Create saml assertion - newbie question

Dear all,

In the sourceid demo, it asks the user to input the username and
password as parameters, then use them to create SAML assertion which is
passed to the Relying party side. I looked at the code but just cannot
find out which classes or servlets really do the job to create the SAML
assertion after taking in the username/password pair..

Could you give me some advices regarding this?

Cheers,
jake