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.





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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




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  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.









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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>.





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  [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: [email protected]


  Sharon Goldberg
  Boston University
  111 Cummington St
  Boston, MA  02215
  United States of America

  Email: [email protected]













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