Thumbnail for video 'Hash functions'

← Back to videos

Hash functions

You might have found out that there is a website that checks if your online accounts have been compromised by hackers. So you enter in your email address and OH NO… You have been pwned! Hackers now know the passwords that you used on all these services…

But do they really know your password?

Well as it turns out: that might not be the case... To understand why, let’s take a look at what options companies have to protect your password and safely store it so that even when hackers get access to their systems, your password stays safe.

There are 3 ways a company can store your password: they store it in plain text, use encryption on it or use what’s called a hash function.

Let’s quickly go over each one of these and let’s start with the most basic one: plain text.

Plain text

This is obviously the most dangerous way of storing passwords. If hackers breach a company’s database, they get to see all the passwords of the users. And since a lot of people have the bad habit of using the same password for multiple accounts, it’s likely that 1 compromised password could lead to more compromised accounts.

You might think that companies aren’t silly and that none of them stores our passwords in plain text. However, you would be very wrong in thinking that. Past breaches have shown us that even top companies and services with millions of users weren’t adequately protecting user passwords.


One possible alternative to plain text storage is encryption. Take the passwords of the users and - before your store them - encrypt them with an encryption key. This would prevent hackers from obtaining the real passwords of users but it’s still quite risky. Underneath the encryption layer is still a plain text password and so if the attacker manages to steal the encryption key as well, he can unlock all passwords.

Encryption is designed to work in two ways: you can encrypt a user’s passwords to keep it safe but you can also decrypt it to reveal the password again. This is very practical when you want to share data in a secure way, but not great if you want to prevent attackers from breaching your password.

And that brings us to the third technique of storing passwords and that is by using a hash function.


How does that work? Well, hash functions take an input, that could be a piece of text like your password or it could be a file and turns that into a string of text that always has the same length.

There are many different hash functions available but here is what the SHA3 hash of “Hello World!” looks like:


Hash functions are very different from encryption because they only work in 1-way. You can calculate the hash of a password but you cannot take a hash and turn it back into the original data. And that’s an interesting property to have. By using hashes, companies can verify that you’re logging in with the correct password, without having to store your actual password.

You can compare hashes to fingerprints. You can take the fingerprint of any person BUT if you find a fingerprint somewhere you can’t identify the person it belongs to unless you’ve seen that print before!

However, they aren’t perfect either. Most hashing algorithms are optimized for speed, the more hashes per second they can calculate, the better. And that makes them vulnerable against brute-force attacks. By simply trying to calculate every possible password, an attacker can reverse the hash function.

A modern GPU can do this with a speed of 292 million hashes per second (292.2 MH/s) so it’s only a matter of time before a hashed password is cracked using this technique. And if that’s not fast enough, attackers can also use Rainbow tables to further accelerate the process. These are lists of precomputed hashes that can be used to quickly find weak and commonly used passwords.

The speed of hashing functions is a positive thing in certain area’s. However, when it comes to storing passwords you don’t want this property. The second problem happens when users share the same password. If both Alice and Bob have the password “qwerty”, the hashes of their passwords will be identical.

So when a hacker cracks of these passwords, he also knows the others. Now you might think: that’s not a big deal because it’s very unlikely that different people will use the same password. Well, think again. The password “qwerty” has been found more than 3 million times in data breaches. To make matters even worse: here’s the top 10 most used password in 2017… Not the strongest of passwords…

Salted hashes

To defend against these attacks we can add what’s called a salt to the password before we hash it. The salt is just some random data but it ensures that the hash of your password will always be unique, even if others are using the same password.

So if Bob and Alice both use the password “qwerty” their hashes will be completely different. So if an attacker cracks Bob’s password, he can’t link that password to Alice and he has to start his cracking attempt again.

salt 1 = VNc4BdR20n
hash 1 = HASH( "qwerty" + salt1)

salt 2 = zWQC6kf6n1
hash 2 = HASH( "qwerty" + salt2)

This technique prevents attackers from cracking a bunch of passwords in 1 go. It makes a brute force attack slower, but still very much possible.


So to solve this, we have to take a look at the third technique, which is using special hash functions that are deliberately being slowed down. Example of these are bcrypt, scrypt or argon2 and they completely neutralize brute force attacks.

These algorithms take a password as input along with a salt and a cost. This last one is very interesting: the cost defines the number of rounds the algorithm goes through and this effectively slows it down. Over time our computers become faster and so brute force attacks against these algorithms becomes easier. That’s because they can simply try more combinations in a shorter timespan. All we have to do to counter this is increase the cost parameter so the algorithm remains resistant against these attacks. Pretty genius!


So that are the 3 options that a company has to store and protect your passwords. But why settle for just one method if we can use multiple? You can’t be greedy enough when it comes to security!

This multi-layer protection is used by Dropbox for instance. They take your password and start by running it through a simple hash function, no salt. This is their first line of defense. They then take the hash and run it through the bcrypt algorithm with a salt and a cost of 10. This prevents brute-force attacks. And finally, the resulting hash is encrypted with the Advanced Encryption Standard or AES. The encryption key for this is not stored in their databases but is instead kept separately.

So if an attackers breach the Dropbox database they will have to peel away each protective layer around your password and that will take a lot of time. In fact, the cracking attempt would likely be more costly than what they’d in return for comprising your account.


So time for a conclusion then: if your account has been compromised, it’s best to change your password immediately. However, depending on the security measures of the company that was compromised, it might be possible that hackers haven’t been able to retrieve your password. That’s thanks to the magic of hash functions and cryptography in general.

So now you know how companies can safely store your passwords. That was it for this video. If you learned something from it: hit the thumbs up button and consider getting subscribed. And as always: thank you very much for watching!


Akhawe, D. (2016, September 21). How Dropbox securely stores your passwords. Dropbox.
bcrypt. (n.d.). Wikipedia.
Friedl, S. (n.d.). An Illustrated Guide to Cryptographic Hashes.
Gosney, J. M. (n.d.). Nvidia GTX 970 cudaHashcat Benchmarks.
Hash function. (n.d.). Wikipedia.
Hunt, T. (n.d.). Pwned websites.
Korosec, K. (2017, December 19). The 25 Most Common Passwords of 2017 Include “Star Wars.” Fortune.
PasswordSecurity. (n.d.).
Random String Generator. (n.d.). Randomness.
Salt (cryptography). (n.d.). Wikipedia.
SHA3-512. (n.d.). Online Tools.