draft-ietf-krb-wg-des-die-die-die-02.txt   draft-ietf-krb-wg-des-die-die-die-03.txt 
Network Working Group L. Hornquist Astrand Network Working Group L. Hornquist Astrand
Internet-Draft Apple, Inc Internet-Draft Apple, Inc
Updates: 1510, 1964, 4120, 4121, 4757 T. Yu Updates: 1510, 1964, 4120, 4121, 4757 T. Yu
(if approved) MIT Kerberos Consortium (if approved) MIT Kerberos Consortium
Intended status: Standards Track February 9, 2012 Intended status: BCP February 16, 2012
Expires: August 12, 2012 Expires: August 19, 2012
Deprecate DES support for Kerberos Deprecate DES, "export strength" RC4, and other weak cryptographic
draft-ietf-krb-wg-des-die-die-die-02 algorithms in Kerberos
draft-ietf-krb-wg-des-die-die-die-03
Abstract Abstract
The Kerberos 5 network authentication protocol, originally specified The Kerberos 5 network authentication protocol, originally specified
in RFC1510, can use the Data Encryption Standard (DES) for in RFC1510, can use the Data Encryption Standard (DES) for
encryption. Almost 30 years after first publishing DES, the National encryption. Almost 30 years after first publishing DES, the National
Institute of Standards and Technology (NIST) finally withdrew the Institute of Standards and Technology (NIST) finally withdrew the
standard in 2005, reflecting a long-established consensus that DES is standard in 2005, reflecting a long-established consensus that DES is
insufficiently secure. By 2008, commercial hardware costing less insufficiently secure. By 2008, commercial hardware costing less
than USD 15,000 could break DES keys in less than a day on average. 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 DES is long past its sell-by date. Accordingly, this document
updates RFC1964, RFC4120, and RFC4121 to deprecate the use of DES in updates RFC1964, RFC4120, RFC4121, and RFC4757 to deprecate the use
Kerberos. Because RFC1510 (obsoleted by RFC4120) supports only DES, of DES, "export strength" RC4, and other weak cryptographic
this document reclassifies RFC1510 as Historic. This document also algorithms in Kerberos. Because RFC1510 (obsoleted by RFC4120)
deprecates the weak "export strength" RC4 enctype of RFC4757. supports only DES, this document reclassifies RFC1510 as Historic.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 12, 2012. This Internet-Draft will expire on August 19, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 26 skipping to change at page 2, line 26
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. Introduction 2. Introduction
The original specification of the Kerberos 5 network authentication The original specification of the Kerberos 5 network authentication
protocol [RFC1510] supports only the Data Encryption Standard (DES) protocol [RFC1510] supports only the Data Encryption Standard (DES)
for encryption. For many years, the cryptographic community has for encryption. For many years, the cryptographic community has
regarded DES as providing inadequate security. Accordingly, this regarded DES as providing inadequate security, mostly because of its
document reclassifies [RFC1510] (obsoleted by [RFC4120]) as Historic, small key size. Accordingly, this document reclassifies [RFC1510]
and updates current Kerberos-related specifications [RFC1964], (obsoleted by [RFC4120]) as Historic, and updates current Kerberos-
[RFC4120], and [RFC4121] to deprecate the use of DES in Kerberos. related specifications [RFC1964], [RFC4120], and [RFC4121] to
This document also deprecates the weak "export strength" RC4 enctype deprecate the use of DES and other weak cryptographic algorithms in
of [RFC4757]. Kerberos, including some unkeyed checksums and hashes, along with the
weak "export strength" RC4 enctype of [RFC4757].
3. Affected specifications 3. Affected specifications
The original IETF specification of Kerberos 5 [RFC1510] only supports The original IETF specification of Kerberos 5 [RFC1510] only supports
DES for encryption. [RFC4120] obsoletes [RFC1510] and updates the DES for encryption. [RFC4120] obsoletes [RFC1510] and updates the
Kerberos specification to include additional cryptographic Kerberos specification to include additional cryptographic
algorithms, but still permits the use of DES. [RFC3961] describes algorithms, but still permits the use of DES. [RFC3961] describes
the Kerberos cryptographic system and includes support for DES the Kerberos cryptographic system and includes support for DES
encryption types, but it does not specify requirement levels for encryption types, but it does not specify requirement levels for
them. them.
The specification of the Kerberos Generic Security Services The specification of the Kerberos Generic Security Services
Application Programming Interface (GSS-API) mechanism [RFC1964] and Application Programming Interface (GSS-API) mechanism [RFC1964] and
its updated version [RFC4121] define checksum and encryption its updated version [RFC4121] define checksum and encryption
mechanisms based on DES. With the existence of newer encryption mechanisms based on DES. With the existence of newer encryption
types for Kerberos GSS-API defined in [RFC4121], Microsoft's RC4-HMAC types for Kerberos GSS-API defined in [RFC4121], Microsoft's RC4-HMAC
based GSS-API mechanism, and MIT's DES3, there is no need to support based GSS-API mechanism, and MIT's DES3 (which is not published as an
the old DES based integrity (SGN) and confidentiality (SEAL) types. 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 [RFC4757] describes the RC4-HMAC encryption types used by Microsoft
Windows, and allows for a 56-bit "export strength" variant. (The Windows, and allows for a 56-bit "export strength" variant. (The
character constant "fortybits" used in the definition is a historical character constant "fortybits" used in the definition is a historical
reference and does not refer to the actual key size of the enctype.) reference and does not refer to the actual key size of the enctype.)
4. DES insecurity 4. DES insecurity
The insecurity of DES has been evident for many years. The National The insecurity of DES has been evident for many years. Even around
Institute of Standards and Technology (NIST) officially withdrew DES the time of its first publication, cryptographers raised the
in 2005 [DES-Withdrawal], and also announced a transition period that possibility that 56 bits was too small a key size for DES. The
ended on May 19, 2007 [DES-Transition-Plan]. The IETF has also National Institute of Standards and Technology (NIST) officially
published its position in [RFC4772], in which the recommendation withdrew DES in 2005 [DES-Withdrawal], and also announced a
summary is very clear: "don't use DES". 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 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 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 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 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 [DES-1day] that cost less than USD 15,000 [DES-crack]. Brute force
key searches of DES will only get faster and cheaper. (The key searches of DES will only get faster and cheaper. (The
aforementioned company markets its device for one-click recovery of 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 lost DES keys.) It is clear that it is well past time to retire the
use of DES in Kerberos. use of DES in Kerberos.
5. Recommendations 5. Recommendations
This document hereby removes the following RECOMMENDED types from This document hereby removes the following RECOMMENDED types from
[RFC4120]: [RFC4120]:
Encryption: DES-CBC-MD5(3) Encryption: DES-CBC-MD5(3)
Checksums: DES-MD5 (8, named RSA-MD5-DES in [RFC3961]). Checksums: DES-MD5 (8, named RSA-MD5-DES in [RFC3961]).
Kerberos implementations and deployments SHOULD NOT implement the Kerberos implementations and deployments SHOULD NOT provide the
following single DES encryption types: DES-CBC-CRC(1), DES-CBC- following single DES encryption types: DES-CBC-CRC(1), DES-CBC-
MD4(2), DES-CBC-MD5(3) (updates [RFC4120]). MD4(2), DES-CBC-MD5(3) (updates [RFC4120]).
Kerberos implementations and deployments SHOULD NOT implement the Kerberos implementations and deployments SHOULD NOT provide the
following "export strength" RC4 encryption type: RC4-HMAC-EXP(24) following "export strength" RC4 encryption type: RC4-HMAC-EXP(24)
(updates [RFC4757]). (updates [RFC4757]). This document does not add any sort of
requirement for conforming implementations to provide RC4-HMAC(23).
Kerberos implementations and deployments SHOULD NOT implement the Kerberos implementations and deployments SHOULD NOT provide the
following checksum types: CRC32(1), RSA-MD4(2), RSA-MD4-DES(3), DES- 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 MAC(4), DES-MAC-K(5), RSA-MD4-DES-K(6), RSA-MD5-DES(8) (updates
[RFC4120]). [RFC4120]).
It is possible to safely use the RSA-MD5(7) checksum type, but only It is possible to safely use the RSA-MD5(7) checksum type, but only
with additional protection, such as the protection that an encrypted with additional protection, such as the protection that an encrypted
Authenticator provides. Implementations MAY use RSA-MD5 inside an Authenticator provides. Implementations MAY use RSA-MD5 inside an
encrypted Authenticator for backward compatibility with systems that encrypted Authenticator for backward compatibility with systems that
do not support newer checksum types (updates [RFC4120]). One example do not support newer checksum types (updates [RFC4120]). One example
is that some legacy systems only support RC4-HMAC(23) [RFC4757] for is that some legacy systems only support RC4-HMAC(23) [RFC4757] for
encryption when DES is not available; these systems use RSA-MD5 encryption when DES is not available; these systems use RSA-MD5
checksums inside Authenticators encrypted with RC4-HMAC. checksums inside Authenticators encrypted with RC4-HMAC.
Kerberos GSS mechanism implementations and deployments SHOULD NOT Kerberos GSS mechanism implementations and deployments SHOULD NOT
implement the following SGN ALG: DES MAC MD5(0000), MD2.5(0100), DES provide the following SGN ALG: DES MAC MD5(0000), MD2.5(0100), DES
MAC(0200) (updates [RFC1964]). MAC(0200) (updates [RFC1964]).
Kerberos GSS mechanism implementations and deployments SHOULD NOT Kerberos GSS mechanism implementations and deployments SHOULD NOT
implement the following SEAL ALG: DES(0000) (updates [RFC1964]). provide the following SEAL ALG: DES(0000) (updates [RFC1964]).
The effect of the two last sentences is that this document deprecates The effect of the two last sentences is that this document deprecates
section 1.2 in [RFC1964]. section 1.2 in [RFC1964].
This document hereby reclassifies [RFC1510] as Historic. This document hereby reclassifies [RFC1510] as Historic.
6. Acknowledgements 6. Acknowledgements
Mattias Amnefelt, Ran Atkinson, Henry Hotz, Jeffrey Hutzelman, Leif Mattias Amnefelt, Ran Atkinson, Henry Hotz, Jeffrey Hutzelman, Leif
Johansson, and Simon Josefsson have read the document and provided Johansson, and Simon Josefsson have read the document and provided
suggestions for improvements. Sam Hartman proposed moving [RFC1510] suggestions for improvements. Sam Hartman proposed moving [RFC1510]
to Historic. to Historic. Michiko Short provided information about the dates of
end of support for Windows releases.
7. Security Considerations 7. Security Considerations
Removing support for single DES improves security, because DES is Removing support for single DES improves security, because DES is
considered to be insecure. 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 Kerberos defines some encryption types that are either underspecified
or that only have number assignments but no specifications. or that only have number assignments but no specifications.
Implementations should make sure that they only implement and enable Implementations should make sure that they only implement and enable
secure encryption types. secure encryption types.
RC4, used in RC4-HMAC, is considered weak; however, the use in The security considerations of [RFC4757] continue to apply to RC4-
Kerberos is vetted and considered secure for now. The main reason to HMAC, including the known weaknesses of RC4 and MD4, and this
not actively discourage the use of RC4-HMAC is that it is the only document does not change the Informational status of [RFC4757] for
encryption type that interoperates with older versions of Microsoft now. The main reason to not actively discourage the use of RC4-HMAC
Windows once DES and RC4-HMAC-EXP are removed. 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 8. IANA Considerations
There are no IANA Considerations for this document. There are no IANA Considerations for this document.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
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