    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
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       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: aanchal4@bu.edu

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

       Email: goldbe@cs.bu.edu

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