Message Authentication Code for the Network Time Protocol
draft-ietf-ntp-mac-05

Versions: (draft-aanchal4-ntp-mac)   00 01 02 03         Standards Track
          04 05                                                         
Internet Engineering Task Force                              A. Malhotra
Internet-Draft                                               S. Goldberg
Updates: 5905 (if approved)                            Boston University
Intended status: Standards Track                         October 3, 2018
Expires: April 6, 2019


       Message Authentication Code for the Network Time Protocol
                         draft-ietf-ntp-mac-05

Abstract

   RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets
   should be authenticated by appending a 128-bit key to the NTP data,
   and hashing the result with MD5 to obtain a 128-bit tag.  This
   document deprecates MD5-based authentication, which is considered to
   be too weak, and recommends the use of AES-CMAC [RFC4493] as a
   replacement.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 6, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   3
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets
   should be authenticated by appending a 128-bit key to the NTP data,
   and hashing the result with MD5 to obtain a 128-bit tag.  This
   document deprecates MD5-based authentication, which is considered to
   be 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Deprecating the use of MD5

   RFC 5905 [RFC5905] defines how the MD5 digest algorithm 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 authentication is implemented, then AES-CMAC as specified in RFC
   4493 [RFC4493] SHOULD 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 at



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   least 128 bits long AES-128 key and the resulting MAC tag MUST be at
   least 128 bits long 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 who 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, AES-CMAC) and the key itself.  All three have to
   match in order to succesfully 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 standard that is available 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.

5.  Test Vectors

   For test vectors and their outputs refer to Section 4 of RFC 4493
   [RFC4493]

6.  Security Considerations

   Refer to the Appendices A, B and C of NIST document [NIST] and
   Security Considerations Section of RFC 4493 [RFC4493] for discussion
   on security guarantees of AES-CMAC.

7.  Acknowledgements

   The authors wish to acknowledge useful discussions with Leen
   Alshenibr, Daniel Franke, Ethan Heilman, Kenny Paterson, Leonid
   Reyzin, Harlan Stenn, and Mayank Varia.







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8.  IANA Considerations

   This memo includes no request to IANA.

9.  References

9.1.  Normative References

   [NIST]     Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: The CMAC Mode for Authentication",
              <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>.

9.2.  Informative References

   [BCK]      Bellare, M., Canetti, R., and H. Krawczyk, "Keyed Hash
              Functions and Message Authentication", in Proceedings of
              Crypto'96, 1996.

   [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>.

   [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>.





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   [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>.

Authors' Addresses

   Aanchal Malhotra
   Boston University
   111 Cummington St
   Boston, MA  02215
   US

   Email: aanchal4@bu.edu


   Sharon Goldberg
   Boston University
   111 Cummington St
   Boston, MA  02215
   US

   Email: goldbe@cs.bu.edu




























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