Fix EnableRelaxedDecoding implementation and add test coverage

Src/Fido2.Models/Converters/Base64UrlConverter.cs

@@ -37,13 +37,29 @@ public override byte[] Read(ref Utf8JsonReader reader, Type typeToConvert, JsonS
         {
             return Base64Url.DecodeFromUtf8(source);
         }
-        catch
+        catch (Exception ex)
         {
             if (Base64.IsValid(source))
             {
+                static byte[] DecodeBase64FromUtf8(scoped ReadOnlySpan<byte> source)
+                {
+                    var rentedBuffer = ArrayPool<byte>.Shared.Rent(Base64.GetMaxDecodedFromUtf8Length(source.Length));
+
+                    try
+                    {
+                        _ = Base64.DecodeFromUtf8(source, rentedBuffer, out _, out int written);
+
+                        return rentedBuffer[0..written];
+                    }
+                    finally
+                    {
+                        ArrayPool<byte>.Shared.Return(rentedBuffer);
+                    }
+                }
+
                 if (EnableRelaxedDecoding)
                 {
-                    return Base64Url.DecodeFromUtf8(source);
+                    return DecodeBase64FromUtf8(source);
                 }
                 else
                 {
@@ -52,7 +68,7 @@ public override byte[] Read(ref Utf8JsonReader reader, Type typeToConvert, JsonS
             }
             else
             {
-                throw new JsonException("Invalid Base64Url data");
+                throw new JsonException(ex.Message, ex);
             }
         }
         finally

Tests/Fido2.Tests/Fido2UserTests.cs

@@ -0,0 +1,36 @@
+using System.Text.Json;
+
+namespace Fido2NetLib.Tests;
+
+public class Base64UrlConverterTests
+{
+    [Fact]
+    public void RelaxedDecodingWorks()
+    {
+        string jsonText =
+            """
+            {
+              "name": "anders",
+              "id": "7w80qsdaWWm5R+KK2O64MO/WSitunFbxH1H8mE9IVORxzbXdxXDY1VRQlayoK/Lmh3MI/p0M59Rh98D8r4EoJw==",
+              "displayName": "anders"
+            }
+            """;
+
+        Base64UrlConverter.EnableRelaxedDecoding = false;
+
+        try
+        {
+            _ = JsonSerializer.Deserialize<Fido2User>(jsonText);
+        }
+        catch (JsonException ex)
+        {
+            Assert.Equal("Expected data to be in Base64Url format, but received Base64 encoding instead.", ex.Message);
+        }
+
+        Base64UrlConverter.EnableRelaxedDecoding = true;
+
+        var user = JsonSerializer.Deserialize<Fido2User>(jsonText);
+
+        Assert.Equal("7w80qsdaWWm5R+KK2O64MO/WSitunFbxH1H8mE9IVORxzbXdxXDY1VRQlayoK/Lmh3MI/p0M59Rh98D8r4EoJw==", Convert.ToBase64String(user.Id));
+    }
+}