calculateSignature function
Implementation
Future<String> calculateSignature(String key, String stringToSign) async {
List<int> secretKey = utf8.encode(key);
List<int> message = utf8.encode(stringToSign);
Hmac hmacSha256 = Hmac(sha256, secretKey);
Digest signature = hmacSha256.convert(message);
return signature.toString();
}