This will allow you to verify and parse the Facebook Javascript Connect cookie for Ruby/Ruby on Rails. Currently working this into a gem: http://github.com/aaronvb/fb_js_connect

fb_js_connect.rb

require 'digest/md5'
class FbJsConnect
  # When using the Facebook Javascript SDK to connect users to your site, 
  # a cookie will be placed in your applications session store. This class
  # will verify your cookie, tell you if you're connected, and provide
  # you with a User and other information

  #
  # cookie = cookies["fbs_YOUR FACEBOOK APP ID"]
  # fb = FbJsConnect.new(cookie)
  # fb.connected => true or false
  # fb.verified? => true or false
  # fb.user => User or nil (Assuming you have a User class with the 
  # attribute fb_uid)
  # fb.uid if fb.connected => uid if connected
  # fb.access_token if fb.connected => access_token if connected
  
  # Verification works by combining cookie key + values, minus sig, 
  # appending app_secret, then MD5 hashing it.
  # The value of the MD5 should equal the sig value.

  attr_accessor :connected
  
  def initialize(cookie)
    @app_id = "" # YOUR FACEBOOK APP ID
    @app_secret = "" # YOUR FACEBOOK APP SECRET KEY
    fb_cookie = cookie
    if fb_cookie
      # remove any quotes in cookie. Facebook puts it's cookie in
      # quotes for some reason
      fb_cookie.gsub!('"','')
      # split cookie values into array by '&' symbol
      fb_cookie = fb_cookie.split("&")
      fb_cookie_hash = Hash.new
      fb_cookie.each do |c, i|
  	 key_value_arr = c.split("=") # split value into kv hash
  	 fb_cookie_hash[key_value_arr[0]] = key_value_arr[1] # add to new hash
      end

      @fb_cookie_hash = fb_cookie_hash # for verification

      # apply connected and fb_cookie_hash to class attributes IF verified
      if self.verified?
        @attributes = fb_cookie_hash 
        self.connected = true
      end
    else
      # no cookie found so return false
      self.connected = false
    end
  end
  
  def verified?
    Rails.logger.debug("\nFB: digested sig: #{process}")
    Rails.logger.debug("FB: sig: #{@sig}\n\n")
    if process == @sig
      return true
    else
      return false
    end
  end
  
  def user
    # find user with UID
    # 
    user = User.find(:first, :conditions => {:fb_uid => 
      @fb_cookie_hash["uid"]})
    if user
      return user # a user was found with UID
    else
      return nil # a user was not found
    end
  end
  
  def method_missing(name, *args)
    attribute = name.to_s
    if attribute =~ /=$/
      @attributes[attribute.chop] = args[0]
    else
      @attributes[attribute]
    end
  end
  
  private
  def process
    @sig = @fb_cookie_hash['sig'] # get sig to compare final hash
    payload = []
    @fb_cookie_hash.each do |k, v|
      payload.push "#{k}=#{v}" unless k == "sig"
    end
    digest = Digest::MD5.hexdigest(payload.sort.to_s + @app_secret)
    return digest
  end
  
end