fix(sample): Implement nonce generation for Facebook sign-in process

Author: Kapusch

samples/Kapusch.Facebook.iOS.Sample/AppDelegate.cs

@@ -1,3 +1,4 @@
+using System.Security.Cryptography;
 using Foundation;
 using Kapusch.Facebook.iOS;
 using UIKit;
@@ -29,10 +30,11 @@ public override bool FinishedLaunching(UIApplication application, NSDictionary?
 
 			try
 			{
+				var rawNonce = GenerateRawNonce();
 				var result = await NativeFacebookLogin.SignInAsync(
 					presenter.Handle,
 					FacebookTrackingMode.Limited,
-					rawNonce: null,
+					rawNonce: rawNonce,
 					CancellationToken.None
 				);
 
@@ -54,6 +56,12 @@ public override bool FinishedLaunching(UIApplication application, NSDictionary?
 		return true;
 	}
 
+	private static string GenerateRawNonce(int size = 32)
+	{
+		var bytes = RandomNumberGenerator.GetBytes(size);
+		return Convert.ToBase64String(bytes).TrimEnd('=').Replace('+', '-').Replace('/', '_');
+	}
+
 	public override bool OpenUrl(UIApplication application, NSUrl url, NSDictionary options) =>
 		NativeFacebookLogin.HandleOpenUrl(
 			application.Handle,