Implement JWT signature verification and Xbox Live checks, close #315

This can be enabled or disabled using the "online-mode" directive in
server.properties.

NOTE: For safety reasons it is enabled by default, since many naive server owners currently believe that authentication is not needed because "the client is forced to sign-in".
Newsflash for readers: the forced authentication is easily bypassed using a LAN proxy.

Un-authenticated LAN connections will still work fine if the online mode is disabled.

Added the following API methods:
- Server->getOnlineMode() : bool
- Server->requiresAuthentication() : bool
- Player->isAuthenticated() : bool

JWT verification is rather expensive, so it is done in an AsyncTask. Make sure you don't hog your worker threads.

src/pocketmine/network/mcpe/VerifyLoginTask.php

@@ -0,0 +1,144 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\network\mcpe;
+
+use pocketmine\network\mcpe\protocol\LoginPacket;
+use pocketmine\Player;
+use pocketmine\scheduler\AsyncTask;
+use pocketmine\Server;
+use pocketmine\utils\MainLogger;
+
+class VerifyLoginTask extends AsyncTask{
+
+	const MOJANG_ROOT_PUBLIC_KEY = "MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAE8ELkixyLcwlZryUQcu1TvPOmI2B7vX83ndnWRUaXm74wFfa5f/lwQNTfrLVHa2PmenpGI6JhIMUJaWZrjmMj90NoKNFSNBuKdm8rYiXsfaz3K36x/1U26HpG0ZxK/V1V";
+
+	/** @var LoginPacket */
+	private $packet;
+
+	/**
+	 * @var bool
+	 * Whether the keychain signatures were validated correctly. This will be set to false if any link in the keychain
+	 * has an invalid signature. If false, the keychain might have been tampered with.
+	 * The player will always be disconnected if this is false.
+	 */
+	private $valid = true;
+	/**
+	 * @var bool
+	 * Whether the player is logged into Xbox Live. This is true if any link in the keychain is signed with the Mojang
+	 * root public key.
+	 */
+	private $authenticated = false;
+
+
+	public function __construct(Player $player, LoginPacket $packet){
+		$this->storeLocal($player);
+		$this->packet = $packet;
+	}
+
+	public function onRun(){
+		$packet = $this->packet; //Get it in a local variable to make sure it stays unserialized
+
+		$currentKey = null;
+
+		foreach($packet->chainData["chain"] as $jwt){
+			if(!$this->validateToken($jwt, $currentKey)){
+				$this->valid = false;
+
+				return;
+			}
+		}
+
+		if(!$this->validateToken($packet->clientDataJwt, $currentKey)){
+			$this->valid = false;
+		}
+	}
+
+	private function validateToken(string $jwt, ?string &$currentPublicKey) : bool{
+		[$headB64, $payloadB64, $sigB64] = explode('.', $jwt);
+
+		$headers = json_decode(base64_decode(strtr($headB64, '-_', '+/'), true), true);
+
+		if($currentPublicKey === null){ //First link, check that it is self-signed
+			$currentPublicKey = $headers["x5u"];
+		}
+
+		$plainSignature = base64_decode(strtr($sigB64, '-_', '+/'), true);
+
+		//OpenSSL wants a DER-encoded signature, so we extract R and S from the plain signature and crudely serialize it.
+
+		assert(strlen($plainSignature) === 96);
+
+		[$rString, $sString] = str_split($plainSignature, 48);
+
+		$rString = ltrim($rString, "\x00");
+		if(ord($rString{0}) >= 128){ //Would be considered signed, pad it with an extra zero
+			$rString = "\x00" . $rString;
+		}
+
+		$sString = ltrim($sString, "\x00");
+		if(ord($sString{0}) >= 128){ //Would be considered signed, pad it with an extra zero
+			$sString = "\x00" . $sString;
+		}
+
+		//0x02 = Integer ASN.1 tag
+		$sequence = "\x02" . chr(strlen($rString)) . $rString . "\x02" . chr(strlen($sString)) . $sString;
+		//0x30 = Sequence ASN.1 tag
+		$derSignature = "\x30" . chr(strlen($sequence)) . $sequence;
+
+		$v = openssl_verify("$headB64.$payloadB64", $derSignature, "-----BEGIN PUBLIC KEY-----\n" . wordwrap($currentPublicKey, 64, "\n", true) . "\n-----END PUBLIC KEY-----\n", OPENSSL_ALGO_SHA384);
+		if($v !== 1){
+			return false; //bad signature, it might have been tampered with
+		}
+
+		if($currentPublicKey === self::MOJANG_ROOT_PUBLIC_KEY){
+			$this->authenticated = true; //we're signed into xbox live
+		}
+
+		$claims = json_decode(base64_decode(strtr($payloadB64, '-_', '+/'), true), true);
+
+		$time = time();
+		if(isset($claims["nbf"]) and $claims["nbf"] > $time){
+			return false; //token can't be used yet
+		}
+
+		if(isset($claims["exp"]) and $claims["exp"] < $time){
+			return false; //token has expired
+		}
+
+		$currentPublicKey = $claims["identityPublicKey"]; //the next link should be signed with this
+
+		return true;
+	}
+
+	public function onCompletion(Server $server){
+		/** @var Player $player */
+		$player = $this->fetchLocal($server);
+		if($player->isClosed()){
+			$server->getLogger()->error("Player " . $player->getName() . " was disconnected before their login could be verified");
+		}else{
+			$player->onVerifyCompleted($this->packet, $this->valid, $this->authenticated);
+		}
+	}
+
+}