A Secure Selection and Filtering Mechanism for the Network Time Protocol
draft-ietf-ntp-chronos-02
Network Working Group N. Rozen-Schiff
Internet-Draft D. Dolev
Intended status: Informational Hebrew University of Jerusalem
Expires: August 25, 2021 T. Mizrahi
Huawei Network.IO Innovation Lab
M. Schapira
Hebrew University of Jerusalem
February 21, 2021
A Secure Selection and Filtering Mechanism for the Network Time Protocol
draft-ietf-ntp-chronos-02
Abstract
The Network Time Protocol version 4 (NTPv4), as defined in RFC 5905,
is the mechanism used by NTP clients to synchronize with NTP servers
across the Internet. This document specifies an extension to the
NTPv4 client, named Chronos, which is used as a "watchdog" alongside
NTPv4, and provides improved security against time shifting attacks.
Chronos involves changes to the NTP client's system process only and
is backwards compatible with NTPv4 servers. Chronos is also
compatible with NTPv5, since it does not affect the wire protocol.
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 August 25, 2021.
Copyright Notice
Copyright (c) 2021 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Terms and Abbreviations . . . . . . . . . . . . . . . . . 4
2.3. Notations . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Extension to the NTP System Process . . . . . . . . . . . . . 5
3.1. Chronos' System Process . . . . . . . . . . . . . . . . . 6
3.2. Chronos' Recommended Parameters . . . . . . . . . . . . . 7
4. Chronos' Pseudocode . . . . . . . . . . . . . . . . . . . . . 7
5. Precision vs. Security . . . . . . . . . . . . . . . . . . . 8
6. Chronos' Threat Model and Security Guarantees . . . . . . . . 8
6.1. Security Analysis Overview . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
NTPv4, as defined in RFC 5905 [RFC5905], is vulnerable to time
shifting attacks, in which the attacker's goal is to shift the local
time at an NTP client. See [Chronos_paper] for details. Time
shifting attacks on NTP are possible even if NTP communication is
encrypted and authenticated. A weaker man-in-the-middle (MitM)
attacker can shift time simply by dropping or delaying packets,
whereas a powerful attacker, who has full control over an NTP server,
can determine the response content. This document introduces a time
shifting mitigation mechanism called Chronos. Chronos is backwards
compatible with NTPv4 and serves as an NTPv4 client's "watchdog" for
time shifting attacks. An NTP client that runs Chronos is
interoperable with [RFC5905]-compatible NTPv4 servers. Chronos is
also compatible with NTPv5, since it does not affect the wire
protocol.
Chronos is a background mechanism that continuously maintains a
virtual "Chronos" clock update and compares it to NTPv4's clock
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update. When the gap between the two updates exceeds a certain
threshold (specified in Section 6), this is interpreted as the client
experiencing a time shifting attack. In this case, Chronos is used
to update the client's clock, and NTPv4 is operated in the background
until the gap between NTPv4 and Chronos' updates are again below this
threshold, and hence NTPv4 is safe to use again.
Due to Chronos operating in the background, the client clock's
precision and accuracy are precisely as in NTPv4 while not
experiencing a time-shifting attack. When under attack, Chronos
prevents the clock from being shifted by the attacker, thus still
preserving high accuracy and precision (as discussed in Section 6).
Chronos achieves accurate synchronization even in the presence of
powerful attackers who are in direct control of a large number of NTP
servers: up to 1/3 of the servers in the pool (where the pool may
consist of hundreds or even thousands of servers). NTPv4 chooses a
small subset of the NTP server pool (e.g. 4 servers), and
periodically queries this subset of servers. Thus, even if only 1/3
of the servers in the pool are compromised, the small subset that is
used by NTPv4 may consist of a majority of faulty servers.
Conversely, Chronos constantly updates the set of servers it queries;
in each poll interval Chronos randomly chooses a different subset of
servers from the pool. Thus, even if an attack is not detected in a
given poll interval, Chronos is bound to detect the attack within a
relatively small number of poll intervals.
A Chronos client iteratively "crowdsources" time queries across NTP
servers and applies a provably secure algorithm for eliminating
"suspicious" responses and for averaging over the remaining
responses. Chronos is carefully engineered to minimize communication
overhead so as to avoid overloading NTP servers. Chronos' security
was evaluated both theoretically and experimentally with a prototype
implementation. These evaluation results indicate that in order to
successfully shift time at a Chronos client by over 100 milliseconds
from the UTC, even a powerful man-in-the-middle attacker requires
over 20 years of effort in expectation. The full paper is available
at [Chronos_paper].
Chronos introduces a watchdog mechanism that is added to the client's
system process and maintains a virtual clock value that is used as a
reference for detecting attacks. The virtual clock value computation
differs from the current NTPv4 in two key aspects. First, a Chronos
client relies on a large number of NTP servers, from which only few
servers to synchronize with are periodically chosen at random, in
order to avoid overloading the servers. Second, the selection
algorithm of the virtual clock uses an approximate agreement
technique to remove outliers, thus limiting the attacker's ability to
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contaminate the "time samples" (offsets) derived from the queried NTP
servers. These two elements of Chronos' design provide provable
security guarantees against both man-in-the-middle attackers and
attackers capable of compromising a large number of NTP servers.
2. Conventions Used in This Document
2.1. Terminology
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].
2.2. Terms and Abbreviations
NTPv4 Network Time Protocol version 4 [RFC5905].
Selection process Clock filter algorithm and system process
[RFC5905].
2.3. Notations
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Describing Chronos algorithm, the following notation are used.
+----------+--------------------------------------------------------+
| Notation | Meaning |
+----------+--------------------------------------------------------+
| n | The number of candidate servers in the pool that |
| | Chronos can query (potentially hundreds) |
| m | The number of servers that NTPv4 queries in each poll |
| | interval (up to tens) |
| w | An upper bound on the distance of the local time from |
| | the UTC at any NTP server with an accurate clock |
| | (termed "truechimer" in [RFC5905]) |
| Cest | The client's estimation for the time that has passed |
| | since its last synchronization to the server pool |
| | (sec) |
| B | An upper bound on the client's time estimation error |
| | (ms/sec) |
| ERR | An upper bound on the client's error regarding his |
| | estimation of the time passed from the last update, |
| | equals to B*Cest (ms) |
| K | Panic trigger - the number of pool re-sampling until |
| | reaches "Panic mode"; |
| tc | The current time [sec], as indicated by the virtual |
| | clock value that is computed by Chronos |
+----------+--------------------------------------------------------+
Table 1: Chronos Notations
The recommended values are discussed in Section 3.2.
3. Extension to the NTP System Process
A client that runs Chronos as a watchdog, uses NTPv4 as in [RFC5905]
and in the background runs a modification to the elements of the
system process described in Section 11.2.1 and 11.2.2 in [RFC5905]
(namely, the Selection Algorithm and the Cluster Algorithm). The
NTPv4 conventional protocol periodically queries m servers in each
poll interval. In parallel the Chronos watchdog periodically queries
a (variable) set of m servers in each Chronos poll interval.
Specifically, in Chronos, after executing the "Clock Filter
Algorithm" as defined in Section 10 in [RFC5905], the client discards
outliers by executing the procedure described in this section and the
next. Then, the NTPv4 "Combine Algorithm" is used for computing the
system peer offset, as specified in Section 11.2.3 in [RFC5905]. In
each poll interval the Chronos virtual clock value is compared with
the NTPv4 clock value, and if the difference exceeds a predetermined
value, an attack is detected. This process holds also for Chronos as
a watchdog of future NTPv5.
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3.1. Chronos' System Process
At the first time the Chronos system process is executed, calibration
is needed. The calibration process generates a local pool of servers
the client can synchronize with, consisting of n servers (up to
hundreds). To this end, the NTP client executes the "Peer Process"
and "Clock Filter Algorithm" as in Sections 9,10 in [RFC5905]
(respectively), on an hourly basis, for 24 consecutive hours, and
generates the union of all received NTP servers' IP addresses.
Importantly, this process can also be executed in the background
periodically, once in a long time (e.g., every few weeks/months).
In each Chronos poll interval the Chronos system process randomly
chooses a set of m servers (where n with magnitude of hundreds and m
of tens) out of the local pool of n servers. Then, out of the time-
samples received from this chosen subset of servers, a lowest third
of the samples' offset values and highest third of the samples'
offset values are discarded.
Chronos checks that the following two conditions hold for the
remaining samples:
o The maximal distance between every two time samples does not
exceed 2w.
o The average value of the remaining samples is at distance at most
ERR+2w from the client's local clock (as computed by Chronos).
(where w, ERR are as described in Table 1. Notice that ERR magnitude
is approximately LAMBDA as defined in [RFC5905]).
In the event that both of these conditions are satisfied, the average
of the remaining samples is the "final offset". Otherwise, a random
partial of the interval is chosen, after which a new subset of
servers is sampled, in the exact same manner. This way, Chronos
client queries are spread across the time interval better in case of
DoS attack on the NTP servers. This resampling process continues in
subsequent Chronos poll intervals until the two conditions are both
satisfied or the number of times the servers are re-sampled exceeds a
"Panic Trigger" (K in Table 1), in which case, Chronos enters a
"Panic Mode". Note that it is configurable whether the client allows
panic mode or not.
In panic mode, Chronos queries all the servers in the local server
pool, orders the collected time samples from lowest to highest and
eliminates the bottom third and the top third of the samples. The
client then averages over the remaining samples, and sets this
average to be the new "final offset".
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As in [RFC5905], the final offset is passed on to the clock
discipline algorithm for the purpose of steering the Chronos virtual
clock to the correct time. The Chronos virtual clock is then
compared to the NTPv4 (or to the future NTPv5) clock as part of the
watchdog process.
3.2. Chronos' Recommended Parameters
According to empirical observations (presented in [Chronos_paper]),
querying 15 servers at each poll interval (i.e., m=15) out of 500
servers (n=500), and setting w to be around 25 milliseconds provides
both high time accuracy and good security. Moreover, empirical
analyses showed that, on average, approximately 83% of the servers'
clocks are at most w-away from the UTC, and within 2w from each
other, satisfying the first condition of Chronos' system process.
Furthermore, according to Chronos security analysis, setting K to be
3 (i.e., if after 3 re-sampling, the two conditions are not
satisfied, then Chronos reaches "panic mode") is both safe when
facing time shifting attacks and the probability of reaching the
"panic mode" is negligible (less than 0.000002).
Chronos effect on precision and accuracy are discussed in Section 5
and Section 6.
4. Chronos' Pseudocode
The pseudocode for Chronos' Time Sampling Scheme, which is invoked in
each Chronos poll interval is as follows:
counter := 0
S = []
T = []
While counter < K do
S := sample(m) //gather samples from (tens of) randomly chosen servers
T := bi-side-trim(S,1/3) //trim the third lowest and highest values
if (max(T) -min(T) <= 2w) and (|avg(T)-tc| < ERR + 2w) Then
return avg(t)
end
counter ++
sleep(rand(0,1)*poll interval)
end
// panic mode
S := sample(n)
T := bi-sided-trim(S,1/3) //trim bottom and top thirds;
return avg(T)
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5. Precision vs. Security
Since NTPv4 (and future NTPv5) updates the clock as long as time-
shifting attacks are not detected, the precision and accuracy of a
Chronos client are the same as NTPv4 when not under attack. Under
attack, Chronos, which changes the list of the sampled servers more
frequently than NTPv4 [Chronos_paper], and does not use some of the
filters in NTPv4's system process, can potentially be less precise
(though provably more secure than NTPv4, which is vulnerable to time-
shifting attacks [RFC5905]).
However, our experimental and empirical analyses of Chronos revealed
that Chornos and NTPv4 exhibit the same level of precision and
accuracy when not under attack, with Chronos maintaining this level
even in the presence of time-shifting attacks.
6. Chronos' Threat Model and Security Guarantees
As explained above, Chronos repeatedly gathers time samples from
small subsets of a large local pool of NTP servers. The following
form of a man-in-the-middle (MitM) Byzantine attacker is considered:
the MitM attacker is assumed to control a subset of the servers in
the local pool of servers and is capable of determining precisely the
values of the time samples gathered by the Chronos client from these
NTP servers. The threat model thus encompasses a broad spectrum of
MitM attackers, ranging from fairly weak (yet dangerous) MitM
attackers only capable of delaying and dropping packets to extremely
powerful MitM attackers who are in control of (even authenticated)
NTP servers. MitM attackers captured by this framework might be, for
example, (1) in direct control of a fraction of the NTP servers
(e.g., by exploiting a software vulnerability), (2) an ISP (or other
Autonomous-System-level attacker) on the default BGP paths from the
NTP client to a fraction of the available servers, (3) a nation state
with authority over the owners of NTP servers in its jurisdiction, or
(4) an attacker capable of hijacking (e.g., through DNS cache
poisoning or BGP prefix hijacking) traffic to some of the available
NTP servers. The details of the specific attack scenario are
abstracted by reasoning about MitM attackers in terms of the fraction
of servers with respect to which the attacker has MitM capabilities.
Chronos detects time-shifting attacks by constantly monitoring
NTPv4's (or NTPv5's) offset and the offset computed by Chronos, as
explained above, and checking whether it exceeds a certain threshold
(10 milliseconds by default).
Analytical results (in [Chronos_paper]) indicate that in order to
succeed in shifting time at a Chronos client by even a small amount
(e.g., 100 milliseconds), even a powerful MitM attacker requires many
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years of effort (e.g., over 20 years in expectation). See a brief
overview of Chronos' security analysis below.
Notably, Chronos provides protection from MitM attacks that cannot be
achieved by cryptographic authentication protocols since even with
such measures in place an attacker can still influence time by
dropping/delaying packets. However, adding an authentication and
crypto-based security layer to Chronos will enhance its security
guarantees and enable the detection of various spoofing and
modification attacks.
Chronos' security analysis is briefly described next.
6.1. Security Analysis Overview
Time-samples that are at most w away from the UTC are considered
"good", whereas other samples are considered "malicious". Two
scenarios are considered:
o Less than 2/3 of the queried servers are under the attacker's
control.
o The attacker controls more than 2/3 of the queried servers.
The first scenario, where there are more than 1/3 good samples,
consists of two sub-cases: (i) there is at least one good sample in
the set of samples not eliminated by Chronos (that is, in the middle
third of samples), and (ii) there are no good samples in the
remaining set of samples. In the first of these two cases (at least
one good sample in the set of samples was not eliminated by Chronos),
the other remaining samples, including those provided by the
attacker, must be close to a good sample (for otherwise, the first
condition of Chronos' system process in Section 3.1 is violated and a
new set of servers is chosen). This implies that the average of the
remaining samples must be close to the UTC. In the second case
(there are no good samples in the set of remaining samples), since
more than a third of the initial samples were good, both the
(discarded) third lowest-value samples and the (discarded) third
highest-value samples must each contain a good sample. Hence, all
the remaining samples are bounded from both above and below by good
samples, and so is their average value, implying that this value is
close to the UTC [RFC5905].
In the second scenario, where the attacker controls more than 2/3 of
the queried servers, the worst possibility for the client is that all
remaining samples are malicious (i.e., more than w away from the
UTC). However, as proved in [Chronos_paper], the probability of this
scenario is extremely low even if the attacker controls a large
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fraction (e.g., 1/4) of the servers in the local pool. The
probability that the attacker repeatedly succeeds in realising this
scenario decays exponentially, rendering the probability of a
significant time shift negligible. See [Chronos_paper] for details.
Beyond evaluating the probability of an attacker successfully
shifting time at the client's clock, we also evaluated the
probability that the attacker succeeds in launching a DoS attack on
the servers by causing many clients to enter panic mode (and so query
all the servers in their local pools). This probability too is
negligible even for an attacker in control of a large number of
servers in clients' local server pools. See [Chronos_paper]for
details.
Further details about Chronos's threat model and security guarantees
can be found in [Chronos_paper].
7. Acknowledgements
The authors would like to thank Erik Kline, Miroslav Lichvar, Danny
Mayer, Karen O'Donoghue, Dieter Sibold, Yaakov. J. Stein, and
Harlan Stenn, for valuable contributions to this document and helpful
discussions and comments.
8. IANA Considerations
This memo includes no request to IANA.
9. References
9.1. Normative References
[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>.
[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
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[Chronos_paper]
Deutsch, O., Schiff, N., Dolev, D., and M. Schapira,
"Preventing (Network) Time Travel with Chronos", 2018,
<https://www.ndss-symposium.org/wp-
content/uploads/2018/02/ndss2018_02A-2_Deutsch_paper.pdf>.
Authors' Addresses
Neta Rozen-Schiff
Hebrew University of Jerusalem
Jerusalem
Israel
Phone: +972 2 549 4599
Email: neta.r.schiff@gmail.com
Danny Dolev
Hebrew University of Jerusalem
Jerusalem
Israel
Phone: +972 2 549 4588
Email: danny.dolev@mail.huji.ac.il
Tal Mizrahi
Huawei Network.IO Innovation Lab
Israel
Email: tal.mizrahi.phd@gmail.com
Michael Schapira
Hebrew University of Jerusalem
Jerusalem
Israel
Phone: +972 2 549 4570
Email: schapiram@huji.ac.il
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