1 files changed, 24 insertions, 0 deletions
diff --git a/source/storage/KeyManager.mc b/source/storage/KeyManager.mc
index ebb0a62..567288f 100644
--- a/source/storage/KeyManager.mc
+++ b/source/storage/KeyManager.mc
@@ -33,6 +33,9 @@ class KeyManager {
 
     private var _store as Persist.Backend;
     private var _deviceKeys as SealedBox.Keys? = null;
+    private var _fingerprintKey as ByteArray? = null;
+
+    static const FINGERPRINT_BYTES = 8;
 
     function initialize(store as Persist.Backend) {
         _store = store;
@@ -55,11 +58,32 @@ class KeyManager {
             _store.setValue(DEVICE_KEY, raw);
         }
         keys = new SealedBox.Keys(raw);
+        _fingerprintKey = Kdf.hkdf(raw, []b, Bytes.fromUtf8("OTPW fp v1"), KEY_BYTES);
         Bytes.wipe(raw);
         _deviceKeys = keys;
         return keys;
     }
 
+    // An 8-byte keyed fingerprint of an entry's kind, type and secret (or
+    // payload): equal for duplicates, meaningless without the device key.
+    // Stored in the readable header so imports can find duplicates of locked
+    // entries while the watch is locked (docs/SECURITY.md).
+    function fingerprint(entry as Entry) as ByteArray {
+        deviceKeys();
+        var input = Bytes.fromUtf8("OTPW-FP-1");
+        input.add(entry.kind);
+        input.add(entry.type);
+        var material = entry.kind == Entry.KIND_OTP ? entry.secret : entry.payload;
+        if (material != null) {
+            input.addAll(material);
+        }
+        var mac = Hmac.nativeSha256(_fingerprintKey as ByteArray, input);
+        Bytes.wipe(input);
+        var out = mac.slice(0, FINGERPRINT_BYTES);
+        Bytes.wipe(mac);
+        return out;
+    }
+
     // True when a PIN has been set.
     function hasPin() as Boolean {
         return _store.getValue(WRAP_KEY) != null;