rsa sign
@param  {String} input             rsa sign input
@param  {String} algorithm         rsa sign algorithm
@param  {String} privateSecret     rsa sign secret
@return {String}                   rsa signature
function rsaSign (input, algorithm, privateSecret) {
  try {
    // sign with rsa_private_key.pem
    return crypto.createSign(algorithm).update(input).sign(privateSecret, 'base64')
  } catch (e) {
    throw new Error(errorMap['003'].message)
  }
}