<?php

/**
 * Provides a mechanism for hashing passwords, like "iterated md5", "bcrypt",
 * "scrypt", etc.
 *
 * Hashers define suitability and strength, and the system automatically
 * chooses the strongest available hasher and can prompt users to upgrade as
 * soon as a stronger hasher is available.
 *
 * @task hasher   Implementing a Hasher
 * @task hashing  Using Hashers
 */
abstract class PhabricatorPasswordHasher extends Phobject {

  const MAXIMUM_STORAGE_SIZE = 128;


/* -(  Implementing a Hasher  )---------------------------------------------- */


  /**
   * Return a human-readable description of this hasher, like "Iterated MD5".
   *
   * @return string Human readable hash name.
   * @task hasher
   */
  abstract public function getHumanReadableName();


  /**
   * Return a short, unique, key identifying this hasher, like "md5" or
   * "bcrypt". This identifier should not be translated.
   *
   * @return string Short, unique hash name.
   * @task hasher
   */
  abstract public function getHashName();


  /**
   * Return the maximum byte length of hashes produced by this hasher. This is
   * used to prevent storage overflows.
   *
   * @return int  Maximum number of bytes in hashes this class produces.
   * @task hasher
   */
  abstract public function getHashLength();


  /**
   * Return `true` to indicate that any required extensions or dependencies
   * are available, and this hasher is able to perform hashing.
   *
   * @return bool True if this hasher can execute.
   * @task hasher
   */
  abstract public function canHashPasswords();


  /**
   * Return a human-readable string describing why this hasher is unable
   * to operate. For example, "To use bcrypt, upgrade to PHP 5.5.0 or newer.".
   *
   * @return string Human-readable description of how to enable this hasher.
   * @task hasher
   */
  abstract public function getInstallInstructions();


  /**
   * Return an indicator of this hasher's strength. When choosing to hash
   * new passwords, the strongest available hasher which is usuable for new
   * passwords will be used, and the presence of a stronger hasher will
   * prompt users to update their hashes.
   *
   * Generally, this method should return a larger number than hashers it is
   * preferable to, but a smaller number than hashers which are better than it
   * is. This number does not need to correspond directly with the actual hash
   * strength.
   *
   * @return float  Strength of this hasher.
   * @task hasher
   */
  abstract public function getStrength();


  /**
   * Return a short human-readable indicator of this hasher's strength, like
   * "Weak", "Okay", or "Good".
   *
   * This is only used to help administrators make decisions about
   * configuration.
   *
   * @return string Short human-readable description of hash strength.
   * @task hasher
   */
  abstract public function getHumanReadableStrength();


  /**
   * Produce a password hash.
   *
   * @param   PhutilOpaqueEnvelope  Text to be hashed.
   * @return  PhutilOpaqueEnvelope  Hashed text.
   * @task hasher
   */
  abstract protected function getPasswordHash(PhutilOpaqueEnvelope $envelope);


  /**
   * Verify that a password matches a hash.
   *
   * The default implementation checks for equality; if a hasher embeds salt in
   * hashes it should override this method and perform a salt-aware comparison.
   *
   * @param   PhutilOpaqueEnvelope  Password to compare.
   * @param   PhutilOpaqueEnvelope  Bare password hash.
   * @return  bool                  True if the passwords match.
   * @task hasher
   */
  protected function verifyPassword(
    PhutilOpaqueEnvelope $password,
    PhutilOpaqueEnvelope $hash) {

    $actual_hash = $this->getPasswordHash($password)->openEnvelope();
    $expect_hash = $hash->openEnvelope();

    return ($actual_hash === $expect_hash);
  }


  /**
   * Check if an existing hash created by this algorithm is upgradeable.
   *
   * The default implementation returns `false`. However, hash algorithms which
   * have (for example) an internal cost function may be able to upgrade an
   * existing hash to a stronger one with a higher cost.
   *
   * @param PhutilOpaqueEnvelope  Bare hash.
   * @return bool                 True if the hash can be upgraded without
   *                              changing the algorithm (for example, to a
   *                              higher cost).
   * @task hasher
   */
  protected function canUpgradeInternalHash(PhutilOpaqueEnvelope $hash) {
    return false;
  }


/* -(  Using Hashers  )------------------------------------------------------ */


  /**
   * Get the hash of a password for storage.
   *
   * @param   PhutilOpaqueEnvelope  Password text.
   * @return  PhutilOpaqueEnvelope  Hashed text.
   * @task hashing
   */
  final public function getPasswordHashForStorage(
    PhutilOpaqueEnvelope $envelope) {

    $name = $this->getHashName();
    $hash = $this->getPasswordHash($envelope);

    $actual_len = strlen($hash->openEnvelope());
    $expect_len = $this->getHashLength();
    if ($actual_len > $expect_len) {
      throw new Exception(
        pht(
          "Password hash '%s' produced a hash of length %d, but a ".
          "maximum length of %d was expected.",
          $name,
          new PhutilNumber($actual_len),
          new PhutilNumber($expect_len)));
    }

    return new PhutilOpaqueEnvelope($name.':'.$hash->openEnvelope());
  }


  /**
   * Parse a storage hash into its components, like the hash type and hash
   * data.
   *
   * @return map  Dictionary of information about the hash.
   * @task hashing
   */
  private static function parseHashFromStorage(PhutilOpaqueEnvelope $hash) {
    $raw_hash = $hash->openEnvelope();
    if (strpos($raw_hash, ':') === false) {
      throw new Exception(
        pht(
          'Malformed password hash, expected "name:hash".'));
    }

    list($name, $hash) = explode(':', $raw_hash);

    return array(
      'name' => $name,
      'hash' => new PhutilOpaqueEnvelope($hash),
    );
  }


  /**
   * Get all available password hashers. This may include hashers which can not
   * actually be used (for example, a required extension is missing).
   *
   * @return list<PhabicatorPasswordHasher> Hasher objects.
   * @task hashing
   */
  public static function getAllHashers() {
    $objects = id(new PhutilSymbolLoader())
      ->setAncestorClass('PhabricatorPasswordHasher')
      ->loadObjects();

    $map = array();
    foreach ($objects as $object) {
      $name = $object->getHashName();

      $potential_length = strlen($name) + $object->getHashLength() + 1;
      $maximum_length = self::MAXIMUM_STORAGE_SIZE;

      if ($potential_length > $maximum_length) {
        throw new Exception(
          pht(
            'Hasher "%s" may produce hashes which are too long to fit in '.
            'storage. %d characters are available, but its hashes may be '.
            'up to %d characters in length.',
            $name,
            $maximum_length,
            $potential_length));
      }

      if (isset($map[$name])) {
        throw new Exception(
          pht(
            'Two hashers use the same hash name ("%s"), "%s" and "%s". Each '.
            'hasher must have a unique name.',
            $name,
            get_class($object),
            get_class($map[$name])));
      }
      $map[$name] = $object;
    }

    return $map;
  }


  /**
   * Get all usable password hashers. This may include hashers which are
   * not desirable or advisable.
   *
   * @return list<PhabicatorPasswordHasher> Hasher objects.
   * @task hashing
   */
  public static function getAllUsableHashers() {
    $hashers = self::getAllHashers();
    foreach ($hashers as $key => $hasher) {
      if (!$hasher->canHashPasswords()) {
        unset($hashers[$key]);
      }
    }
    return $hashers;
  }


  /**
   * Get the best (strongest) available hasher.
   *
   * @return PhabicatorPasswordHasher Best hasher.
   * @task hashing
   */
  public static function getBestHasher() {
    $hashers = self::getAllUsableHashers();
    $hashers = msort($hashers, 'getStrength');

    $hasher = last($hashers);
    if (!$hasher) {
      throw new PhabricatorPasswordHasherUnavailableException(
        pht(
          'There are no password hashers available which are usable for '.
          'new passwords.'));
    }

    return $hasher;
  }


  /**
   * Get the hashser for a given stored hash.
   *
   * @return PhabicatorPasswordHasher Corresponding hasher.
   * @task hashing
   */
  public static function getHasherForHash(PhutilOpaqueEnvelope $hash) {
    $info = self::parseHashFromStorage($hash);
    $name = $info['name'];

    $usable = self::getAllUsableHashers();
    if (isset($usable[$name])) {
      return $usable[$name];
    }

    $all = self::getAllHashers();
    if (isset($all[$name])) {
      throw new PhabricatorPasswordHasherUnavailableException(
        pht(
          'Attempting to compare a password saved with the "%s" hash. The '.
          'hasher exists, but is not currently usable. %s',
          $name,
          $all[$name]->getInstallInstructions()));
    }

    throw new PhabricatorPasswordHasherUnavailableException(
      pht(
        'Attempting to compare a password saved with the "%s" hash. No such '.
        'hasher is known to Phabricator.',
        $name));
  }


  /**
   * Test if a password is using an weaker hash than the strongest available
   * hash. This can be used to prompt users to upgrade, or automatically upgrade
   * on login.
   *
   * @return bool True to indicate that rehashing this password will improve
   *              the hash strength.
   * @task hashing
   */
  public static function canUpgradeHash(PhutilOpaqueEnvelope $hash) {
    if (!strlen($hash->openEnvelope())) {
      throw new Exception(
        pht('Expected a password hash, received nothing!'));
    }

    $current_hasher = self::getHasherForHash($hash);
    $best_hasher = self::getBestHasher();

    if ($current_hasher->getHashName() != $best_hasher->getHashName()) {
      // If the algorithm isn't the best one, we can upgrade.
      return true;
    }

    $info = self::parseHashFromStorage($hash);
    if ($current_hasher->canUpgradeInternalHash($info['hash'])) {
      // If the algorithm provides an internal upgrade, we can also upgrade.
      return true;
    }

    // Already on the best algorithm with the best settings.
    return false;
  }


  /**
   * Generate a new hash for a password, using the best available hasher.
   *
   * @param   PhutilOpaqueEnvelope  Password to hash.
   * @return  PhutilOpaqueEnvelope  Hashed password, using best available
   *                                hasher.
   * @task hashing
   */
  public static function generateNewPasswordHash(
    PhutilOpaqueEnvelope $password) {
    $hasher = self::getBestHasher();
    return $hasher->getPasswordHashForStorage($password);
  }


  /**
   * Compare a password to a stored hash.
   *
   * @param   PhutilOpaqueEnvelope  Password to compare.
   * @param   PhutilOpaqueEnvelope  Stored password hash.
   * @return  bool                  True if the passwords match.
   * @task hashing
   */
  public static function comparePassword(
    PhutilOpaqueEnvelope $password,
    PhutilOpaqueEnvelope $hash) {

    $hasher = self::getHasherForHash($hash);
    $parts = self::parseHashFromStorage($hash);

    return $hasher->verifyPassword($password, $parts['hash']);
  }


  /**
   * Get the human-readable algorithm name for a given hash.
   *
   * @param   PhutilOpaqueEnvelope  Storage hash.
   * @return  string                Human-readable algorithm name.
   */
  public static function getCurrentAlgorithmName(PhutilOpaqueEnvelope $hash) {
    $raw_hash = $hash->openEnvelope();
    if (!strlen($raw_hash)) {
      return pht('None');
    }

    try {
      $current_hasher = PhabricatorPasswordHasher::getHasherForHash($hash);
      return $current_hasher->getHumanReadableName();
    } catch (Exception $ex) {
      $info = self::parseHashFromStorage($hash);
      $name = $info['name'];
      return pht('Unknown ("%s")', $name);
    }
  }


  /**
   * Get the human-readable algorithm name for the best available hash.
   *
   * @return  string                Human-readable name for best hash.
   */
  public static function getBestAlgorithmName() {
    try {
      $best_hasher = PhabricatorPasswordHasher::getBestHasher();
      return $best_hasher->getHumanReadableName();
    } catch (Exception $ex) {
      return pht('Unknown');
    }
  }

}