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Internet Engineering Task Force (IETF)                       A. MalhotraRequest for Comments: 8573                                   S. GoldbergUpdates:5905                                          Boston UniversityCategory: Standards Track                                      June 2019ISSN: 2070-1721Message Authentication Code for the Network Time ProtocolAbstract   The Network Time Protocol (NTP), as described inRFC 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   inRFC 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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 2019Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .22.  Deprecating the Use of MD5  . . . . . . . . . . . . . . . . .23.  Replacement Recommendation  . . . . . . . . . . . . . . . . .24.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .35.  Test Vectors  . . . . . . . . . . . . . . . . . . . . . . . .36.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .37.  Security Considerations . . . . . . . . . . . . . . . . . . .38.  References  . . . . . . . . . . . . . . . . . . . . . . . . .48.1.  Normative References  . . . . . . . . . . . . . . . . . .48.2.  Informative References  . . . . . . . . . . . . . . . . .4   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . .5   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .51.  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 inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.2.  Deprecating the Use of MD5RFC 5905 [RFC5905] defines how the MD5 digest algorithm described inRFC 1321 [RFC1321] can be used as a Message Authentication Code (MAC)   for authenticating NTP packets.  However, as discussed in [BCK] andRFC 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 inRFC 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 inRFC 5905 [RFC5905].  The MAC key for NTP MUST be an   AES-128 key that is 128 bits in length, and the resulting MAC tagMalhotra & 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 ofRFC4493 [RFC4493].  NTP makes this transition possible as it supports   algorithm agility as described inSection 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 toSection 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 ofRFC 4493 [RFC4493] for discussion on security   guarantees of AES-CMAC.Malhotra & Goldberg          Standards Track                    [Page 3]

RFC 8573                       MAC for NTP                     June 20198.  References8.1.  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 inRFC2119 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>.Malhotra & Goldberg          Standards Track                    [Page 4]

RFC 8573                       MAC for NTP                     June 2019   [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.eduMalhotra & Goldberg          Standards Track                    [Page 5]