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      1 /*
      2  * Copyright 2017 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.android.server.net.watchlist;
     18 
     19 import android.privacy.DifferentialPrivacyEncoder;
     20 import android.privacy.internal.longitudinalreporting.LongitudinalReportingConfig;
     21 import android.privacy.internal.longitudinalreporting.LongitudinalReportingEncoder;
     22 import android.util.Slog;
     23 
     24 import com.android.internal.annotations.VisibleForTesting;
     25 
     26 import java.util.HashMap;
     27 import java.util.List;
     28 import java.util.Map;
     29 
     30 /**
     31  * Helper class to apply differential privacy to watchlist reports.
     32  */
     33 class PrivacyUtils {
     34 
     35     private static final String TAG = "PrivacyUtils";
     36     private static final boolean DEBUG = NetworkWatchlistService.DEBUG;
     37 
     38     /**
     39      * Parameters used for encoding watchlist reports.
     40      * These numbers are optimal parameters for protecting privacy with good utility.
     41      *
     42      * TODO: Add links to explain the math behind.
     43      */
     44     private static final String ENCODER_ID_PREFIX = "watchlist_encoder:";
     45     private static final double PROB_F = 0.469;
     46     private static final double PROB_P = 0.28;
     47     private static final double PROB_Q = 1.0;
     48 
     49     private PrivacyUtils() {
     50     }
     51 
     52     /**
     53      * Get insecure DP encoder.
     54      * Should not apply it directly on real data as seed is not randomized.
     55      */
     56     @VisibleForTesting
     57     static DifferentialPrivacyEncoder createInsecureDPEncoderForTest(String appDigest) {
     58         final LongitudinalReportingConfig config = createLongitudinalReportingConfig(appDigest);
     59         return LongitudinalReportingEncoder.createInsecureEncoderForTest(config);
     60     }
     61 
     62     /**
     63      * Get secure encoder to encode watchlist.
     64      *
     65      * Warning: If you use the same user secret and app digest, then you will get the same
     66      * PRR result.
     67      */
     68     @VisibleForTesting
     69     static DifferentialPrivacyEncoder createSecureDPEncoder(byte[] userSecret,
     70             String appDigest) {
     71         final LongitudinalReportingConfig config = createLongitudinalReportingConfig(appDigest);
     72         return LongitudinalReportingEncoder.createEncoder(config, userSecret);
     73     }
     74 
     75     /**
     76      * Get DP config for encoding watchlist reports.
     77      */
     78     private static LongitudinalReportingConfig createLongitudinalReportingConfig(String appDigest) {
     79         return new LongitudinalReportingConfig(ENCODER_ID_PREFIX + appDigest, PROB_F, PROB_P,
     80                 PROB_Q);
     81     }
     82 
     83     /**
     84      * Create a map that stores appDigest, encoded_visitedWatchlist pairs.
     85      */
     86     @VisibleForTesting
     87     static Map<String, Boolean> createDpEncodedReportMap(boolean isSecure, byte[] userSecret,
     88             List<String> appDigestList, WatchlistReportDbHelper.AggregatedResult aggregatedResult) {
     89         if (DEBUG) Slog.i(TAG, "createDpEncodedReportMap start");
     90         final int appDigestListSize = appDigestList.size();
     91         final HashMap<String, Boolean> resultMap = new HashMap<>(appDigestListSize);
     92         for (int i = 0; i < appDigestListSize; i++) {
     93             final String appDigest = appDigestList.get(i);
     94             // Each app needs to have different PRR result, hence we use appDigest as encoder Id.
     95             final DifferentialPrivacyEncoder encoder = isSecure
     96                     ? createSecureDPEncoder(userSecret, appDigest)
     97                     : createInsecureDPEncoderForTest(appDigest);
     98             final boolean visitedWatchlist = aggregatedResult.appDigestList.contains(appDigest);
     99             if (DEBUG) Slog.i(TAG, appDigest + ": " + visitedWatchlist);
    100             // Get the least significant bit of first byte, and set result to True if it is 1
    101             boolean encodedVisitedWatchlist = ((int) encoder.encodeBoolean(visitedWatchlist)[0]
    102                     & 0x1) == 0x1;
    103             resultMap.put(appDigest, encodedVisitedWatchlist);
    104         }
    105         return resultMap;
    106     }
    107 }
    108