Near impossible is a good way to put it, yeah.
Just remember since it is a one-way hash you can't get the values from it. All you can do is one way hash the values you have and see if the two values are equal. You might end up doing something like storing user name in plain text and a one-way hash of the password in the cookie. Then when the user visits the site, you grab the username and hashed password from the cookie, lookup the user in your DB and hash the password you have for that user and see if it matches what is in the cookie. If it does match, you can be relatively sure the user is who they say they are, if not they aren't.