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Goldberg 4 Intended status: Standards Track Boston University 5 Expires: May 3, 2018 October 30, 2017 7 Message Authentication Code for the Network Time Protocol 8 draft-ietf-ntp-mac-03 10 Abstract 12 RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets 13 should be authenticated by appending a 128-bit key to the NTP data, 14 and hashing the result with MD5 to obtain a 128-bit tag. This 15 document deprecates MD5-based authentication, which is considered to 16 be too weak, and recommends the use of AES-CMAC [RFC4493] as a 17 replacement. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at https://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on May 3, 2018. 36 Copyright Notice 38 Copyright (c) 2017 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (https://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 55 2. Deprecating MD5 . . . . . . . . . . . . . . . . . . . . . . . 2 56 3. Replacement Recommendation . . . . . . . . . . . . . . . . . 2 57 4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 5. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 3 59 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 3 60 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 61 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 8.1. Normative References . . . . . . . . . . . . . . . . . . 3 63 8.2. Informative References . . . . . . . . . . . . . . . . . 4 64 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4 66 1. Introduction 68 RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets 69 should be authenticated by appending a 128-bit key to the NTP data, 70 and hashing the result with MD5 to obtain a 128-bit tag. This 71 document deprecates MD5-based authentication, which is considered to 72 be too weak, and recommends the use of AES-CMAC [RFC4493] as a 73 replacement. 75 1.1. Requirements Language 77 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 78 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 79 document are to be interpreted as described in RFC 2119 [RFC2119]. 81 2. Deprecating MD5 83 RFC 5905 [RFC5905] defines how the MD5 digest algorithm in RFC 1321 84 [RFC1321] can be used as a message authentication code (MAC) for 85 authenticating NTP packets. However, as discussed in [BCK] and RFC 86 6151 [RFC6151], this is not a secure MAC and therefore MUST be 87 deprecated. 89 3. Replacement Recommendation 91 If authentication is implemented, then AES-CMAC as specified in RFC 92 4493 [RFC4493] SHOULD be computed over all fields in the NTP header, 93 and any extension fields that are present in the NTP packet as 94 described in RFC 5905 [RFC5905]. The MAC key for NTP SHOULD be 128 95 bits long AES-128 key and the resulting MAC tag SHOULD be 128 bits 96 long as stated in section 2.4 of RFC 4493 [RFC4493]. NTP makes this 97 transition possible as it supports algorithm agility as described in 98 Section 2.1 of RFC 7696 [RFC7696]. 100 4. Motivation 102 AES-CMAC is recommended for the following reasons: 104 1. It is an IETF standard that is available in many open source 105 implementations. 107 2. It is immune to nonce-reuse vulnerabilities (e.g. [Joux]) 108 because it does not use a nonce. 110 3. It has fine performance in terms of latency and throughput. 112 4. It benefits from native hardware support, for instance, Intel's 113 New Instruction set. 115 5. Test Vectors 117 For test vectors and their outputs refer to Section 4 of RFC 4493 118 [RFC4493] 120 6. Acknowledgements 122 The authors wish to acknowledge useful discussions with Leen 123 Alshenibr, Daniel Franke, Ethan Heilman, Kenny Paterson, Leonid 124 Reyzin, Harlan Stenn, and Mayank Varia. 126 7. IANA Considerations 128 This memo includes no request to IANA. 130 8. References 132 8.1. Normative References 134 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 135 Requirement Levels", BCP 14, RFC 2119, 136 DOI 10.17487/RFC2119, March 1997, 137 . 139 [RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The 140 AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June 141 2006, . 143 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 144 "Network Time Protocol Version 4: Protocol and Algorithms 145 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 146 . 148 8.2. Informative References 150 [BCK] Bellare, M., Canetti, R., and H. Krawczyk, "Keyed Hash 151 Functions and Message Authentication", in Proceedings of 152 Crypto'96, 1996. 154 [Joux] Joux, A., "Authentication Failures in NIST version of 155 GCM", 156 . 159 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 160 DOI 10.17487/RFC1321, April 1992, 161 . 163 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations 164 for the MD5 Message-Digest and the HMAC-MD5 Algorithms", 165 RFC 6151, DOI 10.17487/RFC6151, March 2011, 166 . 168 [RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm 169 Agility and Selecting Mandatory-to-Implement Algorithms", 170 BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015, 171 . 173 Authors' Addresses 175 Aanchal Malhotra 176 Boston University 177 111 Cummington St 178 Boston, MA 02215 179 US 181 Email: aanchal4@bu.edu 183 Sharon Goldberg 184 Boston University 185 111 Cummington St 186 Boston, MA 02215 187 US 189 Email: goldbe@cs.bu.edu