Internet Engineering Task Force (IETF)                       A. Malhotra
Internet-Draft
Request for Comments: 8573                                   S. Goldberg
Updates: 5905 (if approved)                                          Boston University
Intended status:
Category: Standards Track                         January 4, 2019
Expires: July 8,                                      June 2019
ISSN: 2070-1721

       Message Authentication Code for the Network Time Protocol
                         draft-ietf-ntp-mac-06

Abstract

   RFC 5905 states that

   The Network Time Protocol (NTP) (NTP), as described in RFC 5905, states
   that NTP packets should be authenticated by appending the NTP data to a
   128-bit key, key 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 as described
   in RFC 4493 as a replacement.

Status of This Memo

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

   Internet-Drafts are working documents an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for a maximum 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 six months this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 8, 2019.
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Deprecating the use Use of MD5  . . . . . . . . . . . . . . . . .   2
   3.  Replacement Recommendation  . . . . . . . . . . . . . . . . .   3   2
   4.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Test Vectors  . . . . . . . . . . . . . . . . . . . . . . . .   3
   6.  Security  IANA Considerations . . . . . . . . . . . . . . . . . . .   3
   7.  Acknowledgements  . . .   3
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   4   3
   8.  IANA Considerations . .  References  . . . . . . . . . . . . . . . . . . .   4
   9.  References  . . . . . . .   3
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     9.1.  Normative
     8.2.  Informative References  . . . . . . . . . . . . . . . . . .   4
     9.2.  Informative References
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . .   4 .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   RFC 5905 [RFC5905] states that

   The Network Time Protocol (NTP) [RFC5905] states that NTP packets should be
   authenticated by appending the NTP data to a 128-bit key, key 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", "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 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 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, 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 128
   bits long an
   AES-128 key that is 128 bits in length, and the resulting MAC tag
   MUST be at least 128 bits long in length, as stated in section 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 that wish to use NTP authentication share a symmetric key
   out-of-band.
   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, (e.g., MD5 and AES-CMAC) and the key itself.  All three have to
   match in order to succesfully 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 standard specification that is available supported in many open source
       implementations.

   2.  It is immune to nonce-reuse vulnerabilities (e.g. (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 outputs, refer to Section 4 of RFC 4493
   [RFC4493]
   [RFC4493].

6.  IANA Considerations

   This document has no IANA actions.

7.  Security Considerations

   Refer to the Appendices A, B B, and C of the NIST document on [NIST] for a
   recommendation for the CMAC mode of authentication [NIST] and authentication; see the 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.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  References

9.1.

8.1.  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>. 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>.

9.2.

   [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, "Keyed "Keying Hash
              Functions and Message Authentication", Advances in Proceedings of
              Crypto'96,
              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", <https://www.intel.com/content/dam/doc/
              white-paper/advanced-encryption-standard-new-instructions-
              set-paper.pdf>. 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>.

   [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>.

   [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>.

Appendix A.  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
   US
   United States of America

   Email: aanchal4@bu.edu

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

   Email: goldbe@cs.bu.edu