Return null, not throw, for stored JSON with no $type discriminator

CredentialCache.Test/UnknownPayloadTests.cs

@@ -0,0 +1,73 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.CredentialCache.Test;
+
+using System.Collections.Concurrent;
+using System.Text;
+using ktsu.CredentialCache.Storage;
+
+/// <summary>
+/// Tests that a stored payload which is not a known credential reads back as "not found"
+/// rather than throwing, whatever shape the JSON takes.
+/// </summary>
+[TestClass]
+public class UnknownPayloadTests
+{
+	[TestMethod]
+	[DataRow("{}")]
+	[DataRow("{\"Token\":\"x\"}")]
+	[DataRow("{\"$type\":\"Bogus\"}")]
+	[DataRow("[]")]
+	[DataRow("123")]
+	public void DeserializeReturnsNullForJsonThatIsNotAKnownCredential(string json) =>
+		Assert.IsNull(CredentialSerialization.Deserialize(Encoding.UTF8.GetBytes(json)));
+
+	[TestMethod]
+	[DataRow("{}")]
+	[DataRow("{\"Token\":\"x\"}")]
+	[DataRow("{\"$type\":\"Bogus\"}")]
+	[DataRow("[]")]
+	[DataRow("123")]
+	public void DeserializeFromStringReturnsNullForJsonThatIsNotAKnownCredential(string json) =>
+		Assert.IsNull(CredentialSerialization.DeserializeFromString(json));
+
+	[TestMethod]
+	[DataRow("{}")]
+	[DataRow("{\"Token\":\"x\"}")]
+	public void TryGetReturnsFalseForAStoredEntryWithoutATypeDiscriminator(string json)
+	{
+		RawBlobCredentialStore store = new();
+		using CredentialCache cache = new(store);
+		PersonaGUID persona = CredentialCache.CreatePersonaGUID();
+		store.Blobs[persona] = Encoding.UTF8.GetBytes(json);
+
+		bool found = cache.TryGet(persona, out Credential? credential);
+
+		Assert.IsFalse(found);
+		Assert.IsNull(credential);
+	}
+}
+
+/// <summary>
+/// A store that holds raw bytes and reads them back the way the native stores do, so an entry
+/// written by another tool, or damaged, can be planted directly.
+/// </summary>
+public sealed class RawBlobCredentialStore : ICredentialStore
+{
+	public ConcurrentDictionary<PersonaGUID, byte[]> Blobs { get; } = new();
+
+	public string Name => "RawBlob";
+
+	public bool TryLoad(PersonaGUID persona, out Credential? credential)
+	{
+		credential = Blobs.TryGetValue(persona, out byte[]? blob)
+			? CredentialSerialization.DeserializeAndScrub([.. blob])
+			: null;
+		return credential is not null;
+	}
+
+	public void Save(PersonaGUID persona, Credential credential) =>
+		Blobs[persona] = CredentialSerialization.Serialize(credential);
+
+	public bool Remove(PersonaGUID persona) => Blobs.TryRemove(persona, out _);
+}

CredentialCache/Storage/CredentialSerialization.cs

@@ -59,6 +59,11 @@ public static string SerializeToString(Credential credential) =>
 		{
 			return null;
 		}
+		catch (NotSupportedException)
+		{
+			// Well-formed JSON with no $type discriminator is not a known credential either
+			return null;
+		}
 	}
 
 	/// <summary>
@@ -123,5 +128,10 @@ internal static void Zero(byte[] buffer)
 		{
 			return null;
 		}
+		catch (NotSupportedException)
+		{
+			// Well-formed JSON with no $type discriminator is not a known credential either
+			return null;
+		}
 	}
 }