diff --git a/server/lib/totp-lockout.js b/server/lib/totp-lockout.js
new file mode 100644
index 0000000..b10db4f
--- /dev/null
+++ b/server/lib/totp-lockout.js
@@ -0,0 +1,30 @@
+'use strict';
+
+// #100 (tightening #2): brute-force lockout for POST /api/auth/totp/verify. A 6-digit
+// code is only 1e6 wide, and an attacker who has the password already holds a valid
+// mfa_pending token - the verify endpoint is the real attack surface. Lock a key
+// (the mfa_pending user id) after MAX_FAILS bad codes, on top of the per-route 10/min
+// rate-limit. Same shape as lib/pair-lockout.js (#87). In-memory; resets on restart.
+
+const MAX_FAILS = 5;                  // consecutive bad codes before lockout
+const LOCKOUT_MS = 15 * 60 * 1000;    // how long the key is then blocked
+
+const failures = new Map(); // key -> { count, lockedUntil }
+
+function isLocked(key, now = Date.now()) {
+  const rec = failures.get(key);
+  return !!(rec && rec.lockedUntil > now);
+}
+
+function recordFailure(key, now = Date.now()) {
+  const rec = failures.get(key) || { count: 0, lockedUntil: 0 };
+  rec.count += 1;
+  if (rec.count >= MAX_FAILS) { rec.lockedUntil = now + LOCKOUT_MS; rec.count = 0; }
+  failures.set(key, rec);
+  return rec;
+}
+
+// A successful verify (or any reason to forgive) clears the key.
+function reset(key) { failures.delete(key); }
+
+module.exports = { isLocked, recordFailure, reset, MAX_FAILS, LOCKOUT_MS };
diff --git a/server/lib/totp.js b/server/lib/totp.js
new file mode 100644
index 0000000..18c912a
--- /dev/null
+++ b/server/lib/totp.js
@@ -0,0 +1,56 @@
+'use strict';
+
+// #100: TOTP (RFC 6238) helper. The shared secret is REVERSIBLE (the server must
+// recompute codes), so it's stored via secretbox (AES-256-GCM) - NOT hashed like the
+// API token / recovery codes. Recovery codes ARE hashed (SHA-256, same discipline as
+// api_tokens) - see generateRecoveryCodes / hashRecoveryCode.
+
+const { authenticator } = require('otplib');
+const crypto = require('crypto');
+const secretbox = require('./secretbox');
+const { hashToken } = require('../middleware/apiToken');
+
+const STEP_SEC = 30;
+const ISSUER = 'ScreenTinker';
+authenticator.options = { window: 1 }; // accept ±1 step (±30s) for clock skew
+
+function generateSecret() { return authenticator.generateSecret(); }            // base32 plaintext
+function keyuri(email, secret) { return authenticator.keyuri(email, ISSUER, secret); } // otpauth:// for the QR
+function encryptSecret(secret) { return secretbox.encrypt(secret); }            // for storage
+function decryptSecret(enc) { return secretbox.decrypt(enc); }                  // for verification
+
+function currentStep(now = Date.now()) { return Math.floor(now / 1000 / STEP_SEC); }
+
+// Verify a 6-digit code against the PLAINTEXT secret, blocking intra-window replay
+// via lastStep. Returns the matched step (always > lastStep) on success, else null.
+// The caller persists the returned step as the user's new totp_last_step.
+function verifyCode(token, secret, lastStep = 0, now = Date.now()) {
+  if (!secret || !/^[0-9]{6}$/.test(String(token || '').trim())) return null;
+  const delta = authenticator.checkDelta(String(token).trim(), secret); // -1|0|1 or null
+  if (delta == null) return null;
+  const step = currentStep(now) + delta;
+  if (step <= lastStep) return null; // a code from an already-consumed step (replay)
+  return step;
+}
+
+// 10 single-use recovery codes. Returns plaintext (shown ONCE) + SHA-256 hashes (stored).
+function generateRecoveryCodes(n = 10) {
+  const plain = [], hashes = [];
+  for (let i = 0; i < n; i++) {
+    const code = crypto.randomBytes(5).toString('hex').toUpperCase(); // 10 hex chars
+    plain.push(code);
+    hashes.push(hashToken(code));
+  }
+  return { plain, hashes };
+}
+
+// Normalize user input (strip spaces/hyphens, uppercase) then hash, so a code typed
+// with stray formatting still matches the stored hash.
+function hashRecoveryCode(input) {
+  return hashToken(String(input || '').toUpperCase().replace(/[^0-9A-F]/g, ''));
+}
+
+module.exports = {
+  generateSecret, keyuri, encryptSecret, decryptSecret,
+  verifyCode, currentStep, generateRecoveryCodes, hashRecoveryCode, STEP_SEC,
+};