gistfile1.rb

# app/views/posts/new.html.erb

<%= form_for(Post.new) do |form| %>
  Title: <%= form.text_field :title %>
  Body:  <%= form.text_field :body %>
<% end %>

...would produce a signature field that can be used to automatically untaint:

  <input type="hidden" name="signature" value="....">

# controller
class PostsController
  def create
    # This will blow up if it contains anything but title + body
    # because it'll then be marked as tainted
    Post.create(params[:post])
  rescue ActiveRecord::TaintedAttributes
    # This would happen if you tried to submit post[published]=1 as
    # part of the request
  end
  
  def update
    # This will work because #permit would untaint the hash
    Post.find(params[:id]).update_attributes(
      params[:post].permit(:title, :body)
    )
  end
end