Double Hashing Passwords: Client-Side & Server-Side

Hey guys! Ever wondered how to securely handle passwords when you're building a service like CoolEncryptedCloudService? We're diving deep into the world of password hashing, specifically focusing on the scenario where you've already hashed a password on the client-side and need to do it again on the server-side. It sounds a bit like password inception, right? Let's break down why you'd do this, the risks involved, and how to do it safely, covering important topics like hashing, key derivation, and PBKDF (Password-Based Key Derivation Function).

The CoolEncryptedCloudService Scenario

So, imagine you're building CoolEncryptedCloudService. Your main goals are to allow users to authenticate and to encrypt their data. This means you'll need to handle user passwords securely. You know you never want to store passwords in plain text, because that's a security nightmare! Instead, you're going to hash them. But here's the twist: you've decided to do some hashing on the client-side before the password even hits your servers. Why would you do this, and how do you handle it?

First off, let's talk about why you might want client-side hashing. It's all about minimizing the risk of a breach. If a hacker manages to intercept a password before it's hashed, they can use it directly. With client-side hashing, even if they intercept the password during transit, they only get the hashed version, which is much harder to crack. This adds an extra layer of security. This approach, however, complicates the server-side process, because now you've got a password that's already been hashed.

The core of the problem revolves around the fact that if you perform client-side hashing, you essentially prevent yourself from storing the original password, and, in effect, you must also treat the client-side hashed version as the password. This presents a challenge: you cannot authenticate users against the original password, because you never receive it. What you receive is the client-side hash. To solve this dilemma, we are forced to rehash the already client-side hashed password on the server-side, which is not an intuitive process. Also, it raises some security concerns, and it's essential to understand the implications of this double-hashing approach. We're going to dive into the best way to do this while maximizing your user's security.

The Problem with Client-Side Hashing Alone

Doing only client-side hashing isn't a silver bullet. If a malicious actor gains access to your server (which, let's face it, is a real possibility), they'll have the client-side hashed passwords. While this is better than plain text, it's not ideal. Attackers can still try to crack the hashes using techniques like rainbow tables or brute-force attacks, especially if the hashing algorithm is weak or poorly configured. To address this, you need a strong server-side hashing strategy as well.

Server-Side Re-Hashing Explained

So, here's the deal: you get a client-side hashed password. Now what? You have to hash that again on the server-side. It might seem odd, but it's a critical step. The goal here is to add another layer of security and to utilize a robust hashing algorithm with key derivation functions, and salts that are under your control. This ensures that even if the client-side hashing is compromised (or if it's less secure than you'd like), your server-side security is still strong.

The basic idea is this: When a user attempts to log in, you receive the client-side hash. You then feed this hash into a server-side hashing algorithm along with a salt. This generates a new hash. You compare this new hash to the hash you've stored in your database. If they match, the user is authenticated. Let's look into how to do that securely.

Key Derivation Functions (KDFs) and Why You Need Them

Okay, guys, here's where things get interesting. You don't want to just use a simple hashing algorithm like SHA-256 for the server-side re-hashing. Instead, you need a Key Derivation Function (KDF). KDFs are specifically designed for password storage because they are designed to be slow and computationally expensive. This is a good thing! The more time it takes to hash a password, the harder it is for an attacker to crack.

Why are KDFs so important? They incorporate techniques like: salt, pepper, and iteration, making it very difficult for attackers to use precomputed tables or parallelize attacks. A KDF takes your client-side hash, a salt (more on that later), and often a number of iterations. The higher the number of iterations, the slower the process, and the more secure the hash.

What is a Salt?

A salt is a random string of characters that is unique to each user's password. It's crucial because it prevents attackers from using precomputed rainbow tables to crack your hashes. Essentially, you take the client-side hash and combine it with the salt before hashing it on the server. The salt ensures that even if two users have the same client-side hashed password, their server-side hashes will be different.

What is Pepper?

Pepper is a secret that is added before your hashing function. Unlike salt, pepper is applied to every password. However, in the context of our double-hashing scenario, it should still be incorporated into the server-side hash.

PBKDF2, Argon2, and bcrypt: The Heavy Hitters

So, what KDFs should you use? The usual suspects are: PBKDF2 (Password-Based Key Derivation Function 2), Argon2, and bcrypt. All three of these are battle-tested and considered secure, but let's compare them:

• PBKDF2: This is a solid, widely supported option. It's been around for a while and is available in many programming languages and libraries. It's generally a good choice if you need broad compatibility, but it's not the most modern option.
• Argon2: Argon2 is the winner of the Password Hashing Competition. It's designed to be memory-hard, which means it's even more resistant to certain types of attacks, like those using specialized hardware. Argon2 is an excellent choice if security is your top priority. The latest research indicates that Argon2 is likely the best choice for password storage because it is resistant to a broad range of attacks.
• bcrypt: bcrypt is another strong contender. It's been around for a while and is also memory-hard. It's known for its adaptive nature: it allows you to increase the computational cost over time as hardware gets faster, making your hashes more resistant to cracking.

Implementation: A Practical Example

Let's get practical with a simplified example. We'll show you the basic steps, but remember to use the appropriate libraries for your programming language. Security is not an area where you should