func getStringToSign(canonicalRequest string, t time.Time, scope string) string {
	stringToSign := signV4Algorithm + "\n" + t.Format(iso8601Format) + "\n"
	stringToSign += scope + "\n"
	canonicalRequestBytes := sha256.Sum256([]byte(canonicalRequest))
	stringToSign += hex.EncodeToString(canonicalRequestBytes[:])
	return stringToSign
}

// getSigningKey hmac seed to calculate final signature.