Password Storage

Never store a password in plain text — if the database ever leaks (and eventually, somewhere, it will), every user's real password leaks with it. Instead, store a hash: a one-way function of the password that's practically impossible to reverse, but easy to re-check.

import hashlib

def hash_password(password, salt):
    return hashlib.sha256((salt + password).encode()).hexdigest()

print(hash_password("hunter2", "random_salt_abc"))

Without a salt, an attacker with a leaked database of hashes can use a rainbow table — a giant precomputed lookup of hash(common_password) -> common_password — to instantly reverse huge numbers of hashes at once, since the same password always produces the same hash. A random, unique salt per user defeats this entirely: even if two users pick the exact same password, their stored hashes come out completely different, because the salt is mixed in before hashing.

def verify_password(password, salt, stored_hash):
    return hash_password(password, salt) == stored_hash

Real production systems go further still — bcrypt, scrypt, and argon2 are DELIBERATELY slow hash functions (unlike plain SHA-256, which is fast — a property that's actually a liability here, since it makes brute-forcing millions of guesses per second easier). This course uses plain SHA-256 to keep the salting CONCEPT clear; a real system should reach for one of those purpose-built password-hashing functions instead.