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n. A solid piece of metal code used to protect your application.

Why another authentication library?

  1. Because most of the other libraries are too huge.
  2. Extending other libraries is a pain.
  3. Writing code is fun :-).

What shield is

  1. Simple (~ 110 lines of Ruby code).
  2. Doesn't get in the way.
  3. Treats you like a grown up.

What shield is not

  • is not a ready-made end-to-end authentication solution.
  • is not biased towards any kind of ORM.

Understanding Shield in 15 minutes


Shield::Model is a very basic protocol for doing authentication against your model. It doesn't assume a lot, apart from the following:

  1. You will implement User.fetch which receives the login string.
  2. You have an attribute crypted_password which is able to store up to 192 characters.

And that's it.

In order to implement the model protocol, you start by including Shield::Model.

class User <, :crypted_password)
  include Shield::Model

  def self.fetch(email)
    user = new(email)
    user.password = "pass1234"

    return user

By including Shield::Model, you get all the general methods needed in order to do authentication.

  1. You get User.authenticate which receives the login string and password as the two parameters.
  2. You get User#password= which automatically converts the clear text password into a hashed form and assigns it into #crypted_password.
u ="")

# A password accessor has been added which manages `crypted_password`.
u.password = "pass1234"

Shield::Password.check("pass1234", u.crypted_password)
# => true

# Since we've hard coded all passwords to pass1234
# we're able to authenticate properly.
nil == User.authenticate("", "pass1234")
# => false

# If we try a different password on the other hand,
# we get `nil`.
nil == User.authenticate("", "wrong")
# => true

Shield uses Armor for encrypting passwords. Armor is a pure ruby implementation of PBKDF2, a password-based key derivation function recommended for the protection of electronically-stored data.

To make Shield work with any ORM, make sure that an .[] method which fetches the user instance by id is implemented.

class User
  include Shield::Model

  # ...

  def self.[](id)
    get id

Logging in with an email and username?

If your requirements dictate that you need to be able to support logging in using either username or email, then you can simply extend User.fetch a bit by doing:

# in Sequel (
class User < Sequel::Model
  include Shield::Model

  def self.fetch(identifier)
    filter(email: identifier).first || filter(username: identifier).first

# in Ohm (
class User < Ohm::Model
  include Shield::Model

  attribute :email
  attribute :username

  unique :email
  unique :username

  def self.fetch(identifier)
    with(:email, identifier) || with(:username, identifier)

If you want to allow case-insensitive logins for some reason, you can simply normalize the values to their lowercase form.


As the name suggests, Shield::Helpers is out there to aid you a bit, but this time it aids you in the context of your Rack application.

Shield::Helpers assumes only the following:

  1. You have included in your application a Session handler, (e.g. Rack::Session::Cookie)
  2. You have an env method which returns the environment hash as was passed in Rack.

Note: As of this writing, Sinatra, Cuba & Rails adhere to having an env method in the handler / controller context. Shield also ships with tests for both Cuba and Sinatra.

require "sinatra"

# Satisfies assumption number 1 above.
use Rack::Session::Cookie

# Mixes `Shield::Helpers` into your routes context.
helpers Shield::Helpers

get "/private" do
  error(401) unless authenticated(User)


get "/login" do
  erb :login

post "/login" do
  if login(User, params[:login], params[:password])
    remember(authenticated(User)) if params[:remember_me]
    redirect(params[:return] || "/")
    redirect "/login"

get "/logout" do
  redirect "/"


@@ login

<form action='/login' method='post'>
<input type='text' name='login' placeholder='Email'>
<input type='password' name='password' placeholder='Password'>
<input type='submit' name='proceed' value='Login'>

Note for the reader: The redirect to params[:return] in the example is vulnerable to URL hijacking. You can whitelist redirectable urls, or simply make sure the URL matches the pattern /\A[\/a-z0-9\-]+\z/i.


If you have a keen eye you might have noticed that instead of redirecting away to the login URL in the example above, we instead chose to do a 401 Unauthorized. In strict HTTP Status code terms, this is the proper approach. The redirection is simply the user experience pattern that has emerged in web applications.

But don't despair! If you want to do redirects simply add Shield::Middleware to your middleware stack like so:

# taken from example above
use Shield::Middleware, "/login"
use Rack::Session::Cookie

# rest of code follows here
# ...

Now when your application responds with a 401, Shield::Middleware will be responsible for doing the redirect to /login.

If you try and do a curl --head http://localhost:4567/private with Shield::Middleware, you'll get a response similar to the following:

HTTP/1.1 302 Found
Location: http://localhost:4567/login?return=%2Fprivate
Content-Type: text/html

Notice that it specifies /private as the return URL.


You can install it using rubygems:

gem install shield