OAuthTokenUtility.cs

public class OAuthTokenUtility {

  private readonly RSACryptoServiceProvider _PublicProvider;
  private readonly RSACryptoServiceProvider _PrivateProvider;
  
  public OAuthTokenUtility(X509Certificate2 certificate) {
    _PublicProvider = (RSACryptoServiceProvider)certificate.PublicKey.Key;
    _PrivateProvider = (RSACryptoServiceProvider)Certificate.PrivateKey;
  }
  public string Token(string username, double numberOfMinutesToExpireIn, IEnumerable<string> tokens) {
    var authServerAccessToken = new AuthorizationServerAccessToken();

    authServerAccessToken.Lifetime = TimeSpan.FromMinutes(2);
    authServerAccessToken.ResourceServerEncryptionKey = _PublicProvider;
    authServerAccessToken.AccessTokenSigningKey = _PrivateProvider;
    var result = new AccessTokenResult(authServerAccessToken);

    var accessToken = result.AccessToken;

    var applyAuthorizationMethod = accessToken.GetType()
     .GetMethods(System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic)
     .FirstOrDefault(f => f.Name == "ApplyAuthorization" && f.GetParameters().Count() == 3);

    var serializeMethod = accessToken.GetType()
     .GetMethod("Serialize", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);

    applyAuthorizationMethod.Invoke(accessToken, new Object[] { tokens, username, TimeSpan.FromMinutes(numberOfMinutesToExpireIn) });

    var output = serializeMethod.Invoke(accessToken, new Object[] { }) as string;
    return output;
  }
}