136 lines
5 KiB
JavaScript
136 lines
5 KiB
JavaScript
/** \file
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* \brief Wrappers for Generic Sensor API
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*
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* \see https://www.w3.org/TR/generic-sensor/
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*
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* \author Copyright (C) 2021 Radek Hranicky
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*
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* \license SPDX-License-Identifier: GPL-3.0-or-later
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*/
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <https://www.gnu.org/licenses/>.
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//
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/** \file
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* \ingroup wrappers
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*
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* MOTIVATION
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* The risk of using Generic Sensor API calls for device fingerprinting is
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* mentioned within the W3C Candidate Recommendation Draft, 29 July 2021
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* (https://www.w3.org/TR/2021/CRD-generic-sensor-20210729/#device-fingerprinting)
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* Documented threats include manufacturing imperfections and differences
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* that are unique to the concrete model of the device and can be used
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* for fingerprinting.
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*
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* We discovered another loophole in the `Sensor.timestamp` attribute. The value
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* describes when the last `Sensor.onreading` event occurred, in millisecond precision.
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* We observed the time origin is not the time of browsing context creation
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* but the last boot time of the device. Exposing such information is dangerous
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* as it allows to fingerprint the user easily. It is unlikely that two different
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* devices will boot at exactly the same time.
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*
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* Tested with the Magnetometer sensor on the following devices:
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* - Samsung Galaxy S21 Ultra; Android 11, kernel 5.4.6-215566388-abG99BXXU3AUE1, Build/RP1A.200720.012.G998BXXU3AUE1
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* Chrome 94.0.4606.71 and Kiwi (Chromium) 94.0.4606.56
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* - Xiaomi Redmi Note 5; Android 9, kernel 4.4.156-perf+, Build/9 PKQ1.180901.001
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* Chrome 94.0.4606.71
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*
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*
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* WRAPPING
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*
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* The wrapper thus protects device by changing the time origin to the browsing context
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* creation time, whereas the timestamp should still uniquely identify the reading.
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* This is achieved in the following way:
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* - At the first reading, we calculate the difference between the original value
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* and performance.now(). This gives us the offset between 1) the device boot
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* and 2) the page context initialization.
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* - On every reading, the offset is subtracted from the original value. The resulting
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* value then uniquely identifies the reading sample without exposing the boot time.
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* - Like in the other time precision wrappers, the resulting timestamp is processed
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* by the mitigation function before return. The mitigation may round and (optionally)
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* add noise to the resulting timestamp.
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*
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*
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* POSSIBLE IMPROVEMENTS
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* in protection level 2, the timestamp origin may be set to a random value based
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* on the session hash. This can serve as a "fake boot time."
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*/
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/*
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* Create private namespace
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*/
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(function() {
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var remember_past_values = `var precision = args[0];
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var doNoise = args[1];
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var pastValues = {};
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${rounding_function}
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${noise_function}
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var mitigationF = rounding_function;
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if (doNoise === true){
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mitigationF = function(value, precision) {
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let params = [value, precision];
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if (params in pastValues) {
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return pastValues[params];
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}
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let result = noise_function(...params);
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pastValues[params] = result;
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return result;
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}
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}
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var offsetCompStartPageStart = undefined;
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`;
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var wrappers = [
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{
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/**
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* \see https://dom.spec.whatwg.org/#ref-for-dom-event-timestamp%E2%91%A0
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*/
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parent_object: "Sensor.prototype",
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parent_object_property: "timestamp",
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wrapped_objects: [],
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helping_code: remember_past_values + `let origGet = Object.getOwnPropertyDescriptor(Sensor.prototype, "timestamp").get`,
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post_wrapping_code: [
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{
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code_type: "object_properties",
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parent_object: "Sensor.prototype",
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parent_object_property: "timestamp",
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wrapped_objects: [],
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/** \brief replaces Sensor.prototype.timestamp getter to create
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* a timestamp with the desired precision.
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*/
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wrapped_properties: [
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{
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property_name: "get",
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property_value: `
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function() {
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orig_val = origGet.call(this);
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if (typeof orig_val != 'number') {
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// Sensor is not available or there is no reading yet.
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return orig_val;
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}
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if (offsetCompStartPageStart === undefined) {
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// The offset has not been set yet so it needs to be calculated.
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offsetCompStartPageStart = orig_val - performance.now();
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}
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return mitigationF(orig_val - offsetCompStartPageStart, precision);
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}`,
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},
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],
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}
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],
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},
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]
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add_wrappers(wrappers);
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})()
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