feat: verify allocated join authorisations with hmac

Game/scripts/match_net.gd

@@ -56,6 +56,7 @@ var require_join_authorisation := false
 var _allowed_join_authorisations: Dictionary = {}
 var _active_join_peers: Dictionary = {} # opaque authorisation -> peer_id
 var _join_authorisation_context: Dictionary = {}
+var _join_signing_key := PackedByteArray()
 
 # Test hook (tests/match_net_smoke.gd): set false before connecting to
 # suppress the automatic real hello, so a test can send a deliberately
@@ -92,10 +93,11 @@ func _on_shutting_down() -> void:
 	_allowed_join_authorisations.clear()
 	_active_join_peers.clear()
 	_join_authorisation_context.clear()
+	_join_signing_key = PackedByteArray()
 	require_join_authorisation = false
 
 
-func configure_join_authorisations(tokens: Array, context: Dictionary) -> bool:
+func configure_join_authorisations(tokens: Array, context: Dictionary, signing_key: PackedByteArray = PackedByteArray()) -> bool:
 	var allowed := {}
 	for token in tokens:
 		if not token is String or String(token).is_empty():
@@ -105,6 +107,7 @@ func configure_join_authorisations(tokens: Array, context: Dictionary) -> bool:
 		return false
 	_allowed_join_authorisations = allowed
 	_join_authorisation_context = context.duplicate(true)
+	_join_signing_key = signing_key.duplicate()
 	require_join_authorisation = true
 	return true
 
@@ -233,6 +236,27 @@ func _valid_join_authorisation(token: String) -> bool:
 	var protocol := str(claims.get("Protocol", ""))
 	var expires_at := str(claims.get("ExpiresAt", ""))
 	var expiry := Time.get_unix_time_from_datetime_string(expires_at)
+	if not _join_signing_key.is_empty():
+		var signature_token := str(envelope["Signature"])
+		var signature := Marshalls.base64_to_raw(signature_token)
+		if signature.size() != 32:
+			return false
+		var canonical := PackedByteArray()
+		var fields := [
+			str(claims.get("MatchID", "")), str(claims.get("ServerID", "")),
+			str(claims.get("PlayerID", "")), str(claims.get("SteamID", "")),
+			str(int(claims.get("Slot", -1))), str(int(claims.get("Team", -1))), protocol,
+			str(int(claims.get("Generation", 0))), expires_at,
+		]
+		for index in fields.size():
+			canonical.append_array(String(fields[index]).to_utf8_buffer())
+			if index < fields.size() - 1:
+				canonical.append(0)
+		var hmac := HMACContext.new()
+		hmac.start(HashingContext.HASH_SHA256, _join_signing_key)
+		hmac.update(canonical)
+		if hmac.finish() != signature:
+			return false
 	return str(claims.get("MatchID", "")) == str(_join_authorisation_context.get("match_id", "")) \
 		and str(claims.get("ServerID", "")) == str(_join_authorisation_context.get("server_id", "")) \
 		and protocol == str(_join_authorisation_context.get("protocol", "")) \

Game/scripts/server_boot.gd

@@ -64,14 +64,16 @@ func _ready() -> void:
 		return
 	if allocated_mode:
 		var roster_file := String(config.get_value("join-authorisations-file"))
+		var key_file := String(config.get_value("join-authorisations-key-file"))
 		var roster_json := FileAccess.get_file_as_string(roster_file)
+		var signing_key := FileAccess.get_file_as_bytes(key_file)
 		var roster_tokens = JSON.parse_string(roster_json)
-		if not roster_tokens is Array or roster_tokens.is_empty() or not MatchNet.configure_join_authorisations(roster_tokens, {
+		if not roster_tokens is Array or roster_tokens.is_empty() or signing_key.is_empty() or not MatchNet.configure_join_authorisations(roster_tokens, {
 			"match_id": String(config.get_value("match-id")),
 			"server_id": String(config.get_value("server-id")),
 			"protocol": str(NetCodec.PROTOCOL_VERSION),
 			"protocol_version": NetCodec.PROTOCOL_VERSION,
-		}):
+		}, signing_key):
 			printerr("cosmic-clash-server: refusing to start with invalid join-authorisations-file")
 			get_tree().quit(1)
 			return

Game/scripts/server_config.gd

@@ -76,6 +76,7 @@ static func specs() -> Array[Spec]:
 	out.append(Spec.new("transport", Kind.STRING, "", "allocation", "Assigned transport: steam_sdr or enet"))
 	out.append(Spec.new("region", Kind.STRING, "", "allocation", "Assigned region: EU or NA"))
 	out.append(Spec.new("join-authorisations-file", Kind.STRING, "", "allocation", "JSON array of control-plane signed join envelopes mounted for this match"))
+	out.append(Spec.new("join-authorisations-key-file", Kind.STRING, "", "allocation", "HMAC-SHA256 key file for verifying mounted join envelopes"))
 	return out
 
 
@@ -269,6 +270,8 @@ func _validate() -> void:
 			errors.append("--assignment-expiry-unix must be in the future")
 		if String(values["join-authorisations-file"]).is_empty():
 			errors.append("--join-authorisations-file is required in allocated mode")
+		if String(values["join-authorisations-key-file"]).is_empty():
+			errors.append("--join-authorisations-key-file is required in allocated mode")
 		var digest := String(values["server-image-digest"])
 		if not _is_sha256_digest(digest):
 			errors.append("--server-image-digest must be sha256:<64 hex characters>")