diff --git a/engine/src/main/kotlin/io/flamenet/e2e/KeyLog.kt b/engine/src/main/kotlin/io/flamenet/e2e/KeyLog.kt
index 6f92e8e..08ba927 100644
--- a/engine/src/main/kotlin/io/flamenet/e2e/KeyLog.kt
+++ b/engine/src/main/kotlin/io/flamenet/e2e/KeyLog.kt
@@ -40,11 +40,21 @@ object KeyLog {
      */
     fun canonicalLeaf(
         kind: String,
-        userId: Long,
+        subject: String,
         deviceId: String,
         ikSigB64: String,
         ikDhB64: String,
-    ): ByteArray = "v1|$kind|$userId|$deviceId|$ikSigB64|$ikDhB64".toByteArray(Charsets.UTF_8)
+    ): ByteArray = "v2|$kind|$subject|$deviceId|$ikSigB64|$ikDhB64".toByteArray(Charsets.UTF_8)
+
+    /**
+     * The subject a leaf names — the account's fnid where one has been
+     * published, and the relay-qualified row number where it has not.
+     *
+     * Shared rather than rebuilt per call site, so the three engines cannot
+     * drift on the fallback format.
+     */
+    fun logSubject(fnid: String?, audience: String, userId: Long): String =
+        if (!fnid.isNullOrEmpty()) fnid else "local:$audience/$userId"
 
     /**
      * Recompute the root from a leaf and its audit path.
diff --git a/engine/src/test/kotlin/io/flamenet/e2e/PublishedVectorsTest.kt b/engine/src/test/kotlin/io/flamenet/e2e/PublishedVectorsTest.kt
index 2cc6de5..bb87599 100644
--- a/engine/src/test/kotlin/io/flamenet/e2e/PublishedVectorsTest.kt
+++ b/engine/src/test/kotlin/io/flamenet/e2e/PublishedVectorsTest.kt
@@ -91,7 +91,7 @@ class PublishedVectorsTest {
 
         val encoded = KeyLog.canonicalLeaf(
             kind = input.getString("kind"),
-            userId = input.getLong("userId"),
+            subject = input.getString("subject"),
             deviceId = input.getString("deviceId"),
             ikSigB64 = input.getString("ikSigB64"),
             ikDhB64 = input.getString("ikDhB64"),