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diff --git a/source/storage/Session.mc b/source/storage/Session.mc
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+// Author:  cirakg & claude ai
+// Summary: The unlocked session: holds the master key in RAM and locks after inactivity or on request.
+//
+// README 7.5: unlocking puts the locked master key (and its derived keys) in
+// RAM; locking zeroes and drops them. The UI calls touch() on every user
+// action and checkExpiry() from its 1 Hz timer; the app calls lock() in
+// onStop. An auto-lock time of 0 means "Immediately": the UI locks as soon as
+// you leave a locked entry, and checkExpiry() locks on its next call.
+
+import Toybox.Lang;
+
+class Session {
+
+    static const DEFAULT_AUTO_LOCK_MS = 60000;
+
+    // Inactivity time before locking, in ms (0 = immediately).
+    var autoLockMs as Number = DEFAULT_AUTO_LOCK_MS;
+
+    private var _time as TimeSource;
+    private var _masterKey as ByteArray? = null;
+    private var _keys as SealedBox.Keys? = null;
+    private var _lastActivity as Number = 0;
+
+    function initialize(time as TimeSource) {
+        _time = time;
+    }
+
+    // Take ownership of the master key (the caller must not wipe or keep it).
+    function unlock(masterKey as ByteArray) as Void {
+        lock();
+        _masterKey = masterKey;
+        _keys = new SealedBox.Keys(masterKey);
+        touch();
+    }
+
+    function isUnlocked() as Boolean {
+        return _keys != null;
+    }
+
+    // The master key's derived keys. Throws VaultError LOCKED when locked.
+    function keys() as SealedBox.Keys {
+        var keys = _keys;
+        if (keys == null) {
+            throw new VaultError(VaultError.LOCKED, "locked");
+        }
+        return keys;
+    }
+
+    // The master key itself (only to re-wrap it under a new PIN). Throws
+    // VaultError LOCKED when locked.
+    function masterKey() as ByteArray {
+        var key = _masterKey;
+        if (key == null) {
+            throw new VaultError(VaultError.LOCKED, "locked");
+        }
+        return key;
+    }
+
+    // Note user activity; restarts the inactivity timeout.
+    function touch() as Void {
+        _lastActivity = _time.uptimeMs();
+    }
+
+    // Lock if the inactivity timeout has passed. Returns true when this call
+    // locked the session, so the UI can close open locked entries.
+    function checkExpiry() as Boolean {
+        if (!isUnlocked()) {
+            return false;
+        }
+        var now = _time.uptimeMs();
+        // A timer that went backwards (watch restart, wrap) counts as expired.
+        if (now < _lastActivity || now - _lastActivity >= autoLockMs) {
+            lock();
+            return true;
+        }
+        return false;
+    }
+
+    // Zero and drop every key. Safe to call when already locked.
+    function lock() as Void {
+        var keys = _keys;
+        if (keys != null) {
+            keys.wipe();
+        }
+        Bytes.wipe(_masterKey);
+        _keys = null;
+        _masterKey = null;
+    }
+}