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, 2012
Expires: August 6, 2012
Deprecate DES support for Kerberos
draft-ietf-krb-wg-des-die-die-die-00
Abstract
The Kerberos 5 network authentication protocol originally used the
Data Encryption Standard (DES) for encryption. Almost 30 years it
introduced the first DES standard, 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.
By 2008, it was possible to buy commercial hardware that breaks 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),
supports only DES, this document also reclassifies RFC1510 as
Historic.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 6, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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described in the Simplified BSD License.
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1. 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].
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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, cryptographic community has regarded
DES as providing inadequate security. This document reclassifies
[RFC1510] as Historic, and updates current Kerberos-related
specifications [RFC1964], [RFC4120], and [RFC4121] to deprecate the
use of DES in Kerberos.
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3. Affected specifications
The initial IETF specification of the Kerberos 5 network
authentication protocol [RFC1510] supports only the Data Encryption
Standard (DES) for encryption. [RFC4120] 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 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.
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)
types.
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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]. 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.
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5. Recommendations
This document removes the following RECOMMENDED types from [RFC4120]:
Encryption: DES-CBC-MD5(3)
Checksums: DES-MD5 (8, RSA-MD5-DES from [RFC3961]).
Kerberos implementation and deployments SHOULD NOT implement the
single DES encryption types: DES-CBC-CRC(1), DES-CBC-MD4(2), DES-CBC-
MD5(3).
Kerberos implementation 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 use
RSA-MD5 for the checksum that is stored inside the encrypted part of
the authenticator. This use of RSA-MD5 is to be considered safe, so
Kerberos implementations MAY allow this usage when used with legacy
systems that can't handle newer checksum types.
Kerberos GSS mechanism implementation and deployments SHOULD NOT
implement the SGN ALG: DES MAC MD5(0000), MD2.5(0100), DES MAC(0200)
(updates [RFC1964]).
Kerberos GSS mechanism implementation 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.
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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
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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.
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8. Security Considerations
Removing support for single DES improves security since DES is
considered to be insecure.
Kerberos defines some encryption types that are either underspecified
or that were only allocated a number in the 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.
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9. IANA Considerations
There are no IANA Considerations for this document
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10. References
10.1. 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.
10.2. 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
workshop 2006", April 2006.
[DES-1day]
SciEngines GmbH, "Break DES in less than a single day", <h
ttp://www.sciengines.com/company/news-a-events/
74-des-in-1-day.html>.
[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
Document 05-9945, 70 FR 28907-28908", May 2005, <http://
www.gpo.gov/fdsys/pkg/FR-2005-05-19/pdf/05-9945.pdf>.
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[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.
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Authors' Addresses
Love Hornquist Astrand
Apple, Inc
Cupertino
USA
Email: lha@apple.com
Tom Yu
MIT Kerberos Consortium
Email: tlyu@mit.edu
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