Network Working Group                               L. Hornquist Astrand
Internet-Draft                                                Apple, Inc
Updates: 1510, 1964, 4120, 4121, 4757                              T. Yu
(if approved)                                    MIT Kerberos Consortium
Intended status: BCP                                   February 16, 27, 2012
Expires: August 19, 30, 2012

Deprecate DES, "export strength" RC4, RC4-HMAC-EXP, and other weak cryptographic algorithms in


   The Kerberos 5 network authentication protocol, originally specified
   in RFC1510, can use the Data Encryption Standard (DES) for
   encryption.  Almost 30 years after first publishing DES, the National
   Institute of Standards and Technology (NIST) finally withdrew the
   standard in 2005, reflecting a long-established consensus that DES is
   insufficiently secure.  By 2008, commercial hardware 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, RFC4121, and RFC4757 to deprecate the use
   of DES, "export strength" RC4, RC4-HMAC-EXP, and other weak cryptographic algorithms in
   Kerberos.  Because RFC1510 (obsoleted by RFC4120) supports only DES,
   this document 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 19, 30, 2012.

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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, mostly because of its
   small key size.  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 and other weak cryptographic algorithms in
   Kerberos, including some unkeyed checksums and hashes, along with the
   weak 56-bit "export strength" RC4 variant enctype of [RFC4757].

3.  Affected specifications

   The original IETF specification of Kerberos 5 [RFC1510] only supports
   DES for encryption.  [RFC4120] obsoletes [RFC1510] and updates the
   Kerberos specification to include additional cryptographic
   algorithms, but still permits the use of DES.  [RFC3961] describes
   the Kerberos cryptographic system and includes support for DES
   encryption types, but it does not specify requirement levels for

   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 RC4-HMAC
   based GSS-API mechanism, and MIT's DES3 (which is not published as an
   RFC), there is no need to support the old DES based integrity (SGN)
   and confidentiality (SEAL) types.

   [RFC4757] describes the RC4-HMAC encryption types used by Microsoft
   Windows, and allows for a 56-bit "export strength" variant.  (The
   character constant "fortybits" used in the definition is a historical
   reference and does not refer to the actual key size of the enctype.)

4.  DES insecurity

   The insecurity of DES has been evident for many years.  Even around
   the time of its first publication, cryptographers raised the
   possibility that 56 bits was too small a key size for DES.  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 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 in [RFC3961]).

   Kerberos implementations and deployments SHOULD NOT provide implement or
   deploy the following single DES encryption types: DES-CBC-CRC(1), DES-CBC-
   DES-CBC-MD4(2), DES-CBC-MD5(3) (updates [RFC4120]).

   Kerberos implementations and deployments SHOULD NOT provide implement or
   deploy the following "export strength" RC4 variant encryption type:
   RC4-HMAC-EXP(24) (updates [RFC4757]).  This document does not add any
   sort of requirement for conforming implementations to provide RC4-HMAC(23). implement RC4-

   Kerberos implementations and deployments SHOULD NOT provide implement or
   deploy the following checksum types: CRC32(1), RSA-MD4(2), RSA-MD4-DES(3), DES-
   MAC(4), RSA-MD4-
   DES(3), DES-MAC(4), DES-MAC-K(5), RSA-MD4-DES-K(6), RSA-MD5-DES(8)
   (updates [RFC4120]).

   It is possible to safely use the RSA-MD5(7) checksum type, but only
   with additional protection, such as the protection that an encrypted
   Authenticator provides.  Implementations MAY use RSA-MD5 inside an
   encrypted Authenticator for backward compatibility with systems that
   do not support newer checksum types (updates [RFC4120]).  One example
   is that some legacy systems only support RC4-HMAC(23) [RFC4757] for
   encryption when DES is not available; these systems use RSA-MD5
   checksums inside Authenticators encrypted with RC4-HMAC.

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

   Kerberos GSS mechanism implementations and deployments SHOULD NOT
   implement or deploy the following SEAL ALG: DES(0000) (updates

   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.  Acknowledgements

   Mattias Amnefelt, Ran Atkinson, Henry Hotz, Jeffrey Hutzelman, Leif
   Johansson, and Simon Josefsson Josefsson, and Martin Rex have read the document and
   provided suggestions for improvements.  Sam Hartman proposed moving
   [RFC1510] to Historic.  Michiko Short provided information about the
   dates of end of support for Windows releases.

7.  Security Considerations

   Removing support for single DES improves security, because DES is
   considered to be insecure.  RC4-HMAC-EXP has a similarly inadequate
   key size, so removing support for it also improves security.

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

   The security considerations of [RFC4757] continue to apply to RC4-
   HMAC, including the known weaknesses of RC4 and MD4, and this
   document does not change the Informational status of [RFC4757] for
   now.  The main reason to not actively discourage the use of RC4-HMAC
   is that it is the only encryption type that interoperates with older
   versions of Microsoft Windows once DES and RC4-HMAC-EXP are removed.
   These older versions of Microsoft Windows will likely be in use until
   at least 2015.

8.  IANA Considerations

   There are no IANA Considerations for this 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.

   [RFC4757]  Jaganathan, K., Zhu, L., and J. Brezak, "The RC4-HMAC
              Kerberos Encryption Types Used by Microsoft Windows",
              RFC 4757, December 2006.

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'06 - Special-purpose Hardware for Attacking
              Cryptographic Systems", April 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", 70 FR 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.

   [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