  Decentralized cryptocurrency exchanges offer compelling security benefits
over centralized exchanges: users control their funds and avoid the risk of an
exchange hack or malicious operator. However, because user assets are fully
accessible by a secret key, decentralized exchanges pose significant internal
security risks for trading firms and automated trading systems, where a
compromised system can result in total loss of funds. Centralized exchanges
mitigate this risk through API key based security policies that allow
professional users to give individual traders or automated systems specific and
customizable access rights such as trading or withdrawal limits. Such policies,
however, are not compatible with decentralized exchanges, where all exchange
operations require a signature generated by the owner's secret key. This paper
introduces a protocol based upon multiparty computation that allows for the
creation of API keys and security policies that can be applied to any existing
decentralized exchange. Our protocol works with both ECDSA and EdDSA signature
schemes and prioritizes efficient computation and communication. We have
deployed this protocol on Nash exchange, as well as around several
Ethereum-based automated market maker smart contracts, where it secures the
trading accounts and wallets of thousands of users.
