How do you use hashing to implement a password authentication system?

1 Hashing basics

Hashing is a one-way function that takes any input and produces a fixed-length output, called a hash or a digest. The same input always generates the same hash, but different inputs produce different hashes. For example, the input "password" might produce the hash "5f4dcc3b5aa765d61d8327deb882cf99", while the input "passw0rd" might produce the hash "6c569aabbf7775ef8fc5705a9f1f9b2f". Hashing is irreversible, meaning that you cannot recover the original input from the hash. This makes hashing suitable for password authentication, as you can store and compare hashes instead of passwords, without revealing the actual passwords.

2 Hashing for authentication

To use hashing for password authentication, you need to apply a hashing function to the password when a user creates or updates their password and store the resulting hash in a database. When the user tries to log in, you compare the resulting hash with the stored hash in the database. If the hashes match, you grant the user access and if they do not match, you deny the user access. This method of authentication enables you to verify a user's identity without storing or transmitting their passwords in plain text, thus reducing the risk of password theft or leakage.

3 Hashing challenges

Hashing is not a perfect solution for password authentication, as it presents some challenges that need to be addressed. Hash collisions occur when two different inputs produce the same hash, which can compromise security and uniqueness of passwords. To avoid hash collisions, use a hashing function with a large output space and low probability of collisions, such as SHA-256 or SHA-3. Brute force attacks are attempts to guess passwords by trying different combinations of characters until finding a match. Since hashing is deterministic, a hacker can use a brute force attack to generate hashes and compare them with stored hashes in the database. To prevent brute force attacks, use a hashing function that is slow and computationally intensive, such as bcrypt or scrypt. Additionally, use a salt (random value added to the password before hashing) and pepper (secret value added to the password before hashing) to increase complexity and diversity of hashes and defend against rainbow tables (pre-computed tables of hashes and their corresponding inputs).

4 Hashing limitations

Hashing is a powerful tool for password authentication, but it is not a silver bullet. It cannot prevent users from choosing weak passwords, nor does it protect passwords from being exposed or stolen if the database or server is compromised. To maximize the security and privacy of your users and their accounts, you should implement password policies, such as minimum length, complexity, and expiration, provide feedback and guidance on password strength and security, encrypt the database and communication channels, use secure protocols and certificates, and monitor and audit the system for suspicious activity.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?