docs: def hexlify_pbkdf2(password, salt, iterations, length, digest=hashlib.sha1):
      """
      >>> hash = hexlify_pbkdf2("not secret", "a salt value", iterations=100, length=16)
      >>> hash == '0b919231515dde16f76364666cf07107'
      True
      """
      # log.debug("hexlify_pbkdf2 with iterations=%i", iterations)
      hash = crypto.pbkdf2(password, salt, iterations=iterations, dklen=length, digest=digest)
      hash = binascii.hexlify(hash)
      hash = six.text_type(hash, "ascii")
      return hash