Raytime: Validating token expiry on an unbounded local time interval
draft-amsuess-t2trg-raytime-02
| Document | Type | Active Internet-Draft (individual) | |
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| Author | Christian Amsüss | ||
| Last updated | 2024-01-11 | ||
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draft-amsuess-t2trg-raytime-02
t2trg C. Amsüss
Internet-Draft 11 January 2024
Intended status: Experimental
Expires: 14 July 2024
Raytime: Validating token expiry on an unbounded local time interval
draft-amsuess-t2trg-raytime-02
Abstract
When devices are deployed in locations with no real-time access to
the Internet, obtaining a trusted time for validation of time limited
tokens and certificates is sometimes not possible. This document
explores the options for deployments in which the trade-off between
availability and security needs to be made in favor of availability.
While considerations are general, terminology and examples primarily
focus on the ACE framework.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-amsuess-t2trg-raytime/.
Discussion of this document takes place on the Thing-to-Thing
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Source for this draft and an issue tracker can be found at
https://gitlab.com/chrysn/raytime.
Status of This Memo
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This Internet-Draft will expire on 14 July 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Motivating example . . . . . . . . . . . . . . . . . . . 3
1.3. Security and availability . . . . . . . . . . . . . . . . 4
1.4. Threat model . . . . . . . . . . . . . . . . . . . . . . 4
1.5. Applicability constraints . . . . . . . . . . . . . . . . 5
2. Raytime . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Using raytime with ACE . . . . . . . . . . . . . . . . . . . 7
3.1. Open questions . . . . . . . . . . . . . . . . . . . . . 7
3.2. Limits to the use of old tokens . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Informative References . . . . . . . . . . . . . . . . . . . 9
Appendix A. Comparison of raytime with prior work . . . . . . . 10
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[ See also abstract. ]
1.1. Terminology
This document uses terminology from the ACE framework ([ACE]) to
describe the typical participants, even in general parts that may
apply beyond ACE. In particular:
* The Resource Server (RS) is a constrained device, also sometimes
called “the device”.
In the context of this document, it has no regular access to the
Internet, and its ability to maintain a concept of local time is
limited by intermittent power supplies.
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* The Authorization Server (AS) is a party trusted by the RS.
In the context of this document, it is connected to the Internet.
(When more private networks are used, the term Internet can be
replaced with the partition of the private networks that contains
the AS). It is considered the source of trusted time in this
document, as distributed time sources are not relevant to its
mechanisms.
* The Client is a device that has obtained time- and scope limited
credentials for the RS from the AS.
In the context of this document, it acquires these credentials
ahead of time before it moves out of communication range of the
Internet and into communication range of the RS.
Beyond these credentials, the client is untrusted in the sense
that no action of the RS should allow it to exceed the permissions
encoded in the credentials it holds.
1.2. Motivating example
Devices that use Internet style connectivity and security have been
deployed far beyond the range of easily accessible connectivity, for
example in remote forests [forest40] and in space [space].
For a concrete example, consider a scientific instrument set up by
students. The instrument is under the administrative control of the
university, which is issuing time limited access tokens that allow
configuring experiments and reading data, but not altering inventory
designations or handing off ownership.
The instrument is set up in a far-off valley without cellular
reception, and visited by students once per week to collect data,
verify that it is still operational, and perform adjustments or
repairs on site as needed. Any repairs require powering down the
device completely.
When following the mechanisms and guidance of this document, the
instrument will be usable by the students even after they had to
power it down completely, and (once used by the new students)
justifiedly reject the tokens of students that used the device
earlier. It will stay usable to malicious students if they
disassemble it and use it after their allowed time on the instrument
has expired, but can only be operated for a total time of about the
validity time span of those users’ tokens.
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1.3. Security and availability
When a device has no current time, no means of acquiring time, and
receives a time limited token to authorize an action, two outcomes
are possible: the action is rejected or allowed. Rejection is the
safe alternative in terms of security, but reduces the availability
of the device, possibly to the point of not providing its function at
all. Conversely, allowing the action maintains availability at the
risk of violating security objectives.
The fundamental trade-off between security and availability is common
around time limited access, and usually manifested in a choice of
expiry times: A token with a validity of one week (or, equivalently,
a revocation list with a maximum age of one week) will be usable
across a day-long outage of the authorization infrastructure. On the
downside, it provides technical authorization to an administratively
deauthorized user for several days after revocation of permissions.
This document concerns itself with getting the most security out of
the case that favors availability after a period of no operational
clock.
1.4. Threat model
Three main threats are considered here:
* Using old tokens.
A user who has obtained a token may attempt to use it after the
token’s lifetime (and the user’s permission) has expired.
* Manipulation of time sources.
An attacker controlling a time source which the device trusts may
make false statements on the current time. Indicating an earlier
time can be used to extend the usability of old tokens.
Indicating a later time can be used to deny service to users of
valid tokens.
* Manipulation of the clock.
An attacker with physical access (possibly only to the device’s
environment) may cut power to the device’s clock.
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Other attacks, such as physical attacks on the device, or exploiting
weaknesses of the cryptographic systems used, are relevant to the
system, but have no different impact on a system following this
document than on a system that requires an operational clock before
it allows access.
1.5. Applicability constraints
The mechanisms of this document are intended only for scenarios
matching the listed prerequisites. If either of them are not met,
there are better solutions available, with suggestions listed along
the condition.
* There is no network connectivity, not even for the client.
If there is, the RS may use communication with the AS to obtain a
current time value (as described in [ace-time]), perform token
validation ([ACE]). Alternatively, it may use other Internet
services such as [roughtime].
Note that due to CoAP’s good proxy support, the connectivity
between the RS and the AS does not need to be in a continuous IP
network.
* The lifetime of tokens is limited.
When tokens of unlimited lifetime are used, there is no need to
employ the methods described in this document.
* Devices have no means of reliably maintaining time throughout
their whole lifetime.
If the power supply of the device’s clock is reliable enough to be
sure to outlive the device, regular evaluation of time bounds can
be done, possibly accounting for an upper bound of clock skew.
* Devices need to stay accessible after local power interruptions,
that is, favor availability over security after a loss of local
time.
Otherwise, the device can issue an AS Request Creation Hint
containing a cnonce (described in [ACE]). The client then needs
to travel back to into communication distance to the AS, and
obtain a token confirming that cnonce. Alternatively, one of the
time synchronization mechanisms mentioned above can be used.
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The applicability is also limited when it comes to requirements of a
certain date being in the past, such as “nbf” (“not before”) claims
described in [JWT]. These claims are notoriously hard to verify
after a loss of time (as discussed in [imperfect]), but fortunately
also rarely needed. The use of such claims is not fundamentally
prohibitive of this document’s raytime mechanism, but its approach
favoring availability over security is inapplicable to them (as a
device would accept any nbf value after power-up).
While data sources such as radio time signal stations (e.g., DCF77,
MSF, WWVB) or GNSSs such as GPS are frequently offered as a solution,
neither is reliable enough for use with security systems: Radio time
signals are effectively unauthenticated, and GPS signals can be
forged with hardware that is now cheap [gps].
2. Raytime
It is relatively common in clock synchronization to treat known time
in interval arithmetic as the earliest and latest possible current
point in time (for an example, see [optimal]).
When a device has been dormant for an indeterminate amount of time,
that interval’s upper bound needs to be set to infinity, creating a
half unbounded interval, or a ray. For lack of a term established in
literature [ i.e., if you happen to find one, please tell and this
will change ], we call devices operating under such conditions to
work “in raytime”. A device in raytime can be sure that some points
in time have passed, but never that they have not.
Devices that require a two-sided bound on some credentials they
accept may use any trusted time synchronization mechanism to
establish an upper bound on current time and switch to interval time
(and fall back to raytime after the next loss of continuous time).
Devices that never evaluate the upper bound on time anyway may only
implement the lower bound, and always operate in raytime.
Maintaining raytime is fundamentally a best-effort undertaking.
Implementations have two mechanisms to advance time:
* Any time the device receives a trusted statement on a time stamp
being in the past that is ahead of its earliest possible current
time, it updates that bound.
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Care has to be taken to limit this mechanism to trusted time
sources. It is recommended to exclusively use statements from the
AS, as they are provided in tokens’ “iat” claim. The build time
of the latest firmware update may also provide such a time source,
provided it is known to be on the same timescale as the AS’s
timescale.
* As time passes, the device advances the lower bound on the current
time. To avoid refusing tokens used quickly after issuance, lower
bound time should be advanced slower than time actually passes,
accounting for the maximum specified clock skew.
Both kinds of advancements need to be recorded persistently, lest
power cycling the device makes all tokens valid under raytime
assumptions. To avoid wearing out limited persistence options,
writing may be delayed as suitable in the application. The trade-off
to consider here are flash write cycles and power consumption of
flash writes versus the additional time malicious users gain for
using the device by removing the power supply at the right time.
Being best-effort, there is no need to transactionally commit the
time seen in tokens – this is not replay protection.
3. Using raytime with ACE
When using raytime with ACE, resource servers accepts tokens with
some [ possibly special, see below ] expiry date unless that expiry
is smaller than its current lower bound on time.
In a sense, it accounts for its own uncertainty in time in favor of
its peer.
[ This section will contain concrete guidance when the open questions
are resolved. ]
3.1. Open questions
* Should the decision on whether raytime is OK to use be encoded in
the token or in the application?
Options are to use “exp” and say that the action will tell whether
or not raytime is good enough to evaluate the credentials’
validity (some actions may require more assurances), or to use an
“exp-besteffort” or similar indicator to describe the whole token.
The latter approach may be limiting in that then not all
permissions can be expressed in a single token, but then again,
that’s also true with different validity times.
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More generally (also for interval time), can a token indicate
whether the clock skew upper bound is used in favor or against the
user? [JWT] talks of “some small leeway”, but is not overly
precise.
* If the device _is_ used online, can it set a cnonce in its
creation hints and the application will still try to use a cnonce-
less token? Or should optional cnonces use a different key?
(Setting a cnonce in online situations does have the advantage
that the device can convert from raytime to interval time. But is
that even useful, unless the tokens used have nbf claims?)
3.2. Limits to the use of old tokens
When raytime is applied to the evaluation of tokens as described,
devices take the risk of accepting expired tokens.
By properly maintaining their raytime as per Section 2, a user of an
expired token (who may be an attacker) is limited by two hard bounds,
and by their interpolation:
* A token limited to some amount of time before it expires can only
be used with the device for that amount of time, plus quantization
effects.
For example, a token issued as part of a five-day rental agreement
can be used arbitrarily late when the rented device is powered
off, but will become unusable after a total of 120 hours of
operation.
* When any new token is used with the device that was issued after
the expiry time of another token, the expired token becomes
unusable.
For example, once the device rented in the last example is found
by someone whose tokens were issued after the five days were over
and interacted with, the expired tokens become unusable.
4. Security Considerations
The use of Raytime diminishes security properties. It is only to be
used when applicable (see Section 1.5) and when the loss of the
properties is tolerated as part of the trade-off described in
Section 1.3.
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The property that is lost is that expired tokens are not rejected
unconditionally any more. Expired tokens (and thus also attacks
enabled by them) are still subject to the practical constraints
described in Section 3.2.
5. Informative References
[ACE] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/rfc/rfc9200>.
[ace-time] Navas, R., Selander, G., and L. Seitz, "Lightweight
Authenticated Time (LATe) Synchronization Protocol", Work
in Progress, Internet-Draft, draft-navas-ace-secure-time-
synchronization-00, 31 October 2016,
<https://datatracker.ietf.org/doc/html/draft-navas-ace-
secure-time-synchronization-00>.
[forest40] Singh, R., Gehlot, A., Vaseem Akram, S., Kumar Thakur, A.,
Buddhi, D., and P. Kumar Das, "Forest 4.0: Digitalization
of forest using the Internet of Things (IoT)", Journal of
King Saud University - Computer and Information
Sciences vol. 34, no. 8, pp. 5587-5601,
DOI 10.1016/j.jksuci.2021.02.009, September 2022,
<https://doi.org/10.1016/j.jksuci.2021.02.009>.
[gps] Sharp, J., Wu, C., and Q. Zeng, "Authentication for drone
delivery through a novel way of using face biometrics",
Proceedings of the 28th Annual International Conference on
Mobile Computing And Networking,
DOI 10.1145/3495243.3560550, October 2022,
<https://doi.org/10.1145/3495243.3560550>.
[imperfect]
Carrol, J. L. and D. B. Carren, "Future Imperfect", Star
Trek: The Next Generation , 1990.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/rfc/rfc7519>.
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[keyinfra18]
Pritikin, M., Richardson, M., Behringer, M. H., Bjarnason,
S., and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructures (BRSKI)", Work in Progress, Internet-
Draft, draft-ietf-anima-bootstrapping-keyinfra-18, 17
January 2019, <https://datatracker.ietf.org/doc/html/
draft-ietf-anima-bootstrapping-keyinfra-18>.
[optimal] Schmid, U. and K. Schossmaier, "Interval-based clock
synchronization with optimal precision", Information and
Computation vol. 186, no. 1, pp. 36-77,
DOI 10.1016/s0890-5401(03)00103-2, October 2003,
<https://doi.org/10.1016/s0890-5401(03)00103-2>.
[roughtime]
Malhotra, A., Langley, A., Ladd, W., and M. Dansarie,
"Roughtime", Work in Progress, Internet-Draft, draft-ietf-
ntp-roughtime-08, 18 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ntp-
roughtime-08>.
[space] Kua, J., Arora, C., Loke, S., Fernando, N., and C.
Ranaweera, "Internet of Things in Space: A Review of
Opportunities and Challenges from Satellite-Aided
Computing to Digitally-Enhanced Space Living",
arXiv article, DOI 10.48550/ARXIV.2109.05971, 2021,
<https://doi.org/10.48550/ARXIV.2109.05971>.
Appendix A. Comparison of raytime with prior work
Draft versions of RFC8995 up to 18 Section 2.6.1 of [keyinfra18] had
explicit guidance for updating a current reasonable date (CRD) from
seen certificates. That CRD is similar to the lower time bound of
raytime.
[ TBD: Go through that again to verify we’re not losing properties.
In particular, look not only at ACE but also MASA interactions. ]
Appendix B. Change log
Since -01:
* Add security considerations
* Add section on the limits to the use of old tokens
* Add short appendix that will later compare this to prior art in
draft-ietf-anima-bootstrapping-keyinfra
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Since -00:
* Write 'raytime' as a single word rather than 'ray time'.
* Editorial fixes provided by Carsten Bormann.
Acknowledgments
TBD: CB
Author's Address
Christian Amsüss
Austria
Email: christian@amsuess.com
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