You don't - SHA-1 and things like it are one-way hashes. The way you
test if an incoming password matches is to do SHA-1 against the incoming password and see if the hashes match.
That way your passwords aren't recoverable if someone compromises your password file/database.
You should also read up on SHA-1 attack methods, there exists a method for producing SHA-1 collisions fairly quickly.
[ October 03, 2006: Message edited by: Tim LeMaster ]