refactor: modularize into core/events/formatters/drivers

src/events/verify.ts

@@ -0,0 +1,43 @@
+const keyCache = new Map<string, CryptoKey>();
+
+async function getHmacKey(secret: string): Promise<CryptoKey> {
+  const cached = keyCache.get(secret);
+  if (cached) return cached;
+  const key = await crypto.subtle.importKey(
+    "raw",
+    new TextEncoder().encode(secret),
+    { name: "HMAC", hash: "SHA-256" },
+    false,
+    ["sign"],
+  );
+  keyCache.set(secret, key);
+  return key;
+}
+
+export async function verifySignature(
+  payload: string,
+  signature: string | undefined,
+  secret: string,
+): Promise<boolean> {
+  if (!signature) return false;
+
+  const encoder = new TextEncoder();
+  const key = await getHmacKey(secret);
+  const sig = await crypto.subtle.sign("HMAC", key, encoder.encode(payload));
+  const expected = `sha256=${Array.from(new Uint8Array(sig))
+    .map((b) => b.toString(16).padStart(2, "0"))
+    .join("")}`;
+
+  try {
+    const a = encoder.encode(signature);
+    const b = encoder.encode(expected);
+    if (a.byteLength !== b.byteLength) return false;
+    let diff = 0;
+    for (let i = 0; i < a.byteLength; i++) {
+      diff |= a[i]! ^ b[i]!;
+    }
+    return diff === 0;
+  } catch {
+    return false;
+  }
+}