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Internet Engineering Task Force (IETF)              L. Hornquist AstrandRequest for Comments: 6649                                   Apple, Inc.BCP: 179                                                           T. YuObsoletes:1510                                  MIT Kerberos ConsortiumUpdates:1964,4120,4121,4757                                July 2012Category: Best Current PracticeISSN: 2070-1721Deprecate DES, RC4-HMAC-EXP, and Other Weak Cryptographic Algorithmsin KerberosAbstract   The Kerberos 5 network authentication protocol, originally specified   inRFC 1510, 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   updatesRFC 1964,RFC 4120,RFC 4121, andRFC 4757 to deprecate the   use of DES, RC4-HMAC-EXP, and other weak cryptographic algorithms in   Kerberos.  BecauseRFC 1510 (obsoleted byRFC 4120) supports only   DES, this document recommends the reclassification ofRFC 1510 as   Historic.Status of This Memo   This memo documents an Internet Best Current Practice.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   BCPs is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6649.Hornquist Astrand & Yu    Best Current Practice                 [Page 1]

RFC 6649                Deprecate DES in Kerberos              July 2012Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Hornquist Astrand & Yu    Best Current Practice                 [Page 2]

RFC 6649                Deprecate DES in Kerberos              July 20121.  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 recommends the   reclassification of [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 encryption type of [RFC4757].2.  Requirements Notation   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].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   them.   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 encryption   type.)Hornquist Astrand & Yu    Best Current Practice                 [Page 3]

RFC 6649                Deprecate DES in Kerberos              July 20124.  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 find a DES key via   brute-force search 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   [RFC4120]:      Encryption: DES-CBC-MD5(3)      Checksums: DES-MD5 (8, named RSA-MD5-DES in [RFC3961]).   Kerberos implementations and deployments SHOULD NOT implement or   deploy the following single DES encryption types: DES-CBC-CRC(1),   DES-CBC-MD4(2), DES-CBC-MD5(3) (updates [RFC4120]).   Kerberos implementations and deployments SHOULD NOT 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 implement   RC4-HMAC(23).   Kerberos implementations and deployments SHOULD NOT implement or   deploy the following checksum types: CRC32(1), RSA-MD4(2),   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 anHornquist Astrand & Yu    Best Current Practice                 [Page 4]

RFC 6649                Deprecate DES in Kerberos              July 2012   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   [RFC1964]).   The effect of the two last sentences is that this document deprecatesSection 1.2 of [RFC1964].   This document hereby recommends the reclassification of [RFC1510] as   Historic.6.  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 2015.7.  Acknowledgements   Mattias Amnefelt, Ran Atkinson, Henry Hotz, Jeffrey Hutzelman, Leif   Johansson, Simon 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.Hornquist Astrand & Yu    Best Current Practice                 [Page 5]

RFC 6649                Deprecate DES in Kerberos              July 20128.  References8.1.  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 2006.8.2.  Informative References   [Break-DES] 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://www.copacobana.org/paper/copacobana_SHARCS2006.pdf>.   [DES-1day]  SciEngines GmbH, "Break DES in less than a single day",               <http://www.sciengines.com/company/news-a-events/74-des-in-1-day.html>.   [DES-Crack] Scott, T., "DES Brute Force Cracking Efforts 1977 to               2010", 2010, <http://www.tjscott.net/security.extras/des.crack.efforts.pdf>.   [DES-Transition-Plan]               National Institute of Standards and Technology, "DES               Transition Plan", May 2005, <http://csrc.nist.gov/groups/STM/common_documents/DESTranPlan.pdf>.Hornquist Astrand & Yu    Best Current Practice                 [Page 6]

RFC 6649                Deprecate DES in Kerberos              July 2012   [DES-Withdrawal]               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               Vol. 70, No. 96, Document 05-9945, 70 FR 28907-28908,               May 2005, <http://www.gpo.gov/fdsys/pkg/FR-2005-05-19/pdf/05-9945.pdf>.   [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.   Cupertino, California   USA   EMail: lha@apple.com   Tom Yu   MIT Kerberos Consortium   77 Massachusetts Ave.   Cambridge, Massachusetts   USA   EMail: tlyu@mit.eduHornquist Astrand & Yu    Best Current Practice                 [Page 7]