Internet DRAFT - draft-ietf-ntp-mac
draft-ietf-ntp-mac
Internet Engineering Task Force A. Malhotra
Internet-Draft S. Goldberg
Updates: 5905 (if approved) Boston University
Intended status: Standards Track January 4, 2019
Expires: July 8, 2019
Message Authentication Code for the Network Time Protocol
draft-ietf-ntp-mac-06
Abstract
RFC 5905 states that Network Time Protocol (NTP) packets should be
authenticated by appending the 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 to be too
weak, and recommends the use of AES-CMAC as in RFC 4493 as a
replacement.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 8, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
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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 . . . . . . . . . . . . . . . . . 3
4. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 3
6. Security Considerations . . . . . . . . . . . . . . . . . . . 3
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
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 the 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 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 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.
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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 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 GUE [GUE].
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 on recommendation
for the CMAC mode of authentication [NIST] and Security
Considerations Section of RFC 4493 [RFC4493] for discussion on
security guarantees of AES-CMAC.
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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. 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.
[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>.
[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>.
[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>.
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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