Custom .NET Membership

I use .net membership but everything what i work i want to be custom.

What i want to do is:

1. Create custom data table [Users] with custom fields


2. Import current data into new table


3. Create custom classes and functions about everything what i need for [Users]

I`m not sure how .net membership works, but maybe it send encrypted cookie then when i use

var user = Membership.GetUser();
 

.Net decrypt user cookie and know which user is.

Here is a screenshot how .net create user AUTH cookie http://prntscr.com/97043

But everytime user logout-login, this value is different.

So what i want to know is:

1. Lets say i want to make 100% custom website, how i can make custom login?


2. Can you tell me all security issues about going for custom membership?

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