Network Working Group                                          D. Nelson
Internet-Draft                                     Elbrys Networks, Inc.
Intended status: Informational                             March 3, 2011
Expires: September 3, 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).

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with
   the provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 3, 2011.

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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Copyright Notice

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Table of Contents

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

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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 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.  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 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 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
   a protocol to adapt to evolving cryptography and security

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   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

   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 protect RADIUS packets and to hide RADIUS Attributes.
   This capability covers all RADIUS message types:
   Access-Request/Response, Accounting-Request/Response and
   and CoA/Disconnect-Request/Response.

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.  The Message-Authenticator
   Attribute utilizes HMAC-MD5 to authenticate and integrity protect
   RADIUS packets.  Various RADIUS attributes support hidden values,
   including: User-Password, Tunnel-Password, and various Vendor-
   Specific Attributes.  Generally speaking, the hiding mechanism uses a
   stream cipher based on a key stream from an MD5 digest.

   Recent work on MD5 collisions does not immediately compromise any of
   these methods, absent knowledge of the RADIUS shared secret.
   However, the progress toward compromise of MD5's basic cryptographic
   assumptions has resulted in the deprecation of MD5 usage in a variety
   of applications.

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] and [RFC3579], this technique is
   already available to those who wish to use it.  Therefore, it is
   expected that proposals will utilize other techniques.

4.2.  Security Services

   Proposals MUST support the negotiation of cryptographic algorithms
   for per-packet integrity/authentication protection.  Support for
   confidentiality of entire RADIUS packets is OPTIONAL.  However,
   proposals MUST support the negotiation of algorithms for encryption

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   (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" attributes
   as well as attributes (such as location attributes) that require

   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 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
   algorithms for each defined mechanism.

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

   Strong, fresh session 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.
         In order to enable a NAS and RADIUS server to transmit keying
         material directly, it is RECOMMENDED that a RADIUS 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

   Prevent the Domino 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.

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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 to determining which set of
   mechanisms is used (with a particular peer) include some kind of
   negotiation, and manual configuration.

   Proposals MUST NOT introduce new capabilities negotation features
   into the RADIUS protocol, but rather MUST use the existing
   mechanisms.  Included in such negotiation techniques are "hint and
   accept" and "hint and reject" mechanisms, where the NAS (RADIUS
   client) provides a list of supported algorithms and the RADIUS server
   selects one.

   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.

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, and CoA/Disconnect

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

   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 negotiation,
   legacy interoperability and the potential for biding down attacks,
   SHOULD be among these discussions.

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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

   o  Organizations already have operational practices to manage
      existing RADIUS shared secrets to address key changes required
      as a result of personnel 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 solutions that use cryptographic modes of operation that
   require frequent key changes.

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5.  IANA Considerations

   This document makes no request of IANA.

6.  Security Considerations

   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

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

8.  Informative References

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

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

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

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

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Author's Address

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


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