Use Certificate for Microsoft Graph Refresh Token in C#

Question

I recently facing to a problem. Because at beginning I used client secret for generating access token and refresh token like that:

  public TokenModel RefreshToken(string refreshToken, string tenantId, string clientId, string clientSecret)
        {
            string url = string.Format(TOKEN_ENDPOINT_URL, tenantId);

            Dictionary<string, string> values = new Dictionary<string, string>
            {
                { "client_id", clientId },
                { "scope", ALL_SCOPE_AUTHORIZATIONS },
                { "client_secret", clientSecret },
                { "grant_type", "refresh_token" },
                { "refresh_token", refreshToken }
            };

            FormUrlEncodedContent data = new FormUrlEncodedContent(values);

            HttpClient client = new HttpClient();
            HttpResponseMessage response = client.PostAsync(url, data).Result;
            string jsonToken = response.Content.ReadAsStringAsync().Result;
            return ExtractToken(jsonToken);
        }

and

  public TokenModel GetAccessTokenByAuthorizationCode(string authorizationCode, string tenantId, string clientId, string clientSecret, string redirectUrl)
        {
            string url = string.Format(TOKEN_ENDPOINT_URL, tenantId);

            Dictionary<string, string> values = new Dictionary<string, string>
            {
                { "client_id", clientId },
                { "scope", ALL_SCOPE_AUTHORIZATIONS },
                { "client_secret", clientSecret },
                { "grant_type", "authorization_code" },
                { "redirect_uri", "https://mycompany.com/" },
                { "code", authorizationCode }
            };

            FormUrlEncodedContent data = new FormUrlEncodedContent(values);

            HttpClient client = new HttpClient();
            HttpResponseMessage response = client.PostAsync(url, data).Result;
            string jsonToken = response.Content.ReadAsStringAsync().Result;
            return ExtractToken(jsonToken);
        }

TokenModel contains the access token and refresh token

But now I need to pass a Certificate and never use a client secret. Any one know how to do this with a HttpClient Request plz? I already read with article: Microsoft Graph: How to get access token with certificate in client credentials flow? (instead of using a client_secret)

But I don't know how to generate the client_assertion in my C# code

Best regards Adrien

A code part for generating access and refresh token

Answer 1

Well, generating a client assertion is quite complex task. I've found some resources where you find more details and examples for C#.

Create signed assertions

Alternative method to create assertion with Microsoft.IdentityModel.JsonWebTokens

Assertion format