Use BCrypt Fool!

Published on:

Almost any application will eventually need to store a collection of passwords or another type of data that has to be stored using a hashing algorithm. Blogs, forums, issue trackers, they all need to store user data and these passwords. This article covers the common mistakes made when dealing with passwords and what you should use instead. In order to fully understand this article some basic knowledge of programming and computers is required, you should also know a bit about the common hashing algorithms such as MD5 and SHA1.

The Problem

When developing applications developers make the common mistake of thinking they have a solid understanding of how hashing works. They think that by doing X they’re done and perfectly safe. Guess what, that’s not the case (not even close). The following mistakes are the most common:

We’ll start with the first problem. Up until a few years ago MD5 was the most common hashing algorithm used for passwords (and other data as well). MD5 was considered to be pretty safe until a group of people managed to prove how weak it really was: they were able to generate a set of collisions in a relatively short amount of time (a few hours or so). This set off a chain reaction and many more flaws were found.

Luckily MD5 isn’t the only hashing algorithm out there, there’s SHA1 and the SHA2 family as well as a few other ones. SHA1-SHA2 are much strong than MD5 and at the time of writing (April 2011) only SHA1 has been compromised. Technically it would take serious amount of time to crack SHA1 but the idea of using an algorithm that can be cracked before humanity is wiped out should be enough for people to not use it for privacy related data.

So why are collisions bad? Can’t we just use a very very long password or use method X (insert your favorite counter measure)? Yes, you can. The problem however isn’t fixed, you’re merely making the process slower rather than fixing the actual root of the problem. Time for an example. Assuming we have a hashing function called “hash” and two strings, A and B (where A and B are unique), our hashing process of these strings would look like the following:

pwd1 = hash(A)
pwd2 = hash(B)

In this case both pwd1 and pwd2 are unique. At this point a lot of people think they’re good to go as they assume nobody is willing to wait for a certain period of time before they’re able to crack the password, this is a very stupid mistake. While trying to crack a password (by bruteforcing it for example) may take a long time on a single computer most hackers can easily boot up a few servers or even worse, use a botnet. All known hashing algorithms (except BCrypt, more on that later) are affected by a single common problem: Moore’s law. Moore’s law states that every two years the amount of transistors that can be put in a computer doubles. This means that the faster computers get the quicker they’re able to crack a password. A hacker merely has to use N computers and the time required to retrieve the original password will be greatly reduced.

Because of this problem developers try to come up with solutions. These solutions don’t actually solve the problem, they just make it harder and require more time. A common “fix” is to hash a password N times and then save it in the database. Developers do this for a few reasons:

The fun thing is that this entire process doesn’t actually make the password more secure. The first reason is pretty easy to bust: simply add more hardware (or better hardware) and you’re good to go. The second reason is a bit harder to bust as it depends on the algorithm that is used. If we look back at our hash() function the process of hashing a hash multiple times would look like the following:

hash = hash( hash(hash(A)) )

In this example there are 3 calls to the hashing function. If A was “yorick” this would look a bit like the following:

hash(yorick)  -> j238103
hash(j238103) -> a9shda9
hash(a9shda9) -> 11s08j1

In this case “11s08j1” is the final hash that will be stored in our database. At this point developers usually lay down their work and take a coffee or a tea thinking they’ve done a good job and are hacker proof. Guess what, they’re not. What just happened is that the process of hashing A multiple times actually increased the possibility of a hash collision. While we do have to crack the hashing process N times for each call to hash() we don’t actually have to start at the very end (with “11s08j1”). The reason for this is that “11s08j1” isn’t directly based on “yorick” but on “a9shda9”. This means that we merely have to find the hash that results in “11s08j1” when using our hash function. If we find a collision we can simply crack it again and we’d end up with our original password.

In order to explain this properly I simplified the process of hashing A N times:

password --> hash 1 --> hash 2 --> final hash

In order to retrieve the original password (“password”) we’d have to find a collision for “hash 2”. We can’t use hash 1 as it’s source (“password”) can be considered totally random and would take more time. However, the source of hash 2 is much easier due one big issue: the entropy (the amount of possible combinations) of the password has been decreased. If we look back at the previous example we know the final hash is “11s08j1” and that the original password is “yorick”. Using various techniques (rainbow tables, bruteforcing, etc) we can quickly identify the source of “final hash”. The value of “hash 2” is “a9shda9”, while in our example this looks more random (it is) than the original password common hashing algorithms only use regular characters (letters and numbers) for their output. A good example of this is the following Ruby example:

require 'digest'

password = 'as9(A*&SD&(@))'
hash     = Digest::SHA1.new.hexdigest(password)

p hash # => "d4c36f9b1f003bee2e5dcafdf6b006110709dfb5"

The hash of the password (which is just something I randomly typed on my keyboard) may be longer but it only uses letters and numbers opposed to all the gibberish in the original password. The same happens with our hash() function and this allows us to quickly retrieve the original password. If we have the original hash of “final hash” we can then simply continue reversing the process until we end up at “yorick”.

The reason why you can’t initially find the source of “hash 2” is because you can’t find out what “hash 1” is because it’s not stored somewhere while “final hash” is.

To cut a long story short, hashing a hash N times doesn’t make your passwords more secure and can actually make it less secure as a hacker can quite easily reverse the process by generating hash collisions.

The Solution

It has already been mentioned before but the solution is to use an algorithm called “BCrypt”. BCrypt is a hashing algorithm based on Blowfish with a small twist: it keeps up with Moore’s law. The idea of BCrypt is quite simple, don’t just use regular characters (and thus increasing the entropy) and make sure password X always takes the same amount of time regardless of how powerful the hardware is that’s used to generate X. I’m not going to cover all the technical details but basically BCrypt requires you to specify a cost/workfactor in order to generate a password. This workfactor not only makes the entire process slower but is also used to generate the end hash. This means that if somebody were to change the workfactor the hash would also be different. In other words, hackers, you’re fucked. In order for a hacker to gain the original password he must use the same workfactor and thus has to wait N times longer than when not using a workfactor.

Time for an example in Ruby:

require 'benchmark'
require 'bcrypt'

password = 'yorick'
amount   = 100

Benchmark.bmbm(20) do |run|

  run.report("Cost of 5") do
    amount.times do
      hash = BCrypt::Password.create(password, :cost => 5)
    end
  end

  run.report("Cost of 10") do
    amount.times do
      hash = BCrypt::Password.create(password, :cost => 10)
    end
  end

  run.report("Cost of 15") do
    amount.times do
      hash = BCrypt::Password.create(password, :cost => 15)
    end
  end

end

For the non Ruby people, this is a simple benchmark script that shows the time it takes to hash “yorick” with BCrypt with a cost/workfactor of 5, 10 and 15 a total of 100 times. The results of this benchmark would look like the following:

Rehearsal -------------------------------------------------------
Cost of 5             0.250000   0.000000   0.250000 (  0.249723)
Cost of 10            7.740000   0.010000   7.750000 (  7.879849)
Cost of 15          247.510000   0.460000 247.970000 (255.346897)
-------------------------------------------- total: 255.970000sec

                          user     system      total        real
Cost of 5             0.250000   0.000000   0.250000 (  0.272549)
Cost of 10            7.750000   0.030000   7.780000 (  8.442511)
Cost of 15          247.530000   0.480000 248.010000 (254.815985)

The column we’re really interested in is the “real” column. As you can see a cost of 5 only takes about 250 miliseconds while a cost of 15 takes a whopping 250 seconds (around 4 minutes).

To cut another long story short: BCrypt adopts to Moore’s law and makes it impossible for a hacker to crack a password using rainbow tables or other techniques.

Implementations

The BCrypt hashing algorithm is implemented in quite a few languages. I’ve collected a list of resources for various languages so you can start using BCrypt right away.

PHP

PHP allows you to use BCrypt passwords using the crypt() function. This works as following:

<?php

$hash = crypt('rasmuslerdorf', '$2a$07$usesomesillystringforsalt$');

Ruby

For Ruby there’s a gem called “bcrypt-ruby” which can be installed using Rubygems:

$ gem install bcrypt-ruby

Once installed you can use it as following:

require 'bcrypt'

hash = BCrypt::Password.create('yorick', :cost => 10)

Perl

For Perl there’s Crypt::Eksblowfish which works as following:

use Crypt::Eksblowfish::Bcrypt qw(bcrypt_hash);

$salt     = '1p23j1-9381-23';
$password = 'yorick';
$hash     = bcrypt_hash({
    key_nul => 1,
    cost    => 10,
    salt    => $salt,
}, $password);

Others

Special Thanks

I’d like to thank the following IRC folks for helping me out (all of them can be found on Freenode):