAA Key Management Requirements .................................7
   4. AAA Key Management Recommendations .............................13
   5. Security Considerations ........................................14
   6. Normative References ...........................................15
   7. Informative References .........................................15
   Appendix: AAA Key Management History ..............................20
   Acknowledgments ...................................................22

1.  Introduction

   This document provides architectural guidance to designers of AAA key
   management protocols.  The guidance is also useful to designers of
   systems and solutions that include AAA key management protocols.

   AAA key management often includes a collection of protocols, one of
   which is the AAA protocol.  Other protocols are used in conjunction
   with the AAA protocol to provide an overall solution.  These other
   protocols often provide authentication and security association
   establishment.

   Given the complexity and difficulty in designing secure, long-lasting
   key management algorithms and protocols by experts in the field, it
   is almost certainly inappropriate for IETF working groups without
   deep expertise in the area to be designing their own key management
   algorithms and protocols based on Authentication, Authorization and
   Accounting (AAA) protocols.  These guidelines apply to documents
   requesting publication as IETF RFCs.  Further, these guidelines will
   be useful to other standards development organizations (SDOs) that
   specify AAA key management that depends on IETF specifications for
   protocols such as Extensible Authentication Protocol (EAP) [RFC3748],
   Remote Authentication Dial-In User Service (RADIUS) [RFC2865], and
   Diameter [RFC3588].

   In March 2003, at the IETF 56 AAA Working Group Session, Russ Housley
   gave a presentation on "Key Management in AAA" [H].  That
   presentation established the vast majority of the requirements
   contained in this document.  Over the last three years, this
   collection of requirements have become known as the "Housley
   Criteria".





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1.1.  Requirements Specification

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in RFC 2119 [RFC2119].

   An AAA key management proposal is not compliant with this
   specification if it fails to satisfy one or more of the MUST or MUST
   NOT statements.  An AAA key management proposal that satisfies all
   the MUST, MUST NOT, SHOULD, and SHOULD NOT statements is said to be
   "unconditionally compliant"; one that satisfies all the MUST and MUST
   NOT statements but not all the SHOULD or SHOULD NOT requirements is
   said to be "conditionally compliant".

1.2.  Mandatory to Implement

   The guidance provided in this document is mandatory to implement.
   However, it is not mandatory to use.  That is, configuration at the
   time of deployment may result in a deployed implementation that does
   not conform with all of these requirements.

   For example, [RFC4072] enables EAP keying material to be delivered
   from a AAA server to an AAA client without disclosure to third
   parties.  Thus, key confidentiality is mandatory to implement in
   Diameter [RFC3588].  However, key confidentiality is not mandatory to
   use.

1.3.  Terminology

   This section defines terms that are used in this document.

      AAA
         Authentication, Authorization, and Accounting (AAA).  AAA
         protocols include RADIUS [RFC2865] and Diameter [RFC3588].

      Authenticator
         The party initiating EAP authentication.  The term
         authenticator is used in [802.1X], and authenticator has the
         same meaning in this document.

      Backend authentication server
         A backend authentication server is an entity that provides an
         authentication service to an authenticator.  This terminology
         is also used in [802.1X].







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      CHAP
         Challenge Handshake Authentication Protocol; a one-way
         challenge/response authentication protocol defined in
         [RFC1994].

      EAP
         Extensible Authentication Protocol, defined in [RFC3748].

      EAP server
         The entity that terminates the EAP authentication method with
         the peer.  In the case where no backend authentication server
         is used, the EAP server is part of the authenticator.  In the
         case where the authenticator operates in pass-through mode, the
         EAP server is located on the backend authentication server.

      Key Wrap
         The encryption of one symmetric cryptographic key in another.
         The algorithm used for the encryption is called a key wrap
         algorithm or a key encryption algorithm.  The key used in the
         encryption process is called a key-encryption key (KEK).

      PAP
         Password Authentication Protocol; a deprecated cleartext
         password PPP authentication protocol, originally defined in
         [RFC1334].

      Party
         A party is a processing entity that can be identified as a
         single role in a protocol.

      Peer
         The end of the link that responds to the authenticator.  In
         [802.1X], this end is known as the supplicant.

      PPP
         Point-to-Point Protocol, defined in [RFC1661], provides support
         for multiprotocol serial datalinks.  PPP is the primary IP
         datalink used for dial-in NAS connection service.

      Secure Association Protocol
         A protocol for managing security associations derived from EAP
         and/or AAA exchanges.  The protocol establishes a security
         association, which includes symmetric keys and a context for
         the use of the keys.  An example of a Secure Association
         Protocol is the 4-way handshake defined within [802.11i].






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      Session Keys
         Keying material used to protect data exchanged after
         authentication has successfully completed, using the negotiated
         ciphersuite.

      Network Access Server (NAS)
         A device that provides an access service for a user to a
         network.  The service may be a network connection, or a value
         added service such as terminal emulation, as described in
         [RFC2881].

      4-Way Handshake
         A Secure Association Protocol, defined in [802.11i], which
         confirms mutual possession of a Pairwise Master Key by two
         parties and distributes a Group Key.

2.  AAA Environment Concerns

   Examples of serious flaws plague the history of key management
   protocol development, starting with the very first attempt to define
   a key management protocol in the open literature, which was published
   in 1978 [NS].  A flaw and a fix were published in 1981 [DS], and the
   fix was broken in 1994 [AN].  In 1995 [L], a new flaw was found in
   the original 1978 version, in an area not affected by the 1981/1994
   issue.  All of these flaws were blindingly obvious once described,
   yet no one spotted them earlier.  Note that the original protocol, if
   it were revised to employ certificates, which of course had yet to be
   invented, was only three messages.  Many proposed AAA key management
   schemes are significantly more complicated.

   This bit of history shows that key management protocols are subtle.
   Experts can easily miss a flaw.  As a result, peer review by multiple
   experts is essential, especially since many proposed AAA key
   management schemes are significantly more complicated.  In addition,
   formal methods can help uncover problems [M].

   AAA-based key management is being incorporated into standards
   developed by the IETF and other standards development organizations
   (SDOs), such as IEEE 802.  However, due to ad hoc development of
   AAA-based key management, AAA-based key distribution schemes have
   poorly understood security properties, even when well-studied
   cryptographic algorithms are employed.  More academic research is
   needed to fully understand the security properties of AAA-based key
   management in the diverse protocol environments where it is being
   employed today.  In the absence of such research results, pragmatic
   guidance based on sound security engineering principles is needed.





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   In addition to the need for interoperability, cryptographic algorithm
   independent solutions are greatly preferable.  Without algorithm
   independence, the AAA-based key management protocol must be changed
   whenever a problem is discovered with any of the selected algorithms.
   As AAA history shows, problems are inevitable.  Problems can surface
   due to age or design failure.

   DES [FIPS46] was a well-designed encryption algorithm, and it
   provided protection for many years.  Yet, the 56-bit key size was
   eventually overcome by Moore's Law.  No significant cryptographic
   deficiencies have been discovered in DES.

   The history of AAA underlines the importance of algorithm
   independence as flaws have been found in authentication mechanisms
   such as CHAP, MS-CHAPv1 [SM1], MS-CHAPv2 [SM2], Kerberos
   [W][BM][DLS], and LEAP [B].  Unfortunately, RADIUS [RFC2865] mandates
   use of the MD5 algorithm for integrity protection, which has known
   deficiencies, and RADIUS has no provisions to negotiate substitute
   algorithms.  Similarly, the vendor-specific key wrap mechanism
   defined in [RFC2548] has no provisions to negotiate substitute
   algorithms.

   The principle of least privilege is an important design guideline.
   This principle requires that a party be given no more privilege than
   necessary to perform the task assigned to them.  Ensuring least
   privilege requires clear identification of the tasks assigned to each
   party, and explicit determination of the minimum set of privileges
   required to perform those tasks.  Only those privileges necessary to
   perform the tasks are granted.  By denying to parties unneeded
   privileges, those denied privileges cannot be used to circumvent
   security policy or enable attackers.  With this principle in mind,
   AAA key management schemes need to be designed in a manner where each
   party has only the privileges necessary to perform their role.  That
   is, no party should have access to any keying material that is not
   needed to perform their own role.  A party has access to a particular
   key if it has access to all of the secret information needed to
   derive it.

   EAP is being used in new ways.  The inclusion of support for EAP
   within Internet Key Exchange Protocol version 2 (IKEv2) and the
   standardization of robust Wireless LAN security [802.11i] based on
   EAP are two examples.  EAP has also been proposed within IEEE 802.16e
   [802.16e] and by the IETF PANA Working Group.  AAA-based key
   management is being incorporated into standards developed by the IETF
   and other standards development organizations (SDOs), such as IEEE
   802.  However, due to ad hoc development of AAA-based key management,
   AAA-based key distribution schemes have poorly understood security
   properties, even when well-studied cryptographic algorithms are



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   employed.  More academic research is needed to fully understand the
   security properties of AAA-based key management in the diverse
   protocol environments where it is being employed today.  In the
   absence of research results, pragmatic guidance based on sound
   security engineering principles is needed.

   EAP selects one end-to-end authentication mechanism.  The mechanisms
   defined in [RFC3748] only support unilateral authentication, and they
   do not support mutual authentication or key derivation.  As a result,
   these mechanisms do not fulfill the security requirements for many
   deployment scenarios, including Wireless LAN authentication
   [RFC4017].

   To ensure adequate security and interoperability, EAP applications
   need to specify mandatory-to-implement algorithms.  As described in
   [RFC3748], EAP methods seeking publication as an IETF RFC need to
   document their security claims.  However, some EAP methods are not
   based on well-studied models, which makes the validity of these
   security claims difficult to determine.

   In the context of EAP, the EAP peer and server are the parties
   involved in the EAP method conversation, and they gain access to key
   material when the conversation completes successfully.  However, the
   lower-layer needs keying material to provide the desired protection
   through the use of cryptographic mechanisms.  As a result, a "pass-
   through" mode is used to provide the keying material, and the lower-
   layer keying material is replicated from the AAA server to the
   authenticator.  The only parties authorized to obtain all of the
   keying material are the EAP peer and server; the authenticator
   obtains only the keying material necessary for its specific role.  No
   other party can obtain direct access to any of the keying material;
   however, other parties may receive keys that are derived from this
   keying material for a specific purpose as long as the requirements
   defined in the next section are met.

3.  AAA Key Management Requirements

   The overall goal of AAA key management is to provide cryptographic
   keying material in situations where key derivation cannot be used by
   the peer and authenticator.  It may not be possible because the
   authenticator lacks computational power, because it lacks the
   resources necessary to implement the various authentication
   mechanisms that might be required, or because it is undesirable for
   each authenticator to engage in a separate key management
   conversation.






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   This section provides guidance to AAA protocol designers, EAP method
   designers, and security association protocol designers.  Acceptable
   solutions MUST meet all of these requirements.

      Cryptographic algorithm independent

         The AAA key management protocol MUST be cryptographic algorithm
         independent.  However, an EAP method MAY depend on a specific
         cryptographic algorithm.  The ability to negotiate the use of a
         particular cryptographic algorithm provides resilience against
         compromise of a particular cryptographic algorithm.  Algorithm
         independence is also REQUIRED with a Secure Association
         Protocol if one is defined.  This is usually accomplished by
         including an algorithm identifier and parameters in the
         protocol, and by specifying the algorithm requirements in the
         protocol specification.  While highly desirable, the ability to
         negotiate key derivation functions (KDFs) is not required.  For
         interoperability, at least one suite of mandatory-to-implement
         algorithms MUST be selected.  Note that without protection by
         IPsec as described in [RFC3579] Section 4.2, RADIUS [RFC2865]
         does not meet this requirement, since the integrity protection
         algorithm cannot be negotiated.

         This requirement does not mean that a protocol must support
         both public-key and symmetric-key cryptographic algorithms.  It
         means that the protocol needs to be structured in such a way
         that multiple public-key algorithms can be used whenever a
         public-key algorithm is employed.  Likewise, it means that the
         protocol needs to be structured in such a way that multiple
         symmetric-key algorithms can be used whenever a symmetric-key
         algorithm is employed.

      Strong, fresh session keys

         While preserving algorithm independence, session keys MUST be
         strong and fresh.  Each session deserves an independent session
         key.  Fresh keys are required even when a long replay counter
         (that is, one that "will never wrap") is used to ensure that
         loss of state does not cause the same counter value to be used
         more than once with the same session key.

         Some EAP methods are capable of deriving keys of varying
         strength, and these EAP methods MUST permit the generation of
         keys meeting a minimum equivalent key strength.  BCP 86
         [RFC3766] offers advice on appropriate key sizes.  The National
         Institute for Standards and Technology (NIST) also offers
         advice on appropriate key sizes in [SP800-57].




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         A fresh cryptographic key is one that is generated specifically
         for the intended use.  In this situation, a secure association
         protocol is used to establish session keys.  The AAA protocol
         and EAP method MUST ensure that the keying material supplied as
         an input to session key derivation is fresh, and the secure
         association protocol MUST generate a separate session key for
         each session, even if the keying material provided by EAP is
         cached.  A cached key persists after the authentication
         exchange has completed.  For the AAA/EAP server, key caching
         can happen when state is kept on the server.  For the NAS or
         client, key caching can happen when the NAS or client does not
         destroy keying material immediately following the derivation of
         session keys.

         Session keys MUST NOT be dependent on one another.  Multiple
         session keys may be derived from a higher-level shared secret
         as long as a one-time value, usually called a nonce, is used to
         ensure that each session key is fresh.  The mechanism used to
         generate session keys MUST ensure that the disclosure of one
         session key does not aid the attacker in discovering any other
         session keys.

      Limit key scope

         Following the principle of least privilege, parties MUST NOT
         have access to keying material that is not needed to perform
         their role.  A party has access to a particular key if it has
         access to all of the secret information needed to derive it.

         Any protocol that is used to establish session keys MUST
         specify the scope for session keys, clearly identifying the
         parties to whom the session key is available.

      Replay detection mechanism

         The AAA key management protocol exchanges MUST be replay
         protected, including AAA, EAP, and Secure Association Protocol
         exchanges.  Replay protection allows a protocol message
         recipient to discard any message that was recorded during a
         previous legitimate dialogue and presented as though it
         belonged to the current dialogue.

      Authenticate all parties

         Each party in the AAA key management protocol MUST be
         authenticated to the other parties with whom they communicate.
         Authentication mechanisms MUST maintain the confidentiality of
         any secret values used in the authentication process.



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         When a secure association protocol is used to establish session
         keys, the parties involved in the secure association protocol
         MUST identify themselves using identities that are meaningful
         in the lower-layer protocol environment that will employ the
         session keys.  In this situation, the authenticator and peer
         may be known by different identifiers in the AAA protocol
         environment and the lower-layer protocol environment, making
         authorization decisions difficult without a clear key scope.
         If the lower-layer identifier of the peer will be used to make
         authorization decisions, then the pair of identifiers
         associated with the peer MUST be authorized by the
         authenticator and/or the AAA server.

         AAA protocols, such as RADIUS [RFC2865] and Diameter [RFC3588],
         provide a mechanism for the identification of AAA clients;
         since the EAP authenticator and AAA client are always co-
         resident, this mechanism is applicable to the identification of
         EAP authenticators.

         When multiple base stations and a "controller" (such as a WLAN
         switch) comprise a single EAP authenticator, the "base station
         identity" is not relevant; the EAP method conversation takes
         place between the EAP peer and the EAP server.  Also, many base
         stations can share the same authenticator identity.  The
         authenticator identity is important in the AAA protocol
         exchange and the secure association protocol conversation.

         Authentication mechanisms MUST NOT employ plaintext passwords.
         Passwords may be used provided that they are not sent to
         another party without confidentiality protection.

      Peer and authenticator authorization

         Peer and authenticator authorization MUST be performed.  These
         entities MUST demonstrate possession of the appropriate keying
         material, without disclosing it.  Authorization is REQUIRED
         whenever a peer associates with a new authenticator.  The
         authorization checking prevents an elevation of privilege
         attack, and it ensures that an unauthorized authenticator is
         detected.

         Authorizations SHOULD be synchronized between the peer, NAS,
         and backend authentication server.  Once the AAA key management
         protocol exchanges are complete, all of these parties should
         hold a common view of the authorizations associated with the
         other parties.





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         In addition to authenticating all parties, key management
         protocols need to demonstrate that the parties are authorized
         to possess keying material.  Note that proof of possession of
         keying material does not necessarily prove authorization to
         hold that keying material.  For example, within an IEEE
         802.11i, the 4-way handshake demonstrates that both the peer
         and authenticator possess the same EAP keying material.
         However, by itself, this possession proof does not demonstrate
         that the authenticator was authorized by the backend
         authentication server to possess that keying material.  As
         noted in RFC 3579 in Section 4.3.7, where AAA proxies are
         present, it is possible for one authenticator to impersonate
         another, unless each link in the AAA chain implements checks
         against impersonation.  Even with these checks in place, an
         authenticator may still claim different identities to the peer
         and the backend authentication server.  As described in RFC
         3748 in Section 7.15, channel binding is required to enable the
         peer to verify that the authenticator claim of identity is both
         consistent and correct.

      Keying material confidentiality and integrity

         While preserving algorithm independence, confidentiality and
         integrity of all keying material MUST be maintained.

      Confirm ciphersuite selection

         The selection of the "best" ciphersuite SHOULD be securely
         confirmed.  The mechanism SHOULD detect attempted roll-back
         attacks.

      Uniquely named keys

         AAA key management proposals require a robust key naming
         scheme, particularly where key caching is supported.  The key
         name provides a way to refer to a key in a protocol so that it
         is clear to all parties which key is being referenced.  Objects
         that cannot be named cannot be managed.  All keys MUST be
         uniquely named, and the key name MUST NOT directly or
         indirectly disclose the keying material.  If the key name is
         not based on the keying material, then one can be sure that it
         cannot be used to assist in a search for the key value.

      Prevent the Domino effect

         Compromise of a single peer MUST NOT compromise keying material
         held by any other peer within the system, including session
         keys and long-term keys.  Likewise, compromise of a single



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         authenticator MUST NOT compromise keying material held by any
         other authenticator within the system.  In the context of a key
         hierarchy, this means that the compromise of one node in the
         key hierarchy must not disclose the information necessary to
         compromise other branches in the key hierarchy.  Obviously, the
         compromise of the root of the key hierarchy will compromise all
         of the keys; however, a compromise in one branch MUST NOT
         result in the compromise of other branches.  There are many
         implications of this requirement; however, two implications
         deserve highlighting.  First, the scope of the keying material
         must be defined and understood by all parties that communicate
         with a party that holds that keying material.  Second, a party
         that holds keying material in a key hierarchy must not share
         that keying material with parties that are associated with
         other branches in the key hierarchy.

         Group keys are an obvious exception.  Since all members of the
         group have a copy of the same key, compromise of any one of the
         group members will result in the disclosure of the group key.

      Bind key to its context

         Keying material MUST be bound to the appropriate context.  The
         context includes the following.

            o  The manner in which the keying material is expected to be
               used.

            o  The other parties that are expected to have access to the
               keying material.

            o  The expected lifetime of the keying material.  Lifetime
               of a child key SHOULD NOT be greater than the lifetime of
               its parent in the key hierarchy.

         Any party with legitimate access to keying material can
         determine its context.  In addition, the protocol MUST ensure
         that all parties with legitimate access to keying material have
         the same context for the keying material.  This requires that
         the parties are properly identified and authenticated, so that
         all of the parties that have access to the keying material can
         be determined.









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         The context will include the peer and NAS identities in more
         than one form.  One (or more) name form is needed to identify
         these parties in the authentication exchange and the AAA
         protocol.  Another name form may be needed to identify these
         parties within the lower layer that will employ the session
         key.

4.  AAA Key Management Recommendations

   Acceptable solutions SHOULD meet all of these requirements.

      Confidentiality of identity

         In many environments, it is important to provide
         confidentiality protection for identities.  However, this is
         not important in other environments.  For this reason, EAP
         methods are encouraged to provide a mechanism for identity
         protection of EAP peers, but such protection is not a
         requirement.

      Authorization restriction

         If peer authorization is restricted, then the peer SHOULD be
         made aware of the restriction.  Otherwise, the peer may
         inadvertently attempt to circumvent the restriction.  For
         example, authorization restrictions in an IEEE 802.11
         environment include:

            o  Key lifetimes, where the keying material can only be used
               for a certain period of time;

            o  SSID restrictions, where the keying material can only be
               used with a specific IEEE 802.11 SSID;

            o  Called-Station-ID restrictions, where the keying material
               can only be used with a single IEEE 802.11 BSSID; and

            o  Calling-Station-ID restrictions, where the keying
               material can only be used with a single peer IEEE 802 MAC
               address.











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5.  Security Considerations

   This document provides architectural guidance to designers of AAA key
   management protocols.  The guidance is also useful to designers of
   systems and solutions that include AAA key management protocols.

   In some deployment scenarios, more than one party in the AAA key
   management protocol can reside on the same host.  For example, the
   EAP authenticator and AAA client are expected to reside on the same
   entity.  Colocation enables a single unique authenticator identity to
   be sent by the authenticator to the AAA server as well as by the
   authenticator to the EAP peer.  Use of the same identity in both
   conversations enables the peer and AAA server to confirm that the
   authenticator is consistent in its identification, avoiding potential
   impersonation attacks.  If the authenticator and AAA client are not
   colocated, then the authenticator and AAA client identities will
   differ, and the key scope will not be synchronized between the EAP
   peer, authenticator, and server.  Lack of key scope synchronization
   enables a number of security vulnerabilities, including
   impersonation.  For this reason, a design needs to include mechanisms
   to ensure that the key scope and key naming are unambiguous.

   The AAA server is a trusted entity.  When keying material is present
   at all, it establishes keying material with the peer and distributes
   keying material to the authenticator using the AAA protocol.  It is
   trusted to only distribute keying material to the authenticator that
   was established with the peer, and it is trusted to provide that
   keying material to no other parties.  In many systems, keying
   material established by the EAP peer and EAP server are combined with
   publicly available data to derive other keys.  The AAA server is
   trusted to refrain from deriving these same keys even though it has
   access to the secret values that are needed to do so.

   The authenticator is also a trusted party.  The authenticator is
   trusted not to distribute keying material provided by the AAA server
   to any other parties.  If the authenticator uses a key derivation
   function to derive additional keying material, the authenticator is
   trusted to distribute the derived keying material only to the
   appropriate party that is known to the peer, and no other party.
   When this approach is used, care must be taken to ensure that the
   resulting key management system meets all of the principles in this
   document, confirming that keys used to protect data are to be known
   only by the peer and authenticator.

   EAP is used to authenticate the peer to the AAA/EAP server.
   Following successful authentication, the AAA/EAP server authorizes
   the peer.  In many situations, this is accomplished by sending keying
   material to the authenticator and the peer in separate protocol



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   messages.  The authenticator is not directly authenticated to the
   peer.  Rather, the peer determines that the authenticator has been
   authorized by the AAA/EAP server by confirming that the authenticator
   has the same AAA/EAP server-provided keying material.  In some
   systems, explicit authenticator and peer mutual authentication is
   possible.  This is desirable since it greatly improves
   accountability.

   When MIB modules are developed for AAA protocols or EAP methods,
   these MIB modules might include managed objects for keying material.
   The existence of managed objects associated with keying material
   offers an additional avenue for key compromise if these objects
   include the keying material itself.  Therefore, these MIB modules
   MUST NOT include objects for private keys or symmetric keys.
   However, these MIB modules MAY include management objects that expose
   names and context associated with keys, and they MAY provide a means
   to delete keys.

6.  Normative References

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

7.  Informative References

   [802.1X]   IEEE Standards for Local and Metropolitan Area Networks:
              Port based Network Access Control, IEEE Std 802.1X-2004,
              December 2004.

   [802.11i]  Institute of Electrical and Electronics Engineers,
              "Supplement to Standard for Telecommunications and
              Information Exchange Between Systems -- LAN/MAN Specific
              Requirements - Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications:
              Specification for Enhanced Security", IEEE 802.11i, July
              2004.

   [802.16e]  Institute of Electrical and Electronics Engineers,
              "Supplement to Standard for Telecommunications and
              Information Exchange Between Systems -- LAN/MAN Specific
              Requirements - Part 16: Air Interface for Fixed and Mobile
              Broadband Wireless Access Systems -- Amendment for
              Physical and Medium Access Control Layers for Combined
              Fixed and Mobile Operation in Licensed Bands", Draft, IEEE
              802.16e/D8, May 2005.






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RFC 4962            Guidance for AAA Key Management            July 2007


   [AN]       M. Abadi and R. Needham, "Prudent Engineering Practice for
              Cryptographic Protocols", Proc. IEEE Computer Society
              Symposium on Research in Security and Privacy, May 1994.

   [B]        Brewin, B., "LEAP attack tool author says he wants to
              alert users to risks", Computerworld, October 17, 2003.

   [BM]       Bellovin, S. and M. Merrit, "Limitations of the Kerberos
              authentication system", Proceedings of the 1991 Winter
              USENIX Conference, pp. 253-267, 1991.

   [DDNN39.2] DCA DDN Program Management Office, "MILNET TAC Access
              Control", Defense Data Network Newsletter, DDN News 39,
              Special Issue, 26 Apr 1985, <http://www.isi.edu/
              in-notes/museum/ddn-news/ddn-news.n39.2>.

   [DLS]      Dole, B., Lodin, S. and E. Spafford, "Misplaced trust:
              Kerberos 4 session keys", Proceedings of the Internet
              Society Network and Distributed System Security Symposium,
              pp. 60-70, March 1997.

   [DS]       D. Denning and G. Sacco.  "Timestamps in key distributed
              protocols", Communication of the ACM, 24(8):533--535,
              1981.

   [FIPS46]   Federal Information Processing Standards Publication (FIPS
              PUB) 46, Data Encryption Standard, 1977 January 15.

   [H]        Housley, R., "Key Management in AAA", Presentation to the
              AAA WG at IETF 56, March 2003, <http://www.ietf.org/
              proceedings/03mar/slides/aaa-5/index.html>.

   [L]        G. Lowe.  "An attack on the Needham-Schroeder public key
              authentication protocol", Information Processing Letters,
              56(3):131--136, November 1995.

   [M]        Meadows, C., "Analysis of the Internet Key Exchange
              Protocol using the NRL Protocol Analyser", Proceedings of
              the 1999 IEEE Symposium on Security & Privacy, Oakland,
              CA, USA, IEEE Computer Society, May 1999,
              <http://chacs.nrl.navy.mil/publications/CHACS/1999/
              1999meadows-IEEE99.pdf>.

   [NS]       R. Needham and M. Schroeder. "Using encryption for
              authentication in large networks of computers",
              Communications of the ACM, 21(12), December 1978.





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RFC 4962            Guidance for AAA Key Management            July 2007


   [RFC0927]  Anderson, B.A., "TACACS user identification Telnet
              option", RFC 927, December 1984.

   [RFC1334]  Lloyd, B. and B. Simpson, "PPP Authentication Protocols",
              RFC 1334, October 1992, Obsoleted by RFC 1994.

   [RFC1492]  Finseth, C., "An Access Control Protocol, Sometimes Called
              TACACS", RFC 1492, July 1993.

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC1968]  Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968,
              June 1996.

   [RFC1994]  Simpson, W., "PPP Challenge Handshake Authentication
              Protocol (CHAP)", RFC 1994, August 1996.

   [RFC2284]  Blunk, L. and J. Vollbrecht, "PPP Extensible
              Authentication Protocol (EAP)", RFC 2284, March 1998.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [RFC2419]  Sklower, K. and G. Meyer, "The PPP DES Encryption
              Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998.

   [RFC2420]  Hummert, K., "The PPP Triple-DES Encryption Protocol
              (3DESE)", RFC 2420, September 1998.

   [RFC2433]  Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC
              2433, October 1998.

   [RFC2548]  Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
              RFC 2548, March 1999.

   [RFC2637]  Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
              W., and G.  Zorn, "Point-to-Point Tunneling Protocol
              (PPTP)", RFC 2637, July 1999.

   [RFC2716]  Aboba, B. and D. Simon, "PPP EAP TLS Authentication
              Protocol", RFC 2716, October 1999.

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






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

   [RFC2881]  Mitton, D. and M. Beadles, "Network Access Server
              Requirements Next Generation (NASREQNG) NAS Model", RFC
              2881, July 2000.

   [RFC3078]  Pall, G. and G. Zorn, "Microsoft Point-To-Point Encryption
              (MPPE) Protocol", RFC 3078, March 2001.

   [RFC3079]  Zorn, G., "Deriving Keys for use with Microsoft Point-to-
              Point Encryption (MPPE)", RFC 3079, March 2001.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

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

   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strength for Public
              Keys Used For Exchanging Symmetric Keys", BCP 86, RFC
              3766, April 2004.

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, March 2005.

   [RFC4072]  Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter
              Extensible Authentication Protocol (EAP) Application", RFC
              4072, August 2005.

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

   [SM1]      Schneier, B. and Mudge, "Cryptanalysis of Microsoft's
              Point-to-Point Tunneling Protocol", Proceedings of the 5th
              ACM Conference on Communications and Computer Security,
              ACM Press, November 1998.

   [SM2]      Schneier, B. and Mudge, "Cryptanalysis of Microsoft's PPTP
              Authentication Extensions (MS-CHAPv2)", CQRE 99,
              Springer-Verlag, 1999, pp. 192-203.



Housley & Aboba          Best Current Practice                 [Page 18]

RFC 4962            Guidance for AAA Key Management            July 2007


   [SP800-57] National Institute of Standards and Technology,
              "Recommendation for Key Management", Special Publication
              800-57, May 2006.

   [W]        Wu, T., "A Real-World Analysis of Kerberos Password
              Security", Proceedings of the 1999 ISOC Network and
              Distributed System Security Symposium,
              <http://www.isoc.org/isoc/conferences/ndss/99/
              proceedings/papers/wu.pdf>.










































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RFC 4962            Guidance for AAA Key Management            July 2007


Appendix: AAA Key Management History

   Protocols for Authentication, Authorization, and Accounting (AAA)
   were originally developed to support deployments of Network Access
   Servers (NASes).  In the ARPAnet, the Terminal Access Controller
   (TAC) provided a means for "dumb terminals" to access the network,
   and the TACACS [RFC0927][RFC1492] AAA protocol was designed by BBN
   under contract to the Defense Data Network Program Management Office
   (DDN PMO) for this environment.  [RFC1492] documents a later version
   of TACACS, not the original version that was widely deployed in
   ARPAnet and MILNET [DDNN39.2].

   Later, additional AAA protocols were developed to support deployments
   of NASes providing access to the Internet via PPP [RFC1661].  In
   deployments supporting more than a modest number of users, it became
   impractical for each NAS to contain its own list of users and
   associated credentials.  As a result, additional AAA protocols were
   developed, including RADIUS [RFC2865] and Diameter [RFC3588].  These
   protocols enabled a central AAA server to authenticate users
   requesting network access, as well as providing authorization and
   accounting.

   While PPP [RFC1661] originally supported only PAP [RFC1334] and CHAP
   [RFC1661] authentication, the limitations of these authentication
   mechanisms became apparent.  For example, both PAP and CHAP are
   unilateral authentication schemes supporting only authentication of
   the PPP peer to the NAS.  Since PAP is a cleartext password scheme,
   it is vulnerable to snooping by an attacker with access to the
   conversation between the PPP peer and NAS.  In addition, the use of
   PAP creates vulnerabilities within RADIUS as described in Section 4.3
   of [RFC3579].  As a result, use of PAP is deprecated.  While CHAP, a
   challenge-response scheme based on MD5, offers better security than
   cleartext passwords, it does not provide for mutual authentication,
   and CHAP is vulnerable to dictionary attack.

   With the addition of the Encryption Control Protocol (ECP) to PPP
   [RFC1968] as well as the definition of PPP ciphersuites in [RFC2419],
   [RFC2420], and [RFC3078], the need arose to provide keying material
   for use with link layer ciphersuites.  As with user authentication,
   provisioning of static keys on each NAS did not scale well.

   Additional vendor-specific PPP authentication protocols such as
   MS-CHAP [RFC2433] and MS-CHAPv2 [RFC2759] were developed to provide
   mutual authentication as well as key derivation [RFC3079] for use
   with negotiated ciphersuites, and they were subsequently adapted for
   use with PPP-based VPNs [RFC2637].  As with PAP and CHAP, flaws were
   subsequently found in these new mechanisms [SM1][SM2].




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   Even though PPP provided for negotiation of authentication
   algorithms, addressing the vulnerabilities found in authentication
   mechanisms still proved painful, since new code needed to be deployed
   on PPP peers as well as on the AAA server.  In order to enable more
   rapid deployment of new authentication mechanisms, as well as fixes
   for vulnerabilities found in existing methods, the Extensible
   Authentication Protocol (EAP) [RFC3748] was developed, along with
   support for centralized authentication via RADIUS/EAP [RFC3579].

   By enabling "pass through" authentication on the NAS, EAP enabled
   deployment of new authentication methods or updates to existing
   methods by revising code only on the EAP peer and AAA server.  The
   initial authentication mechanisms defined in [RFC2284] (MD5-
   Challenge, One-Time Password (OTP), and Generic Token Card (GTC))
   only supported unilateral authentication, and these mechanisms do not
   support key derivation.  Subsequent authentication methods such as
   EAP-TLS [RFC2716] supported mutual authentication and key derivation.

   In order to support the provisioning of dynamic keying material for
   link layer ciphersuites in an environment supporting centralized
   authentication, a mechanism was needed for the transport of keying
   material between the AAA server and NAS.  Vendor-specific RADIUS
   attributes were developed for this purpose [RFC2548].
   Vulnerabilities were subsequently found in the key wrap technique, as
   described in Section 4.3 of [RFC3579].

   In theory, public key authentication mechanisms such as EAP-TLS are
   capable of supporting mutual authentication and key derivation
   between the EAP peer and NAS without requiring AAA key distribution.
   However, in practice, such pure two-party schemes are rarely
   deployed.  Operation of a centralized AAA server significantly
   reduces the effort required to deploy certificates to NASes, and even
   though an AAA server may not be required for key derivation and
   possibly authentication, its participation is required for service
   authorization and accounting.

   "Pass-through" authentication and AAA key distribution has retained
   popularity even in the face of rapid improvements in processor and
   memory capabilities.  In addition to producing NAS devices of
   increased capability for enterprise and carrier customers,
   implementers have also produced low-cost/high-volume NAS devices such
   as 802.11 Access Points, causing the resources available on an
   average NAS to increase more slowly than Moore's law.  Despite
   widespread support for certificate handling and sophisticated key
   derivation mechanisms such as IKEv1 [RFC2409] within host operating
   systems, these security capabilities are rarely deployed on low-end
   NASes and clients.




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RFC 4962            Guidance for AAA Key Management            July 2007


   Even on more capable NASes, such as VPN servers, centralized
   authentication and AAA key management has proven popular.  For
   example, one of the major limitations of IKEv1 [RFC2409] was the lack
   of integration with EAP and AAA, requiring proprietary extensions to
   enable use of IPsec VPNs by organizations deploying password or
   authentication tokens.  These limitations were addressed in IKEv2
   [RFC4306], which while handling key derivation solely between the VPN
   client and server, supports EAP methods for user authentication.  In
   order to enable cryptographic binding of EAP user authentication to
   keys derived within the IKEv2 exchange, the transport of EAP-derived
   keys within AAA is required where the selected EAP method supports
   key derivation.

Acknowledgments

   Many thanks to James Kempf, Sam Hartman, and Joe Salowey for their
   quality review and encouragement.

   Thanks to the IETF AAA Working Group and the IETF EAP Working Group
   for their review and comment.  The document is greatly improved by
   their contribution.

Authors' Addresses

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA
   EMail: housley@vigilsec.com
   Phone: +1 703-435-1775
   Fax:   +1 703-435-1274

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052
   USA
   EMail: bernarda@microsoft.com
   Phone: +1 425-706-6605
   Fax:   +1 425-936-7329










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RFC 4962            Guidance for AAA Key Management            July 2007


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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