Internet Engineering Task Force (IETF) L. Hornquist Astrand
Request for Comments: 6649 Apple, Inc.
BCP: 179 T. Yu
Obsoletes: 1510 MIT Kerberos Consortium
Updates: 1964, 4120, 4121, 4757 June 2012
Category: Best Current Practice
ISSN: 2070-1721
Deprecate DES, RC4-HMAC-EXP, and Other Weak Cryptographic Algorithms
in Kerberos
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 (c) 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
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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. 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", <h
ttp://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: lha@apple.com
Tom Yu
MIT Kerberos Consortium
77 Massachusetts Ave.
Cambridge, Massachusetts
USA
EMail: tlyu@mit.edu