bcrypt-ruby

An easy way to keep your users' passwords secure.

• http://github.com/codahale/bcrypt-ruby/tree/master

Why you should use bcrypt()

If you store user passwords in the clear, then an attacker who steals a copy of your database has a giant list of emails and passwords. Some of your users will only have one password -- for their email account, for their banking account, for your application. A simple hack could escalate into massive identity theft.

It's your responsibility as a web developer to make your web application secure -- blaming your users for not being security experts is not a professional response to risk.

bcrypt() allows you to easily harden your application against these kinds of attacks.

Note: JRuby versions of bcrypt <= 2.1.3 had a security vulnerability that was fixed in >= 2.1.4. If you used a vulnerable version to hash passwords with international characters in them, you will need to re-hash those passwords. This vulnerability only affected the JRuby gem.

How to install bcrypt

gem install bcrypt

The bcrypt gem is available on the following ruby platforms:

• JRuby
• RubyInstaller 1.8, 1.9, and 2.0 builds on win32
• Any 1.8, 1.9, or 2.0 ruby on a BSD/OSX/Linux system with a compiler

How to use bcrypt() in your Rails application

Note: Rails versions >= 3 ship with ActiveModel::SecurePassword which uses bcrypt-ruby. has_secure_password docs
implements a similar authentication strategy to the code below.

The User model

require 'bcrypt'

class User < ActiveRecord::Base
  # users.password_hash in the database is a :string
  include BCrypt

  def password
    @password ||= Password.new(password_hash)
  end

  def password=(new_password)
    @password = Password.create(new_password)
    self.password_hash = @password
  end
end

Creating an account

def create
  @user = User.new(params[:user])
  @user.password = params[:password]
  @user.save!
end

Authenticating a user

def login
  @user = User.find_by_email(params[:email])
  if @user.password == params[:password]
    give_token
  else
    redirect_to home_url
  end
end

If a user forgets their password?

# assign them a random one and mail it to them, asking them to change it
def forgot_password
  @user = User.find_by_email(params[:email])
  random_password = Array.new(10).map { (65 + rand(58)).chr }.join
  @user.password = random_password
  @user.save!
  Mailer.create_and_deliver_password_change(@user, random_password)
end

How to use bcrypt-ruby in general

require 'bcrypt'

my_password = BCrypt::Password.create("my password")
  #=> "$2a$10$vI8aWBnW3fID.ZQ4/zo1G.q1lRps.9cGLcZEiGDMVr5yUP1KUOYTa"

my_password.version              #=> "2a"
my_password.cost                 #=> 10
my_password == "my password"     #=> true
my_password == "not my password" #=> false

my_password = BCrypt::Password.new("$2a$10$vI8aWBnW3fID.ZQ4/zo1G.q1lRps.9cGLcZEiGDMVr5yUP1KUOYTa")
my_password == "my password"     #=> true
my_password == "not my password" #=> false

Check the rdocs for more details -- BCrypt, BCrypt::Password.

How bcrypt() works

bcrypt() is a hashing algorithm designed by Niels Provos and David Mazières of the OpenBSD Project.

Background

Hash algorithms take a chunk of data (e.g., your user's password) and create a "digital fingerprint," or hash, of it. Because this process is not reversible, there's no way to go from the hash back to the password.

In other words:

hash(p) #=> <unique gibberish>

You can store the hash and check it against a hash made of a potentially valid password:

<unique gibberish> =? hash(just_entered_password)

Rainbow Tables

But even this has weaknesses -- attackers can just run lists of possible passwords through the same algorithm, store the results in a big database, and then look up the passwords by their hash:

PrecomputedPassword.find_by_hash(<unique gibberish>).password #=> "secret1"

Salts

The solution to this is to add a small chunk of random data -- called a salt -- to the password before it's hashed:

hash(salt + p) #=> <really unique gibberish>

The salt is then stored along with the hash in the database, and used to check potentially valid passwords:

<really unique gibberish> =? hash(salt + just_entered_password)

bcrypt-ruby automatically handles the storage and generation of these salts for you.

Adding a salt means that an attacker has to have a gigantic database for each unique salt -- for a salt made of 4 letters, that's 456,976 different databases. Pretty much no one has that much storage space, so attackers try a different, slower method -- throw a list of potential passwords at each individual password:

hash(salt + "aadvark") =? <really unique gibberish>
hash(salt + "abacus")  =? <really unique gibberish>
etc.

This is much slower than the big database approach, but most hash algorithms are pretty quick -- and therein lies the problem. Hash algorithms aren't usually designed to be slow, they're designed to turn gigabytes of data into secure fingerprints as quickly as possible. bcrypt(), though, is designed to be computationally expensive:

Ten thousand iterations:
             user     system      total        real
md5      0.070000   0.000000   0.070000 (  0.070415)
bcrypt  22.230000   0.080000  22.310000 ( 22.493822)

If an attacker was using Ruby to check each password, they could check ~140,000 passwords a second with MD5 but only ~450 passwords a second with bcrypt().

Cost Factors

In addition, bcrypt() allows you to increase the amount of work required to hash a password as computers get faster. Old passwords will still work fine, but new passwords can keep up with the times.

The default cost factor used by bcrypt-ruby is 10, which is fine for session-based authentication. If you are using a stateless authentication architecture (e.g., HTTP Basic Auth), you will want to lower the cost factor to reduce your server load and keep your request times down. This will lower the security provided you, but there are few alternatives.

To change the default cost factor used by bcrypt-ruby, use BCrypt::Engine.cost = new_value:

BCrypt::Password.create('secret').cost
  #=> 10, the default provided by bcrypt-ruby

# set a new default cost
BCrypt::Engine.cost = 8
BCrypt::Password.create('secret').cost
  #=> 8

The default cost can be overridden as needed by passing an options hash with a different cost:

BCrypt::Password.create('secret', :cost => 6).cost  #=> 6

More Information

bcrypt() is currently used as the default password storage hash in OpenBSD, widely regarded as the most secure operating system available.

For a more technical explanation of the algorithm and its design criteria, please read Niels Provos and David Mazières' Usenix99 paper: http://www.usenix.org/events/usenix99/provos.html

If you'd like more down-to-earth advice regarding cryptography, I suggest reading <i>Practical Cryptography</i> by Niels Ferguson and Bruce Schneier: http://www.schneier.com/book-practical.html

Etc

• Author :: Coda Hale coda.hale@gmail.com
• Website :: http://blog.codahale.com