Internet DRAFT - draft-ietf-curdle-des-des-des-die-die-die
draft-ietf-curdle-des-des-des-die-die-die
Network Working Group B. Kaduk
Internet-Draft Akamai
Updates: 3961,4120 (if approved) M. Short
Intended status: Best Current Practice Microsoft Corporation
Expires: March 22, 2018 September 18, 2017
Deprecate 3DES and RC4 in Kerberos
draft-ietf-curdle-des-des-des-die-die-die-05
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. Accordingly, RFC 4757 is moved to
Historic status, as none of the encryption types it specifies should
be used, and RFC 3961 is updated to note the deprecation of the
triple-DES encryption types.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 22, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 . . . . . . . . . . . . . . . . 5
5.4. Interoperability Concerns . . . . . . . . . . . . . . . . 5
6. 3DES Weakness . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Password-based Keys . . . . . . . . . . . . . . . . . . . 6
6.2. Block Size . . . . . . . . . . . . . . . . . . . . . . . 6
6.3. Interoperability . . . . . . . . . . . . . . . . . . . . 6
7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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. Accordingly, RFC 4757 is moved to
Historic status, as none of the encryption types it specifies should
be used, and RFC 3961 is updated to note the deprecation of the
triple-DES encryption types.
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.
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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.
Though the RC4 and 3DES 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 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 ([RFC7465]). At least some of these attacks 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
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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 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 hash 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 hash 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.
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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,
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 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 deployments 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
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, respectively; MIT Kerberos [MITKRB5] has fully supported
AES (including the GSSAPI mechanism) since 2004 with the release of
version 1.3.2; 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 aministrators of such environments are expected to
be capable of devising workarounds.
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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 its use. As is the case for the RC4 enctypes, the
string2key algorithm is weak. Additionally, the 3DES 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, in contrast to RC4, but a brute-force dictionary attack
on common passwords may still be feasible.
6.2. 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 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
The triple-DES encryption types were implemented by MIT Kerberos
early in its development (ca. 1999) and present in the 1.2 release,
but were superseded when encryption types 17 and 18 (AES) were
implemented by 2003 and present in the 1.3 release. The Heimdal
Kerberos implementation also provided a version of 3DES in 1999
(though the GSSAPI portions remained non-interoperable with MIT for
some time after that), and gained 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 3DES but not AES are quite rare.
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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.
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],
[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],
[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.
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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 [IANA-KRB] 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 [IANA-KRB] 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,
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>.
10.2. Informative References
[HEIMDAL] Heimdal Project, "Heimdal Kerberos Implementation", April
2017, <https://www.h5l.org/>.
[IANA-KRB]
Internet Assigned Numbers Authority, "IANA Kerberos
Parameters Registry", March 2017,
<https://www.iana.org/assignments/kerberos-parameters/
kerberos-parameters.xhtml>.
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[MITKRB5] MIT, "MIT Kerberos Implementation", March 2017,
<https://web.mit.edu/kerberos/>.
[MS-NLMP] Microsoft Corporation, "[MS-NLMP]: NT LAN Manager (NTLM)
Authentication Protocol", May 2014,
<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>.
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
Akamai Technologies
Email: kaduk@mit.edu
Michiko Short
Microsoft Corporation
Email: michikos@microsoft.com
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