hmacSHA256 method
Implementation
String hmacSHA256(String secret, String toSign) {
if (secret.isEmpty || toSign.isEmpty) {
return "";
}
var key = utf8.encode(secret);
var bytes = utf8.encode(toSign);
var hmacSha256 = new Hmac(sha256, key);
var digest = hmacSha256.convert(bytes);
return digest.toString();
}