feat: HMAC-signed finalist confirmation token

src/lib/finalist-token.ts

@@ -0,0 +1,45 @@
+import { createHmac, timingSafeEqual } from 'crypto'
+
+export type FinalistTokenPayload = {
+  confirmationId: string
+  /** Unix seconds. Token is rejected after this. */
+  exp: number
+}
+
+function getSecret(): string {
+  const s = process.env.NEXTAUTH_SECRET
+  if (!s) throw new Error('NEXTAUTH_SECRET is not set; cannot sign finalist tokens')
+  return s
+}
+
+function hmac(payloadB64: string): string {
+  return createHmac('sha256', getSecret()).update(payloadB64).digest('hex')
+}
+
+export function signFinalistToken(payload: FinalistTokenPayload): string {
+  const payloadB64 = Buffer.from(JSON.stringify(payload)).toString('base64url')
+  const sig = hmac(payloadB64)
+  return `${payloadB64}.${sig}`
+}
+
+export function verifyFinalistToken(token: string): FinalistTokenPayload {
+  const parts = token.split('.')
+  if (parts.length !== 2) throw new Error('Invalid finalist token: malformed')
+  const [payloadB64, sig] = parts
+  const expected = hmac(payloadB64)
+  const a = Buffer.from(sig, 'hex')
+  const b = Buffer.from(expected, 'hex')
+  if (a.length !== b.length || !timingSafeEqual(a, b)) {
+    throw new Error('Invalid finalist token: signature mismatch')
+  }
+  let payload: FinalistTokenPayload
+  try {
+    payload = JSON.parse(Buffer.from(payloadB64, 'base64url').toString('utf-8'))
+  } catch {
+    throw new Error('Invalid finalist token: payload not parseable')
+  }
+  if (typeof payload.exp !== 'number' || payload.exp < Math.floor(Date.now() / 1000)) {
+    throw new Error('Invalid finalist token: expired')
+  }
+  return payload
+}