Detailed descriptions of some parts of the bitcoin code that are not documented well are contained in this paper. Those trying to understand how the bitcoin code works should read this paper to start their journey. However, I am concerned about the knowledge and “hacking” techniques described in the paper for deanonymization of clients. Because core developers are changing the code regularly and can make this paper obsolete quickly, even elegant stochastic processes to measure different limits and bounds may produce different results as the code base is changing. At the same time, I understand that the authors’ goal is to make this paper obsolete as quickly as possible for the safety and security of bitcoin.

This is a novel approach to deanonymize clients while they are behind firewalls or network address translation (NAT). This technique will be useful in other networks and applications. The attack also needs a limited amount of resources, and will also work if “bitcoin encrypts the connection.” The techniques and probing used in this paper relate to the usage of the GETADDR, ADDR, and INVENTORY messages and that of the time stamps by the bitcoin protocol. The deanonymization process described in the paper has four steps. In step 1, it gets the list of bitcoin servers. In step 2, it composes the nodes it wants to deanonymize. In step 3, it maps clients to their entry nodes using some knowledge about the topology of the network. In step 4, transactions are mapped to entry nodes running in parallel to steps 1 to 3. This paper also describes “how to decrease block mining difficulty by creating an alternative blockchain.”

The attack described in the paper prohibits bitcoin servers from accepting connections via Tor or other similar services (section 3). This is not very practical, and parties looking to stay anonymous may stop using the system until they have access to such a service. Interestingly, once such a service is available, the proposed attack may not be able to deanonymize clients who are not using Tor or other similar services. Nevertheless, this paper is a good contribution toward making the bitcoin network more secure.