[wtracz]

They can use Shamir's Secret Sharing to achieve this with no plaintext.

see http://willtracz.co.uk/shamir-secret-sharing-and-passwords

[nxzero]

>> "This post is intended to introduce concepts and practically demonstrate a method. The implementation provided is not a reliable/hardened solution (i.e. there are vulnerabilities because finite fields are not used) and should not be used anywhere near a production system."

Above is on the page you're linking to.

[wtracz]

Yes, I wrote it and it is intended as a demo only. I have not put in the finite field logic in that implementation. It shows the concepts though.