add advert parser and verifier

src/advert.js

@@ -0,0 +1,44 @@
+import { ed25519 } from "@noble/curves/ed25519";
+import BufferReader from "./buffer_reader.js";
+import Packet from "./packet.js";
+
+class Advert {
+
+    constructor(publicKey, timestamp, signature, appData) {
+        this.publicKey = publicKey;
+        this.timestamp = timestamp;
+        this.signature = signature;
+        this.appData = appData;
+    }
+
+    getTimestamp() {
+        return (new BufferReader(this.timestamp)).readUInt32LE();
+    }
+
+    isVerified() {
+        return ed25519.verify(this.signature, Buffer.concat([
+            this.publicKey,
+            this.timestamp,
+            this.appData,
+        ]), this.publicKey);
+    }
+
+    static fromPacketBytes(bytes) {
+
+        // parse packet from bytes
+        const packet = Packet.fromBytes(bytes);
+
+        // read packet payload
+        const bufferedReader = new BufferReader(packet.payload);
+        const publicKey = bufferedReader.readBytes(32);
+        const timestamp = bufferedReader.readBytes(4); // read as bytes for signature
+        const signature = bufferedReader.readBytes(64);
+        const appData = bufferedReader.readRemainingBytes();
+
+        return new Advert(publicKey, timestamp, signature, appData);
+
+    }
+
+}
+
+export default Advert;