verify existing oracle password before changing

I am creating a web based application using the oracle application server toolkit. After a user has already logged on to the system, they will have the ability to change their password using a simple web page. This is easy enough to do with the alter user statement, but for security reasons I would like to prompt them for their original password first and then verify that password against "dba_users" before allowing the change.

I understand that oracle guards their password encryption/hash routines, but is there a function call I could call in a pl/sql program that would check to see if the original password is correct?

Thanks for your Help,
Ed

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