code: function getSigningKey(date, region, secretKey) {
    if (!isObject(date)) {
      throw new TypeError('date should be of type "object"')
    }
    if (!isString(region)) {
      throw new TypeError('region should be of type "string"')
    }
    if (!isString(secretKey)) {
      throw new TypeError('secretKey should be of type "string"')
    }
    var dateLine = makeDateShort(date),
      hmac1 = Crypto.createHmac('sha256', 'AWS4' + secretKey).update(dateLine).digest(),
      hmac2 = Crypto.createHmac('sha256', hmac1).update(region).digest(),
      hmac3 = Crypto.createHmac('sha256', hmac2).update('s3').digest()
    return Crypto.createHmac('sha256', hmac3).update('aws4_request').digest()
  }
docs: returns the key used for calculating signature