Message Authentication Code for the Network Time Protocol
RFC 8573

Document Type RFC - Proposed Standard (June 2019; No errata)
Updates RFC 5905
Last updated 2019-06-23
Replaces draft-aanchal4-ntp-mac
Stream IETF
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Stream WG state Submitted to IESG for Publication
Document shepherd Karen O'Donoghue
Shepherd write-up Show (last changed 2018-05-30)
IESG IESG state RFC 8573 (Proposed Standard)
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Send notices to Karen O'Donoghue <odonoghue@isoc.org>
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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/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.

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 [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

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 [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
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