ASP Membership database

I am working on a conversion of a website with user authentication. The new version now uses the ASP Membership login control and database. I want to transfer the users of the old site into the ASPMembership database. The password field of the current user table is not encrypted, so I cannot insert the usernames and passwords through a query into the table asp_membership the login, I would need the encryption algorythm that Microsoft put behind the login control. How can I de- and encrypt passwords before passing them into the table?

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