User##password
http://www.4chan.org/faq
What is a "secure tripcode"?
A secure tripcode can be generated by placing two octothorpes in the [Name] field, as opposed to one as with a normal tripcode (ex. "User##password"). Secure tripcodes use a secret key file on the server to help obscure their password. The previous example would display "User !!Oo43raDvH61" after being posted.