diff --git a/SVSimLoader/InstanceIdentityStore.cs b/SVSimLoader/InstanceIdentityStore.cs
new file mode 100644
index 0000000..219fa9c
--- /dev/null
+++ b/SVSimLoader/InstanceIdentityStore.cs
@@ -0,0 +1,51 @@
+using System.Collections.Generic;
+using System.IO;
+
+namespace SVSimLoader
+{
+    /// <summary>
+    /// File-backed store for the three account-identity PlayerPrefs keys (UDID, VIEWER_ID,
+    /// SHORT_UDID), so a second client instance on the same machine gets its own identity
+    /// instead of sharing the per-Windows-user registry store. Only these three keys are
+    /// redirected; everything else (RES_VER, language, asset cache) stays in the shared store.
+    /// Persisted as dependency-free key=value lines — the values are a GUID and two integers,
+    /// which never contain '=' or newlines.
+    /// </summary>
+    public static class InstanceIdentityStore
+    {
+        public static readonly string[] Keys = { "UDID", "VIEWER_ID", "SHORT_UDID" };
+
+        private static string _path;
+        private static readonly Dictionary<string, string> _values = new Dictionary<string, string>();
+
+        public static void Initialize(string path)
+        {
+            _path = path;
+            _values.Clear();
+            if (!File.Exists(_path)) return;
+            foreach (var line in File.ReadAllLines(_path))
+            {
+                int eq = line.IndexOf('=');
+                if (eq <= 0) continue;
+                string key = line.Substring(0, eq);
+                string val = line.Substring(eq + 1);
+                if (System.Array.IndexOf(Keys, key) >= 0) _values[key] = val;
+            }
+        }
+
+        public static bool TryGet(string key, out string value) => _values.TryGetValue(key, out value);
+
+        public static void Set(string key, string value)
+        {
+            _values[key] = value;
+            Save();
+        }
+
+        private static void Save()
+        {
+            var lines = new List<string>();
+            foreach (var kv in _values) lines.Add(kv.Key + "=" + kv.Value);
+            File.WriteAllLines(_path, lines.ToArray());
+        }
+    }
+}