Internet Engineering Task Force (IETF) B. Kaduk
Request for Comments: 8429 Akamai
BCP: 218 M. Short
Updates: 3961, 4120 Microsoft Corporation
Category: Best Current Practice September 2018
ISSN: 2070-1721
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 (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2
3. Affected Specifications . . . . . . . . . . . . . . . . . . . 3
4. Affected Encryption Types . . . . . . . . . . . . . . . . . . 3
5. RC4 Weakness . . . . . . . . . . . . . . . . . . . . . . . . 3
5.1. Statistical Biases . . . . . . . . . . . . . . . . . . . 4
5.2. Password Hash . . . . . . . . . . . . . . . . . . . . . . 4
5.3. Cross-Protocol Key Reuse . . . . . . . . . . . . . . . . 5
5.4. Interoperability Concerns . . . . . . . . . . . . . . . . 5
6. Triple-DES Weakness . . . . . . . . . . . . . . . . . . . . . 6
6.1. Password-Based Keys . . . . . . . . . . . . . . . . . . . 6
6.2. Block Size . . . . . . . . . . . . . . . . . . . . . . . 6
6.3. Interoperability Concerns . . . . . . . . . . . . . . . . 7
7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
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
ocurs 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. 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(0400).
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