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Versions: 00 01

Network Working Group                                           B. Kaduk
Internet-Draft                                                   MIT KIT
Obsoletes: 4757 (if approved)                                   M. Short
Updates: 3961 (if approved)                        Microsoft Corporation
Intended status: Informational                             March 6, 2015
Expires: September 7, 2015


                   Deprecate 3DES and RC4 in Kerberos
             draft-kaduk-kitten-des-des-des-die-die-die-00

Abstract

   The 3DES and RC4 encryption types are steadily weakening in
   cryptographic strength, and the deprecation process should be begun
   for their use in Kerberos.

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 September 7, 2015.

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   described in the Simplified BSD License.



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Table of Contents

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

1.  Introduction

   The 3DES and RC4 encryption types are steadily weakening in
   cryptographic strength, and the deprecation process should be begun
   for their use in Kerberos.

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 RC4 Kerberos encryption types are specified in [RFC4757], which
   is moved to historic.

   The des3-cbc-sha1-kd encryption type is specified in [RFC3961].
   Additional 3DES encryption types are in use with no formal
   specification, in particular des3-cbc-md5 and des3-cbc-sha1.  These
   unspecified encryption types are also deprecated by this document.






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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, and these statistical biases
   cause concern for any consumer of the RC4 cipher.  However, the RC4
   Kerberos enctypes have additional flaws which reduce the security of
   applications using them, 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 using
   RC4, such as TLS ([XXX]).  These attacks seem to rely on repeated
   encryptions of thousands of copies of the same plaintext; whereas it
   is easy for malicious javascript in a website to cause such traffic,
   it is unclear that 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.






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5.2.  Password Hash

   Kerberos long-term keys can either be random (as might be used in a
   service's keytab) or derived from a password (usable 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 PBKDF2 algorithm, which 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 a value which is unique to both the user
   and the realm of authentication as input to the has function; 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, which is a single iteration of the MD4 HMAC function
   applied to the UTF-16 encoded password, with no salt at all.  The MD4
   hash function is very old, and is 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, which are applicable to all principals in all
   Kerberos realms.  Given the prevalance of poor-quality user-selected
   password, 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 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 was chosen to be compatible with the NTLM scheme,
   this means that these application servers also possess the long-term



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   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 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 which allows two parties to use Kerberos to
   communicate.  In particular, the Kerberos implementions 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 situations where the cross-realm key
   was initially established when one peer only supported RC4, or where
   machines only supporting RC4 will need to obtain a cross-realm TGT.
   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.

   However, Windows XP is already out of its official support period,
   and the support period for Windows Server 2003 ends on July 14, 2015.
   At that point, machines that might be broken by disabling RC4 will be
   unsupported, and concerns about breaking them will be reduced.  That
   should facilitate the removal of RC4 from common use.

6.  3DES 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 their use.  As is the case for the RC4 enctypes, the
   string2key algorithm is weak.  Additionally, the 3DES encryption
   types were never implemented in all Kerberos implementations, and the
   64-bit blocksize may be problematic in some environments.

6.1.  Password-based Keys

   The string2key function used by the des-cbc-sha1-kd encryption type
   is essentially just the same n-fold algorithm used by the single-DES
   family of enctypes.  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 des-cbc-sha1-kd's




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   string2key, in contrast to RC4, but a brute-force dictionary attack
   on common passwords may still be feasible.

6.2.  Interoperability

   The triple-DES encryption types were implemented by MIT Kerberos
   early in its development, but encryption types 17 and 18 (AES)
   quickly followed, so there are only a small number of such
   deployments which support 3DES but not AES.  Similarly, the Heimdal
   Kerberos implementation provided 3DES shortly followed by AES, and
   has provided AES for nearly ten years.

   The Kerberos implementation in Microsoft Windows does not currently
   and has never implemented the 3DES 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 encryption types.

   The 3DES encryption type offers very slow encryption, especially
   compared to the performance of AES using the hardware accelleration
   available in modern CPUs.  There are no areas where it offers
   advantages over other encryption types except in the rare case where
   AES is not available.

6.3.  Block Size

   Because triple-DES is based on the single-DES primitive, just using
   additional key material and nested encryption, 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 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.

7.  Recommendations

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

   Encryption:  DES3-CBC-SHA1-KD

   Checksum:  HMAC-SHA1-DES3-KD





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   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 [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 [RFC4120]).

   Kerberos GSS mechanism implementations and deployments SHOULD NOT
   implement or deploy the following SGN_ALGs: HMAC MD5(1100) and HMAC
   SHA1 DES3 KD (updates [RFC4757]).

   Kerberos GSS mechanism implementations and deployments SHOULD NOT
   implement or deploy the following SEAL_ALGs: RC4(1000) and
   DES3KD(0400).

   This document recommends the reclassification of [RFC4757] as
   Historic.

8.  Security Considerations

   This document is entirely about security considerations, namely that
   the use of the 3DES and RC4 Kerberos encryption types is not secure,
   and they should not be used.

9.  IANA Considerations

   IANA is requested to update the registry of Kerberos Encryption Type
   Numbers to note that encryption types 1, 2, 3, and 24 are deprecated,
   with RFC 6649 ([RFC6649]) as the reference, and that encryption types
   5, 7, 16, and 23 are deprecated, with this document as the reference.

   Similarly, IANA Is requested to update the registry of Kerberos
   Checksum Type Numbers to note that checksum types 1, 2, 3, 4, 5, 6,
   and 8 are deprecated, with RFC 6649 as the reference, and that
   checksum types 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, March 1997.




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   [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.

   [RFC6150]  Turner, S. and L. Chen, "MD4 to Historic Status", RFC
              6150, March 2011.

10.2.  Informative References

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

   [RFC6649]  Hornquist Astrand, L. and T. Yu, "Deprecate DES, RC4-HMAC-
              EXP, and Other Weak Cryptographic Algorithms in Kerberos",
              BCP 179, RFC 6649, July 2012.

   [MS-NLMP]  Microsoft Corporation, "[MS-NLMP]: NT LAN Manager (NTLM)
              Authentication Protocol", May 2014.

Appendix A.  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
   MIT Consortium for Kerberos and Internet Trust

   Email: kaduk@mit.edu


   Michiko Short
   Microsoft Corporation

   Email: michikos@microsoft.com










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