return user

I use ASP.Net template to build a sample application.
I tried to register a user that the system create membership tables and enter one entry in the database.
The data are encrypted.
I need to retrieve the informaiton for the user. how would I know which return user come back visit.
Are there any way to decryp the data or get some variable from the form or the system?
Your help and information is great appreciated,
Regards,
 
inungh,
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