Ray time: Validating token expiry on an unbounded local time interval
draft-amsuess-t2trg-raytime-00

t2trg                                                          C. Amsüss
Internet-Draft                                               6 July 2023
Intended status: Experimental                                           
Expires: 7 January 2024

 Ray time: Validating token expiry on an unbounded local time interval
                     draft-amsuess-t2trg-raytime-00

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.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Thing-to-Thing
   Research Group mailing list (t2trg@irtf.org), which is archived at
   https://mailarchive.ietf.org/arch/search/?email_list=t2trg.

   Source for this draft and an issue tracker can be found at
   https://gitlab.com/chrysn/raytime.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on 7 January 2024.

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Copyright Notice

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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.  Ray time  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Using ray time with ACE . . . . . . . . . . . . . . . . . . .   7
     3.1.  Open questions  . . . . . . . . . . . . . . . . . . . . .   7
   4.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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.

   *  The Authorization Server (AS) is a party trusted by the RS.

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      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.

      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 and in space.

   For 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 requirements.

   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, 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 life time (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
   weakenesses of the used cryptographic systems, are relevant to the
   system, but have no different impact on a system following this
   document than 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 violated,
   there are better solutions available, with suggestens 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 life time of tokens is limited.

      When tokens of unlimited life time 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 ray time 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 DCT77 or GNSSs such as GPS are frequently
   offered as a solution, neither is reliable enough for use with
   security systems: DCT77 is effectively unencrypted, and GPS signals
   can be forged with cheap hardware [gps].

2.  Ray time

   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 ray time”. A device in ray time 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 ray time 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 ray time.

   Maintaining ray time 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 past that is ahead of its earliest possible current time, it
      updates that bound.

      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 time scale as the AS’s time
      scale.

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   *  As time passes, the device advances the lower bound on the current
      time.  To avoid refusing fast used tokens, time should be advanced
      slower than it 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 ray time
   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 ray time with ACE

   [ This section will contain concrete guidance when the open questions
   are resolved. ]

3.1.  Open questions

   *  Should the decision on whether ray time 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 ray time 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.

      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 ray time to interval time.  But
      is that even useful, unless the tokens used have nbf claims?)

4.  Informative References

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   [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>.

   [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>.

   [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-07, 26 September 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ntp-
              roughtime-07>.

Acknowledgments

   TBD.

Author's Address

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   Christian Amsüss
   Austria
   Email: christian@amsuess.com

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