Abstract

The Kerberos 5 network authentication protocol, originally specified in RFC 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 updates RFC 1964, RFC 4120, RFC 4121, and RFC 4757 to deprecate the use of DES, RC4-HMAC-EXP, and other weak cryptographic algorithms in Kerberos. Because RFC 1510 (obsoleted by RFC 4120) supports only DES, this document recommends the reclassification of RFC 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 in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6649.

Copyright Notice

Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 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.

1. 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.)

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 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 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 [RFC1964]).

The effect of the two last sentences is that this document deprecates Section 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.

8. References

8.1. 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>.

[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:

          [email protected]

Tom Yu
MIT Kerberos Consortium
77 Massachusetts Ave.
Cambridge, Massachusetts
USA

EMail:

          [email protected]