Network Working Group                               L. Hornquist Astrand
Internet-Draft                                                Apple, Inc
Updates: 1510, 1964, 3961, 4120, 4121                                    T. Yu
(if approved)                                    MIT Kerberos Consortium
Intended status: Standards Track                        February 3, 8, 2012
Expires: August 6, 11, 2012

                   Deprecate DES support for Kerberos


   The Kerberos 5 network authentication protocol protocol, originally used specified
   in RFC1510, can use the Data Encryption Standard (DES) for
   encryption.  Almost 30 years it
   introduced the after first DES standard, publishing DES, the National
   Institute of Standards and Technology (NIST) finally withdrew the DES
   standard in 2005, reflecting a long-established consensus that DES no longer
   provides adequate security.  In 2006, it cost less than EUR 10,000 to
   build a machine to brute force DES keys in an average of 8.7 days. is
   insufficiently secure.  By 2008, it was possible to buy commercial hardware that breaks costing less
   than USD 15,000 could break DES keys in less than a day on average.
   DES is long past its sell-by date.  Accordingly, this document
   updates RFC1964, RFC4120, and RFC4121 to deprecate the use of DES in
   Kerberos.  Because the
   original Kerberos 5 specification, RFC1510 (obsoleted by RFC 4120), RFC4120) supports only DES,
   this document also reclassifies RFC1510 as Historic.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 6, 11, 2012.

Copyright Notice

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   described in the Simplified BSD License.

1.  Requirements Notation

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

2.  Introduction

   The original specification of the Kerberos 5 network authentication
   protocol [RFC1510] supports only the Data Encryption Standard (DES)
   for encryption.  For many years, the cryptographic community has
   regarded DES as providing inadequate security.  This  Accordingly, this
   document reclassifies [RFC1510] (obsoleted by [RFC4120]) as Historic,
   and updates current Kerberos-related specifications [RFC1964],
   [RFC4120], and [RFC4121] to deprecate the use of DES in Kerberos.

3.  Affected specifications

   The initial original IETF specification of the Kerberos 5 network
   authentication protocol [RFC1510] supports only the Data Encryption
   Standard (DES) supports
   DES for encryption.  [RFC4120] obsoletes [RFC1510] and updates the
   Kerberos specification to include additional cryptographic algorithms.
   [RFC3961] describes the Kerberos cryptographic system and includes
   support for Data Encryption Standard (DES) encryption types.

   DES no longer provides adequate cryptographic protection.  This
   document updates [RFC1964], [RFC4120], and [RFC4121] to deprecate
   algorithms, but still permits the use of DES in Kerberos.  Because the version of Kerberos specified in
   [RFC1510] supports only DES, and [RFC4120] supersedes it, this
   document reclassifies [RFC1510] as Historic. DES.

   The specification of the Kerberos Generic Security Services
   Application Programming Interface (GSS-API) mechanism [RFC1964] and
   its updated version [RFC4121] define checksum and encryption
   mechanisms based on DES.  With the existence of newer encryption
   types for Kerberos GSS-API defined in [RFC4121], Microsoft's ARCFOUR-
   HMAC based GSS-API mechanism, and MIT's DES3, there is no need to
   support the old DES based integrity (SGN) and confidentiality (SEAL)

4.  DES insecurity

   The insecurity of DES has been evident for many years.  The National
   Institute of Standards and Technology (NIST) officially withdrew DES
   in 2005 [DES-Withdrawal], and also announced a transition period that
   ended on May 19, 2007 [DES-Transition-Plan].  The IETF has also
   published its position in [RFC4772], in which the recommendation
   summary is very clear: "don't use DES".

   In 2006, researchers demonstrated the ability to brute force a DES
   key in an average of less than 9 days using less than EUR 10,000
   worth of hardware [Break-DES].  By 2008, a company was offering
   hardware capable of breaking a DES key in less than a day on average
   [DES-1day] that cost less than USD 15,000 [DES-crack].  Brute force
   key searches of DES will only get faster and cheaper.  (The
   aforementioned company markets its device for one-
   click one-click recovery of
   lost DES keys.)  It is clear that it is well past time to retire the
   use of DES in Kerberos.

5.  Recommendations

   This document hereby removes the following RECOMMENDED types from
      Encryption: DES-CBC-MD5(3)
      Checksums: DES-MD5 (8, named RSA-MD5-DES from in [RFC3961]).

   Kerberos implementation implementations and deployments SHOULD NOT implement the
   single DES encryption types: DES-CBC-CRC(1), DES-CBC-MD4(2), DES-CBC-

   Kerberos implementation implementations and deployments SHOULD NOT implement the
   checksum types: CRC32(1), RSA-MD4(2), RSA-MD4-DES(3), DES-MAC(4),
   DES-MAC-K(5), RSA-MD4-MAC-K(6), DES-MD5(7), RSA-MD5-DES(8).

   Note that RSA-MD5 might be used with non-DES encryption types, for
   example, when doing a TGS-REQ with ARCFOUR-HMAC-MD5, some clients

   It is possible to safely use
   RSA-MD5 for the RSA-MD5(7) checksum that is stored inside type, but only
   with additional protection, such as the protection that an encrypted part of
   the authenticator.  This
   Authenticator provides.  Implementations MAY use of RSA-MD5 is to be considered safe, so
   Kerberos implementations MAY allow this usage when used inside an
   encrypted Authenticator for backward compatibility with legacy systems that can't handle
   do not support newer checksum types.  One example is that some legacy
   systems only support ARCFOUR-HMAC-MD5 for encryption when DES is not
   available; these systems use RSA-MD5 checksums inside Authenticators
   encrypted with ARCFOUR-HMAC-MD5.

   Kerberos GSS mechanism implementation implementations and deployments SHOULD NOT
   implement the SGN ALG: DES MAC MD5(0000), MD2.5(0100), DES MAC(0200)
   (updates [RFC1964]).

   Kerberos GSS mechanism implementation implementations and deployments SHOULD NOT
   implement the SEAL ALG: DES(0000) (updates [RFC1964]).

   The effect of the two last sentences is that this document deprecates
   section 1.2 in [RFC1964].

   This document hereby reclassifies [RFC1510] as Historic.

6.  Other old enctypes

   The following older encryption types and checksum types should not be
   deployed and can be disabled by default by implementations:

   XXX add list here

7.  Acknowledgements

   Jeffrey Hutzelman, Simon Josefsson, Mattias Amnefelt, Leif Johansson,
   and Ran Atkinson have read the document and provided suggestions for
   improvements.  Sam Hartman proposed moving [RFC1510] to Historic.


7.  Security Considerations

   Removing support for single DES improves security since security, because DES is
   considered to be insecure.

   Kerberos defines some encryption types that are either underspecified
   or that were only allocated a have number in the assignments but no specifications.
   Implementations should make sure that they only implement and enable
   secure encryption types.

   RC4, used in ARCFOUR-HMAC, is considered weak; however, the use in
   Kerberos is vetted and considered secure for now.  The main reason to
   not actively discourage the use of ARCFOUR-HMAC is that it is the
   only encryption type that interoperates with older versions of
   Microsoft Windows once DES is removed.


8.  IANA Considerations

   There are no IANA Considerations for this document

10. document.

9.  References


9.1.  Normative References

   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
              RFC 1964, June 1996.

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

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              July 2005.


9.2.  Informative References

              Kumar, S., Paar, C., Pelzl, J., Pfeiffer, G., Rupp, A.,
              and M. Schimmler, "How to break DES for EUR 8,980 - SHARCS
              workshop 2006",
              SHARCS'06 - Special-purpose Hardware for Attacking
              Cryptographic Systems", April 2006. 2006, <http://

              SciEngines GmbH, "Break DES in less than a single day", <h

              National Institute of Standards and Technology, "DES
              Transition Plan", May 2005, <

              National Institute of Standards and Technology,
              "Announcing Approval of the Withdrawal of Federal
              Information Processing Standard (FIPS) 46-3, Data
              Encryption Standard (DES); FIPS 74, Guidelines for
              Implementing and Using the NBS Data Encryption Standard;
              and FIPS 81, DES Modes of Operation - Federal Register
              Document 05-9945, 05-9945", 70 FR 28907-28908", 28907-28908, May 2005, <http://

              Scott, T., "DES Brute Force Cracking Efforts 1977 to
              2010", 2010, <

   [RFC1510]  Kohl, J. and B. Neuman, "The Kerberos Network
              Authentication Service (V5)", RFC 1510, September 1993.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC4772]  Kelly, S., "Security Implications of Using the Data
              Encryption Standard (DES)", RFC 4772, December 2006.

Authors' Addresses

   Love Hornquist Astrand
   Apple, Inc


   Tom Yu
   MIT Kerberos Consortium
   77 Massachusetts Ave
   Cambridge, Massachusetts