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Anonymous Lottery in the Proof-of-Stake setting

Published in CSF, 2020

When Proof-of-Stake (PoS) underlies a consensus protocol, parties who are eligible to participate in the protocol are selected via a public selection function that depends on the stake they own. Identity and stake of the selected parties must then be disclosed in order to allow verification of their eligibility, and this can raise privacy concerns. In this paper, we present a modular approach for addressing the identity leaks of selection functions, decoupling the problem of implementing an anonymous selection of the participants, from the problem of implementing others task, e.g. consensus. We present an ideal functionality for anonymous selection that can be more easily composed with other protocols. We then show an instantiation of our anonymous selection functionality based on the selection function of Algorand.

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On the Anonymity Guarantees of Anonymous Proof-of-Stake Protocols

Published in S&P, 2021

In proof-of-stake (PoS) blockchains, stakeholders that extend the chain are selected according to the amount of stake they own. In S&P 2019 the “Ouroboros Crypsinous” system of Kerber et al. (and concurrently Ganesh et al. in EUROCRYPT 2019) presented a mechanism that hides the identity of the stakeholder when adding blocks, hence preserving anonymity of stakeholders both during payment and mining in the Ouroboros blockchain. They focus on anonymizing the messages of the blockchain protocol, but suggest that potential identity leaks from the networklayer can be removed as well by employing anonymous broadcast channels. In this work we show that this intuition is flawed. Even ideal anonymous broadcast channels do not suffice to protect the identity of the stakeholder who proposes a block. We make the following contributions. First, we show a formal network-attack against Ouroboros Crypsinous, where the adversary can leverage network delays to distinguish who is the stakeholder that added a block on the blockchain. Second, we abstract the above attack and show that whenever the adversary has control over the network delay – within the synchrony bound – loss of anonymity is inherent for any protocol that provides liveness guarantees. We do so, by first proving that it is impossible to devise a (deterministic) state-machine replication protocol that achieves basic liveness guarantees and better than (1 − 2f) anonymity at the same time (where f is the fraction of corrupted parties). We then connect this result to the PoS setting by presenting the tagging and reverse tagging attack that allows an adversary, across several executions of the PoS protocol, to learn the stake of a target node, by simply delaying messages for the target. We demonstrate that our assumption on the delaying power of the adversary is realistic by describing how our attack could be mounted over the Zcash blockchain network (even when Tor is used). We conclude by suggesting approaches that can mitigate such attacks.

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