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