  We address the Threshold Information Disclosure (TID) problem on Ethereum: An
arbitrary number of users commit to the scheduled disclosure of their
individual messages recorded on the Ethereum blockchain if and only if all such
messages are disclosed. Before a disclosure, only the original sender of each
message should know its contents. To accomplish this, we task a small council
with executing a distributed generation and threshold sharing of an asymmetric
key pair. The public key can be used to encrypt messages which only become
readable once the threshold-shared decryption key is reconstructed at a
predefined point in time and recorded on-chain. With blockchains like Ethereum,
it is possible to coordinate such procedures and attach economic stakes to the
actions of participating individuals. In this paper, we present ETHTID, an
Ethereum smart contract application to coordinate Threshold Information
Disclosure. We base our implementation on ETHDKG [1], a smart contract
application for distributed key generation and threshold sharing, and adapt it
to fit our differing use case as well as add functionality to oversee a
scheduled reconstruction of the decryption key. For our main cost saving
optimisation, we show that the security of the underlying cryptographic scheme
is maintained. We evaluate how the execution costs depend on the size of the
council and the threshold and show that the presented protocol is deployable on
Ethereum with a council of more than 200 members with gas savings of 20-40%
compared to ETHDKG.
