When the cookie meets the blockchain: Privacy risks of web payments via\n cryptocurrencies

Abstract

We show how third-party web trackers can deanonymize users of\ncryptocurrencies. We present two distinct but complementary attacks. On most\nshopping websites, third party trackers receive information about user\npurchases for purposes of advertising and analytics. We show that, if the user\npays using a cryptocurrency, trackers typically possess enough information\nabout the purchase to uniquely identify the transaction on the blockchain, link\nit to the user's cookie, and further to the user's real identity. Our second\nattack shows that if the tracker is able to link two purchases of the same user\nto the blockchain in this manner, it can identify the user's entire cluster of\naddresses and transactions on the blockchain, even if the user employs\nblockchain anonymity techniques such as CoinJoin. The attacks are passive and\nhence can be retroactively applied to past purchases. We discuss several\nmitigations, but none are perfect.\n