FairDrop: a Confidential Fair Exchange Protocol for Media Workers

In recent years, the asymmetry between open societies and regimes that control their media has increased, leading to the number of murdered journalists more than doubling worldwide. Even in countries in which freedom of the press is publicly recognized, the number of journalists jailed, assaulted, or criminally charged is relevant and growing. These attacks on media workers usually want to limit or control information regarding critical topics. In this context, the necessity of a system that allows reporters to publish their works without risking their own life is evident. Some systems to share information with newspapers while keeping the source anonymous exist. An example is SecureDrop, developed and maintained by the Freedom of the Press Foundation, and widely adopted by all major international newspapers. What limits them from extensively using this type of system is the lack of credibility in the information exchanged, which represents the main problem for the publisher's reputation. In this thesis, we present FairDrop, a system that allows the exchange of information between two untrusted parties and proposes a tradeoff between the anonymity of the source and the credibility of the information exchanged. We present a fair exchange protocol based on blockchain that allows sharing of a digital good fairly and confidentially. We also define the guidelines for a system based on ring signatures to measure the credibility of the exchanged information. All our design decisions are made taking into account the requirements of a journalist-newspaper communication, and the guidelines for anonymous sources applied by major newspapers around the world. We test the system in a real-world blockchain testnet, considering multi-seller and buyer situations, and introducing economic incentives for sources to use the system.