🔐 TOTP Generator — 5-Digit Code (Letter-Protocol Based)

This module generates a 5-digit Time-Based One-Time Password (TOTP) using a letter-based secret protocol.
It follows standard TOTP logic (RFC 6238) but outputs a shorter numeric code ideal for POS devices, embedded systems, and lightweight authentication.

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✨ Features

• Outputs a 5-digit numeric TOTP (00000–99999)
• Uses a letter-protocol secret (A–Z converted to bytes)
• 30-second code validity window
• HMAC-SHA1 hashing with dynamic truncation
• Lightweight and easy to integrate

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⚙️ How It Works

1. Letter-Protocol Secret
   The secret is provided as a sequence of letters (A–Z).
   Each letter is converted to a byte and used as the shared secret.

2. Time Step (30s)
   Current Unix time is divided into 30-second intervals.
   Each interval produces a unique one-time password.

3. HMAC Calculation
   The time step is hashed with the secret using HMAC-SHA1.

4. Dynamic Truncation
   The HMAC result is truncated to a 31-bit integer, following TOTP standards.

5. Final 5-Digit Code
   The value is reduced modulo 100000 to generate the final 5-digit OTP.

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📌 Example Output

TOTP: 48291 Valid for: 30 seconds

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📦 Use Cases

• POS activation & linking
• Offline device verification
• Quick identity confirmation
• Embedded systems with low UI requirements