'RSA' does not contain a definition for 'ImportPkcs8PrivateKey'

Hi all,

i'm trying to setup a backend system able to send push notification to mobile app (not developed in creatio) through FCM rest API.

The first hurdle is obtaining a valid token to authenticate to notification api exposed by firebase. In order to obtain this token i do have to call another rest api passing a jwt token generated by me and signed by a private key downloaded from fcm.

I've got a code (pasted below) that manages to generate this encrypted token and it's working in visual studio. But if i try to use it in a script task i got the exception 

'RSA' does not contain a definition for 'ImportPkcs8PrivateKey' and no accessible extension method 'ImportPkcs8PrivateKey' accepting a first argument of type 'RSA' could be found (are you missing a using directive or an assembly reference?)

As far as i know this exception is thrown if .net core being used is version 5 or below. But i'm on a demo instance with creation 8.2.0.4183 which should be already using net core 6 right?

Do you have any suggestion? (the flow is already configured to import System.Security.Cryptography)

--code--

   var now = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
var header = new Dictionary
       {
           { "alg", "RS256" },
           { "typ", "JWT" }
       };
string headerJson = JsonConvert.SerializeObject(header);
string encodedHeader = Convert.ToBase64String(Encoding.UTF8.GetBytes(headerJson)).TrimEnd('=').Replace('+', '-').Replace('/', '_');

var payload = new Dictionary
       {
           { "iss", CLIENT_EMAIL },
           { "scope", SCOPE },
           { "aud", TOKEN_URI },
           { "iat", now },
           { "exp", now + 3600 }
       };
string payloadJson = JsonConvert.SerializeObject(payload);
string encodedPayload = Convert.ToBase64String(Encoding.UTF8.GetBytes(payloadJson)).TrimEnd('=').Replace('+', '-').Replace('/', '_');
string unsignedJwt = $"{encodedHeader}.{encodedPayload}";
byte[] dataBytes = Encoding.UTF8.GetBytes(unsignedJwt);

// Decode PEM -> PKCS#8 bytes
string cleanKey = PRIVATE_KEY
   .Replace("-----BEGIN PRIVATE KEY-----", "")
   .Replace("-----END PRIVATE KEY-----", "")
   .Replace("\\n", "\n")  // ← decodifica reale da stringa JSON
   .Trim();

byte[] privateKeyBytes = Convert.FromBase64String(cleanKey);

// Firma con RSA-SHA256
byte[] signature;
using (var rsa = RSA.Create())
{
   rsa.ImportPkcs8PrivateKey(privateKeyBytes, out _);
   signature = rsa.SignData(dataBytes, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
}
string encodedSignature = Convert.ToBase64String(signature)
   .TrimEnd('=').Replace('+', '-').Replace('/', '_');

string jwt = $"{unsignedJwt}.{encodedSignature}";

return true;