Protection against SPAM using pre-challenges

Spam turns out to be an increasingly serious problem to email users. A number of anti-spam schemes have been proposed and deployed, but the problem has yet been well addressed. One of those schemes is challenge-response, in which a challenge is imposed on an email sender. However, such a scheme introduces new problems for the users, e.g., delay of service and denial of service attacks. In this paper, we introduce a pre-challenge scheme that avoids those problems. It assumes each user has a challenge that is defined by the user himself/herself and associated with his/her email address, in such a way that an email sender can simultaneously retrieve a new receiver's email address and challenge before sending an email in the first contact. Some new mechanisms are employed to reach a good balance between security against spam and convenience to email users.