The effectiveness of synthetic coordinate systems against DoS and spam stems from the fact that, while changing or hiding a logical address is easier, changing the location of the spammer inside the network should be harder. But synthetic coordinate systems are limited by the fact that malicious nodes can easily lie about their position or introduce additional delays which have an immediate impact on it. For this purpose, we enhance the synthetic coordinate system provided by Vivaldi with secure nonces that are periodically broadcasted by trusted servers to achieve a provable location claim within the overlay network. As we advocate in this work, secure localization can help in choosing the hardness of PoWs since locations from which more malicious traffic originates can receive PoWs with higher difficulties.
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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