app/src/main/kotlin/io/flamenet/messenger/app/Session.kt

@@ -989,6 +989,53 @@ class Session(
         return out
     }
 
+    /**
+     * Add or remove members of a sealed group.
+     *
+     * 🔴 **Owner only, and the relay checks a signature rather than an
+     * account.** We hold the owner's private key iff we created the group; a
+     * member who is not the owner has nothing to sign with, and is refused here
+     * rather than sent to be rejected.
+     *
+     * ⚠️ The version must increase. A roster signature stays valid for ever, so
+     * a stale roster could otherwise be replayed to re-add somebody who was just
+     * removed — with a signature that genuinely verifies.
+     */
+    fun updateSealedGroupMembers(gid: String, add: List<Long>, remove: List<Long>) {
+        val secrets = vault.groupSecrets(gid)
+        val ownerSeed = secrets?.ownerPrivate
+            ?: throw IllegalStateException("only the group's owner can change its members")
+
+        val current = sealedGroups().firstOrNull { it.id == gid }
+            ?: throw IllegalStateException("unknown group")
+        val roster = (current.memberIds + add - remove.filter { it != userId }.toSet())
+            .distinct().sorted()
+        require(roster.size <= 32) { "groups are capped at 32 members" }
+
+        // Existing members keep their tags; only newcomers get fresh ones.
+        // Reusing a tag would hand somebody a token another member had held.
+        val previous = runCatching {
+            SealedRoster.open(
+                relay.fetchSealedGroups(listOf(secrets.tag)).first { it.id == gid }.rosterBlob,
+                secrets.key)
+        }.getOrNull()?.tags ?: emptyMap()
+        val tags = roster.associate { m ->
+            m.toString() to (previous[m.toString()] ?: SealedRoster.newTag())
+        }
+
+        val name = vault.groupName(gid) ?: ""
+        val version = secrets.version + 1
+        val blob = SealedRoster.seal(SealedRoster.Contents(roster, tags, name), secrets.key)
+        val sig = SealedRoster.signRoster(ownerSeed, gid, version, blob)
+
+        relay.replaceRoster(gid, blob, version, sig, tags.values.sorted())
+        vault.saveGroupSecrets(gid, secrets.copy(version = version))
+
+        // Anyone newly added must be told their tag and the key, or they cannot
+        // fetch the group at all.
+        announceSealedGroup(gid, tags, secrets.key, name, add.filter { it in roster })
+    }
+
     /** Hand each member their tag and the group key, over the ratchet. */
     private fun announceSealedGroup(
         gid: String, tags: Map<String, String>, key: ByteArray,

engine/src/main/kotlin/io/flamenet/e2e/Vault.kt

@@ -1219,6 +1219,18 @@ class Vault(private val storage: Storage, private val key: SecretKey) {
             // of groups all called "Group" — the exact loss this file exists to
             // prevent.
             .also { p -> readJson(GROUP_NAMES)?.let { p.put("groupNames", it) } }
+            // 🔴 **Without these a restored device loses its sealed groups for
+            // good.** The relay cannot say what you are a member of — that is
+            // the point of sealing — so the tag is the only way to fetch a group
+            // and the key the only way to read it. A backup carrying the
+            // messages but not these restores a device that can see old
+            // conversations and can never rejoin the groups they came from.
+            //
+            // ⚠️ The most sensitive thing this export holds: a group key opens
+            // every roster it ever sealed. The export is already encrypted under
+            // a passphrase-derived key, which is what makes carrying it
+            // defensible at all.
+            .also { p -> readJson(GROUP_SECRETS)?.let { p.put("groupSecrets", it) } }
     }
 
     data class ImportCounts(val messages: Int, val stories: Int)
@@ -1329,6 +1341,19 @@ class Vault(private val storage: Storage, private val key: SecretKey) {
             }
         }
 
+        // §8.9b — gaps only, like the names. A backup is older than the device
+        // by definition, and a roster version that went backwards would be
+        // refused by the relay on the next rewrite anyway.
+        payload.optJSONObject("groupSecrets")?.let { imported ->
+            val existing = readJson(GROUP_SECRETS) ?: JSONObject()
+            var changed = false
+            for (gid in imported.keys()) {
+                if (existing.has(gid)) continue
+                imported.optJSONObject(gid)?.let { existing.put(gid, it); changed = true }
+            }
+            if (changed) writeJson(GROUP_SECRETS, existing)
+        }
+
         return ImportCounts(newMessages, newStories)
     }
 

engine/src/test/kotlin/io/flamenet/e2e/SealedRosterTest.kt

@@ -18,6 +18,19 @@ import kotlin.test.assertTrue
  */
 class SealedRosterTest {
 
+    private class MapStorage : Vault.Storage {
+        private val records = mutableMapOf<String, ByteArray>()
+        override fun read(name: String) = records[name]
+        override fun write(name: String, bytes: ByteArray) { records[name] = bytes }
+        override fun delete(name: String) { records.remove(name) }
+        override fun names() = records.keys.toList()
+    }
+
+    private fun vault(): Vault = Vault(
+        MapStorage(),
+        javax.crypto.KeyGenerator.getInstance("AES").apply { init(256) }.generateKey(),
+    ).also { it.saveIdentity(DeviceIdentity.create()) }
+
     private fun contents() = SealedRoster.Contents(
         members = listOf(41L, 42L, 43L),
         tags = mapOf("41" to SealedRoster.newTag(), "42" to SealedRoster.newTag()),
@@ -112,6 +125,46 @@ class SealedRosterTest {
             SealedRoster.open(SealedRoster.seal(b, key), key).tags)
     }
 
+    /**
+     * 🔴 **A backup that loses the tags loses the groups for good.** The relay
+     * cannot say what you are a member of, so the tag is the only way to fetch
+     * a group and the key the only way to read it. Restoring the messages
+     * without these gives a device that can read old conversations and can
+     * never rejoin the groups they came from.
+     */
+    @Test
+    fun `an exported backup carries sealed group secrets`() {
+        val old = vault()
+        val secrets = Vault.GroupSecrets(
+            tag = SealedRoster.newTag(), key = SealedRoster.newGroupKey(),
+            ownerPrivate = null, version = 3)
+        old.saveGroupSecrets("g1", secrets)
+        val payload = old.exportPayload()
+
+        val fresh = vault()
+        assertEquals(null, fresh.groupSecrets("g1"))
+        fresh.mergeImportedPayload(payload, 0.0)
+
+        assertEquals(secrets, fresh.groupSecrets("g1"))
+    }
+
+    /** ⚠️ Gaps only — a backup is older than the device by definition. */
+    @Test
+    fun `a restored backup does not overwrite secrets we already hold`() {
+        val old = vault()
+        old.saveGroupSecrets("g1", Vault.GroupSecrets(
+            tag = "old", key = SealedRoster.newGroupKey(), version = 1))
+        val payload = old.exportPayload()
+
+        val current = vault()
+        val live = Vault.GroupSecrets(
+            tag = "live", key = SealedRoster.newGroupKey(), version = 9)
+        current.saveGroupSecrets("g1", live)
+        current.mergeImportedPayload(payload, 0.0)
+
+        assertEquals(live, current.groupSecrets("g1"))
+    }
+
     /** Two groups minted by the same person share nothing. */
     @Test
     fun `two groups share no key material`() {