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Internet Engineering Task Force (IETF)
Request for Comments: 8429
BCP: 218
Updates: 3961, 4120
Category: Best Current Practice
ISSN: 2070-1721
B. Kaduk
Akamai
M. Short
Microsoft Corporation
October 2018

Deprecate Triple-DES (3DES) and RC4 in Kerberos

Abstract

The triple-DES (3DES) and RC4 encryption types are steadily weakening in cryptographic strength, and the deprecation process should begin for their use in Kerberos. Accordingly, RFC 4757 has been moved to Historic status, as none of the encryption types it specifies should be used, and RFC 3961 has been updated to note the deprecation of the triple-DES encryption types. RFC 4120 is likewise updated to remove the recommendation to implement triple-DES encryption and checksum types.

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 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8429.

Copyright Notice

Copyright © 2018 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 (https://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 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   3
   3.  Affected Specifications . . . . . . . . . . . . . . . . . . .   3
   4.  Affected Encryption Types . . . . . . . . . . . . . . . . . .   4
   5.  RC4 Weakness  . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Statistical Biases  . . . . . . . . . . . . . . . . . . .   4
     5.2.  Password Hash . . . . . . . . . . . . . . . . . . . . . .   5
     5.3.  Cross-Protocol Key Reuse  . . . . . . . . . . . . . . . .   5
     5.4.  Interoperability Concerns . . . . . . . . . . . . . . . .   6
   6.  Triple-DES Weakness . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Password-Based Keys . . . . . . . . . . . . . . . . . . .   7
     6.2.  Block Size  . . . . . . . . . . . . . . . . . . . . . . .   7
     6.3.  Interoperability Concerns . . . . . . . . . . . . . . . .   7
   7.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1. Introduction

The triple-DES (3DES) and RC4 encryption types (enctypes) are steadily weakening in cryptographic strength, and the deprecation process should begin for their use in Kerberos. Accordingly, RFC 4757 has been moved to Historic status, as none of the encryption types it specifies should be used, and RFC 3961 has been updated to note the deprecation of the triple-DES encryption types. RFC 4120 is likewise updated to remove the recommendation to implement triple-DES encryption and checksum types.

2. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Affected Specifications

The RC4 Kerberos encryption types (including rc4-hmac) are specified in [RFC4757], which has been moved to Historic status.

The des3-cbc-sha1-kd encryption type is specified in [RFC3961]. Additional triple-DES encryption type codepoints are in use and in the IANA registry with no formal specification, in particular des3-cbc-md5 and des3-cbc-sha1. These unspecified encryption types are also deprecated by this document.

The Kerberos specification ([RFC4120]) includes recommendations for which encryption and checksum types to implement; the deprecated encryption and checksum types are now disrecommended to implement.

Though the RC4 and triple-DES encryption types are still in use in some deployments, the above status changes are made to discourage their use.

4. Affected Encryption Types

The following encryption types are deprecated. The numbers are the official identifiers; the names are only for convenience. 

               +----------------+--------------------------+
               | enctype number | enctype convenience name |
               +----------------+--------------------------+
               |       5        |       des3-cbc-md5       |
               |                |                          |
               |       7        |      des3-cbc-sha1       |
               |                |                          |
               |       16       |     des3-cbc-sha1-kd     |
               |                |                          |
               |       23       |         rc4-hmac         |
               +----------------+--------------------------+

5. RC4 Weakness

RC4's weakness as a TLS cipher due to statistical biases in the keystream has been well publicized [RFC7465], and these statistical biases cause concern for any consumer of the RC4 cipher. However, the RC4 Kerberos enctypes have additional flaws. These flaws reduce the security of applications that use the enctypes; the weakening occurs for various reasons, including the weakness of the password hashing algorithm, the reuse of key material across protocols, and the lack of a salt when hashing the password.

5.1. Statistical Biases

The RC4 stream cipher is known to have statistical biases in its output, which have led to practical attacks against protocols such as TLS that use RC4 [RFC7465]. At least some of these attacks rely on repeated encryptions of thousands of copies of the same plaintext; although it is easy for malicious javascript in a website to cause such traffic, it is unclear whether there is an easy way to induce a kerberized application to generate such repeated encryptions. The statistical biases are most pronounced for earlier bits in the output stream, which is somewhat mitigated by the use of a confounder in Kerberos messages: the first 64 bits of plaintext are a random confounder, and are thus of no use to an attacker who can retrieve them.

Nonetheless, the statistical biases in the RC4 keystream extend well past 64 bits and provide potential attack surface to an attacker. Continuing to use a known weak algorithm is inviting further development of attacks.

5.2. Password Hash

Kerberos long-term keys can be either random (as might be used in a service's keytab) or derived from a password (e.g., for individual users to authenticate to a system). The specification for a Kerberos encryption type must include a "string2key" algorithm for generating a raw crypto key from a string (i.e., password). Modern encryption types, such as those using the AES and Camellia block ciphers, use a string2key function based on the Password-Based Key Derivation Function 2 (PBKDF2) algorithm. This algorithm involves many iterations of a cryptographic hash function, designed to increase the computational effort required to perform a brute-force password- guessing attack. There is an additional option to specify an increased iteration count for a given principal, providing some modicum of adaptability for increases in computing power.

It is also best practice, when deriving cryptographic secrets from user passwords, to include as input to the hash function a value that is unique to both the user and the realm of authentication; this user-specific input is known as a "salt". The default salt for Kerberos principals includes both the name of the principal and the name of the realm, in accordance with these best practices. However, the RC4 encryption types ignore the salt input to the string2key function; the function itself is a single iteration of the MD4 hash function applied to the UTF-16 encoded password, with no salt at all. The MD4 hash function is very old and considered to be weak and unsuitable for new cryptographic applications at this time [RFC6150].

The omission of a salt input to the hash is contrary to cryptographic best practices and allows an attacker to construct a "rainbow table" of password hashes; such tables are applicable to all principals in all Kerberos realms. Given the prevalence of poor-quality user- selected passwords, it is likely that a rainbow table derived from a database of common passwords would be able to compromise a sizable number of Kerberos principals in any realm using RC4 encryption types for password-derived keys.

5.3. Cross-Protocol Key Reuse

The selection of unsalted MD4 as the Kerberos string2key function was deliberate, since it allowed systems to be converted in-place from the old NT LAN Manager (NTLM) logon protocol [MS-NLMP] to use Kerberos.

Unfortunately, there still exist systems using NTLM for authentication to applications, which can result in application servers possessing the NT password hash of user passwords. Because the RC4 string2key function was chosen to be compatible with the NTLM scheme, these application servers also possess the long-term Kerberos key for those users, even though the password is unknown. The cross- protocol use of the long-term key/password hash was convenient for migrating to Kerberos, but it now provides a vulnerability in Kerberos as NTLM continues to be used.

5.4. Interoperability Concerns

The RC4 Kerberos encryption type remains in use in many environments because of interoperability requirements. In those sites, RC4 is the strongest enctype that allows two parties to use Kerberos to communicate. In particular, the Kerberos implementations included with Windows XP and Windows Server 2003 support only single-DES and RC4. Since single-DES is deprecated [RFC6649], machines running those operating systems must use RC4.

Similarly, there are cross-realm deployments in which the cross-realm key was initially established when one peer only supported RC4, or machines only supporting RC4 need to obtain a cross-realm Ticket- Granting Ticket. It can be difficult to inventory all clients in a Kerberos realm and know what implementations will be used by those client principals; this leads to concerns that disabling RC4 will cause breakage on machines that are unknown to the realm administrators.

Fortunately, modern (i.e., supported) Kerberos implementations support a secure alternative to RC4 in the form of AES. Windows has supported AES since 2007-2008 with the release of Windows Vista and Server 2008. MIT Kerberos [MITKRB5] has fully supported AES enctypes since 2004 with the release of version 1.3.2, including the Kerberos mechanism for the Generic Security Service Application Program Interface (GSSAPI). Heimdal [HEIMDAL] has fully supported AES since 2005 with the release of version 0.7. Though there may still be issues running ten-year-old unsupported software in mixed environments with new software, issues of that sort seem unlikely to be unique to Kerberos, and the administrators of such environments are expected to be capable of devising workarounds.

6. Triple-DES Weakness

The flaws in triple-DES as used for Kerberos are not quite as damning as those in RC4, but there is still ample justification for deprecating its use. As is the case for the RC4 enctypes, the string2key algorithm is weak. Additionally, the triple-DES encryption types were not implemented in all Kerberos implementations, and the 64-bit block size may be problematic in some environments.

6.1. Password-Based Keys

The n-fold-based string2key function used by the des3-cbc-sha1-kd encryption type is an ad hoc construction that should not be considered cryptographically sound. It is known to not provide effective mixing of the input bits and is computationally easy to evaluate. As such, it does not slow down brute-force attacks in the way that the computationally demanding PBKDF2 algorithm used by more modern encryption types does. The salt is used by des3-cbc-sha1-kd's string2key function, in contrast to RC4, but a brute-force dictionary attack on common passwords may still be feasible.

6.2. Block Size

Triple-DES is based on the single-DES primitive, simply using additional key material and nested encryption. Therefore, it inherits the 64-bit cipher block size from single-DES. As a result, an attacker who can collect approximately 2**32 blocks of ciphertext has a good chance of finding a cipher block collision (the "birthday attack"), which would potentially reveal a couple of blocks of plaintext.

A cipher block collision would not necessarily cause the key itself to be leaked, so the plaintext revealed by such a collision would be limited. For some sites, that may be an acceptable risk, but it is still considered a weakness in the encryption type.

6.3. Interoperability Concerns

The triple-DES encryption types were implemented by MIT Kerberos early in its development (ca. 1999) and present in the 1.2 release, but they were superseded when encryption types 17 and 18 (AES) were implemented (by 2003); the AES enctypes were present in the 1.3 release. The Heimdal Kerberos implementation also provided a version of triple-DES in 1999 (though the GSSAPI portions remained non- interoperable with MIT for some time after that), gaining support for AES in 2005 with its 0.7 release. Both Heimdal and MIT krb5 have supported the AES enctypes for some 12 years, and it is expected that deployments that support triple-DES but not AES are quite rare.

The Kerberos implementation in Microsoft Windows has never implemented the triple-DES encryption type. Support for AES was introduced with Windows Vista and Windows Server 2008; older versions such as Windows XP and Windows Server 2003 only supported the RC4 and single-DES encryption types.

The triple-DES encryption type offers very slow encryption, especially compared to the performance of AES using the hardware acceleration available in modern CPUs. There are no areas where triple-DES offers advantages over other encryption types except in the rare case where AES is not available.

7. Recommendations

This document hereby removes the following RECOMMENDED types from [RFC4120]: 

      Encryption: DES3-CBC-SHA1-KD
      
      Checksum: HMAC-SHA1-DES3-KD
   
   Kerberos implementations and deployments SHOULD NOT implement or
   deploy the following triple-DES encryption types: DES3-CBC-MD5(5),
   DES3-CBC-SHA1(7), and DES3-CBC-SHA1-KD(16) (updates [RFC3961] and
   [RFC4120]).

Kerberos implementations and deployments SHOULD NOT implement or deploy the RC4 encryption type RC4-HMAC(23). 

   Kerberos implementations and deployments SHOULD NOT implement or
   deploy the following checksum types: RSA-MD5(7), RSA-MD5-DES3(9),
   HMAC-SHA1-DES3-KD(12), and HMAC-SHA1-DES3(13) (updates [RFC3961] and
   [RFC4120]).

Kerberos GSS mechanism implementations and deployments SHOULD NOT implement or deploy the following SGN_ALGs: HMAC MD5(1100) and HMAC SHA1 DES3 KD(0400). (With all its content now deprecated, [RFC4757] has been made Historic by this document.)

Kerberos GSS mechanism implementations and deployments SHOULD NOT implement or deploy the following SEAL_ALGs: RC4(1000) and DES3KD(0200).

Per this document, [RFC4757] has been reclassified as Historic.

8. Security Considerations

This document is entirely about security considerations, namely that the use of the triple-DES and RC4 Kerberos encryption types is not secure, and they should not be used.

9. IANA Considerations

IANA has updated the "Kerberos Encryption Type Numbers" registry [IANA-KRB] to note that 1) encryption types 1, 2, 3, and 24 are deprecated, with [RFC6649] as the reference and that 2) encryption types 5, 7, 16, and 23 are deprecated, with this document as the reference.

Similarly, IANA has updated the "Kerberos Checksum Type Numbers" registry [IANA-KRB] to note that 1) checksum type values 1, 2, 3, 4, 5, 6, and 8 are deprecated, with [RFC6649] as the reference, and that 2) checksum type values 7, 12, and 13 are deprecated, with this document as the reference.

10. References

10.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
              2005, <https://www.rfc-editor.org/info/rfc3961>.
   
   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <https://www.rfc-editor.org/info/rfc4120>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2. Informative References

   [HEIMDAL]  Heimdal Project, "The Heimdal Kerberos 5, PKIX, CMS, GSS-
              API, SPNEGO, NTLM, Digest-MD5 and, SASL implementation",
              <https://www.h5l.org/>.

[IANA-KRB] 

              IANA, "Kerberos Parameters",
              <https://www.iana.org/assignments/kerberos-parameters/>.
   
   [MITKRB5]  MIT, "Kerberos: The Network Authentication Protocol",
              <https://web.mit.edu/kerberos/>.
   
   [MS-NLMP]  Microsoft Corporation, "[MS-NLMP]: NT LAN Manager (NTLM)
              Authentication Protocol", September 2017,
              <https://msdn.microsoft.com/en-us/library/cc236621.aspx>.
   
   [RFC4757]  Jaganathan, K., Zhu, L., and J. Brezak, "The RC4-HMAC
              Kerberos Encryption Types Used by Microsoft Windows",
              RFC 4757, DOI 10.17487/RFC4757, December 2006,
              <https://www.rfc-editor.org/info/rfc4757>.
   
   [RFC6150]  Turner, S. and L. Chen, "MD4 to Historic Status",
              RFC 6150, DOI 10.17487/RFC6150, March 2011,
              <https://www.rfc-editor.org/info/rfc6150>.
   
   [RFC6649]  Hornquist Astrand, L. and T. Yu, "Deprecate DES, RC4-HMAC-
              EXP, and Other Weak Cryptographic Algorithms in Kerberos",
              BCP 179, RFC 6649, DOI 10.17487/RFC6649, July 2012,
              <https://www.rfc-editor.org/info/rfc6649>.
   
   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <https://www.rfc-editor.org/info/rfc7465>.

Acknowledgements

Many people have contributed to the understanding of the weaknesses of these encryption types over the years, and they cannot all be named here.

Authors' Addresses

Benjamin Kaduk
Akamai Technologies

Email: 

          kaduk@mit.edu

Michiko Short
Microsoft Corporation

Email: 

          michikos@microsoft.com