Internet Engineering Task Force A. Malhotra
Internet-Draft S. Goldberg
Intended status: Standards Track Boston University
Expires: January 9, 2017 July 8, 2016
Message Authentication Codes for the Network Time Protocol
draft-aanchal4-ntp-mac-00
Abstract
The Network Time Protocol (NTP) RFC 5905 [RFC5905] uses a message
authentication code (MAC) to cryptographically authenticate its UDP
packets. Currently, NTP packets are authenticated by appending a
128-bit key to the NTP data, and hashing the result with MD5 to
obtain a 128-bit tag. However, as discussed in [BCK] and [RFC6151],
this not a secure MAC. As such, this draft considers different
secure MAC algorithms for use with NTP, and evaluates their
performance. Given the security concerns, we also suggest
deprecating the use of MD5 as defined in [RFC5905] for authenticating
NTP packets.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. MAC Algorithms . . . . . . . . . . . . . . . . . . . . . . . 2
3. Performance Requirements . . . . . . . . . . . . . . . . . . 3
4. Performance Results . . . . . . . . . . . . . . . . . . . . . 3
5. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
NTP uses a message authentication code (MAC) to authenticate its
packets. Currently, NTP packets are authenticated by appending a
128-bit key to the NTP data, and hashing the result with MD5 to
obtain a 128-bit tag. However, as discussed in [BCK] and [RFC6151],
this not a secure MAC. As such, this draft considers different
secure MAC algorithms for use with NTP, and evaluates their
performance. Given the security concerns, we also suggest
deprecating the use of MD5 as defined in [RFC5905] for authenticating
NTP packets.
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. MAC Algorithms
We consider five diverse MAC algorithms, which encompass hash-based
HMAC-MD5 and HMAC-SHA224 [RFC2104], block cipher-based CMAC-AES
[RFC4493], and universal hashing-based Galois MAC (GMAC) [RFC4543]
and Poly1305(ChaCha20) as in section 2.6 of [RFC7539]. For
completeness we also benchmark the legacy MD5(key||message) from
[RFC5905].
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+--------------------+--------------+----------------+--------------+
| Algorithm | Input Key | Output Tag | Security |
| | Length | Length (Bytes) | Level (bits) |
| | (Bytes) | | |
+--------------------+--------------+----------------+--------------+
| legacy MD5 | 16 | 16 | NA |
| HMAC-MD5 | 16 | 16 | NA |
| HMAC-SHA224 | 16 | 28 | 112 |
| CMAC(AES) | 16 | 16 | 128 |
| GMAC(AES) | 16 | 16 | 128 |
| Poly1305(ChaCha20) | 32 | 16 | 128 |
+--------------------+--------------+----------------+--------------+
The choice of algorithms evaluated here is motivated, in part, by
standardization and availablity of open source implementation. Four
out of five algorithms are at least available in the OpenSSL library
and are standardized. The Poly1305(ChaCha20) algorithm is
implemented in LibreSSL, a fork of OpenSSL and also in BoringSSL,
Google's implementation of OpenSSL.
3. Performance Requirements
In order to accurately compute the time, NTP ideally requires MAC
algorithms to have a constant computational latency. However, this
is generally not possible, since latency depends on the CPU load,
temperature, and other uncontrollable factors. Instead, a MAC
algorithm that requires fewer clock cycles for computation is
prefered over one that requires more clock cycles, as this directly
translates to a reduction in jitter (i.e., the variance of the
latency for computing the MAC).
Throughput is another important consideration. NTP servers may have
to deal with thousands of client requests per second. A study [NIST]
on the usage analysis of NIST's NTP stratum 1 servers shows these
servers caters to 28,000 requests/second on an average, per server.
Most of the Internet is served by stratum 2 and stratum 3 servers,
some of which are part of voluntary NTP pool. These machines may be
running old hardware. So we benchmark performance on a range of
software and hardware platforms.
4. Performance Results
The NTP header is 48 bytes long. We therefore consider the latency
and throughput for several secure message authentication code (MAC)
algorithms when computed over 48-byte messages.
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We customize the in-built speed utility of OpenSSL-1.0.2g (03 May
2016) version to compute the latency and throughput for each MAC as
shown in the tables below. OpenSSL, however, does not implement
stream-cipher ChaCha20-based Poly1305 MAC algorithm. To speed test
this MAC, we use LibreSSL 2.3.1, a fork of OpenSSL implementation.
OpenSSL and LibreSSL are the most widely used cryptographic libraries
and are used by the current NTP implementations.
Since the introduction of New Instruction (NI) set for hardware
support in Intel chips, certain MACs like CMAC and GMAC have
performance advantage on such machines. Based on this, we perform
two different benchmarks once with AES-NI enabled and the other time
disabled on an x86_64, Intel(R) Xeon(R) CPU E5-2676 v3 @ 2.40GHz with
one core CPU.
This table shows throughput in terms of number of 48-byte NTP payload
processed per second.
+--------------------+-------------+-----------------+
| Algorithm | with AES-NI | without AES-NI |
+--------------------+-------------+-----------------+
| legacy MD5 | 3118K | 3165K |
| HMAC-MD5 | 2742K | 2749K |
| HMAC-SHA224 | 1265K | 1267K |
| CMAC(AES) | 7567K | 4388K |
| GMAC(AES) | 16612K | 4627K |
| Poly1305(ChaCha20) | 2598K | 2398K |
+--------------------+-------------+-----------------+
This table shows latency in terms of number of CPU cycles per byte
(cpb) when processing a 48-byte NTP payload.
+--------------------+-------------+-----------------+
| Algorithm | with AES-NI | without AES-NI |
+--------------------+-------------+-----------------+
| legacy MD5 | 16.03 | 15.7 |
| HMAC-MD5 | 18.2 | 18.1 |
| HMAC-SHA224 | 39.4 | 39 |
| CMAC(AES) | 6.6 | 11.3 |
| GMAC(AES) | 3.009 | 10.8 |
| Poly1305(ChaCha20) | 14.4 | 15 |
+--------------------+-------------+-----------------+
TODO: Test on other types of hardware.
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5. Recommendation
We suggest that use of GMAC(AES) because it has the best latency and
throughput performance.
6. Security Considerations
The MD5 (key||message) "message authentication code" specified in
[RFC5905] is vulnerable to length extension attacks, and uses the
insecure MD5 hash function, and therefore should be deprecated.
The output of HMAC-SHA224 is 28 bytes, but we truncate it to 16 bytes
as in section 4 of [RFC7630] to fit into the NTP packet. As noted in
section 6 of [RFC2104] it is safe to truncate the output of MACs as
long as the truncated length is greater than 80-bits and not less
than half the length of the hash output.
TO DO: Not finished yet. Following factors will be considered for
security comparison.
1. Output length of tag.
2. Input Key length.
3. Strength of the underlying cryptographic hash function or cipher.
4. Size and number of messages MACd using the same key.
7. Acknowledgements
The authors wish to acknowledge useful discussions with Harlan Stenn,
Mayank Varia, Daniel Franke, Ethan Heilman, and Leen Alshenibr.
8. References
8.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, <http://www.rfc-editor.org/info/rfc4493>.
[RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message
Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
DOI 10.17487/RFC4543, May 2006,
<http://www.rfc-editor.org/info/rfc4543>.
[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,
<http://www.rfc-editor.org/info/rfc5905>.
[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,
<http://www.rfc-editor.org/info/rfc6151>.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<http://www.rfc-editor.org/info/rfc7539>.
[RFC7630] Merkle, J., Ed. and M. Lochter, "HMAC-SHA-2 Authentication
Protocols in the User-based Security Model (USM) for
SNMPv3", RFC 7630, DOI 10.17487/RFC7630, October 2015,
<http://www.rfc-editor.org/info/rfc7630>.
8.2. Informative References
[BCK] Bellare, M., Canetti, R., and H. Krawczyk, "Keyed Hash
Functions and Message Authentication", in Proceedings of
Crypto'96, 1996.
[NIST] Sherman, J. and J. Levine, "Usage Analysis of the NIST
Internet Time Service", in Journal of Research of the
National Institute of Standards and Technology, 2016.
Authors' Addresses
Aanchal Malhotra
Boston University
111 Cummington St
Boston, MA 02215
US
Email: aanchal4@bu.edu
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Sharon Goldberg
Boston University
111 Cummington St
Boston, MA 02215
US
Email: goldbe@cs.bu.edu
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