  We proposed a new construction for contract wallets, smart contract
applications that allow users to control their crypto assets. Users can
manipulate their crypto assets by simply sending emails with no need to manage
keys. These emails are verified using zero-knowledge proof (ZKP) along with
their attached digital signatures that the sender domain server (SDS) generates
according to DomainKeys Identified Mail. Unless the SDS forges the emails, the
crypto assets remain secure in the proposed system. Moreover, the existing SDSs
can be used as is by outsourcing additional work to a third party that is not
necessarily trusted. The system supports various functions to manipulate crypto
assets. We produced a tool for variable-regex mapping (VRM) that enables
developers to build a new function without ZKP skills. For example, using the
tool, we built a demo application where users can exchange crypto assets via
Uniswap only with emails. The published version of this paper is available at
https://doi.org/10.1109/ICBC56567.2023.10174932.
