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ICBC 2022

In this paper, we propose a moderation framework, RedactChain, that allows small and periodically replaced juries to redact transactions from a Bitcoin-like blockchain, e.g., when users report illicit content being engraved on the blockchain. In contrast to previous schemes, our approach enables the swift and transparent removal of such content while keeping per-redaction overheads low. Furthermore, RedactChain defines rules to handle the redaction of manipulated transactions, whose outputs hold currency and might be spendable, without affecting the transaction graph. Namely, spendable outputs are obfuscated such that their content cannot be retrieved anymore, but the output’s associated coins remain spendable.
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IEEE Trans. Netw. Service Manag.

In this paper, we present the full design of CoinPrune, our block-pruning protocol that is retrofittable to Bitcoin via a velvet fork. This work extends our initial paper presented at IFIP Networking 2020 by enabling CoinPrune to obfuscate most objectionable content stored in the UTXO set and introducing an additional store for application-level data. Furthermore, we extend our security discussion, our discussion of related work, we updated our performance evaluation, and we released a prototype implementation of CoinPrune.

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ACM ASIACCS 2020

We present AnonBoot, an architecture for securely bootstrapping anonymity services seizing a public blockchain as a trust anchor. Through periodic peer advertisements, we create a Sybil-resistant repository of privacy peers that can be directly utilized to establish circuits for onion routing networks, or that can be elected to establish small distributed anonymity services such as mixnets or cryptotumblers. Our proof-of-concept implementation shows how AnonBoot can operate even on simple public blockchains such as Bitcoin.
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IEEE ICBC 2019

One central promise of blockchain systems is to mediate interactions of mutually distrusting parties without the need for a trusted third party. SmartJudge is our lightweight framework to extend existing two-party protocols such as fair exchange or cross-blockchain trades with a conditional dispute resolution procedure. The rationale behind SmartJudge is that honest parties can jointly execute a protocol at minimal costs, and simultaneously are fully protected via a conditionally callable verifier smart contract in case of a dispute.
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BTA 2018

Since the insertion of illicit content into public blockchains can have severe consequences for users, we explore and discuss the design space for preventing this insertion. Our findings show that firewall-like scanning of Bitcoin transactions poses no viable solution to the problem. However, while content insertion cannot entirely be prevented by technical means, self-verifying blockchain identifiers make currently simple manipulations hard. Finally, the introduction of mandatory minimum fees can be used to disincentivize content insertion additionally.
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FGCS 80:448-466

CoinParty is a fully decentralized and thus secure Bitcoin mixing service. Our work uses threshold cryptography to combine the adavantages of centralized and decentralized mixers: high usability and strong security guarantees.
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FC'18

We survey methods enabling the insertion of arbitrary content into Bitcoin’s blockchain, quantitatively analyze their utilization, both directly and via content insertion services, and assess potential consequences of being forced to keep potentially illicit content on the users' hard disks. Our findings show that arguably objectionable content has already been inserted to Bitcoin’s blockchain and that it can lead to legal liability for users in jurisdictions such as Germany.
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Journal Reviews

  • 2021: IEEE TII (Transactions on Industrial Informatics)
  • 2021: IEEE TNSM (Transactions on Network and Service Management)
  • 2020: IEEE TIFS (Transactions on Information Forensics and Security)
  • 2020: Pervasive and Mobile Computing
  • 2019: PETs (Privacy Enhancing Technologies Symposium)
  • 2019: Frontiers in Blockchain
  • 2018: Pervasive and Mobile Computing
  • 2017: Future Generation Computer Systems
  • 2015: Transactions on Wireless Communications

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  • 2018: IEEE TMA (Network Traffic Measurement and Analysis Conference)

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