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// Copyright (c) 2013-2023 xipki. All rights reserved.
// License Apache License 2.0

package org.xipki.ca.gateway;

import org.bouncycastle.crypto.digests.SHA256Digest;
import org.bouncycastle.crypto.generators.PKCS5S2ParametersGenerator;
import org.bouncycastle.crypto.params.KeyParameter;
import org.xipki.util.Args;
import org.xipki.util.RandomUtil;
import org.xipki.util.StringUtil;

import java.math.BigInteger;

/**
 * PBKDF2 salted password hashing.
 * @author havoc AT defuse.ca, www: http://crackstation.net/hashing-security.htm
 */
public class PasswordHash {

  // see 'http://stackoverflow.com/questions/22580853/reliable-implementation-of-pbkdf2-hmac-sha256-for-java'
  // public static final String PBKDF2_ALGORITHM = "PBKDF2WithHmacSHA1";

  // The following constants may be changed without breaking existing hashes.
  public static final int SALT_BYTE_SIZE = 24;
  public static final int DERIVED_KEY_SIZE = 32;
  public static final int PBKDF2_ITERATIONS = 10000;

  public static final int ITERATION_INDEX = 0;
  public static final int SALT_INDEX = 1;
  public static final int PBKDF2_INDEX = 2;

  private static final PKCS5S2ParametersGenerator GEN;

  static {
    GEN = new PKCS5S2ParametersGenerator(new SHA256Digest());
  }

  private PasswordHash() {
  }

  /**
   * Returns a salted PBKDF2 hash of the password.
   *
   * @param password - the password to hash
   * @return a salted PBKDF2 hash of the password
   *
   */
  public static String createHash(String password) {
    Args.notBlank(password, "password");
    return createHash(StringUtil.toUtf8Bytes(password));
  }

  /**
   * Returns a salted PBKDF2 hash of the password.
   *
   * @param password - the password to hash
   * @return a salted PBKDF2 hash of the password
   */
  public static String createHash(byte[] password) {
    return createHash(password, SALT_BYTE_SIZE, PBKDF2_ITERATIONS, DERIVED_KEY_SIZE);
  }

  /**
   * Returns a salted PBKDF2 hash of the password.
   *
   * @param password - the password to hash
   * @param saltSize - the size of salt in bytes
   * @param iterations - the iteration count (slowness factor)
   * @param dkSize - the length of the derived key
   * @return a salted PBKDF2 hash of the password
   */
  public static String createHash(byte[] password, int saltSize, int iterations, int dkSize) {
    Args.notNull(password, "password");
    // Generate a random salt
    byte[] salt = RandomUtil.nextBytes(saltSize);

    // Hash the password
    byte[] hash = pbkdf2(password, salt, iterations, dkSize);
    // format iterations:salt:hash
    return iterations + ":" + toHex(salt) + ":" + toHex(hash);
  }

  /**
   * Validates a password using a hash.
   *
   * @param password - the password to check
   * @param correctHash - the hash of the valid password
   * @return true if the password is correct, false if not
   */
  public static boolean validatePassword(String password, String correctHash) {
    Args.notBlank(password, "password");
    return validatePassword(StringUtil.toUtf8Bytes(password), correctHash);
  }

  /**
   * Validates a password using a hash.
   *
   * @param password - the password to check
   * @param correctHash - the hash of the valid password
   * @return true if the password is correct, false if not
   */
  public static boolean validatePassword(byte[] password, String correctHash) {
    Args.notNull(password, "password");
    // Decode the hash into its parameters
    String[] params = correctHash.split(":");
    int iterations = Integer.parseInt(params[ITERATION_INDEX]);
    byte[] salt = fromHex(params[SALT_INDEX]);
    byte[] hash = fromHex(params[PBKDF2_INDEX]);
    // Compute the hash of the provided password, using the same salt,
    // iteration count, and hash length
    byte[] testHash = pbkdf2(password, salt, iterations, hash.length);
    // Compare the hashes in constant time. The password is correct if
    // both hashes match.
    return slowEquals(hash, testHash);
  }

  /**
   * Compares two byte arrays in length-constant time. This comparison method
   * is used so that password hashes cannot be extracted from an on-line
   * system using a timing attack and then attacked off-line.
   *
   * @param arrayA - the first byte array
   * @param arrayB - the second byte array
   * @return true if both byte arrays are the same, false if not
   */
  private static boolean slowEquals(byte[] arrayA, byte[] arrayB) {
    int diff = arrayA.length ^ arrayB.length;
    for (int i = 0; i < arrayA.length && i < arrayB.length; i++) {
      diff |= arrayA[i] ^ arrayB[i];
    }
    return diff == 0;
  }

  /**
   * Computes the PBKDF2 hash of a password.
   *
   * @param password - the password to hash.
   * @param salt - the salt
   * @param iterations - the iteration count (slowness factor)
   * @param bytes - the length of the hash to compute in bytes
   * @return the PBDKF2 hash of the password
   */
  public static byte[] pbkdf2(byte[] password, byte[] salt, int iterations, int bytes) {
    synchronized (GEN) {
      GEN.init(password, salt, iterations);
      return ((KeyParameter) GEN.generateDerivedParameters(bytes * 8)).getKey();
    }
  }

  /**
   * Converts a string of hexadecimal characters into a byte array.
   *
   * @param hex - the hex string
   * @return the hex string decoded into a byte array
   */
  private static byte[] fromHex(String hex) {
    byte[] binary = new byte[hex.length() / 2];
    for (int i = 0; i < binary.length; i++) {
      binary[i] = (byte) Integer.parseInt(hex.substring(2 * i, 2 * i + 2), 16);
    }
    return binary;
  }

  /**
   * Converts a byte array into a hexadecimal string.
   *
   * @param array - the byte array to convert
   * @return a length*2 character string encoding the byte array
   */
  private static String toHex(byte[] array) {
    BigInteger bi = new BigInteger(1, array);
    String hex = bi.toString(16);
    int paddingLength = (array.length * 2) - hex.length();
    return (paddingLength > 0) ? String.format("%0" + paddingLength + "d", 0) + hex : hex;
  }

}