How to check password before changing it?

Hello, I've installed acts as authenticated plugin and I'm trying to change it a little bit. I've created form where I can change my password. There are 3 inputs: old pass, new and confirm new pass. In user model there is:

(..) attr_accessor :password (..) validates_presence_of :password, :if => :password_required? validates_presence_of :password_confirmation, :if => :password_required? validates_length_of :password, :within => 4..40, :if => :password_required? validates_confirmation_of :password, :if => :password_required? (..)

What should I do to check if old password is correct? Is it possible to use validates_ do do it?

Cheers, Stan

if Digest::SHA1.hexdigest(params[:oldpassword] == current_user.crypted_password