content: /*
   * Copyright (C) 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.backup.utils;
  
  import static com.android.server.backup.BackupManagerService.TAG;
  
  import android.util.Slog;
  
  import java.security.Key;
  import java.security.NoSuchAlgorithmException;
  import java.security.spec.InvalidKeySpecException;
  import java.security.spec.KeySpec;
  
  import javax.crypto.SecretKey;
  import javax.crypto.SecretKeyFactory;
  import javax.crypto.spec.PBEKeySpec;
  
  /**
   * Passwords related utility methods.
   */
  public class PasswordUtils {
      // Configuration of PBKDF2 that we use for generating pw hashes and intermediate keys
      public static final int PBKDF2_HASH_ROUNDS = 10000;
      private static final int PBKDF2_KEY_SIZE = 256;     // bits
      public static final int PBKDF2_SALT_SIZE = 512;    // bits
      public static final String ENCRYPTION_ALGORITHM_NAME = "AES-256";
  
      /**
       * Creates {@link SecretKey} instance from given parameters.
       *
       * @param algorithm - key generation algorithm.
       * @param pw - password.
       * @param salt - salt.
       * @param rounds - number of rounds to run in key generation.
       * @return {@link SecretKey} instance or null in case of an error.
       */
      public static SecretKey buildPasswordKey(String algorithm, String pw, byte[] salt, int rounds) {
          return buildCharArrayKey(algorithm, pw.toCharArray(), salt, rounds);
      }
  
      /**
       * Generates {@link SecretKey} instance from given parameters and returns it's hex
       * representation.
       *
       * @param algorithm - key generation algorithm.
       * @param pw - password.
       * @param salt - salt.
       * @param rounds - number of rounds to run in key generation.
       * @return Hex representation of the generated key, or null if generation failed.
       */
      public static String buildPasswordHash(String algorithm, String pw, byte[] salt, int rounds) {
          SecretKey key = buildPasswordKey(algorithm, pw, salt, rounds);
          if (key != null) {
              return byteArrayToHex(key.getEncoded());
          }
          return null;
      }
  
      /**
       * Creates hex string representation of the byte array.
       */
      public static String byteArrayToHex(byte[] data) {
          StringBuilder buf = new StringBuilder(data.length * 2);
          for (int i = 0; i < data.length; i++) {
              buf.append(Byte.toHexString(data[i], true));
          }
          return buf.toString();
      }
  
      /**
       * Creates byte array from it's hex string representation.
       */
      public static byte[] hexToByteArray(String digits) {
          final int bytes = digits.length() / 2;
          if (2 * bytes != digits.length()) {
              throw new IllegalArgumentException("Hex string must have an even number of digits");
          }
  
          byte[] result = new byte[bytes];
          for (int i = 0; i < digits.length(); i += 2) {
              result[i / 2] = (byte) Integer.parseInt(digits.substring(i, i + 2), 16);
          }
          return result;
      }
  
      /**
       * Generates {@link SecretKey} instance from given parameters and returns it's checksum.
       *
       * Current implementation returns the key in its primary encoding format.
       *
       * @param algorithm - key generation algorithm.
       * @param pwBytes - password.
       * @param salt - salt.
       * @param rounds - number of rounds to run in key generation.
       * @return Hex representation of the generated key, or null if generation failed.
       */
      public static byte[] makeKeyChecksum(String algorithm, byte[] pwBytes, byte[] salt,
              int rounds) {
          char[] mkAsChar = new char[pwBytes.length];
          for (int i = 0; i < pwBytes.length; i++) {
              mkAsChar[i] = (char) pwBytes[i];
          }
  
          Key checksum = buildCharArrayKey(algorithm, mkAsChar, salt, rounds);
          return checksum.getEncoded();
      }
  
      private static SecretKey buildCharArrayKey(String algorithm, char[] pwArray, byte[] salt,
              int rounds) {
          try {
              SecretKeyFactory keyFactory = SecretKeyFactory.getInstance(algorithm);
              KeySpec ks = new PBEKeySpec(pwArray, salt, rounds, PBKDF2_KEY_SIZE);
              return keyFactory.generateSecret(ks);
          } catch (InvalidKeySpecException e) {
              Slog.e(TAG, "Invalid key spec for PBKDF2!");
          } catch (NoSuchAlgorithmException e) {
              Slog.e(TAG, "PBKDF2 unavailable!");
          }
          return null;
      }
  }
  