content: /*
   * Copyright 2017 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.net.watchlist;
  
  import android.privacy.DifferentialPrivacyEncoder;
  import android.privacy.internal.longitudinalreporting.LongitudinalReportingConfig;
  import android.privacy.internal.longitudinalreporting.LongitudinalReportingEncoder;
  import android.util.Slog;
  
  import com.android.internal.annotations.VisibleForTesting;
  
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  
  /**
   * Helper class to apply differential privacy to watchlist reports.
   */
  class PrivacyUtils {
  
      private static final String TAG = "PrivacyUtils";
      private static final boolean DEBUG = NetworkWatchlistService.DEBUG;
  
      /**
       * Parameters used for encoding watchlist reports.
       * These numbers are optimal parameters for protecting privacy with good utility.
       *
       * TODO: Add links to explain the math behind.
       */
      private static final String ENCODER_ID_PREFIX = "watchlist_encoder:";
      private static final double PROB_F = 0.469;
      private static final double PROB_P = 0.28;
      private static final double PROB_Q = 1.0;
  
      private PrivacyUtils() {
      }
  
      /**
       * Get insecure DP encoder.
       * Should not apply it directly on real data as seed is not randomized.
       */
      @VisibleForTesting
      static DifferentialPrivacyEncoder createInsecureDPEncoderForTest(String appDigest) {
          final LongitudinalReportingConfig config = createLongitudinalReportingConfig(appDigest);
          return LongitudinalReportingEncoder.createInsecureEncoderForTest(config);
      }
  
      /**
       * Get secure encoder to encode watchlist.
       *
       * Warning: If you use the same user secret and app digest, then you will get the same
       * PRR result.
       */
      @VisibleForTesting
      static DifferentialPrivacyEncoder createSecureDPEncoder(byte[] userSecret,
              String appDigest) {
          final LongitudinalReportingConfig config = createLongitudinalReportingConfig(appDigest);
          return LongitudinalReportingEncoder.createEncoder(config, userSecret);
      }
  
      /**
       * Get DP config for encoding watchlist reports.
       */
      private static LongitudinalReportingConfig createLongitudinalReportingConfig(String appDigest) {
          return new LongitudinalReportingConfig(ENCODER_ID_PREFIX + appDigest, PROB_F, PROB_P,
                  PROB_Q);
      }
  
      /**
       * Create a map that stores appDigest, encoded_visitedWatchlist pairs.
       */
      @VisibleForTesting
      static Map<String, Boolean> createDpEncodedReportMap(boolean isSecure, byte[] userSecret,
              List<String> appDigestList, WatchlistReportDbHelper.AggregatedResult aggregatedResult) {
          if (DEBUG) Slog.i(TAG, "createDpEncodedReportMap start");
          final int appDigestListSize = appDigestList.size();
          final HashMap<String, Boolean> resultMap = new HashMap<>(appDigestListSize);
          for (int i = 0; i < appDigestListSize; i++) {
              final String appDigest = appDigestList.get(i);
              // Each app needs to have different PRR result, hence we use appDigest as encoder Id.
              final DifferentialPrivacyEncoder encoder = isSecure
                      ? createSecureDPEncoder(userSecret, appDigest)
                      : createInsecureDPEncoderForTest(appDigest);
              final boolean visitedWatchlist = aggregatedResult.appDigestList.contains(appDigest);
              if (DEBUG) Slog.i(TAG, appDigest + ": " + visitedWatchlist);
              // Get the least significant bit of first byte, and set result to True if it is 1
              boolean encodedVisitedWatchlist = ((int) encoder.encodeBoolean(visitedWatchlist)[0]
                      & 0x1) == 0x1;
              resultMap.put(appDigest, encodedVisitedWatchlist);
          }
          return resultMap;
      }
  }
  