Encrypted password method

Hi all
I am using a standard implementation of the membership provider. I however, need to compare a supplied  password (from a textbox) with a saved encrypted password. This is for a new security policy at work.
So I need to do something like:
bool passwordExists = false; int previousPasswordCount = Users.OldPasswords.GetPasswordCount(Guid userID, string password) // this supplied password needs to be encrypted? How do I access the method? if previousPasswordCount > 0 { passwordExists = true; }

Thanks in advance.
David

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