Dictionary attack

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A dictionary attack is form of lookup attack used to crack passwords even when the passwords have been obscured with a hash function.

I learned about dictionary attacks in a programming class in college. And took home three important lessons:

  1. Always use unique passwords. If you're not sure if your password is unique, see if you can find it on a list of commonly used passwords.
  2. Just because your data is encrypted, that doesn't make it safe. Always keep your data to yourself and away from clever people.
  3. Be aware of how people crack passwords so you will know how to protect against them.


Step 1: Steal a Password Table

This is the most difficult part of the attack, as system administrators don't just give away user's passwords. Obtaining a password table means being able to defeat a system's security or buying a password table from someone who has. Since it is a security flaw for even admins to know a user's password, the passwords in the table are almost always obfuscated through a one-way hash function. So, when you look at a password table, it may look something like this:

User Password
smithr 5EBE2294ECD0E0F08EAB7690D2A6EE69
jonesd 0CC175B9C0F1B6A831C399E269772661
doej 92EB5FFEE6AE2FEC3AD71C777531578F
johnsonk 25D55AD283AA400AF464C76D713C07AD
williamsd 21232F297A57A5A743894A0E4A801FC3

In this case, the passwords have been run through an MD5 hash function which cannot be reversed. When a user logs in, the computer will first apply the hash to the password they typed in, and compare it with the one on-file to ensure a match. This way, the plaintext password is only ever known by the user. So then, how can a password cracker ever determine a user's password without a Brute Force Attack? This is where the dictionary attack comes into play.

Step 2: Obtain a List of Commonly Used Passwords

This is particularly easy since many crackers have already done the hard work and compiled lists of commonly used passwords. Here is an example list:

Common Password

Step 3: Run the Same Hash On the List of Common Passwords

Hash functions used to obfuscate passwords cannot be reversed, but they can be repeated. To do this, a cracker must know the exact same hash function used to obfuscate the passwords in the stolen password table. This isn't too difficult, since only a few are commonly used. From here, the cracker runs all the commonly used passwords through the same hash function and gets a list of the hashes for each of the passwords.

Common Password MD5 Hash
12345678 25D55AD283AA400AF464C76D713C07AD
admin 21232F297A57A5A743894A0E4A801FC3
god A4757D7419FF3B48E92E90596F0E7548
password 5F4DCC3B5AA765D61D8327DEB882CF99
secret 5EBE2294ECD0E0F08EAB7690D2A6EE69

I should point out that the MD5 hash function is not a cryptographically secure hash function and should never be used to store passwords. I'm just using it for this example.

Step 4: Compare Against the Password Table

Once the dictionary of hashes has been generated, the cracker need simply look for matches in the hashes between the stolen password table and their dictionary. A match indicates that the user chose the password in the table.

User Password Match From Dictionary
smithr 5EBE2294ECD0E0F08EAB7690D2A6EE69 secret
jonesd 0CC175B9C0F1B6A831C399E269772661 -no match-
doej 92EB5FFEE6AE2FEC3AD71C777531578F -no match-
johnsonk 25D55AD283AA400AF464C76D713C07AD 12345678
williamsd 21232F297A57A5A743894A0E4A801FC3 admin

This method will identify every password in the stolen password table that matches one in the dictionary of commonly used passwords, which, in common practice, is often over half.

Since computers are very fast at generating hash functions and comparing values between two tables, a dictionary attack with millions of commonly used passwords can be carried out against a table of millions of user passwords in a matter of minutes.



In order to combat dictionary attacks, most modern password tables first apply a salt to passwords before sending them through a hash function. A salt is a modification to a password so that it will yield a completely different hash. For example, the password below have been salted by adding a question mark to the beginning of them before being hashed, which gives an MD5 hash that is totally different from the hash without the question mark.

Common Password MD5 Hash Salted Hash
12345678 25D55AD283AA400AF464C76D713C07AD 9F7128DB15B794862C8E96A819379D94
admin 21232F297A57A5A743894A0E4A801FC3 C0D28ABB7D0C94329B81AA112518ADA0
god A4757D7419FF3B48E92E90596F0E7548 27DE47E3C4C4205ED02216C2A51AF071
password 5F4DCC3B5AA765D61D8327DEB882CF99 1007980A168F839AA8C4689C7FBDDB0E
secret 5EBE2294ECD0E0F08EAB7690D2A6EE69 D3A4FE3D71CD8546AA9BEC89F0D686DF

When using a salt, the program that handles user passwords must not only apply the salt when storing the password into the password table, but it must also apply the salt to the password each time a user logs in to make sure it will match the hash in the password table.

A salted password table adds a layer of security because, in order for a dictionary attack to work, the dictionary of hashes must be generated with the passwords and the salt. So, if the cracker doesn't know the salt, all of the password will fail to find a match. However, if a cracker defeated a system's security well enough to make a copy of the password table, they probably also made a copy of the program that adds the salt. So, a salt will not stop a cracker, because they will be able to determine how the salt was added and apply it to their password dictionary, but it will slow them down.