How to use the bcrypt-pbkdf.pbkdf function in bcrypt-pbkdf

return new Error('Malformed OpenSSH private key');
    if (kdfOptions.length) {
      switch (kdfName) {
        case 'none':
          return new Error('Malformed OpenSSH private key');
        case 'bcrypt':
          /*
            string salt
            uint32 rounds
          */
          var salt = utils.readString(kdfOptions, 0);
          if (salt === false || kdfOptions._pos + 4 > kdfOptions.length)
            return new Error('Malformed OpenSSH private key');
          var rounds = readUInt32BE(kdfOptions, kdfOptions._pos);
          var gen = Buffer.allocUnsafe(encInfo.keyLen + encInfo.ivLen);
          var r = bcrypt_pbkdf(passphrase,
                               passphrase.length,
                               salt,
                               salt.length,
                               gen,
                               gen.length,
                               rounds);
          if (r !== 0)
            return new Error('Failed to generate information to decrypt key');
          cipherKey = gen.slice(0, encInfo.keyLen);
          cipherIV = gen.slice(encInfo.keyLen);
          break;
      }
    } else if (kdfName !== 'none') {
      return new Error('Malformed OpenSSH private key');
    }