Deanonymisation techniques for Tor and Bitcoin

This thesis is devoted to low-resource off-path deanonymisation techniques for two popular systems, Tor and Bitcoin. Tor is a software and an anonymity network which in order to confuse an observer encrypts and re-routes traffic over random pathways through several relays before it reaches the destination. Bitcoin is a distributed payment system in
which payers and payees can hide their identities behind pseudonyms (public keys) of their choice. The estimated number of daily Tor users is 2,000,000 which makes it arguable the most used anonymity network.
Bitcoin is the most popular cryptocurrency with market capitalization about 3.5 billion USD. In the first part of the thesis we study the Tor network. At the beginning we show how to remotely find out which Tor relays are connected. This effectively allows for an attacker to reduce Tor users' anonymity by ruling out impossible paths in the network. Later we analyze the security of Tor Hidden Services. We look at them from
different attack perspectives and provide a systematic picture of what information can be obtained with very inexpensive means. We expose flaws both in the design and implementation of Tor Hidden Services that allow an attacker to measure the popularity of arbitrary hidden services, efficiently collect hidden service descriptors (and thus get a global picture of all hidden services in Tor), take down hidden services and deanonymize hidden services. In the second part we study Bitcoin anonymity. We describe a generic method to deanonymize a significant fraction of Bitcoin users and correlate their pseudonyms with their public IP addresses. We discover that using Bitcoin through Tor not only provides limited level of anonymity but also exposes the user to man-in-the middle attacks in which an attacker controls which Bitcoin blocks and transactions the user is aware of. We show how to fingerprint Bitcoin users by setting an "address cookie" on their computers. This can be used to correlate the
same user across different sessions, even if he uses Tor, hidden-services or multiple proxies.
Finally, we describe a new anonymous decentralized micropayments scheme in which clients do not pay services with electronic cash directly but submit proof of work shares which the services can resubmit to a
crypto-currency mining pool. Services credit users with tickets that can later be used to purchases enhanced services.