feat(webhooks): add GitHub webhook signature verifier

packages/backend/convex/githubWebhookVerify.ts

@@ -0,0 +1,38 @@
+const toHex = (buffer: ArrayBuffer): string =>
+  Array.from(new Uint8Array(buffer))
+    .map((b) => b.toString(16).padStart(2, '0'))
+    .join('');
+
+export const timingSafeEqualHex = (a: string, b: string): boolean => {
+  if (a.length !== b.length) return false;
+  let mismatch = 0;
+  for (let i = 0; i < a.length; i += 1) {
+    mismatch |= a.charCodeAt(i) ^ b.charCodeAt(i);
+  }
+  return mismatch === 0;
+};
+
+export const verifyGithubSignature = async (
+  secret: string,
+  rawBody: string,
+  signatureHeader: string | null,
+): Promise<boolean> => {
+  if (!signatureHeader || !signatureHeader.startsWith('sha256=')) return false;
+  // Web Crypto's importKey throws on a zero-length HMAC key; guard so the
+  // verifier never throws on attacker-controlled input (returns false instead).
+  if (!secret) return false;
+  const provided = signatureHeader.slice('sha256='.length);
+  const key = await crypto.subtle.importKey(
+    'raw',
+    new TextEncoder().encode(secret),
+    { name: 'HMAC', hash: 'SHA-256' },
+    false,
+    ['sign'],
+  );
+  const mac = await crypto.subtle.sign(
+    'HMAC',
+    key,
+    new TextEncoder().encode(rawBody),
+  );
+  return timingSafeEqualHex(toHex(mac), provided);
+};

packages/backend/tests/unit/github-webhook-verify.test.ts

@@ -0,0 +1,73 @@
+import { describe, expect, test } from 'vitest';
+
+import {
+  timingSafeEqualHex,
+  verifyGithubSignature,
+} from '../../convex/githubWebhookVerify';
+
+const SECRET = "It's a Secret to Everybody";
+const BODY = 'Hello, World!';
+const VALID_SIGNATURE =
+  'sha256=757107ea0eb2509fc211221cce984b8a37570b6d7586c22c46f4379c8b043e17';
+
+describe('verifyGithubSignature', () => {
+  test('verifies the GitHub known vector', async () => {
+    expect(await verifyGithubSignature(SECRET, BODY, VALID_SIGNATURE)).toBe(
+      true,
+    );
+  });
+
+  test('rejects a tampered body', async () => {
+    expect(
+      await verifyGithubSignature(SECRET, 'Hello, World?', VALID_SIGNATURE),
+    ).toBe(false);
+  });
+
+  test('rejects a wrong secret', async () => {
+    expect(
+      await verifyGithubSignature('wrong secret', BODY, VALID_SIGNATURE),
+    ).toBe(false);
+  });
+
+  test('rejects a header missing the sha256= prefix', async () => {
+    expect(
+      await verifyGithubSignature(
+        SECRET,
+        BODY,
+        '757107ea0eb2509fc211221cce984b8a37570b6d7586c22c46f4379c8b043e17',
+      ),
+    ).toBe(false);
+  });
+
+  test('rejects a null header', async () => {
+    expect(await verifyGithubSignature(SECRET, BODY, null)).toBe(false);
+  });
+
+  test('rejects an empty header', async () => {
+    expect(await verifyGithubSignature(SECRET, BODY, '')).toBe(false);
+  });
+
+  test('rejects a correct-format but wrong-length signature', async () => {
+    expect(
+      await verifyGithubSignature(SECRET, BODY, 'sha256=deadbeef'),
+    ).toBe(false);
+  });
+
+  test('rejects an empty secret', async () => {
+    expect(await verifyGithubSignature('', BODY, VALID_SIGNATURE)).toBe(false);
+  });
+});
+
+describe('timingSafeEqualHex', () => {
+  test('returns true for equal strings', () => {
+    expect(timingSafeEqualHex('abcd', 'abcd')).toBe(true);
+  });
+
+  test('returns false for equal-length but differing strings', () => {
+    expect(timingSafeEqualHex('abcd', 'abce')).toBe(false);
+  });
+
+  test('returns false for different lengths', () => {
+    expect(timingSafeEqualHex('abcd', 'abcde')).toBe(false);
+  });
+});