chore: add rollback-safe password rollout window

api/src/auth/password-hasher.ts

@@ -5,9 +5,11 @@ const DEFAULT_N = 32768;
 const DEFAULT_R = 8;
 const DEFAULT_P = 1;
 const MAX_MEMORY = 64 * 1024 * 1024;
+const MAX_LEGACY_WRITE_WINDOW_MS = 2 * 60 * 60 * 1000;
 const LEGACY_SHA256 = /^[a-f0-9]{64}$/i;
 
 export async function hashPassword(password: string) {
+  if (legacyTransitionWriteEnabled()) return legacyHashPassword(password);
   const salt = randomBytes(16);
   const derived = await deriveScrypt(password, salt, KEY_LENGTH, {
     N: DEFAULT_N,
@@ -18,6 +20,13 @@ export async function hashPassword(password: string) {
   return `$scrypt$v=${VERSION}$N=${DEFAULT_N},r=${DEFAULT_R},p=${DEFAULT_P}$${salt.toString('base64url')}$${derived.toString('base64url')}`;
 }
 
+function legacyTransitionWriteEnabled() {
+  if (process.env.PASSWORD_HASH_LEGACY_TRANSITION !== 'true') return false;
+  const until = Date.parse(process.env.PASSWORD_HASH_LEGACY_WRITE_UNTIL ?? '');
+  const remaining = until - Date.now();
+  return Number.isFinite(until) && remaining > 0 && remaining <= MAX_LEGACY_WRITE_WINDOW_MS;
+}
+
 export async function verifyPassword(password: string, encoded: string) {
   if (isLegacySha256(encoded)) {
     const candidate = Buffer.from(legacyHashPassword(password), 'hex');