PIN Code Security: Best Practices

TL;DR

Building secure PIN code authorization isn’t just about asking for a 4-digit number. It involves careful design to prevent common attacks like brute-force guessing, shoulder surfing, and replay attacks. This guide outlines practical steps to improve your PIN code security implementation.

PIN Code Security: A Step-by-Step Guide

1. Choose a Suitable Length
   • Minimum 4 digits is recommended, but 6 or more is better. Longer PINs significantly increase the time needed for brute-force attacks.
   • Avoid allowing very short PINs (e.g., less than 4 digits) as they are easily guessable.
2. Restrict Input Types
   • Don’t allow leading zeros. This reduces the possible combinations.
   • Consider disallowing sequential numbers (1234, 5678) or repeating digits (1111, 2222). These are common guesses. You can implement this in your application logic.
3. Implement Rate Limiting
   • Limit the number of incorrect PIN attempts allowed within a specific timeframe. This prevents brute-force attacks.
   • After too many failed attempts, lock the account or require additional verification (e.g., email confirmation).
   • Example using a simple bash script for rate limiting (for demonstration only – production systems need more robust solutions):

     #!/bin/bash
     ATTEMPTS=3
     TIME_WINDOW=60 # seconds
     IP=$1
     
     if [ -f "/tmp/failed_pins_${IP}" ]; then
       LAST_ATTEMPT=$(cat /tmp/failed_pins_${IP})
       CURRENT_TIME=$(date +%s)
       if (( $(($CURRENT_TIME - $LAST_ATTEMPT)) < $TIME_WINDOW )); then
         echo "Too many attempts from this IP address."
         exit 1
       fi
     fi
     
     touch /tmp/failed_pins_${IP}
     echo $CURRENT_TIME > /tmp/failed_pins_${IP}
     

4. Secure PIN Storage
   • Never store PINs in plain text. Always use a strong hashing algorithm (e.g., bcrypt, Argon2) with a unique salt for each PIN.
   • Example using Python and the bcrypt library:

     import bcrypt
     
     pin = "1234"
     salt = bcrypt.gensalt()
     hashed_pin = bcrypt.hashpw(pin.encode('utf-8'), salt)
     
     print(f"Hashed PIN: {hashed_pin.decode('utf-8')}")
     
     # To verify:
     if bcrypt.checkpw(pin.encode('utf-8'), hashed_pin):
       print("PIN matches!")
     else:
       print("Incorrect PIN.")
     

   • Consider using a Hardware Security Module (HSM) for highly sensitive applications to protect the hashing keys.
5. Protect Against Shoulder Surfing
   • Use a visual masking technique on input fields. Replace each digit entered with an asterisk (*) or other placeholder character.
   • Consider using a randomized PIN pad layout to make it harder for observers to guess the PIN.
6. Prevent Replay Attacks (if applicable)
   • If your system transmits PINs over a network, use encryption and timestamps to prevent attackers from re-using captured PIN data.
   • Implement session tokens or one-time codes in addition to the PIN for sensitive operations.
7. Regular Security Audits
   • Periodically review your PIN code implementation and security measures.
   • Conduct penetration testing to identify vulnerabilities.