diff --git a/src/Room.c b/src/Room.c
index bdd6398..331a17a 100644
--- a/src/Room.c
+++ b/src/Room.c
@@ -599,9 +599,101 @@ AuthoriseAliasV1(PduV1 pdu)
 static bool
 AuthorizeInviteMembershipV1(Room * room, PduV1 pdu)
 {
-    /* TODO */
-    (void) room;
-    (void) pdu;
+    int64_t invite_level;
+    int64_t pdu_level;
+    JsonValue *third_pi;
+    /* Step 5.3.1: If content has a third_party_invite property */
+    if ((third_pi = JsonGet(pdu.content, 1, "third_party_invite")))
+    {
+        JsonValue *signed_val, *mxid, *token;
+        HashMap *third_pi_obj = JsonValueAsObject(third_pi), *signed_obj;
+        HashMap *state, *third_pi_event;
+        char *third_pi_id, *thirdpi_event_sender;
+        /* Step 5.3.1.1: If target user is banned, reject. */
+        if (RoomUserHasMembership(room, pdu.event_id, pdu.state_key, "ban"))
+        {
+            return false;
+        }
+        /* Step 5.3.1.2: If content.third_party_invite does not have a signed 
+         * property, reject. */
+        if (!(signed_val = JsonGet(third_pi_obj, 1, "signed")))
+        {
+            return false;
+        }
+        signed_obj = JsonValueAsObject(signed_val);
+
+        /* Step 5.3.1.3: If signed does not have mxid and token properties, 
+         * reject. */
+        if (!(mxid = JsonGet(signed_obj, 1, "mxid")))
+        {
+            return false;
+        }
+        if (!(token = JsonGet(signed_obj, 1, "token")))
+        {
+            return false;
+        }
+
+        /* Step 5.3.1.4: If mxid does not match state_key, reject. */
+        if (!StrEquals(JsonValueAsString(mxid), pdu.state_key))
+        {
+            return false;
+        }
+
+        /* Step 5.3.1.5: If there is no m.room.third_party_invite event 
+         * in the current room state with state_key matching token, reject. */
+        state = RoomStateGetID(room, pdu.event_id);
+        if (!(third_pi_id = StateGet(
+                state, 
+                "m.room.third_party_invite", JsonValueAsString(token))))
+        {
+            StateFree(state);
+            return false;
+        }
+        third_pi_event = RoomEventFetch(room, third_pi_id);
+        StateFree(state);
+
+        /* Step 5.3.1.6: If sender does not match sender of the 
+         * m.room.third_party_invite, reject. */
+        thirdpi_event_sender = JsonValueAsString(JsonGet(third_pi_event, 1, "sender"));
+        if (!StrEquals(thirdpi_event_sender, pdu.sender))
+        {
+            return false;
+        }
+
+        /* TODO:
+         * Step 5.3.1.7: If any signature in signed matches any public key in 
+         * the m.room.third_party_invite event, allow. 
+         *
+         * The public keys are in content of m.room.third_party_invite as:
+         *  - A single public key in the public_key property.
+         *  - A list of public keys in the public_keys property. */
+
+        /* Step 5.3.1.8: Otherwise, reject. */
+        return false;
+    }
+
+    /* Step 5.3.2: If the sender's current membership state is not join, 
+     * reject. */
+    if (!RoomUserHasMembership(room, pdu.event_id, pdu.sender, "join"))
+    {
+        return false;
+    }
+    /* Step 5.3.3: If target user’s current membership state is join or ban, reject. */
+    if (RoomUserHasMembership(room, pdu.event_id, pdu.state_key, "join") ||
+        RoomUserHasMembership(room, pdu.event_id, pdu.state_key, "join"))
+    {
+        return false;
+    }
+
+    /* Step 5.3.4: If the sender's power level is greater than or equal to the 
+     * invite level, allow. */
+    invite_level = RoomMinPL(room, pdu.event_id, NULL, "invite");
+    pdu_level = RoomUserPL(room, pdu.event_id, pdu.sender);
+    if (pdu_level >= invite_level)
+    {
+        return true;
+    }
+    /* Step 5.3.5: Otherwise, reject. */
     return false;
 }
 static bool