Preventing Timing Attacks

1. Understanding Timing Attack Vectors

Vector	Leak
String compare (==)	Returns early on mismatch — reveals prefix
User existence	Different response times for valid vs invalid user
Cache hits	Faster response reveals state
DB query	Index hit vs miss

2. Implementing Constant-Time Comparisons

Example: Per-language API

Node:    crypto.timingSafeEqual(a, b)            // equal-length Buffers
Python:  hmac.compare_digest(a, b)
Go:      subtle.ConstantTimeCompare(a, b) == 1
Java:    MessageDigest.isEqual(a, b)
Ruby:    ActiveSupport::SecurityUtils.secure_compare

3. Using Secure String Comparison

Warning: Even constant-time compare leaks length. Hash both sides (HMAC with random key) before compare to mask length.

4. Handling Username Enumeration

Endpoint	Mitigation
Login	Same error for unknown user vs wrong password
Signup	Generic "If account exists, check email"
Password reset	Always respond 200; send email only if exists
Timing	Hash dummy password even when user not found

5. Implementing Consistent Response Times

Example: Pad to fixed minimum

async function login(email, pw) {
  const start = Date.now();
  const user = await User.findByEmail(email);
  const hash = user?.passwordHash ?? "$argon2id$v=19$m=65536,t=3,p=4$xxxxxx";
  const ok = await argon2.verify(hash, pw);
  // Pad to min 300ms
  const elapsed = Date.now() - start;
  if (elapsed < 300) await new Promise(r => setTimeout(r, 300 - elapsed));
  return ok && user;
}

6. Using Dummy Operations

Perform equivalent CPU work (e.g., dummy hash) when account does not exist so request timing is indistinguishable.

7. Handling Cache Timing Attacks

Mitigation	Detail
Cache by category	Same cache key class for valid/invalid
Random delay	Add jitter to mask cache hits
Pre-warm	Avoid cold-vs-warm signal

8. Implementing Response Time Randomization

Warning: Random delay alone is statistically defeatable. Combine with constant-time + dummy ops.

9. Using Rate Limiting to Mask Timing

Aggressive per-IP rate limits prevent attacker from collecting enough samples to extract a timing signal.

10. Preventing Information Leakage

Source	Mitigation
Error messages	Generic, identical for similar failures
HTTP status	Same code (e.g., always 401, never 404)
Response size	Pad responses
Logs	Don't echo sensitive input

Related Cheat Sheets

High Level Design