In order to increase the resilience against spam, we design and implement a protocol based on cryptographic puzzles for an open-source web based e-mail client. Our proposal is compatible with existing e-mail infrastructure and does not require modifications on the server side. The only add-on is a stand-alone ticketing server that is used to deliver the current cost to each sender. The puzzles that we use are time-lock puzzles which have the benefit that they cannot be subject to a parallel attack due to their intrinsic sequential nature. Thus an adversary that gains control over multiple hosts cannot use them to solve a puzzle, the risk of a directed attack against some receiver being reduced. Also, the protocol allows the sender of the e-mail to generate the puzzle himself, releasing the e-mail server from an additional computational task. We analyze the efficiency of the proposed solutions in terms of computational costs and the results are satisfactory.
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