Makes it dead easy to do HTTP Basic authentication.

Simple Basic example:

class PostsController < ApplicationController
  USER_NAME, PASSWORD = "dhh", "secret"
  before_filter :authenticate, :except => [ :index ]
  def index
    render :text => "Everyone can see me!"
  end
  def edit
    render :text => "I'm only accessible if you know the password"
  end
  private
    def authenticate
      authenticate_or_request_with_http_basic do |user_name, password|
        user_name == USER_NAME && password == PASSWORD
      end
    end
end

Here is a more advanced Basic example where only Atom feeds and the XML API is protected by HTTP authentication, the regular HTML interface is protected by a session approach:

class ApplicationController < ActionController::Base
  before_filter :set_account, :authenticate
  protected
    def set_account
      @account = Account.find_by_url_name(request.subdomains.first)
    end
    def authenticate
      case request.format
      when Mime::XML, Mime::ATOM
        if user = authenticate_with_http_basic { |u, p| @account.users.authenticate(u, p) }
          @current_user = user
        else
          request_http_basic_authentication
        end
      else
        if session_authenticated?
          @current_user = @account.users.find(session[:authenticated][:user_id])
        else
          redirect_to(login_url) and return false
        end
      end
    end
end

In your integration tests, you can do something like this:

def test_access_granted_from_xml
  get(
    "/notes/1.xml", nil,
    :authorization => ActionController::HttpAuthentication::Basic.encode_credentials(users(:dhh).name, users(:dhh).password)
  )
  assert_equal 200, status
end

Simple Digest example:

require 'digest/md5'
class PostsController < ApplicationController
  REALM = "SuperSecret"
  USERS = {"dhh" => "secret", #plain text password
           "dap" => Digest:MD5::hexdigest(["dap",REALM,"secret"].join(":"))  #ha1 digest password
  before_filter :authenticate, :except => [:index]
  def index
    render :text => "Everyone can see me!"
  end
  def edit
    render :text => "I'm only accessible if you know the password"
  end
  private
    def authenticate
      authenticate_or_request_with_http_digest(REALM) do |username|
        USERS[username]
      end
    end
end

NOTE: The authenticate_or_request_with_http_digestblock must return the user's password or the ha1 digest hash so the framework can appropriately

hash to check the user's credentials. Returning +nil+ will cause authentication to fail.
Storing the ha1 hash: MD5(username:realm:password), is better than storing a plain password. If
the password file or database is compromised, the attacker would be able to use the ha1 hash to
authenticate as the user at this +realm+, but would not have the user's password to try using at
other sites.

On shared hosts, Apache sometimes doesn't pass authentication headers to FCGI instances. If your environment matches this description and you cannot authenticate, try this rule in your Apache setup:

RewriteRule ^(.*)$ dispatch.fcgi [E=X-HTTP_AUTHORIZATION:%{HTTP:Authorization},QSA,L]
Namespace
Methods
A
D
E
U
Instance Public methods
authenticate(controller, &login_procedure)
# File actionpack/lib/action_controller/http_authentication.rb, line 124
def authenticate(controller, &login_procedure)
  unless authorization(controller.request).blank?
    login_procedure.call(*user_name_and_password(controller.request))
  end
end
authentication_request(controller, realm)
# File actionpack/lib/action_controller/http_authentication.rb, line 149
def authentication_request(controller, realm)
  controller.headers["WWW-Authenticate"] = %Q(Basic realm="#{realm.gsub(/"/, "")}")
  controller.__send__ :render, :text => "HTTP Basic: Access denied.\n", :status => :unauthorized
end
authorization(request)
# File actionpack/lib/action_controller/http_authentication.rb, line 134
def authorization(request)
  request.env['HTTP_AUTHORIZATION']   ||
  request.env['X-HTTP_AUTHORIZATION'] ||
  request.env['X_HTTP_AUTHORIZATION'] ||
  request.env['REDIRECT_X_HTTP_AUTHORIZATION']
end
decode_credentials(request)
# File actionpack/lib/action_controller/http_authentication.rb, line 141
def decode_credentials(request)
  ActiveSupport::Base64.decode64(authorization(request).split(' ', 2).last || '')
end
encode_credentials(user_name, password)
# File actionpack/lib/action_controller/http_authentication.rb, line 145
def encode_credentials(user_name, password)
  "Basic #{ActiveSupport::Base64.encode64("#{user_name}:#{password}")}"
end
user_name_and_password(request)
# File actionpack/lib/action_controller/http_authentication.rb, line 130
def user_name_and_password(request)
  decode_credentials(request).split(/:/, 2)
end