Themis is a decentralized system which provides third-party escrow services and dispute resolution services to bring fairness to exchanges in which one or both parties are using digital currencies as subject matter. Technically, Themis should satisfy following requirements:

Fairness : After exchange, either both seller and buyer can obtain all the goods (digital currencies, digital assets, physical goods) they want, or they can obtain nothing (All-or-nothing);

Security : None of the parities can transfer the digital funds during the period of exchange;
Passivity : If no dispute arises, there is no need for the third-party to take part;

Correctness : Ensure transactions and settlement of disputes are executed by the protocol agreed in advance;

Dependability : Mitigate single point failure and DoS attacks;

Privacy : In case of no disputes, the third-party can’t be aware of if the transaction completes, and only related parties can be aware of if disputes arise.

We developed Themis 1, a fair exchange system for digital currencies. Based on blockchain technology, Themis provides a decentralized digital currency escrow service to provide fair exchange services among digital currencies, digital assets, and physical goods. Themis leverages a novel group escrow protocol, along with threshold encryption, anonymous reputation mechanisms, non-interactive zero-knowledge proof, high performance digital signature algorithms, in order to provide atomic exchange with guaranteed fairness. Also, Themis is able to mitigate DoS attacks, and provide privacy for all peers.