  Recent work has demonstrated significant anonymity vulnerabilities in
Bitcoin's networking stack. In particular, the current mechanism for
broadcasting Bitcoin transactions allows third-party observers to link
transactions to the IP addresses that originated them. This lays the groundwork
for low-cost, large-scale deanonymization attacks. In this work, we present
Dandelion++, a first-principles defense against large-scale deanonymization
attacks with near-optimal information-theoretic guarantees. Dandelion++ builds
upon a recent proposal called Dandelion that exhibited similar goals. However,
in this paper, we highlight simplifying assumptions made in Dandelion, and show
how they can lead to serious deanonymization attacks when violated. In
contrast, Dandelion++ defends against stronger adversaries that are allowed to
disobey protocol. Dandelion++ is lightweight, scalable, and completely
interoperable with the existing Bitcoin network. We evaluate it through
experiments on Bitcoin's mainnet (i.e., the live Bitcoin network) to
demonstrate its interoperability and low broadcast latency overhead.
