Internet DRAFT - draft-ietf-krb-wg-des-die-die-die
draft-ietf-krb-wg-des-die-die-die
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 27, 2012
Expires: August 30, 2012
Deprecate DES, RC4-HMAC-EXP, and other weak cryptographic algorithms in
Kerberos
draft-ietf-krb-wg-des-die-die-die-04
Abstract
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, 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
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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This Internet-Draft will expire on August 30, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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described in the Simplified BSD License.
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].
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
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
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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
[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).
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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 in [RFC1964].
This document hereby reclassifies [RFC1510] as Historic.
6. 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.
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.
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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
[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://
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Internet-Draft Deprecate DES in Kerberos February 2012
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-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>.
[DES-crack]
Scott, T., "DES Brute Force Cracking Efforts 1977 to
2010", 2010, <http://www.tjscott.net/security.extras/
des.crack.efforts.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
USA
Email: lha@apple.com
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Tom Yu
MIT Kerberos Consortium
77 Massachusetts Ave
Cambridge, Massachusetts
USA
Email: tlyu@mit.edu
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