Daniel Malinowski

Fair Two-Party Computations via Bitcoin Deposits

We show how the Bitcoin currency system (with a small modification) can be used to obtain fairness in any two-party secure computation protocol in the following sense: if one party aborts the protocol after learning the output then the other party gets a financial compensation (in bitcoins). One possible application of such protocols is the fair contract signing: each party is forced to complete the protocol, or to pay to the other one a fine.

On the Malleability of Bitcoin Transactions

We study the problem of malleability of Bitcoin transactions. Our first two contributions can be summarized as follows: (i) we perform practical experiments on Bitcoin that show that it is very easy to maul Bitcoin transactions with high probability, and (ii) we analyze the behavior of the popular Bitcoin wallets in the situation when their transactions are mauled; we conclude that most of them are to some extend not able to handle this situation correctly.

Efficient Zero-Knowledge Contingent Payments in Cryptocurrencies Without Scripts

One of the most promising innovations offered by the cryptographic currencies (like Bitcoin) are the so-called smart contracts, which can be viewed as financial agreements between mutually distrusting participants. Their execution is enforced by the mechanics of the currency, and typically has monetary consequences for the parties. The rules of these contracts are written in the form of so-called “scripts”, which are pieces of code in some “scripting language”. Although smart contracts are believed to have a huge potential, for the moment they are not widely used in practice. In particular, most of Bitcoin miners allow only to post standard transactions (i.e.: those without the non-trivial scripts) on the blockchain. As a result, it is currently very hard to create non-trivial smart contracts in Bitcoin.

Modeling bitcoin contracts by timed automata

Bitcoin is a peer-to-peer cryptographic currency system. Since its introduction in 2008, Bitcoin has gained noticeable popularity, mostly due to its following properties: (1) the transaction fees are very low, and (2) it is not controlled by any central authority, which in particular means that nobody can “print” the money to generate inflation. Moreover, the transaction syntax allows to create the so-called contracts, where a number of mutually-distrusting parties engage in a protocol to jointly perform some financial task, and the fairness of this process is guaranteed by the properties of Bitcoin. Although the Bitcoin contracts have several potential applications in the digital economy, so far they have not been widely used in real life. This is partly due to the fact that they are cumbersome to create and analyze, and hence risky to use.

Disproving the Conjectures from “On the Complexity of Scrypt and Proofs of Space in the Parallel Random Oracle Model”

In the paper “On the Complexity of Scrypt and Proofs of Space in the Parallel Random Oracle Model” (Eurocrypt 2016) Joel Alwen et al. focused on proving a lower bound of the complexity of a general problem that underlies both proofs of space protocols [Dziembowski et al. CRYPTO 2015] as well as data-dependent memory-hard functions like \(\mathsf {scrypt}\) — a key-derivation function that is used e.g. as proofs of work in cryptocurrencies like Litecoin.