⚙️ 1. Why OTP Systems Are Easy to Build, but Hard to Secure

Most developers implement OTP like this:

if (userInput === storedOTP) return true;

Sounds simple. But this approach breaks under real-world pressure:

• ❌ Users can brute-force OTPs (999999 → success eventually).
• ❌ OTPs stored in plain text can be leaked.
• ❌ No protection against expired or reused OTPs.
• ❌ Attackers can spam resend requests endlessly.

So, in production systems (like AWS, Auth0, or banking apps), OTP management is treated as a critical security primitive.

🧩 2. The Architecture — OTPs as Ephemeral, Stateful Entities

We treat each OTP as a short-lived state machine:

1. Created → Stored securely (hashed) in Redis.
2. Used → Deleted immediately after success.
3. Failed → Attempts increment, TTL unchanged.
4. Exceeded → User blocked temporarily.

This gives us:

✅ Rate limiting,

✅ Expiry control,

✅ Replay prevention,