/*
* Copyright (C) 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.server.location;
import java.io.FileDescriptor;
import java.io.PrintWriter;
import java.security.SecureRandom;
import android.content.Context;
import android.database.ContentObserver;
import android.location.Location;
import android.os.Handler;
import android.os.SystemClock;
import android.provider.Settings;
import android.util.Log;
/**
* Contains the logic to obfuscate (fudge) locations for coarse applications.
*
* <p>The goal is just to prevent applications with only
* the coarse location permission from receiving a fine location.
*/
public class LocationFudger {
private static final boolean D = false;
private static final String TAG = "LocationFudge";
/**
* Default coarse accuracy in meters.
*/
private static final float DEFAULT_ACCURACY_IN_METERS = 2000.0f;
/**
* Minimum coarse accuracy in meters.
*/
private static final float MINIMUM_ACCURACY_IN_METERS = 200.0f;
/**
* Secure settings key for coarse accuracy.
*/
private static final String COARSE_ACCURACY_CONFIG_NAME = "locationCoarseAccuracy";
/**
* This is the fastest interval that applications can receive coarse
* locations.
*/
public static final long FASTEST_INTERVAL_MS = 10 * 60 * 1000; // 10 minutes
/**
* The duration until we change the random offset.
*/
private static final long CHANGE_INTERVAL_MS = 60 * 60 * 1000; // 1 hour
/**
* The percentage that we change the random offset at every interval.
*
* <p>0.0 indicates the random offset doesn't change. 1.0
* indicates the random offset is completely replaced every interval.
*/
private static final double CHANGE_PER_INTERVAL = 0.03; // 3% change
// Pre-calculated weights used to move the random offset.
//
// The goal is to iterate on the previous offset, but keep
// the resulting standard deviation the same. The variance of
// two gaussian distributions summed together is equal to the
// sum of the variance of each distribution. So some quick
// algebra results in the following sqrt calculation to
// weigh in a new offset while keeping the final standard
// deviation unchanged.
private static final double NEW_WEIGHT = CHANGE_PER_INTERVAL;
private static final double PREVIOUS_WEIGHT = Math.sqrt(1 - NEW_WEIGHT * NEW_WEIGHT);
/**
* This number actually varies because the earth is not round, but
* 111,000 meters is considered generally acceptable.
*/
private static final int APPROXIMATE_METERS_PER_DEGREE_AT_EQUATOR = 111000;
/**
* Maximum latitude.
*
* <p>We pick a value 1 meter away from 90.0 degrees in order
* to keep cosine(MAX_LATITUDE) to a non-zero value, so that we avoid
* divide by zero fails.
*/
private static final double MAX_LATITUDE = 90.0 -
(1.0 / APPROXIMATE_METERS_PER_DEGREE_AT_EQUATOR);
private final Object mLock = new Object();
private final SecureRandom mRandom = new SecureRandom();
/**
* Used to monitor coarse accuracy secure setting for changes.
*/
private final ContentObserver mSettingsObserver;
/**
* Used to resolve coarse accuracy setting.
*/
private final Context mContext;
// all fields below protected by mLock
private double mOffsetLatitudeMeters;
private double mOffsetLongitudeMeters;
private long mNextInterval;
/**
* Best location accuracy allowed for coarse applications.
* This value should only be set by {@link #setAccuracyInMetersLocked(float)}.
*/
private float mAccuracyInMeters;
/**
* The distance between grids for snap-to-grid. See {@link #createCoarse}.
* This value should only be set by {@link #setAccuracyInMetersLocked(float)}.
*/
private double mGridSizeInMeters;
/**
* Standard deviation of the (normally distributed) random offset applied
* to coarse locations. It does not need to be as large as
* {@link #COARSE_ACCURACY_METERS} because snap-to-grid is the primary obfuscation
* method. See further details in the implementation.
* This value should only be set by {@link #setAccuracyInMetersLocked(float)}.
*/
private double mStandardDeviationInMeters;
public LocationFudger(Context context, Handler handler) {
mContext = context;
mSettingsObserver = new ContentObserver(handler) {
@Override
public void onChange(boolean selfChange) {
setAccuracyInMeters(loadCoarseAccuracy());
}
};
mContext.getContentResolver().registerContentObserver(Settings.Secure.getUriFor(
COARSE_ACCURACY_CONFIG_NAME), false, mSettingsObserver);
float accuracy = loadCoarseAccuracy();
synchronized (mLock) {
setAccuracyInMetersLocked(accuracy);
mOffsetLatitudeMeters = nextOffsetLocked();
mOffsetLongitudeMeters = nextOffsetLocked();
mNextInterval = SystemClock.elapsedRealtime() + CHANGE_INTERVAL_MS;
}
}
/**
* Get the cached coarse location, or generate a new one and cache it.
*/
public Location getOrCreate(Location location) {
synchronized (mLock) {
Location coarse = location.getExtraLocation(Location.EXTRA_COARSE_LOCATION);
if (coarse == null) {
return addCoarseLocationExtraLocked(location);
}
if (coarse.getAccuracy() < mAccuracyInMeters) {
return addCoarseLocationExtraLocked(location);
}
return coarse;
}
}
private Location addCoarseLocationExtraLocked(Location location) {
Location coarse = createCoarseLocked(location);
location.setExtraLocation(Location.EXTRA_COARSE_LOCATION, coarse);
return coarse;
}
/**
* Create a coarse location.
*
* <p>Two techniques are used: random offsets and snap-to-grid.
*
* <p>First we add a random offset. This mitigates against detecting
* grid transitions. Without a random offset it is possible to detect
* a users position very accurately when they cross a grid boundary.
* The random offset changes very slowly over time, to mitigate against
* taking many location samples and averaging them out.
*
* <p>Second we snap-to-grid (quantize). This has the nice property of
* producing stable results, and mitigating against taking many samples
* to average out a random offset.
*/
private Location createCoarseLocked(Location fine) {
Location coarse = new Location(fine);
// clean all the optional information off the location, because
// this can leak detailed location information
coarse.removeBearing();
coarse.removeSpeed();
coarse.removeAltitude();
coarse.setExtras(null);
double lat = coarse.getLatitude();
double lon = coarse.getLongitude();
// wrap
lat = wrapLatitude(lat);
lon = wrapLongitude(lon);
// Step 1) apply a random offset
//
// The goal of the random offset is to prevent the application
// from determining that the device is on a grid boundary
// when it crosses from one grid to the next.
//
// We apply the offset even if the location already claims to be
// inaccurate, because it may be more accurate than claimed.
updateRandomOffsetLocked();
// perform lon first whilst lat is still within bounds
lon += metersToDegreesLongitude(mOffsetLongitudeMeters, lat);
lat += metersToDegreesLatitude(mOffsetLatitudeMeters);
if (D) Log.d(TAG, String.format("applied offset of %.0f, %.0f (meters)",
mOffsetLongitudeMeters, mOffsetLatitudeMeters));
// wrap
lat = wrapLatitude(lat);
lon = wrapLongitude(lon);
// Step 2) Snap-to-grid (quantize)
//
// This is the primary means of obfuscation. It gives nice consistent
// results and is very effective at hiding the true location
// (as long as you are not sitting on a grid boundary, which
// step 1 mitigates).
//
// Note we quantize the latitude first, since the longitude
// quantization depends on the latitude value and so leaks information
// about the latitude
double latGranularity = metersToDegreesLatitude(mGridSizeInMeters);
lat = Math.round(lat / latGranularity) * latGranularity;
double lonGranularity = metersToDegreesLongitude(mGridSizeInMeters, lat);
lon = Math.round(lon / lonGranularity) * lonGranularity;
// wrap again
lat = wrapLatitude(lat);
lon = wrapLongitude(lon);
// apply
coarse.setLatitude(lat);
coarse.setLongitude(lon);
coarse.setAccuracy(Math.max(mAccuracyInMeters, coarse.getAccuracy()));
if (D) Log.d(TAG, "fudged " + fine + " to " + coarse);
return coarse;
}
/**
* Update the random offset over time.
*
* <p>If the random offset was new for every location
* fix then an application can more easily average location results
* over time,
* especially when the location is near a grid boundary. On the
* other hand if the random offset is constant then if an application
* found a way to reverse engineer the offset they would be able
* to detect location at grid boundaries very accurately. So
* we choose a random offset and then very slowly move it, to
* make both approaches very hard.
*
* <p>The random offset does not need to be large, because snap-to-grid
* is the primary obfuscation mechanism. It just needs to be large
* enough to stop information leakage as we cross grid boundaries.
*/
private void updateRandomOffsetLocked() {
long now = SystemClock.elapsedRealtime();
if (now < mNextInterval) {
return;
}
if (D) Log.d(TAG, String.format("old offset: %.0f, %.0f (meters)",
mOffsetLongitudeMeters, mOffsetLatitudeMeters));
// ok, need to update the random offset
mNextInterval = now + CHANGE_INTERVAL_MS;
mOffsetLatitudeMeters *= PREVIOUS_WEIGHT;
mOffsetLatitudeMeters += NEW_WEIGHT * nextOffsetLocked();
mOffsetLongitudeMeters *= PREVIOUS_WEIGHT;
mOffsetLongitudeMeters += NEW_WEIGHT * nextOffsetLocked();
if (D) Log.d(TAG, String.format("new offset: %.0f, %.0f (meters)",
mOffsetLongitudeMeters, mOffsetLatitudeMeters));
}
private double nextOffsetLocked() {
return mRandom.nextGaussian() * mStandardDeviationInMeters;
}
private static double wrapLatitude(double lat) {
if (lat > MAX_LATITUDE) {
lat = MAX_LATITUDE;
}
if (lat < -MAX_LATITUDE) {
lat = -MAX_LATITUDE;
}
return lat;
}
private static double wrapLongitude(double lon) {
lon %= 360.0; // wraps into range (-360.0, +360.0)
if (lon >= 180.0) {
lon -= 360.0;
}
if (lon < -180.0) {
lon += 360.0;
}
return lon;
}
private static double metersToDegreesLatitude(double distance) {
return distance / APPROXIMATE_METERS_PER_DEGREE_AT_EQUATOR;
}
/**
* Requires latitude since longitudinal distances change with distance from equator.
*/
private static double metersToDegreesLongitude(double distance, double lat) {
return distance / APPROXIMATE_METERS_PER_DEGREE_AT_EQUATOR / Math.cos(Math.toRadians(lat));
}
public void dump(FileDescriptor fd, PrintWriter pw, String[] args) {
pw.println(String.format("offset: %.0f, %.0f (meters)", mOffsetLongitudeMeters,
mOffsetLatitudeMeters));
}
/**
* This is the main control: call this to set the best location accuracy
* allowed for coarse applications and all derived values.
*/
private void setAccuracyInMetersLocked(float accuracyInMeters) {
mAccuracyInMeters = Math.max(accuracyInMeters, MINIMUM_ACCURACY_IN_METERS);
if (D) {
Log.d(TAG, "setAccuracyInMetersLocked: new accuracy = " + mAccuracyInMeters);
}
mGridSizeInMeters = mAccuracyInMeters;
mStandardDeviationInMeters = mGridSizeInMeters / 4.0;
}
/**
* Same as setAccuracyInMetersLocked without the pre-lock requirement.
*/
private void setAccuracyInMeters(float accuracyInMeters) {
synchronized (mLock) {
setAccuracyInMetersLocked(accuracyInMeters);
}
}
/**
* Loads the coarse accuracy value from secure settings.
*/
private float loadCoarseAccuracy() {
String newSetting = Settings.Secure.getString(mContext.getContentResolver(),
COARSE_ACCURACY_CONFIG_NAME);
if (D) {
Log.d(TAG, "loadCoarseAccuracy: newSetting = \"" + newSetting + "\"");
}
if (newSetting == null) {
return DEFAULT_ACCURACY_IN_METERS;
}
try {
return Float.parseFloat(newSetting);
} catch (NumberFormatException e) {
return DEFAULT_ACCURACY_IN_METERS;
}
}
}
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