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 android.privacy.internal.rappor;
  
  import android.privacy.DifferentialPrivacyEncoder;
  
  import com.google.android.rappor.Encoder;
  
  import java.nio.ByteBuffer;
  import java.nio.charset.StandardCharsets;
  import java.security.MessageDigest;
  import java.security.NoSuchAlgorithmException;
  import java.security.SecureRandom;
  import java.util.Random;
  
  /**
   * Differential privacy encoder by using
   * <a href="https://research.google.com/pubs/pub42852.html">RAPPOR</a>
   * algorithm.
   *
   * @hide
   */
  public class RapporEncoder implements DifferentialPrivacyEncoder {
  
      // Hard-coded seed and secret for insecure encoder
      private static final byte[] INSECURE_SECRET = new byte[]{
              (byte) 0xD7, (byte) 0x68, (byte) 0x99, (byte) 0x93,
              (byte) 0x94, (byte) 0x13, (byte) 0x53, (byte) 0x54,
              (byte) 0xFE, (byte) 0xD0, (byte) 0x7E, (byte) 0x54,
              (byte) 0xFE, (byte) 0xD0, (byte) 0x7E, (byte) 0x54,
              (byte) 0xD7, (byte) 0x68, (byte) 0x99, (byte) 0x93,
              (byte) 0x94, (byte) 0x13, (byte) 0x53, (byte) 0x54,
              (byte) 0xFE, (byte) 0xD0, (byte) 0x7E, (byte) 0x54,
              (byte) 0xFE, (byte) 0xD0, (byte) 0x7E, (byte) 0x54,
              (byte) 0xD7, (byte) 0x68, (byte) 0x99, (byte) 0x93,
              (byte) 0x94, (byte) 0x13, (byte) 0x53, (byte) 0x54,
              (byte) 0xFE, (byte) 0xD0, (byte) 0x7E, (byte) 0x54,
              (byte) 0xFE, (byte) 0xD0, (byte) 0x7E, (byte) 0x54
      };
      private static final SecureRandom sSecureRandom = new SecureRandom();
  
      private final RapporConfig mConfig;
  
      // Rappor encoder
      private final Encoder mEncoder;
      // True if encoder is secure (seed is securely randomized)
      private final boolean mIsSecure;
  
  
      private RapporEncoder(RapporConfig config, boolean secureEncoder, byte[] userSecret) {
          mConfig = config;
          mIsSecure = secureEncoder;
          final Random random;
          if (secureEncoder) {
              // Use SecureRandom as random generator.
              random = sSecureRandom;
          } else {
              // To have deterministic result by hard coding encoder id as seed.
              random = new Random(getInsecureSeed(config.mEncoderId));
              userSecret = INSECURE_SECRET;
          }
          mEncoder = new Encoder(random, null, null,
                  userSecret, config.mEncoderId, config.mNumBits,
                  config.mProbabilityF, config.mProbabilityP, config.mProbabilityQ,
                  config.mNumCohorts, config.mNumBloomHashes);
      }
  
      private long getInsecureSeed(String input) {
          try {
              MessageDigest digest = MessageDigest.getInstance("SHA-256");
              byte[] bytes = digest.digest(input.getBytes(StandardCharsets.UTF_8));
              return ByteBuffer.wrap(bytes).getLong();
          } catch (NoSuchAlgorithmException e) {
              // Should not happen
              throw new AssertionError("Unable generate insecure seed");
          }
      }
  
      /**
       * Create {@link RapporEncoder} with {@link RapporConfig} and user secret provided.
       *
       * @param config     Rappor parameters to encode input.
       * @param userSecret Per device unique secret key.
       * @return {@link RapporEncoder} instance.
       */
      public static RapporEncoder createEncoder(RapporConfig config, byte[] userSecret) {
          return new RapporEncoder(config, true, userSecret);
      }
  
      /**
       * Create <strong>insecure</strong> {@link RapporEncoder} with {@link RapporConfig} provided.
       * Should not use it to process sensitive data.
       *
       * @param config Rappor parameters to encode input.
       * @return {@link RapporEncoder} instance.
       */
      public static RapporEncoder createInsecureEncoderForTest(RapporConfig config) {
          return new RapporEncoder(config, false, null);
      }
  
      @Override
      public byte[] encodeString(String original) {
          return mEncoder.encodeString(original);
      }
  
      @Override
      public byte[] encodeBoolean(boolean original) {
          return mEncoder.encodeBoolean(original);
      }
  
      @Override
      public byte[] encodeBits(byte[] bits) {
          return mEncoder.encodeBits(bits);
      }
  
      @Override
      public RapporConfig getConfig() {
          return mConfig;
      }
  
      @Override
      public boolean isInsecureEncoderForTest() {
          return !mIsSecure;
      }
  }
  