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IETF RFC 8573
Last modified on Monday, June 24th, 2019
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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/RFC 8573.
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.
Malhotra & Goldberg Standards Track PAGE 1 
RFC 8573 MAC for NTP June 2019
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 [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 [RFC 4493] 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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
2. Deprecating the Use of MD5
RFC 5905 [RFC 5905] defines how the MD5 digest algorithm described in
RFC 1321 [RFC 1321] can be used as a Message Authentication Code (MAC)
for authenticating NTP packets. However, as discussed in [BCK] and
RFC 6151 [RFC 6151], 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 [RFC 4493] 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 [RFC 5905]. The MAC key for NTP MUST be an
AES-128 key that is 128 bits in length, and the resulting MAC tag
Malhotra & Goldberg Standards Track PAGE 2
RFC 8573 MAC for NTP June 2019
MUST be at least 128 bits in length, as stated in Section 2.4 of RFC
4493 [RFC 4493]. NTP makes this transition possible as it supports
algorithm agility as described in Section 2.1 of RFC 7696 [RFC 7696].
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
[RFC 4493].
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 [RFC 4493] for discussion on security
guarantees of AES-CMAC.
Malhotra & Goldberg Standards Track PAGE 3
RFC 8573 MAC for NTP June 2019
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>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC 4493, June
2006, <https://www.rfc-editor.org/info/RFC 4493>.
[RFC 5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC 5905, June 2010,
<https://www.rfc-editor.org/info/RFC 5905>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
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>.
Malhotra & Goldberg Standards Track PAGE 4
RFC 8573 MAC for NTP June 2019
[RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC 1321, April 1992,
<https://www.rfc-editor.org/info/RFC 1321>.
[RFC 6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC 6151, March 2011,
<https://www.rfc-editor.org/info/RFC 6151>.
[RFC 7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC 7696, November 2015,
<https://www.rfc-editor.org/info/RFC 7696>.
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
Malhotra & Goldberg Standards Track PAGE 5
RFC TOTAL SIZE: 10055 bytes
PUBLICATION DATE: Monday, June 24th, 2019
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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