From 22eb41b0bc7a0377ab946d336457dc9e8bcfb362 Mon Sep 17 00:00:00 2001
From: devilreef <devilreef@femboy.page>
Date: Wed, 29 Apr 2026 12:14:17 +0600
Subject: [PATCH] feat: validate telegram initData via hmac-sha256

---
 src/lib/initdata.test.ts | 54 ++++++++++++++++++++++++++++++++
 src/lib/initdata.ts      | 67 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 121 insertions(+)
 create mode 100644 src/lib/initdata.test.ts
 create mode 100644 src/lib/initdata.ts

diff --git a/src/lib/initdata.test.ts b/src/lib/initdata.test.ts
new file mode 100644
index 0000000..dbeccaa
--- /dev/null
+++ b/src/lib/initdata.test.ts
@@ -0,0 +1,54 @@
+import assert from 'node:assert/strict'
+import { createHmac } from 'node:crypto'
+import { describe, it } from 'node:test'
+import { InitDataError, validateInitData } from '@/lib/initdata.js'
+
+const BOT = '123456:test-token'
+const NOW = 1_700_000_000
+
+function sign(fields: Record<string, string>): string {
+  const sp = new URLSearchParams(fields)
+  const sorted = [...sp.entries()].sort(([a], [b]) => a.localeCompare(b))
+  const dcs = sorted.map(([k, v]) => `${k}=${v}`).join('\n')
+  const secret = createHmac('sha256', 'WebAppData').update(BOT).digest()
+  const hash = createHmac('sha256', secret).update(dcs).digest('hex')
+  sp.set('hash', hash)
+  return sp.toString()
+}
+
+describe('validateInitData', () => {
+  it('accepts a valid signature and returns userId', () => {
+    const raw = sign({ auth_date: String(NOW - 10), user: '{"id":42}' })
+    const result = validateInitData(raw, BOT, 86400, NOW)
+    assert.equal(result.userId, 42n)
+  })
+
+  it('rejects a tampered hash', () => {
+    const raw = sign({ auth_date: String(NOW), user: '{"id":1}' }).replace(/hash=([0-9a-f]+)/, 'hash=$1aa')
+    assert.throws(() => validateInitData(raw, BOT, 86400, NOW), { code: 'bad-hash' })
+  })
+
+  it('rejects an expired auth_date', () => {
+    const raw = sign({ auth_date: String(NOW - 100_000), user: '{"id":1}' })
+    assert.throws(() => validateInitData(raw, BOT, 86400, NOW), { code: 'expired' })
+  })
+
+  it('rejects missing user', () => {
+    const raw = sign({ auth_date: String(NOW) })
+    assert.throws(() => validateInitData(raw, BOT, 86400, NOW), { code: 'malformed' })
+  })
+
+  it('rejects missing hash', () => {
+    assert.throws(
+      () => validateInitData(`auth_date=${NOW}&user=%7B%22id%22%3A1%7D`, BOT, 86400, NOW),
+      (e: unknown) => e instanceof InitDataError && e.code === 'malformed',
+    )
+  })
+
+  it('handles bigint user ids', () => {
+    const big = '12345678901234567890'
+    const raw = sign({ auth_date: String(NOW), user: `{"id":${big}}` })
+    const result = validateInitData(raw, BOT, 86400, NOW)
+    assert.equal(result.userId, BigInt(big))
+  })
+})
diff --git a/src/lib/initdata.ts b/src/lib/initdata.ts
new file mode 100644
index 0000000..5c02fa6
--- /dev/null
+++ b/src/lib/initdata.ts
@@ -0,0 +1,67 @@
+import { Buffer } from 'node:buffer'
+import { createHmac, timingSafeEqual } from 'node:crypto'
+
+const USER_ID_RE = /"id"\s*:\s*(-?\d+)/
+
+export class InitDataError extends Error {
+  constructor(public readonly code: 'malformed' | 'bad-hash' | 'expired') {
+    super(code)
+  }
+}
+
+export interface InitDataResult {
+  userId: bigint
+  authDate: number
+}
+
+export function validateInitData(
+  raw: string,
+  botToken: string,
+  maxAgeSeconds: number,
+  now: number = Math.floor(Date.now() / 1000),
+): InitDataResult {
+  const params = new URLSearchParams(raw)
+  const hash = params.get('hash')
+  if (!hash)
+    throw new InitDataError('malformed')
+  params.delete('hash')
+
+  const sorted = [...params.entries()].sort(([a], [b]) => a.localeCompare(b))
+  const dataCheckString = sorted.map(([k, v]) => `${k}=${v}`).join('\n')
+
+  const secret = createHmac('sha256', 'WebAppData').update(botToken).digest()
+  const expected = createHmac('sha256', secret).update(dataCheckString).digest('hex')
+
+  const expectedBuf = Buffer.from(expected, 'hex')
+  let givenBuf: Buffer
+  try {
+    givenBuf = Buffer.from(hash, 'hex')
+  }
+  catch {
+    throw new InitDataError('bad-hash')
+  }
+  if (expectedBuf.length !== givenBuf.length || !timingSafeEqual(expectedBuf, givenBuf))
+    throw new InitDataError('bad-hash')
+
+  const authDate = Number(params.get('auth_date'))
+  if (!Number.isFinite(authDate) || now - authDate > maxAgeSeconds)
+    throw new InitDataError('expired')
+
+  const userField = params.get('user')
+  if (!userField)
+    throw new InitDataError('malformed')
+
+  // extract id as raw string before JSON.parse to avoid float precision loss on large ints
+  const idMatch = userField.match(USER_ID_RE)
+  if (!idMatch)
+    throw new InitDataError('malformed')
+
+  try {
+    JSON.parse(userField)
+  }
+  catch {
+    throw new InitDataError('malformed')
+  }
+
+  return { userId: BigInt(idMatch[1]), authDate }
+}