RADEXT Working Group                                  D. Nelson
Internet-Draft (Editor)
INTERNET-DRAFT                                     Elbrys Networks, Inc.
Intended status:
Category: Informational                             March 3, 2011
Expires: September 3, 12, 2011
12 March 2011

  Crypto-Agility Requirements for Remote Dial-In User Service (RADIUS)


   This memo describes the requirements for a crypto-agility solution
   for Remote Authentication Dial-In User Service (RADIUS).

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  General . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.2   Requirements Language . . . . . . . . . . . . . . . . . . . 3
     1.3.  The Charge  . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  A Working Definition of Crypto-Agility  . . . . . . . . . . . . 3 4
   3.  The Current State of RADIUS Security  . . . . . . . . . . . . . 4 5
   4.  The Requirements  . . . . . . . . . . . . . . . . . . . . . . . 4 5
     4.1.  Overall Solution Approach . . . . . . . . . . . . . . . . . 4 5
     4.2.  Security Services . . . . . . . . . . . . . . . . . . . . . 4 6
     4.3.  Backwards Compatibility . . . . . . . . . . . . . . . . . . 6 7
     4.4.  Interoperability and Change Control . . . . . . . . . . . . 6 8
     4.5.  Scope of Work . . . . . . . . . . . . . . . . . . . . . . . 6 8
     4.6.  Applicability of Automated Key Management Requirements  . . 7 8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8 9
   7.  Acknowledgements  Acknowledgments   . . . . . . . . . . . . . . . . . . . . . . . 8  10
   8.  Informative References  . . . . . . . . . . . . . . . . . . . . 8  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9  11

1.  Introduction

1.1.  General

   This memo describes the requirements for a crypto-agility solution
   for Remote Authentication Dial-In User Service (RADIUS).  This memo,
   when approved, reflects the consensus of the RADIUS Extensions
   Working Group of the IETF (RADEXT) as to the features, properties and
   limitations of the crypto-agility work item for RADIUS.  It also
   defines the term "crypto-agility" as used in this context, and
   provides the motivations for undertaking and completing this work.

   The requirements defined in this memo have previously been expressed
   in e-mail developed based on e-
   mail messages posted to the RADEXT WG mailing list, which may be
   found in the archives of that list.  The purpose of framing the
   requirements in this memo is to formalize and memorialize them for
   future reference, and to bring them explicitly to the attention of
   the IESG and the IETF Community, as we proceed with this work.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   A RADIUS crypto-agility solution is not compliant with this
   specification if it fails to satisfy one or more of the MUST or MUST
   NOT statements.  A solution 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".

1.3.  The Charge

   At the IETF-66 meeting, the RADEXT WG was asked by members of the
   Security Area Directorate to undertake the action item to prepare a
   formal description of a crypto-agility work item, and corresponding
   milestones in the RADEXT Charter.  After consultation with one of the
   Security Area Directors, Russ Housley, text was initially proposed on
   the RADEXT WG mailing list on October 26, 2006.  That text reads as

   The RADEXT WG will review the security requirements for crypto-
   agility in IETF protocols, and identify the deficiencies of the
   existing RADIUS protocol specifications against these requirements.
   Specific attention will be paid to RFC 4962 [RFC4962].

   The RADEXT WG will propose one or more Internet Drafts specifications to remediate
   any identified deficiencies in the crypto-agility properties of the
   RADIUS protocol.  The known deficiencies include the issue of
   negotiation of substitute algorithms for the message digest
   functions, the key-wrap functions, and the password-hiding function.
   Additionally, at least one mandatory to implement cryptographic
   algorithm will be defined in each of these areas, as required.

2.  A Working Definition of Crypto-Agility

   A generalized definition of crypto-agility was offered up at the
   RADEXT WG session during IETF-68.  Crypto-Agility is the ability for of a
   protocol to adapt to evolving cryptography and security requirements.
   This may include the provision of a modular mechanism to allow
   cryptographic algorithms to be updated without substantial disruption
   to fielded implementations.  It may provide for the dynamic
   negotiation and installation of cryptographic algorithms within
   protocol implementations (think of Dynamic-Link Libraries (DLL)).

   In the specific context of the RADIUS protocol and RADIUS
   implementations, crypto-agility may be better defined as the ability
   of RADIUS implementations to automatically negotiate cryptographic
   algorithms for use in RADIUS exchanges, including the algorithms used
   to integrity protect and authenticate RADIUS packets and to hide
   RADIUS Attributes.  This capability covers all RADIUS message types:
   Access-Request/Response, Accounting-Request/Response and Accounting-Request/Response, CoA/Disconnect-
   Request/Response, and CoA/Disconnect-Request/Response.

3.  The Current State Status-Server.  Negotiation of cryptographic
   algorithms MAY occur within the RADIUS Security

   RADIUS packets, as defined in [RFC2865], are protected by an MD5
   message integrity check (MIC), protocol, or within a lower
   layer such as the Authenticator field of transport layer.

   Since RADIUS packets other than Access-Request.  The Message-Authenticator
   Attribute utilizes HMAC-MD5 is a request/response protocol, the ability to authenticate and integrity protect negotiate
   cryptographic algorithms within a single RADIUS packets.  Various exchange is
   inherently limited.  While a RADIUS attributes support hidden values,
   including: User-Password, Tunnel-Password, and various Vendor-
   Specific Attributes.  Generally speaking, the request can provide a list of
   supported cryptographic algorithms which can selected for use within
   a response, prior to the receipt of a response, the cryptographic
   algorithms utilized to provide security services within the request
   will need to be determined a-priori.

   Proposals MUST NOT introduce new capabilities negotiation features
   into the RADIUS protocol and crypto-agility solutions SHOULD NOT
   require changes to the RADIUS operational model as defined in "RADIUS
   Design Guidelines" [RFC6158] Section 3.1 and Appendix A.4.
   Similarly, a proposal SHOULD focus on the crypto-agility problem and
   nothing else.  For example, proposals SHOULD NOT require new
   attribute formats and SHOULD be compatible with the guidance provided
   in [RFC6158] Section 2.3.

   Acceptable solutions for determining the mechanisms to be used within
   a request include manual configuration as well as backward compatible
   negotiation techniques such as those described in Section 4.3.
   Solutions for determining the mechanisms to be used in a response
   include manual configuration and "advertise and select" mechanisms
   (e.g. selection within the response from mechanisms advertised in a

3.  The Current State of RADIUS Security

   RADIUS packets, as defined in [RFC2865], are protected by an MD5
   message integrity check (MIC), within the Authenticator field of
   RADIUS packets other than Access-Request [RFC2865] and Status-Server
   [RFC5997].  The Message-Authenticator Attribute utilizes HMAC-MD5 to
   authenticate and integrity protect RADIUS packets.

   While RADIUS does not support confidentiality of entire packets,
   various RADIUS attributes support encrypted (also known as "hidden")
   values, including: User-Password (defined in [RFC2865] Section 5.2),
   Tunnel-Password (defined in [RFC2868] Section 3.5), and various
   Vendor-Specific Attributes, such as the MS-MPPE-Send-Key and MS-MPPE-
   Recv-Key attributes (defined in [RFC2548] Section 2.4).  Generally
   speaking, the hiding mechanism uses a stream cipher based on a key
   stream from an MD5 digest.

   Recent work on  Attacks against this mechanism are
   described in [RFC3579] Section 4.3.4.

   "Updated Security Considerations for the MD5 Message-Digest and the
   HMAC-MD5 Algorithms" [RFC6151] discusses security considerations for
   use of the MD5 and HMAC-MD5 algorithms.  While the advances in MD5
   collisions does do not immediately compromise any the use of
   these methods, absent MD5 or HMAC-MD5
   for the purposes used within RADIUS Eabsent knowledge of the RADIUS
   shared secret.
   However, secret, the progress toward compromise of MD5's basic
   cryptographic assumptions has resulted in the deprecation of MD5
   usage in a variety of applications.  As noted in [RFC6151] Section 2:

      MD5 is no longer acceptable where collision resistance is required
      such as digital signatures.  It is not urgent to stop using MD5 in
      other ways, such as HMAC-MD5; however, since MD5 must not be used
      for digital signatures, new protocol designs should not employ

4.  The Requirements

4.1.  Overall Solution Approach

   RADIUS crypto-agility solutions are not restricted to utilizing
   technology described in existing RFCs.  Since RADIUS over IPsec is
   already described in [RFC3162] Section 5 and [RFC3579], [RFC3579] Section 4.2,
   this technique is already available to those who wish to use it.

   Therefore, it is expected that proposals will utilize other

4.2.  Security Services

   Proposals MUST support the negotiation of cryptographic algorithms
   for per-packet integrity/authentication protection.  Support  It is
   RECOMMENDED that solutions provide support for
   confidentiality confidentiality,
   either by supporting encryption of entire RADIUS packets is OPTIONAL.  However,
   proposals MUST or by
   encrypting individual RADIUS attributes.  This includes providing
   support for improving the negotiation confidentiality of algorithms for encryption existing encrypted
   (sometimes referred to as "hiding") of RADIUS attributes.  It is
   RECOMMENDED for proposals to provide for the encryption
   of existing attributes.  This includes existing "hidden" "hidden") attributes as well as encrypting
   attributes (such as location attributes) that require
   confidentiality. are currently
   transmitted in cleartext.  Proposals supporting confidentiality MUST
   support the negotiation of cryptographic algorithms for encryption.

   Proposals MUST support per-packet replay protection for all RADIUS
   message types.

   Crypto-agility solutions MUST avoid security compromise, even in
   situations where the existing cryptographic algorithms utilized by
   RADIUS implementations are shown to be weak enough to provide little
   or no security (e.g. in event of compromise of the legacy RADIUS
   shared secret).  Included in this would be protection against bidding
   down attacks.  In analyzing the resilience of a crypto-agility
   solution, it can be assumed that the RADIUS server requesters and responders can
   be configured to require the use of new secure algorithms in the
   event of a compromise of existing cryptographic algorithms or the
   legacy RADIUS shared secret.

   Crypto-agility solutions MUST specify mandatory-to-implement
   cryptographic algorithms for each defined mechanism.  Guidance on
   acceptable algorithms can be found in [NIST-SP800-131A].  In
   particular, it is RECOMMENDED that mandatory-to-implement
   cryptographic algorithms be chosen from among those classified as
   "Acceptable" with no known deprecation date within [NIST-SP800-131A].

   In addition to the above goals, goals referred to above, [RFC4962] Section 2
   describes additional security requirements, which translate into the
   following requirements for RADIUS crypto-agility solutions:

Strong, fresh session keys. keys
     RADIUS crypto-agility solutions are REQUIRED to generate fresh
     session keys for use between the RADIUS client and server.  In
     order to prevent the disclosure of one session key from aiding an
     attacker in discovering other session keys,  RADIUS crypto-agility
     solutions are RECOMMENDED to support Perfect Forward Secrecy (PFS)
     with respect to session keys negotiated between the RADIUS client
     and server.

Limit key scope. scope
     In order to enable a NAS and RADIUS server to transmit keying
     material directly, it is RECOMMENDED that a RADIUS crypto-
         agility crypto-agility
     solution be compatible with NAI-based Dynamic Peer Discovery
     [RADYN] as well as that it support the use of public key
     credentials for authentication between the NAS and RADIUS server.
     For compatibility with existing operations, RADIUS crypto-agility
     solutions SHOULD also support pre-shared key credentials.

Prevent the Domino effect. effect
     In order to prevent the domino effect, RADIUS crypto-agility
     solutions MUST enable each RADIUS client and server pair to
     authenticate utilizing unique credentials.

4.3.  Backwards Compatibility

   Solutions to the problem MUST demonstrate backward compatibility with
   existing RADIUS implementations.  That is, an implementation that
   supports both the crypto-agility solution and legacy mechanisms MUST
   be able to talk with legacy RADIUS clients and servers (using the
   legacy mechanisms). Acceptable solutions

   While backward compatibility is needed to determining which set ease the transition between
   legacy RADIUS and crypto-agile RADIUS, use of legacy mechanisms is used (with a particular peer) include some kind
   only appropriate prior to the compromise of
   negotiation, and manual configuration.

   Proposals those mechanisms.  After
   legacy mechanisms have been compromised, secure algorithms MUST NOT introduce new capabilities negotation features
   into be
   used, so that backward compatibility is no longer possible.

   In order to enable a request to be handled both by legacy as well as
   crypto-agile implementations, a request can be secured with legacy
   algorithms as well as more secure algorithms.  While this approach
   permits the RADIUS protocol, but rather MUST initial use of legacy algorithms between crypto-agile
   implementations, if the existing
   mechanisms.  Included responder indicates support for crypto-
   agility, future requests can omit use of legacy algorithms, instead
   utilizing mechanisms indicated in such negotiation techniques are "hint and
   accept" and "hint the response.

   This approach minimizes the response delay from both legacy and reject" mechanisms,
   crypto-agile implementations.  However, it is viable only where
   legacy hidden attributes (e.g. User-Password) are not included within

   Probing techniques can avoid the NAS (RADIUS
   client) provides a list use of supported legacy algorithms between
   crypto-agile implementations.  An initial request can omit use of
   legacy mechanisms, and if a response is received, then the RADIUS server
   selects one.

   Crypto-agility solutions SHOULD NOT require changes recipient
   can be assumed to the RADIUS
   operational model as defined in "RADIUS Design Guidelines" [RFC6158]
   Section 3.1 be crypto-agile and Appendix A.4. future requests to that
   recipient can utilize secure mechanisms.  Similarly, a proposal SHOULD focus on the responder
   can assume that the requester supports crypto-agility problem and nothing else.  For example, proposals
   SHOULD NOT require can
   prohibit use of legacy mechanisms in future requests.

   If a response is not received, in the absence of information
   indicating responder support for crypto-agility (such as pre-
   configuration or previous receipt of a crypto-agile response), a new attribute formats
   request can be composed utilizing legacy mechanisms.

   Since legacy implementations not supporting crypto-agility will
   silently discard requests not protected by legacy algorithms rather
   than returning an error, repeated requests may be required to
   distinguish lack of support for crypto-agility from packet loss or
   other failure conditions.  As a result, probing techniques can delay
   initial communication between crypto-agile requesters and SHOULD legacy
   responders.  This can be compatible
   with addressed by upgrading the guidance provided in [RFC6158] Section 2.3. responders (e.g.
   RADIUS servers) first.

4.4.  Interoperability and Change Control

   Proposals MUST indicate a willingness to cede change control to the

   Crypto-agility solutions MUST be interoperable between independent
   implementations based purely on the information provided in the

4.5.  Scope of Work

   Crypto-agility solutions MUST apply to all RADIUS packet types,
   including Access-Request, Access-Challenge, Access-Reject, Access-
   Accept, Accounting-Request, Accounting-Response, Status-Server and
   CoA/Disconnect messages.

   Since it is expected that the work will occur purely within RADIUS or
   in the transport, message data exchanged with Diameter SHOULD NOT be

   Proposals MUST discuss any inherent assumptions about, or limitations
   on, client/server operations or deployment and SHOULD provide
   recommendations for transition of deployments from legacy RADIUS to
   crypto-agile RADIUS.  Issues regarding ciper-suite cipher-suite negotiation,
   legacy interoperability and the potential for biding bidding down attacks,
   SHOULD be among these discussions.

4.6.  Applicability of Automated Key Management Requirements

   [RFC4107] provides guidelines for when automated key management is
   necessary.  At the IETF-70 meeting, and leading up to that meeting,
   the RADEXT WG debated whether or not RFC 4107 would require a RADIUS
   Crypto-Agility solution to feature Automated Key Management (AKM).
   The working group determined that AKM was not inherently required for
   RADIUS based on the following points:

   o  RFC 4107 requires AKM for protocols that involve O(n^2) keys.
      This does not apply to RADIUS deployments, which require O(n) keys

   o  Requirements for session key freshness can be met without AKM,
      for example, by utilizing a pre-shared key along with an exchange
      of nonces.

   o  RADIUS does not require the encryption of large amounts of data in
      a short time time.

   o  Organizations already have operational practices to manage
      existing RADIUS shared secrets to address key changes required
      as a result of personnel changes changes.

   o  The crypto-agility solution can avoid use cryptographic modes of
      operation such as a counter mode cipher that require frequent key

   However, the same time, it is recognized that features recommended in
   Section 4.2 such as support for PFS and direct transport of keys
   between a NAS and RADIUS server, can only be provided by a solution
   supporting AKM.  As a result, support for Automated Key Management is
   RECOMMENDED within a RADIUS crypto-agility solution.

   Also, automated key management is REQUIRED for RADIUS crypto
   agility crypto-agility
   solutions that use cryptographic modes of operation that require
   frequent key changes.

5.  IANA Considerations

   This document makes no request of IANA.

6.  Security Considerations

   Potential attacks against the RADIUS protocol are described in RFC
   3579 [RFC3579] Section 4.1, and details of known exploits as well as
   potential mitigations are discussed in [RFC3579] Section 4.3.

   This specification describes the requirements for new cryptographic
   protection mechanisms, including the modular selection of algorithms
   and modes.  Therefore, the subject matter of this memo is all about

7.  Acknowledgements  Acknowledgments

   Thanks to all the reviewers and contributors, inclding including Bernard
   Aboba, Joe Salowey and Glen Zorn.

8.  Informative References

           Barker, E. and A. Roginsky, "Transitions: Recommendation for
           Transitioning the Use of Cryptographic Algorithms and Key
           Lengths", NIST SP-800-131A, January 2011.

[RADYN]     Winter, S. and M. McCauley, "NAI-based Dynamic Peer
           Discovery for RADIUS over TLS and DTLS", Internet draft, work
           in progress, March 2010.

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

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

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

[RFC2868]  Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M.
           and I. Goyret, "RADIUS Attributes for Tunnel Protocol
           Support", RFC 2868, June 2000.

[RFC3162]  Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC
           3162, August 2001.

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

[RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
           Key Management", BCP 107, RFC 4107, June 2005.

[RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
           Authorization, and Accounting (AAA) Key Management", BCP 132,
           RFC 4962, July 2007.

[RFC5997]  DeKok, A., "Use of Status-Server Packets in the Remote
           Authentication Dialin User Service (RADIUS) Protocol", RFC
           5997, August 2010.

[RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations for
           the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC
           6151, March 2011.

[RFC6158]  DeKok, A., "RADIUS Design Guidelines", BCP X, 158, RFC 6158,
           March 2011.

Author's Address

   David B. Nelson
   Elbrys Networks, Inc.
   282 Corporate Drive, Unit 1
   Portsmouth, NH  03801