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                                July 2012
Category: Best Current Practice
ISSN: 2070-1721
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                                July 2012
Category: Best Current Practice
ISSN: 2070-1721

Deprecate DES, RC4-HMAC-EXP, and Other Weak Cryptographic Algorithms in Kerberos




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.

最初在RFC 1510中指定的Kerberos 5网络身份验证协议可以使用数据加密标准(DES)进行加密。在DES首次发布近30年后,国家标准与技术研究所(NIST)终于在2005年撤销了该标准,这反映了长期以来的共识,即DES不够安全。到2008年,成本低于15000美元的商用硬件平均不到一天就可以破解DES密钥。DES早已过时了。因此,本文档更新了RFC 1964、RFC 4120、RFC 4121和RFC 4757,以反对在Kerberos中使用DES、RC4-HMAC-EXP和其他弱加密算法。由于RFC 1510(已被RFC 4120淘汰)仅支持DES,本文件建议将RFC 1510重新分类为历史。

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.

本文件是互联网工程任务组(IETF)的产品。它代表了IETF社区的共识。它已经接受了公众审查,并已被互联网工程指导小组(IESG)批准出版。有关BCP的更多信息,请参见RFC 5741第2节。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at


Copyright Notice


Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

版权所有(c)2012 IETF信托基金和确定为文件作者的人员。版权所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( 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.

本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(自本文件出版之日起生效。请仔细阅读这些文件,因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本,并提供简化BSD许可证中所述的无担保。

1. Introduction
1. 介绍

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].

Kerberos 5网络身份验证协议[RFC1510]的原始规范仅支持用于加密的数据加密标准(DES)。多年来,密码学界一直认为DES提供的安全性不足,主要是因为其密钥较小。因此,本文件建议将[RFC1510](已被[RFC4120]淘汰)重新归类为历史性的,并更新当前Kerberos相关规范[RFC1964]、[RFC4120]和[RFC4121],以反对在Kerberos中使用DES和其他弱加密算法,包括一些未知校验和和哈希,以及弱56位“导出强度”RC4变体加密类型[RFC4757]。

2. Requirements Notation
2. 需求符号

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
3. 受影响的规范

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.

Kerberos 5[RFC1510]的原始IETF规范仅支持DES进行加密。[RFC4120]淘汰[RFC1510]并更新Kerberos规范以包括其他加密算法,但仍允许使用DES。[RFC3961]描述了Kerberos加密系统,并包括对DES加密类型的支持,但未指定它们的要求级别。

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.

Kerberos通用安全服务应用程序编程接口(GSS-API)机制规范[RFC1964]及其更新版本[RFC4121]定义了基于DES的校验和加密机制。由于[RFC4121]中定义的Kerberos GSS-API存在较新的加密类型、基于微软RC4 HMAC的GSS-API机制和麻省理工学院的DES3(未发布为RFC),因此无需支持旧的基于DES的完整性(SGN)和机密性(SEAL)类型。

[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.)

[RFC4757]介绍了Microsoft Windows使用的RC4-HMAC加密类型,并允许56位“导出强度”变体。(定义中使用的字符常量“fortybits”是一个历史引用,而不是加密类型的实际密钥大小。)

4. DES Insecurity
4. 不安全感

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.

2006年,研究人员展示了使用价值不到10000欧元的硬件在平均不到9天的时间内通过暴力搜索找到DES密钥的能力[Break DES]。到2008年,一家公司提供的硬件能够在平均不到一天的时间内[DES-1天]破解DES密钥,成本不到15000美元[DES CRAK]。DES的暴力密钥搜索只会变得更快、更便宜。(前面提到的公司将其设备用于一键恢复丢失的DES密钥。)显然,在Kerberos中停止使用DES的时间已经过去了。

5. Recommendations
5. 建议

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机制实现和部署不应实现或部署以下SGN ALG:DES MAC MD5(0000)、MD2.5(0100)、DES MAC(0200)(更新[RFC1964])。

Kerberos GSS mechanism implementations and deployments SHOULD NOT implement or deploy the following SEAL ALG: DES(0000) (updates [RFC1964]).

Kerberos GSS机制实现和部署不应实现或部署以下SEAL ALG:DES(0000)(更新[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
6. 安全考虑

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.

[RFC4757]的安全注意事项继续适用于RC4-HMAC,包括RC4和MD4的已知弱点,本文件暂时不改变[RFC4757]的信息状态。不积极阻止使用RC4-HMAC的主要原因是,一旦删除DES和RC4-HMAC-EXP,它是唯一可以与旧版Microsoft Windows交互操作的加密类型。这些较旧版本的Microsoft Windows可能会在2015年之前投入使用。

7. Acknowledgements
7. 致谢

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.

Mattias Amnefelt、Ran Atkinson、Henry Hotz、Jeffrey Hutzelman、Leif Johansson、Simon Josefsson和Martin Rex阅读了该文件,并提供了改进建议。Sam Hartman建议将[RFC1510]移至历史性项目。Michiko Short提供了有关Windows版本支持终止日期的信息。

8. References
8. 工具书类
8.1. Normative References
8.1. 规范性引用文件

[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996.

[RFC1964]Linn,J.,“Kerberos版本5 GSS-API机制”,RFC19641996年6月。

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2119]Bradner,S.,“RFC中用于表示需求水平的关键词”,BCP 14,RFC 2119,1997年3月。

[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005.

[RFC3961]Raeburn,K.,“Kerberos 5的加密和校验和规范”,RFC 3961,2005年2月。

[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.

[RFC4121]Zhu,L.,Jaganathan,K.,和S.Hartman,“Kerberos版本5通用安全服务应用程序接口(GSS-API)机制:版本2”,RFC 41212005年7月。

[RFC4757] Jaganathan, K., Zhu, L., and J. Brezak, "The RC4-HMAC Kerberos Encryption Types Used by Microsoft Windows", RFC 4757, December 2006.

[RFC4757]Jaganathan,K.,Zhu,L.,和J.Brezak,“微软Windows使用的RC4-HMAC Kerberos加密类型”,RFC 4757,2006年12月。

8.2. Informative References
8.2. 资料性引用

[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://>.

[Break DES]Kumar,S.,Paar,C.,Pelzl,J.,Pfeiffer,G.,Rupp,A.,和M.Schimmler,“如何为8980欧元打破DES”,SHARCS'06-攻击密码系统的专用硬件,2006年4月,<>。

[DES-1day] SciEngines GmbH, "Break DES in less than a single day", < 74-des-in-1-day.html>.

[DES-1day]SCIENGES GmbH,“在不到一天的时间内完成DES”< 74-des-in-1-day.html>。

[DES-Crack] Scott, T., "DES Brute Force Cracking Efforts 1977 to 2010", 2010, < des.crack.efforts.pdf>.

[DES Crack]Scott,T.,“1977年至2010年的暴力破解工作”,2010年< des.crack.efforces.pdf>。

[DES-Transition-Plan] National Institute of Standards and Technology, "DES Transition Plan", May 2005, < STM/common_documents/DESTranPlan.pdf>.

[DES过渡计划]国家标准与技术研究所,“DES过渡计划”,2005年5月< 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, < FR-2005-05-19/pdf/05-9945.pdf>.

[DES撤销]国家标准与技术研究所,“宣布批准撤销联邦信息处理标准(FIPS)46-3,数据加密标准(DES);FIPS 74,实施和使用NBS数据加密标准的指南;FIPS 81,DES操作模式”,联邦公报第70卷,第96号,文件05-9945,70 FR 28907-28908,2005年5月< 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.

[RFC4772]Kelly,S.,“使用数据加密标准(DES)的安全影响”,RFC 47722006年12月。

Authors' Addresses


Love Hornquist Astrand Apple, Inc. Cupertino, California USA

Love Hornquist Astrand Apple,Inc.美国加利福尼亚州库比蒂诺市


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

美国马萨诸塞州剑桥市马萨诸塞大道77号Tom Yu