Connection: Implement sequence numbering system to avoid replay attacks

src/lib/transport/Connection.mjs

@@ -45,6 +45,9 @@ class Connection extends EventEmitter {
 		this.rekey_interval = this.rekey_interval_base + crypto.randomInt(0, 15 * 60 * 1000);
 		this.rekey_in_progress = false;
 		
+		this.sequence_count_receive = 0;
+		this.sequence_count_send = 0;
+		
 		this.session_key = Buffer.from(secret_join, "base64");
 	}
 	
@@ -93,8 +96,13 @@ class Connection extends EventEmitter {
 			this.session_key = await rekey(this, this.session_key);
 			this.rekey_interval = this.rekey_interval_base + crypto.randomInt(0, 15 * 60 * 1000);
 			this.rekey_last = new Date();
+			// Also reset the sequence counters. This will help avoid integer overflow issues - however unlikely they may be
+			this.sequence_count_send = 0;
+			this.sequence_count_receive = 0;
 			/**
 			 * The session key has been re-exchanged.
+			 * This event is fired with the rekeying process
+			 * is complete.
 			 * @event	Connection#rekey
 			 * @type	{void}
 			 */
@@ -144,9 +152,21 @@ class Connection extends EventEmitter {
 	}
 	
 	async handle_message(msg_text) {
+		// If this JSON.parse() call fails, we kill the connection 'cause the
+		// catch part of the above try..catch will trigger in this.handle_frame.
+		// This is very important, because if it didn't kill the connection
+		// we would no doubt be open to all sorts of unpleasant attacks
+		// given that we wouldn't have a chance to check the sequence number
+		// (see below).
 		const msg = JSON.parse(msg_text);
 		
-		l.debug(`RECEIVE:${msg.event}`, msg.message);
+		l.debug(`RECEIVE ${msg.sequence}:${msg.event}`, msg.message);
+		
+		if(msg.sequence !== this.sequence_count_receive) {
+			l.warn(`Killing connection due to invalid sequence number in received message: expected ${this.sequence_count_receive}, but got ${msg.sequence}.`);
+			this.destroy();
+		}
+		this.sequence_count_receive++;
 		
 		if(msg.event == "rekey" && !this.rekey_in_progress) {
 			// Set and forget here
@@ -178,12 +198,12 @@ class Connection extends EventEmitter {
 		/*
 		TODO: Consider anonymous TLS, with jpake for mututal authentication
 		TODO: Consider https://devdocs.io/node/crypto#crypto.createCipheriv() - which lets us use any openssl ciphers we like - e.g. ChaCha20-Poly1305
-		TODO: We're currently vulnerable to a replay attack. We need to mitigate this somehow - probably by maintaining a sequence number. Instead of sending the sequence number though we should instead compute a MAC that also includes the message length and a bunch of other things etc. Of course, we will also need to make sure we don't fall afoul of mac-then-encrypt, encrypt-then-mac, etc issues...
+		Note here that we do *not* need another manual MAC to make this
-		Ref https://www.rfc-editor.org/rfc/rfc4346#appendix-F.2 for the sequence counter in TLS
+		authenticated, as tweetnacl's secretbox uses xsalsa20-poly1305,
-		We do *not* need another manual MAC, as tweetnacl's secretbox uses xsalsa20-poly1305, which is an *authenticated* encryption algorithm. Thus, all we need do is prepend the plaintext with a sequence number.
+		which is an *authenticated* encryption algorithm.
 		*/
 		
-		let payload = JSON.stringify({ event, message });
+		let payload = JSON.stringify({ event, message, sequence: this.sequence_count_send++ });
 		payload = encrypt_bytes(
 			this.session_key,
 			Buffer.from(payload, "utf-8")