RFC 8573 PROPOSED STANDARD NTP · SECURITY · AUTHENTICATION
NTP MAC: Message Authentication Code for the Network Time Protocol
NTP packet authentication moves from MD5 to AES-CMAC, because MD5 is not a secure MAC.
RFC EDITOR
Malhotra, A. and S. Goldberg, "Message Authentication Code for the Network Time Protocol", RFC 8573, DOI 10.17487/RFC8573, June 2019, <https://www.rfc-editor.org/info/rfc8573>.
BIBTEX
@misc{rfc8573,
series = {Request for Comments},
number = 8573,
howpublished = {RFC 8573},
publisher = {RFC Editor},
doi = {10.17487/RFC8573},
url = {https://www.rfc-editor.org/info/rfc8573},
author = {A. Malhotra and S. Goldberg},
title = {{Message Authentication Code for the Network Time Protocol}},
pagetotal = 5,
year = 2019,
month = jun,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) A. Malhotra
Request for Comments: 8573 S. Goldberg
Updates: 5905 Boston University
Category: Standards Track June 2019
ISSN: 2070-1721
Message Authentication Code for the Network Time Protocol
Abstract
The Network Time Protocol (NTP), as described in RFC 5905, states
that NTP packets should be authenticated by appending NTP data to a
128-bit key and hashing the result with MD5 to obtain a 128-bit tag.
This document deprecates MD5-based authentication, which is
considered too weak, and recommends the use of AES-CMAC as described
in RFC 4493 as a replacement.
Status of This Memo
This is an Internet Standards Track document.
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
Internet Standards 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/rfc8573.
Copyright Notice
Copyright (c) 2019 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Deprecating the Use of MD5 . . . . . . . . . . . . . . . . . 2
3. Replacement Recommendation . . . . . . . . . . . . . . . . . 2
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 3
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
7. Security Considerations . . . . . . . . . . . . . . . . . . . 3
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
8.1. Normative References . . . . . . . . . . . . . . . . . . 4
8.2. Informative References . . . . . . . . . . . . . . . . . 4
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
The Network Time Protocol [RFC5905] states that NTP packets should be
authenticated by appending NTP data to a 128-bit key and hashing the
result with MD5 to obtain a 128-bit tag. This document deprecates
MD5-based authentication, which is considered too weak, and
recommends the use of AES-CMAC [RFC4493] as a replacement.
1.1. Requirements Language
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.
2. Deprecating the Use of MD5
RFC 5905 [RFC5905] defines how the MD5 digest algorithm described in
RFC 1321 [RFC1321] can be used as a Message Authentication Code (MAC)
for authenticating NTP packets. However, as discussed in [BCK] and
RFC 6151 [RFC6151], this is not a secure MAC and therefore MUST be
deprecated.
3. Replacement Recommendation
If NTP authentication is implemented, then AES-CMAC as specified in
RFC 4493 [RFC4493] MUST be computed over all fields in the NTP header
and any extension fields that are present in the NTP packet as
described in RFC 5905 [RFC5905]. The MAC key for NTP MUST be an
AES-128 key that is 128 bits in length, and the resulting MAC tag
MUST be at least 128 bits in length, as stated in Section 2.4 of RFC
4493 [RFC4493]. NTP makes this transition possible as it supports
algorithm agility as described in Section 2.1 of RFC 7696 [RFC7696].
The hosts that wish to use NTP authentication share a symmetric key
out of band. So they MUST implement AES-CMAC and share the
corresponding symmetric key. A symmetric key is a triplet of ID,
type (e.g., MD5 and AES-CMAC) and the key itself. All three have to
match in order to successfully authenticate packets between two
hosts. Old implementations that don't support AES-CMAC will not
accept and will not send packets authenticated with such a key.
4. Motivation
AES-CMAC is recommended for the following reasons:
1. It is an IETF specification that is supported in many open source
implementations.
2. It is immune to nonce-reuse vulnerabilities (e.g., [Joux])
because it does not use a nonce.
3. It has fine performance in terms of latency and throughput.
4. It benefits from native hardware support, for instance, Intel's
New Instruction set GUE [GUE].
5. Test Vectors
For test vectors and their outputs, refer to Section 4 of RFC 4493
[RFC4493].
6. IANA Considerations
This document has no IANA actions.
7. Security Considerations
Refer to Appendices A, B, and C of the NIST document [NIST] for a
recommendation for the CMAC mode of authentication; see the Security
Considerations of RFC 4493 [RFC4493] for discussion on security
guarantees of AES-CMAC.
8. References
8.1. Normative References
[NIST] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: The CMAC Mode for Authentication", NIST Special
Publication 800-38B, DOI 10.6028/NIST.SP.800-38B, October
2016, <https://www.nist.gov/publications/recommendation-
block-cipher-modes-operation-cmac-mode-authentication-0>.
[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>.
[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, <https://www.rfc-editor.org/info/rfc4493>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[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>.
8.2. Informative References
[BCK] Bellare, M., Canetti, R., and H. Krawczyk, "Keying Hash
Functions and Message Authentication", Advances in
Cryptology - Crypto 96 Proceedings, Lecture Notes in
Computer Science, Vol. 1109, N. Koblitz ed, Springer-
Verlag, 1996.
[GUE] Geuron, S., "Intel Advanced Encryption Standard (AES) New
Instructions Set", May 2010,
<https://www.intel.com/content/dam/doc/white-paper/
advanced-encryption-standard-new-instructions-set-
paper.pdf>.
[Joux] Joux, A., "Authentication Failures in NIST version of
GCM",
<http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/
comments/800-38_Series-Drafts/GCM/Joux_comments.pdf>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/info/rfc7696>.
Acknowledgements
The authors wish to acknowledge useful discussions with Leen
Alshenibr, Daniel Franke, Ethan Heilman, Kenny Paterson, Leonid
Reyzin, Harlan Stenn, and Mayank Varia.
Authors' Addresses
Aanchal Malhotra
Boston University
111 Cummington St
Boston, MA 02215
United States of America
Email: aanchal4@bu.edu
Sharon Goldberg
Boston University
111 Cummington St
Boston, MA 02215
United States of America
Email: goldbe@cs.bu.edu
AI ELI5
On this page
PROBLEMNTP authenticated packets by hashing the data with a 128-bit key using MD5, which is not a secure message authentication code.
MECHANISMMD5-based authentication is deprecated. When NTP authentication is used, AES-CMAC with a 128-bit AES key is computed over the NTP header and any extension fields.
WHY IT MATTERSHosts share a symmetric key out of band and old implementations without AES-CMAC cannot use such keys. The unit metadata says it updates RFC 5905 and is updated by RFC 9748.
§1Why MD5 Goes
RFC 5905 says NTP packets should be authenticated by appending the NTP data to a 128-bit key and hashing with MD5 to make a 128-bit tag. RFC 8573 says that, as discussed in the cited research and in RFC 6151, this is not a secure MAC and therefore MUST be deprecated.
The old stamp can be copied by clever people, so we stop using it.
§2AES-CMAC as the Replacement
If NTP authentication is implemented, AES-CMAC as specified in RFC 4493 MUST be computed over all fields in the NTP header and any extension fields present. The MAC key MUST be a 128-bit AES-128 key, and the resulting tag MUST be at least 128 bits long.
NTP can make this transition because it supports algorithm agility, as described in RFC 7696.
The new stamp uses a secret key that only the two clocks know, and it covers the whole message.
§3Shared Keys
Hosts that want NTP authentication share a symmetric key out of band, and MUST implement AES-CMAC. A key is a triplet of ID, type (for example MD5 or AES-CMAC) and the key itself, and all three must match for packets between two hosts to authenticate.
Old implementations that do not support AES-CMAC will not accept and will not send packets authenticated with such a key.
Both clocks must hold the same secret, with the same name and the same kind of stamp, or they will not trust each other.
§4Why AES-CMAC and Where to Look
The RFC gives four reasons: it is an IETF specification supported in many open source implementations; it is immune to nonce-reuse vulnerabilities because it uses no nonce; it has fine latency and throughput; and it benefits from native hardware support, such as Intel's New Instruction set.
Test vectors are in Section 4 of RFC 4493. There are no IANA actions. Security discussion is left to the NIST CMAC document and the security considerations of RFC 4493.
It is a well-known method, it does not need a throwaway number that could be reused by mistake, and computers can do it quickly.
§5References
[RFC4493]RFC 4493Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The AES-CMAC Algorithm", RFC 4493, June 2006.June 2006[RFC5905]RFC 5905Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010.June 2010[RFC6151]RFC 6151Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, March 2011.March 2011[RFC1321]RFC 1321Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992.April 1992