Network Working Group                                        S. Bellovin
Request for Comments: 4107                           Columbia University
BCP: 107                                                      R. Housley
Category: Best Current Practice                           Vigil Security
                                                               June 2005

              Guidelines for Cryptographic 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 Internet Society (2005).

Abstract

   The question often arises of whether a given security system requires
   some form of automated key management, or whether manual keying is
   sufficient.  This memo provides guidelines for making such decisions.
   When symmetric cryptographic mechanisms are used in a protocol, the
   presumption is that automated key management is generally but not
   always needed.  If manual keying is proposed, the burden of proving
   that automated key management is not required falls to the proposer.

1.  Introduction

   The question often arises of whether or not a given security system
   requires some form of automated key management, or whether manual
   keying is sufficient.

   There is not one answer to that question; circumstances differ.  In
   general, automated key management SHOULD be used.  Occasionally,
   relying on manual key management is reasonable; we propose some
   guidelines for making that judgment.

   On the other hand, relying on manual key management has significant
   disadvantages, and we outline the security concerns that justify the
   preference for automated key management.  However, there are
   situations in which manual key management is acceptable.

1.1.  Terminology

   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 [B].

2.  Guidelines

   These guidelines are for use by IETF working groups and protocol
   authors who are determining whether to mandate automated key
   management and whether manual key management is acceptable.  Informed
   judgment is needed.

   The term "key management" refers to the establishment of
   cryptographic keying material for use with a cryptographic algorithm
   to provide protocol security services, especially integrity,
   authentication, and confidentiality.  Automated key management
   derives one or more short-term session keys.  The key derivation
   function may make use of long-term keys to incorporate authentication
   into the process.  The manner in which this long-term key is
   distributed to the peers and the type of key used (pre-shared
   symmetric secret value, RSA public key, DSA public key, and others)
   is beyond the scope of this document.  However, it is part of the
   overall key management solution.  Manual key management is used to
   distribute such values.  Manual key management can also be used to
   distribute long-term session keys.

   Automated key management and manual key management provide very
   different features.  In particular, the protocol associated with an
   automated key management technique will confirm the liveness of the
   peer, protect against replay, authenticate the source of the short-
   term session key, associate protocol state information with the
   short-term session key, and ensure that a fresh short-term session
   key is generated.  Further, an automated key management protocol can
   improve interoperability by including negotiation mechanisms for
   cryptographic algorithms.  These valuable features are impossible or
   extremely cumbersome to accomplish with manual key management.

   For some symmetric cryptographic algorithms, implementations must
   prevent overuse of a given key.  An implementation of such algorithms
   can make use of automated key management when the usage limits are
   nearly exhausted, in order to establish replacement keys before the
   limits are reached, thereby maintaining secure communications.

   Examples of automated key management systems include IPsec IKE and
   Kerberos.  S/MIME and TLS also include automated key management
   functions.

   Key management schemes should not be designed by amateurs; it is
   almost certainly inappropriate for working groups to design their
   own.  To put it in concrete terms, the very first key management
   protocol in the open literature was published in 1978 [NS].  A flaw
   and a fix were published in 1981 [DS], and the fix was cracked 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 obvious once described -- yet no one spotted them
   earlier.  Note that the original protocol (translated to employ
   certificates, which had not been invented at that time) was only
   three messages.

   Key management software is not always large or bloated.  Even IKEv1
   [HC] can be done in less than 200 Kbytes of object code, and TLS [DA]
   in half that space.  Note that this TLS estimate includes other
   functionality as well.

   A session key is used to protect a payload.  The nature of the
   payload depends on the layer where the symmetric cryptography is
   applied.

   In general, automated key management SHOULD be used to establish
   session keys.  Strong justification is needed in the security
   considerations section of a proposal that makes use of manual key
   management.

2.1.  Automated Key Management

   Automated key management MUST be used if any of these conditions
   hold:

      A party will have to manage n^2 static keys, where n may become
      large.

      Any stream cipher (such as RC4 [TK], AES-CTR [NIST], or AES-CCM
      [WHF]) is used.

      An initialization vector (IV) might be reused, especially an
      implicit IV.  Note that random or pseudo-random explicit IVs are
      not a problem unless the probability of repetition is high.

      Large amounts of data might need to be encrypted in a short time,
      causing frequent change of the short-term session key.

      Long-term session keys are used by more than two parties.
      Multicast is a necessary exception, but multicast key management
      standards are emerging in order to avoid this in the future.
      Sharing long-term session keys should generally be discouraged.

      The likely operational environment is one where personnel (or
      device) turnover is frequent, causing frequent change of the
      short-term session key.

2.2.  Manual Key Management

   Manual key management may be a reasonable approach in any of these
   situations:

      The environment has very limited available bandwidth or very high
      round-trip times.  Public key systems tend to require long
      messages and lots of computation; symmetric key alternatives, such
      as Kerberos, often require several round trips and interaction
      with third parties.

      The information being protected has low value.

      The total volume of traffic over the entire lifetime of the long-
      term session key will be very low.

      The scale of each deployment is very limited.

   Note that assertions about such things should often be viewed with
   skepticism.  The burden of demonstrating that manual key management
   is appropriate falls to the proponents -- and it is a fairly high
   hurdle.

   Systems that employ manual key management need provisions for key
   changes.  There MUST be some way to indicate which key is in use to
   avoid problems during transition.  Designs SHOULD sketch plausible
   mechanisms for deploying new keys and replacing old ones that might
   have been compromised.  If done well, such mechanisms can later be
   used by an add-on key management scheme.

   Lack of clarity about the parties involved in authentication is not a
   valid reason for avoiding key management.  Rather, it tends to
   indicate a deeper problem with the underlying security model.

2.3.  Key Size and Random Values

   Guidance on cryptographic key size for public keys that are used for
   exchanging symmetric keys can be found in BCP 86 [OH].

   When manual key management is used, long-term shared secret values
   SHOULD be at least 128 bits.

   Guidance on random number generation can be found in BCP 106 [ESC].

   When manual key management is used, long-term shared secrets MUST be
   unpredictable "random" values, ensuring that an adversary will have
   no greater expectation than 50% of finding the value after searching
   half the key search space.

3.  Security Considerations

   This document provides guidance to working groups and protocol
   designers.  The security of the Internet is improved when automated
   key management is employed.

   The inclusion of automated key management does not mean that an
   interface for manual key management is prohibited.  In fact, manual
   key management is very helpful for debugging.  Therefore,
   implementations ought to provide a manual key management interface
   for such purposes, even if it is not specified by the protocol.

4.  References

   This section contains normative and informative references.

4.1.  Normative References

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

   [ESC]  Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness
          Requirements for Security", BCP 106, RFC 4086, June 2005.

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

4.2.  Informative References

   [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.

   [DA]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
          2246, January 1999.

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

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

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

   [NIST] National Institute of Standards and Technology.
          "Recommendation for Block Cipher Modes of Operation -- Methods
          and Techniques," NIST Special Publication SP 800-38A, December
          2001.

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

   [TK]   Thayer, R. and K. Kaukonen.  "A Stream Cipher Encryption
          Algorithm", Work in Progress.

   [WHF]  Whiting, D., Housley, R., and N. Ferguson , "Counter with
          CBC-MAC (CCM)", RFC 3610, September 2003.

Authors' Addresses

   Steven M. Bellovin
   Department of Computer Science
   Columbia University
   1214 Amsterdam Avenue, M.C. 0401
   New York, NY 10027-7003

   Phone: +1 212-939-7149
   EMail: bellovin@acm.org

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170

   Phone: +1 703-435-1775
   EMail: housley@vigilsec.com

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