interop/src/main/kotlin/io/flamenet/interop/Main.kt

@@ -12,6 +12,7 @@ import io.flamenet.e2e.PeerBundle
 import io.flamenet.e2e.Ed25519
 import io.flamenet.e2e.Hkdf
 import io.flamenet.e2e.KeyLog
+import io.flamenet.e2e.Label
 import io.flamenet.e2e.SafetyNumber
 import io.flamenet.e2e.SealedSender
 import io.flamenet.e2e.X25519
@@ -124,6 +125,8 @@ private fun handle(req: JSONObject): JSONObject = when (req.getString("cmd")) {
         req.bytes("key"), req.bytes("box"), req.bytes("aad")), Charsets.UTF_8))
     "seal" -> sealSender(req)
     "open_sealed" -> openSealed(req)
+    "label_encode" -> labelEncode(req)
+    "label_dominates" -> labelDominates(req)
     "content_encode" -> contentEncode(req)
     "content_parse" -> contentParse(req)
     "verify" -> JSONObject().put("ok", Ed25519.verify(
@@ -595,3 +598,41 @@ private fun contentParse(req: JSONObject): JSONObject {
     out.put("control", parsed.isControl)
     return out
 }
+
+// ---------------------------------------------------------------------------
+// The security lattice — §1 of the Files specification, and the core of
+// relay#49's "labels and security levels are to be kept".
+//
+// 🔴 **The ten-byte encoding is authenticated data for every construction that
+// uses a label.** An encoding that differs between platforms does not warn: it
+// produces a decryption failure, or — worse — plaintext that quietly belongs to
+// the wrong sensitivity. Both engines have implemented this independently since
+// the Files apps shipped, and nothing has ever compared them.
+//
+// ⚠️ Each side's own unit tests pin their vectors, which is necessary and not
+// sufficient. Two engines that each pass their own vectors can still disagree,
+// and the disagreement here is silent by construction.
+// ---------------------------------------------------------------------------
+
+private fun labelFrom(json: JSONObject?): Label {
+    val o = json ?: JSONObject()
+    val bits = o.optJSONArray("compartments")?.let { a ->
+        (0 until a.length()).map { a.getInt(it) }
+    } ?: emptyList()
+    return Label(o.optInt("classification", 0).toShort(), bits.toSet())
+}
+
+private fun labelEncode(req: JSONObject): JSONObject {
+    val label = labelFrom(req)
+    return JSONObject()
+        .put("hex", label.bytes().joinToString("") { "%02x".format(it) })
+        // ⚠️ As a STRING. Bit 63 makes the mask a negative Long, and a JSON
+        // number would arrive on the other side as a double and lose the low
+        // bits — turning the one compartment most likely to be mis-shifted
+        // into the one this check cannot see.
+        .put("mask", label.mask.toULong().toString())
+}
+
+private fun labelDominates(req: JSONObject): JSONObject =
+    JSONObject().put("dominates",
+        labelFrom(req.optJSONObject("a")).dominates(labelFrom(req.optJSONObject("b"))))