Password Strength & Breach Checker

This Python script calculates the entropy of a password and checks if it has been exposed in a data breach using the Have I Been Pwned (HIBP) API.

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Features

• Entropy Calculation — Estimates password strength using character variety
• Strength Rating — Categorizes passwords from Very Weak to Very Strong
• Breach Detection — Checks if the password has been exposed in real-world leaks
• Safe Lookup — Uses k-anonymity; password is never fully sent to any server

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Entropy Formula

The entropy (E) is calculated using the formula:

E = log₂(R^L) = L × log₂(R)

Where:

• E = entropy in bits
• R = number of possible characters (charset size)
• L = length of the password

Character Set Contributions

Character Type	Contribution to R
Lowercase a–z	26
Uppercase A–Z	26
Digits 0–9	10
Special characters	30

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Strength Ratings

Entropy (bits)	Strength
< 28	Very Weak
28 – 35	Weak
36 – 59	Moderate
60 – 79	Good
80 – 127	Strong
128+	Very Strong

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Breach Check Logic

This script uses the Have I Been Pwned "Pwned Passwords" API.

1. Password is hashed with SHA-1
2. The first 5 characters of the hash are sent to the API
3. The API returns a list of hashes that match that prefix
4. The suffix is locally checked to determine if the password is compromised

(This preserves privacy using k-anonymity.)

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Requirements

• Python 3.x
• requests module

  pip install requests

Example Output:

python3 password_check.py

Enter password to calculate entropy: Hunter2!

Entropy: 55.5 bits

Password Strength: Moderate

Your password is safe.

(Disclaimer: This tool is for educational and informational use only. Do not use it to test real passwords unless you trust your environment.)

Authors: Developed by Ria Singh, Lavanya Joshi, Anika Atluri