Towards K-Anonymous Payments in Ethereum

It is well known that pseudonymity in blockchains such as Bitcoin or Ethereum does not really hide the identity of the author of a payment. A number of attacks have been documented in the literature to deanonymize blockchain transactions. This is also the case of fully anonymous blockchains such as Monero and ZCash for which traffic analysis methods can be used. The goal of this paper is to outline a solution to the above problem in the most general (and difficult) case, namely that of pseudonymous blockchains. We restrict to the case of blockchains supporting smart contracts, with specific reference to Ethereum. Borrowing an approach used in the context of anonymous communication networks, we design a solution supporting κ-anonymous payments against every eavesdropper, including the network adversary. Roughly, the idea is to organize users in rings of cover transactions, through which users indistinguishably exchange actual data or random noise and the initiator is hidden within the ring. Importantly, no off-chain communication is required.