= bcrypt-ruby

An easy way to keep your users' passwords secure.

== 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.

== How to install bcrypt

  sudo gem install bcrypt-ruby
  
You'll need a working compiler. (Win32 folks should use Cygwin or um, something else.)

== How to use bcrypt in your Rails application

=== 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.

== 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