Finding Tracking Tags
draft-fossaceca-dult-finding-00
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
| Document | Type | Active Internet-Draft (candidate for dult WG) | |
|---|---|---|---|
| Authors | Christine Fossaceca , Eric Rescorla | ||
| Last updated | 2024-10-01 (Latest revision 2024-09-29) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Call For Adoption By WG Issued | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-fossaceca-dult-finding-00
Detecting Unwanted Location Trackers C. Fossaceca
Internet-Draft Microsoft
Intended status: Informational E. Rescorla
Expires: 2 April 2025 Independent
29 September 2024
Finding Tracking Tags
draft-fossaceca-dult-finding-00
Abstract
Lightweight location tracking tags are in wide use to allow users to
locate items. These tags function as a component of a crowdsourced
tracking network in which devices belonging to other network users
(e.g., phones) report which tags they see and their location, thus
allowing the owner of the tag to determine where their tag was most
recently seen. This document defines the protocol by which devices
report tags they have seen and by which owners look up their
location.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://ekr.github.io/draft-fossaceca-dult-finding/draft-fossaceca-
dult-finding.html. Status information for this document may be found
at https://datatracker.ietf.org/doc/draft-fossaceca-dult-finding/.
Discussion of this document takes place on the Detecting Unwanted
Location Trackers Working Group mailing list (mailto:unwanted-
trackers@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/unwanted-trackers/.
Subscribe at https://www.ietf.org/mailman/listinfo/unwanted-
trackers/.
Source for this draft and an issue tracker can be found at
https://github.com/ekr/draft-fossaceca-dult-finding.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Fossaceca & Rescorla Expires 2 April 2025 [Page 1]
Internet-Draft Finding Tracking Tags September 2024
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 April 2025.
Copyright Notice
Copyright (c) 2024 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 (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Stalking Prevention . . . . . . . . . . . . . . . . . . . 4
2.2. Prior Research . . . . . . . . . . . . . . . . . . . . . 5
3. Conventions and Definitions . . . . . . . . . . . . . . . . . 6
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Reporting Device Leakage . . . . . . . . . . . . . . . . 8
4.2. Non-compliant Accessories . . . . . . . . . . . . . . . . 9
4.2.1. Rate Limiting and Attestation . . . . . . . . . . . . 9
5. Protocol Definition . . . . . . . . . . . . . . . . . . . . . 10
5.1. System Stages . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Partial Blind Signature Scheme . . . . . . . . . . . . . 11
5.3. Initial Pairing / Accessory Setup . . . . . . . . . . . . 12
5.3.1. Authenticity Verification . . . . . . . . . . . . . . 12
5.3.2. Key Generation and Signing with Partial Blind
Signatures . . . . . . . . . . . . . . . . . . . . . 12
5.3.3. Accessory in Nearby Owner Mode . . . . . . . . . . . 13
5.3.4. Accessory in Separated (Lost) Mode . . . . . . . . . 13
5.4. Finder Device creates a Location Report . . . . . . . . . 14
5.5. Owner Device queries the Crowdsourced Network . . . . . . 14
Fossaceca & Rescorla Expires 2 April 2025 [Page 2]
Internet-Draft Finding Tracking Tags September 2024
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6.1. Effectiveness of Rate Limiting via Blind Signatures . . . 15
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
DISCLAIMER: This draft is work-in-progress and has not yet seen
significant (or really any) security analysis. It should not be used
as a basis for building production systems.
Lightweight location tracking tags are a mechanism by which users can
track their personal items. These tags function as a component of a
crowdsourced tracking network in which devices belonging to other
network users (e.g., phones) report on the location of tags they have
seen. At a high level, location tracking this works as follows:
* Tags ("Accessories") broadcast an advertisement payload containing
accessory-specific information. The payload also indicates
whether the accessory is separated from its owner and thus
potentially lost.
* Devices belonging to other users ("Non-Owner Devices" or "Finder
Devices") observe those payloads and if the payload is in a
separated mode, reports its location to some central service
("Crowdsourced Network").
* The owner ("Owner Device") queries the central service
("Crowdsourced Network") for the location of their accessory.
A naive implementation of this design exposes users to considerable
privacy risk. In particular:
* If accessories simply have a fixed identifier that is reported
back to the tracking network, then the central server is able to
track any accessory without the user's assistance, which is
clearly undesirable.
* Any attacker who can guess or determine a tag ID can query the
central server for its location.
* An attacker can surreptitiously plant an accessory on a target and
thus track them by tracking their "own" accessory.
Fossaceca & Rescorla Expires 2 April 2025 [Page 3]
Internet-Draft Finding Tracking Tags September 2024
Section 6 provides a more detailed description of the desired
security privacy properties, but briefly, we would like to have a
system in which:
* Nobody other than the owner of an accessory would be able to learn
anything about the location of a given accessory.
* It is possible to detect when an accessory is being used to track
you.
* It is not possible for accessories that do not adhere to the
protocol to use the crowdsourced network protocol.
* It is not possible for unverified accessories to use the
crowdsourced network protocol.
A number of manufacturers have developed their own proprietary
tracking protocols, including Apple (see [WhoTracks] and [Heinrich]),
Samsung (see [Samsung]), and Tile, CUBE, Chipolo, Pebblebee and
TrackR (see [GMCKV21]), with varying security and privacy properties.
This document defines a cryptographic reporting and finding protocol
which is intended to minimize the above privacy risks. It is
intended to work in concert with the requirements defined in
[I-D.detecting-unwanted-location-trackers], which facilitate
detection of unwanted tracking tags. This protocol design is based
on existing academic research surrounding the security and privacy of
bluetooth location tracking accessories on the market today, as
described in [BlindMy] and [GMCKV21] and closely follows the design
of [BlindMy].
2. Motivations
2.1. Stalking Prevention
This work has been inspired by the negative security and privacy
implications that were introduced by lightweight location tracking
tags, and defined in part by
[I-D.detecting-unwanted-location-trackers]. The full threat model is
described in detail in [DultDoc4], however, a significant element of
the threat model lies in part with the security of the Crowdsourced
Network, which will be discussed in detail here.
Fossaceca & Rescorla Expires 2 April 2025 [Page 4]
Internet-Draft Finding Tracking Tags September 2024
In addition to its designed uses, the Crowdsourced Network also
provided stalkers with a means to track others by planting a tracking
tag on them and then using the CN to locate the tracker. Thus, this
document outlines the requirements and responsibilities of the
Crowdsourced Network to verify the authenticity of the participants,
while also preserving user privacy.
* First, the Crowdsourced Network should ensure that only authentic
Finding Devices are sending reports to the Crowdsourced Network,
and this should occur via an authenticated and encrypted channel.
This will help prevent malicious actors from interfering with
location reporting services.
* Second, the Crowdsourced Network should ensure that only
authorized Owner Devices are able to download location reports,
and this should occur via an authenticated and encrypted channel.
This will prevent malicious actors from unauthorized access of
location data.
* Third, the Crowdsourced Network must follow basic security
principles, such as storing location reports in an encrypted
manner
_(The benefits of this requirement are self explanatory.)_
* Fourth, the Crowdsourced Network must validate that the accessory
registered to an owner is valid. This wil prevent malicious
actors from leveraging counterfeit accessories.
* Fifth, users should should be able to opt-out of their devices
participating in the Crowdsourced Network.
2.2. Prior Research
There is substantial research into stalking via the FindMy protocol
and overall crowdsourced network protocol deficiencies have been
described in multiple bodies of work, such as:
* [GMCKV21]
* [Heinrich]
* [WhoTracks]
* [BlindMy]
* [Beck]
Fossaceca & Rescorla Expires 2 April 2025 [Page 5]
Internet-Draft Finding Tracking Tags September 2024
and others.
There are some suggested improvements, such as the security
properties described in [GMCKV21] above. The authors of [GMCKV21]
also suggested fusing a private key into the ACC to make it more
difficult to spoof, and requiring that location updates be signed.
[Heinrich] and [WhoTracks] pointed out early deficiencies in the
protocol, which [BlindMy] set out to solve. By introducing a Blind
Signature scheme, the authors sought to overcome an attacker
leveraging a large amount of keys to avoid triggering the anti-
tracking framework. In this implementation, keys were predetermined
for a set interval, and signed by the server, such that a specific,
presigned key can only be used during a pre-determined interval. The
drawback of this approach is that the authentication is left to the
OD and the CN, and the CN does not do any authentication with the FD,
so it still could store forged location reports. Additionally, the
FD does not do any authentication with the ACC, which means that it
could potentially interact with counterfeit ACC devices.
[Beck] introduces the idea of Multi-Dealer Secret Sharing (MDSS) as a
privacy preserving protocol that should also be considered.
3. Conventions and Definitions
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.
Section 1.2 of [I-D.detecting-unwanted-location-trackers] provides
definitions of the various system components.
Accessory (ACC): This is the device which will be tracked. It is
assumed to lack direct internet access and GPS, but will possess
Bluetooth Low Energy capabilities, which it uses to send
advertisement messages. The accessory protocol is defined in
[DultDoc3].
Advertisement: This is the message that is sent over the BLE Protocol
from the Accessory
Crowdsourced Network (CN): This is the network that provides the
location reporting upload and download services for Owner Devices and
Finder Devices.
Fossaceca & Rescorla Expires 2 April 2025 [Page 6]
Internet-Draft Finding Tracking Tags September 2024
Finder Device (FD): This is a device that is a non-owner device that
contributes information about an accessory to the crowdsourced
network.
Owner Device (OD): This is the device which owns the accessory, and
to which it is paired. There can be multiple owner devices, however,
the security of that implementation is outside of the scope of this
document.
4. Protocol Overview
Figure 1 provides an overall view of the protocol.
In this protocol, the Accessory communicates to Finder Devices or
FDs(such as phones) solely via Bluetooth, and the FDs communicate to
a centralized service on the Crowdsourced Network CN. Only during
the setup phase is the Owner Device OD able to act as a relay between
the Accessory ACC and the Crowdsourced Network CN. In this
implementation, the CN is able to act as a verifier and signer by
leveraging Blind Signatures, which allows the OD to obtain a
signature from the signer CN without revealing the input to the CN.
╭――――――――――――――――――╮
│ o │
│ ╭──────────────╮ │
│ │ │ │
│ │ │ │ .-~~~-.
│ │ │ │ .- ~ ~-( )_ _
o o │ │ │ │ / ~ -.
o o │ │ │ │ | \
o o -------> │ │ │ │ -------> \ .'
o o │ │ │ │ ~- . _____________ . -~
o o │ │ │ │
o o │ │ │ │
│ │ │ │
│ ╰──────────────╯ │
│ (_) │
╰――――――――――――――――――╯
Accessory BLE Finder Device Location CN Server
Advertisement Upload
Figure 1: Protocol Overview
Fossaceca & Rescorla Expires 2 April 2025 [Page 7]
Internet-Draft Finding Tracking Tags September 2024
As part of the setup phase (Section 5.3) the accessory and owning
device are paired, establishing a shared key SK which is known to
both the accessory and the owning device. The rest of the protocol
proceeds as follows.
* The accessory periodically sends out an advertisement which
contains an ephemeral public key Y_i where i is the epoch the key
is valid for (e.g., a one hour window). Y_i and its corresponding
private key X_i are generated in a deterministic fashion from SK
and the epoch i (conceptually as a X_i = PRF(SK, i)).
* In order to report an accessory's location at time i a non-owning
device FD encrypts it under Y_i and transmits the pair ( E(Y_i,
location), Y_i ) to the central service CN.
* In order to locate an accessory at time i, the owner uses SK to
compute (X_i, Y_i) and then sends Y_i to the central service. The
central service responds with all the reports it has for Y_i, and
the owner decrypts them with X_i.
This design provides substantially improved privacy properties over a
naive design:
1. Nobody but the owner can learn the reported location of an
accessory because it is encrypted under Y_i. This includes the
central service, which just sees encrypted reports.
2. It is not possible to correlate the public keys broadcast across
multiple epochs without knowing the shared key SK, which is only
know to the owner. However, an observer who sees multiple
beacons within the same epoch can correlate them, as they will
have the same Y_i. However, fast key rotation also makes it more
difficult to detect unwanted tracking, which relies on multiple
observations of the same identifier over time.
However, there are a number of residual privacy threats, as described
below.
4.1. Reporting Device Leakage
If the central server is able to learn the identity of the device
reporting an accessory or the identity of the owner requesting the
location of an accessory, then it can infer information about that
accessory's behavior. For instance:
Fossaceca & Rescorla Expires 2 April 2025 [Page 8]
Internet-Draft Finding Tracking Tags September 2024
* If device A reports accessories X and Y owned by different users
and they both query for their devices, then the central server may
learn that those users were in the same place, or at least their
accessories were.
* If devices A and B both report tag X, then the server learns that
A and B were in the same place.
* If the central server is able to learn where a reporting device is
(e.g., by IP address) and then the user queries for that
accessory, then the server can infer information about where the
user was, or at least where they lost the accessory.
These issues can be mitigated by concealing the identity and/or IP
address of network elements communicating with the central server
using techniques such as Oblivious HTTP [RFC9458] or MASQUE
[RFC9298].
4.2. Non-compliant Accessories
The detection mechanisms described in
[I-D.detecting-unwanted-location-trackers] depend on correct behavior
from the tracker. For instance, Section 3.5.1 of
[I-D.detecting-unwanted-location-trackers] requires that accessories
use a rotation period of 24 hours when in the "separated" state:
When in a separated state, the accessory SHALL rotate its address
every 24 hours. This duration allows a platform's unwanted tracking
algorithms to detect that the same accessory is in proximity for some
period of time, when the owner is not in physical proximity.
However, if an attacker were to make their own accessory that was
generated the right beacon messages or modify an existing one, they
could cause it to rotate the MAC address and public key Y_i more
frequently, thus evading detection algorithms. The following section
describes a mechanism which is intended to mitigate this attack.
4.2.1. Rate Limiting and Attestation
Because evading detection requires rapidly changing keys, evasion can
be made more difficult by limiting the rate at which keys can change.
This rate limiting works as follows:
1. Instead of allowing the accessory to publish an arbitrary key Y_i
it instead must pre-generate a set of keys, one for each time
window.
Fossaceca & Rescorla Expires 2 April 2025 [Page 9]
Internet-Draft Finding Tracking Tags September 2024
2. During the setup/pairing phase, the accessory and owning device
interact with the central service, which signs each temporal key
using a blind signature scheme. The owning device stores the
signatures for each key Y_i.
3. When it wishes to retrieve the location for a given accessory the
owning device provides the central service with the corresponding
signature, thus proving that it is retrieving location for a pre-
registered key; the central service will refuse to provide
results for unsigned keys.
Note that this mechanism _does not_ prevent the accessory from
broadcasting arbitrary keys, but it cannot retrieve location reports
corresponding to those keys.
This is not a complete defense: it limits an attacker who owns a
single accessory to a small number of keys per time window, but an
attacker who purchases N devices can then use N times that many keys
per window, potentially coordinating usage across spatially separated
devices to reduce the per-device cost. [[OPEN ISSUE: Can we do
better than this?]]
5. Protocol Definition
This section provides a detailed description of the DULT Finding
Protocol.
5.1. System Stages
The there are 5 stages that will be outlined, taking into account
elements from both [BlindMy] and [GMCKV21]. These stages are as
follows:
1) *Initial Pairing / Accessory Setup*
In this phase, the Accessory ACC is paired with the Owner Device OD,
and verified as authentic with the Crowdsourced Network CN
2) *Accessory in Nearby Owner Mode*
In this phase, the Accessory ACC is within Bluetooth range of the
Owner Device OD. In this phase, Finder Devices FDs SHALL NOT
generate location reports to send to the Crowdsourced Network CN.
The Accessory SHALL behave as defined in [DultDoc3]. [[OPEN ISSUE:
Need to make sure that walking around with an AirTag in Nearby Mode
does not allow for stalking]]
3) *Accessory in Separated (Lost) Mode*
Fossaceca & Rescorla Expires 2 April 2025 [Page 10]
Internet-Draft Finding Tracking Tags September 2024
In this phase, the Accessory ACC is not within Bluetooth raange fo
the Owner Device OD, therefore, the accessory must generate "lost"
messages to be received by Finder Devices FD, as described in
[DultDoc3].
4) *Finder Device creates a location report*
Finder Device FD receives a Bluetooth packet, and uploads a location
report to the Crowdsourced Network CN if and only if it is confirmed
to be a valid location report.
[[OPEN ISSUE: Should this be confirmed by the FD, or the CN? or
Both?]]
[[OPEN ISSUE: Should there be auth between FD and ACC as well as FD
and CN]]
5) *Owner Device queries the Crowdsourced Network*
Owner Device OD queries the Crowdsourced Network CN for the encrypted
location report.
5.2. Partial Blind Signature Scheme
[[OPEN ISSUE: Which blind signature scheme to use.]]
In order to verify the parties involved in the protocol, we rely on a
partially blind signature scheme. [RFC9474] describes a blind
signature scheme as follows:
The RSA Blind Signature Protocol is a two-party protocol between a
client and server where they interact to compute sig = Sign(sk,
input_msg), where input_msg = Prepare(msg) is a prepared version of
the private message msg provided by the client, and sk is the private
signing key provided by the server. See Section 6.2 for details on
how sk is generated and used in this protocol. Upon completion of
this protocol, the server learns nothing, whereas the client learns
sig. In particular, this means the server learns nothing of msg or
input_msg and the client learns nothing of sk.
The Finding Protocol uses a partially blind signature scheme in which
the signature also covers an additional info value which is not kept
secret from the signing server.
Fossaceca & Rescorla Expires 2 April 2025 [Page 11]
Internet-Draft Finding Tracking Tags September 2024
5.3. Initial Pairing / Accessory Setup
During the pairing process, the Accessory ACC pairs with the Owner
Device OD over Bluetooth. In this process, the ACC and OD must
generate cryptographically secure keys that will allow for the OD to
decrypt the ACC location reports.
5.3.1. Authenticity Verification
Upon the initial pairing of the the ACC and OD, before the key
generation process, the OD must facilitate communication with the CN
to verify the authenticity of the ACC.
The precise details of this communication are implementation-
dependent, but at the end of this process the CN must be able to
verify that:
1. The ACC is a legitimate (i.e., authorized) device.
2. The ACC has not already been registered.
For instance, each ACC might be provisioned with a unique serial
number which is digitally signed by the manufacturer, thus allowing
the CN to verify legitimacy. The CN could use a database of
registered serial numbers to prevent multiple registrations. Once
registration is complete, there must be some mechanism for the OD to
maintain continuity of authentication; this too is implementation
specific.
5.3.2. Key Generation and Signing with Partial Blind Signatures
The ACC must periodically be provisioned with new temporal keys which
FDs can then use to encrypt reports. Each temporal key is associated
with a given timestamp value,
Once the ACC has been authorized, the ACC (or OD on its behalf) needs
to generate its temporal encryption keys Y_i. It then generates a
signing request for the blinded version of each key.
contains two values:
blindedKey An opaque string representing the key to be signed,
computed as below.
timestamp The time value for the first time when the key will be
used in seconds since the UNIX epoch
blindedKey = Blind(pk, Y_i, info)
Fossaceca & Rescorla Expires 2 April 2025 [Page 12]
Internet-Draft Finding Tracking Tags September 2024
With the following inputs:
pk The public key for CN
Y_i The temporal key to be signed
info The timestamp value serialized as an unsigned 64-bit integer in
network byte order.
Prior to signing the key, the CN must ensure the acceptability of the
timestamp. While the details are implementation dependent, this
generally involves enforcing rate limits on how many keys can be
signed with timestamps within a given window. Once the CN is
satisfied with the submission it constructs a blind signature as
shown below and returns it to the OD.
[[OPEN ISSUE: Is it safe for ACC to hold all of the precomputed keys?
Or does this create a privacy issue? ]]
BlindSign(sk, blindedKey, info)
With the following inputs
sk The secret key for CN
blindedKey The raw bytes of the blinded key provided by CN
Upon receiving the signed blinded key, the OD unblinds the signature
and stores it. If the OD generated Y_i, it must also transfer it to
the ACC. Note that ACC does not need a copy of the signature.
5.3.3. Accessory in Nearby Owner Mode
After pairing, when the Accessory ACC is in Bluetooth range of OD, it
will follow the protocol as decribed in [DultDoc3].
5.3.4. Accessory in Separated (Lost) Mode
After pairing, when the Accessory ACC no longer in the Bluetooth
range of OD, it will follow the protocol as decribed below:, which
should correspond to the behavior outlined in [DultDoc3]:
ACC periodically sends out an Advertisement which contains the then
current ephemeral public key Y_i. The full payload format of the
Advertisement is defined in [DultDoc3].
Fossaceca & Rescorla Expires 2 April 2025 [Page 13]
Internet-Draft Finding Tracking Tags September 2024
5.4. Finder Device creates a Location Report
The Finder Device FD receives the advertisement via Bluetooth. FD
should have a mechanism by which to authenticate that this is a valid
public key with CN. *
In order to report an accessory's location at time i, FD extracts the
elliptic curve public key from the advertisement, and records it own
location data, a timestamp, and a confidence value as described in
[Heinrich].
FD performs ECDH with the public key Y_i and uses it to encrypt the
location data using HPKE Seal [RFC9180]. It sends the result to the
CN along with the hash of the current public key and the current
time. [[OPEN ISSUE: Should we work in terms of hashes or the public
keys. What we send has to be what's looked up.]]. CN stores the
resulting values indexed under the hash of the public key.
5.5. Owner Device queries the Crowdsourced Network
ODs can retrieve the location of a paired ACC by querying the CN.
In order to query for a given time period i it presents:
* The public key Y_i [or hash of the public key]
* The CN's signature over Y_i as well as the associated info value.
The CN then proceeds as follows:
1. Verify the signature over the key [hash]
2. Verify that the timestamp in the info value is within an
acceptable period of time (e.g., one week) from the current time
[[OPEN ISSUE: Why do we need this step?]]
3. Retrieve all reports matching the provided Y_i
4. Remove all reports which have timestamps that are not within the
acceptable time use window for the key, as indicated by the key's
timestamp.
5. Return the remaining reports to OD.
Finally, OD uses HPKE Open to decrypt the resulting reports, thus
recovering the location data for report.
Fossaceca & Rescorla Expires 2 April 2025 [Page 14]
Internet-Draft Finding Tracking Tags September 2024
6. Security Considerations
TODO Security - as described in [DultDoc4]?. This section still
mostly needs to be written.
6.1. Effectiveness of Rate Limiting via Blind Signatures
The blind signature mechanism described here (adapted from [BlindMy])
helps to limit the damage of noncompliant devices.
Because the CN will only generate signatures when the request is
associated with a valid device, an attacker cannot obtain a key
directly for a noncompliant device. However, this does not
necessarily mean that the attacker cannot provision noncompliant
devices. Specifically, if the OD sees the public keys (it need not
know the private keys, as described below) when they are sent to the
CN for signature, then it can provision them to a noncompliant
device.
Even an attacker who can provision invalid devices can only obtain
one key per time window per valid device. Because key use windows
overlap, it is possible to rotate keys more frequently than the
window, but in order to rotate keys significantly more frequently
than this, the attacker must purchase multiple devices. However,
they may be able to provision the keys from multiple valid devices
onto the same device, thus achieving a rotation rate increase at
linear cost.
Note that enforcement of this rate limit happens only on the CN: the
FD does not check. An attacker can generate advertisements with
unsigned keys -- and thus at any rotation rate it chooses -- and the
FD will duly send valid reports encrypted under those keys. The CN
will store them but because the attacker will not be able to produce
valid signatures, they will not be able to retrieve those reports.
As noted above, the ACC does not need to prove that it knows the
corresponding private keys for a given public key. The ACC simply
broadcasts the public keys; it is the OD which needs to know the
private keys in order to decrypt the reports.
7. Privacy Considerations
TODO Privacy - as described in [DultDoc4]?
8. IANA Considerations
This document has no IANA actions.
Fossaceca & Rescorla Expires 2 April 2025 [Page 15]
Internet-Draft Finding Tracking Tags September 2024
9. References
9.1. Normative References
[I-D.detecting-unwanted-location-trackers]
Ledvina, B., Eddinger, Z., Detwiler, B., and S. P.
Polatkan, "Detecting Unwanted Location Trackers", Work in
Progress, Internet-Draft, draft-detecting-unwanted-
location-trackers-01, 20 December 2023,
<https://datatracker.ietf.org/doc/html/draft-detecting-
unwanted-location-trackers-01>.
[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/rfc/rfc2119>.
[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/rfc/rfc8174>.
[RFC9180] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.
9.2. Informative References
[Beck] Gabrielle Beck, Harry Eldridge, Matthew Green, Nadia
Heninger, and Abishek Jain, "Abuse-Resistant Location
Tracking: Balancing Privacy and Safety in the Offline
Finding Ecosystem", 2023,
<https://eprint.iacr.org/2023/1332.pdf>.
[BlindMy] Travis Mayberry, Erik-Oliver Blass, and Ellis Fenske,
"Blind My — An Improved Cryptographic Protocol to Prevent
Stalking in Apple’s Find My Network", 2023,
<https://petsymposium.org/popets/2023/popets-
2023-0006.pdf>.
[DultDoc3] Brent Ledvina, Lazarov, D., Detwiler, B., and S. P.
Polatkan, "Detecting Unwanted Location Trackers Accessory
Protocol", 2024, <https://www.ietf.org/archive/id/draft-
ledvina-dult-accessory-protocol-00.html>.
[DultDoc4] Maggie Delano and Jessie Lowell, "DRAFT Dult Threat
Model", 2024, <https://datatracker.ietf.org/doc/html/
draft-delano-dult-threat-model>.
Fossaceca & Rescorla Expires 2 April 2025 [Page 16]
Internet-Draft Finding Tracking Tags September 2024
[GMCKV21] Chinmay Garg, Aravind Machiry, Andrea Continella,
Christopher Kruegel, and Giovanni Vigna, "Toward a secure
crowdsourced location tracking system", 2021,
<https://dl.acm.org/doi/10.1145/3448300.3467821>.
[Heinrich] Alexander Heinrich, Milan Stute, Tim Kornhuber, and
Matthias Hollick, "Who Can Find My Devices? Security and
Privacy of Apple's Crowd-Sourced Bluetooth Location
Tracking System", 2021,
<https://petsymposium.org/popets/2021/popets-
2021-0045.pdf>.
[RFC9298] Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
DOI 10.17487/RFC9298, August 2022,
<https://www.rfc-editor.org/rfc/rfc9298>.
[RFC9458] Thomson, M. and C. A. Wood, "Oblivious HTTP", RFC 9458,
DOI 10.17487/RFC9458, January 2024,
<https://www.rfc-editor.org/rfc/rfc9458>.
[RFC9474] Denis, F., Jacobs, F., and C. A. Wood, "RSA Blind
Signatures", RFC 9474, DOI 10.17487/RFC9474, October 2023,
<https://www.rfc-editor.org/rfc/rfc9474>.
[Samsung] Tingfeng Yu, James Henderson, Alwen Tiu, and Thomas
Haines, "Privacy Analysis of Samsung’s Crowd-Sourced
Bluetooth Location Tracking System", 2023,
<https://www.usenix.org/system/files/sec23winter-prepub-
498-yu.pdf>.
[WhoTracks]
Travis Mayberry, Ellis Fenske, Dane Brown, Christine
Fossaceca, Sam Teplov, Lucas Foppe, Jeremey Martin, and
Erik Rye, "Who Tracks the Trackers?", 2021,
<https://dl.acm.org/doi/10.1145/3463676.3485616>.
Acknowledgments
TODO acknowledge.
Authors' Addresses
Christine Fossaceca
Microsoft
Email: cfossaceca@microsoft.com
Fossaceca & Rescorla Expires 2 April 2025 [Page 17]
Internet-Draft Finding Tracking Tags September 2024
Eric Rescorla
Independent
Email: ekr@rtfm.com
Fossaceca & Rescorla Expires 2 April 2025 [Page 18]