  Offchain protocols aim at bypassing the scalability and privacy limitations
of classic blockchains by allowing a subset of participants to execute multiple
transactions outside the blockchain. While existing solutions like payment
networks and factories depend on a complex routing protocol, other solutions
simply require participants to build a \emph{childchain}, a secondary
blockchain where their transactions are privately executed. Unfortunately, all
childchain solutions assume either synchrony or a trusted execution
environment.
  In this paper, we present Platypus a childchain that requires neither
synchrony nor a trusted execution environment. Relieving the need for a trusted
execution environment allows Platypus to ensure privacy without trusting a
central authority, like Intel, that manufactures dedicated hardware chipset,
like SGX. Relieving the need for synchrony means that no attacker can steal
coins by leveraging clock drifts or message delays to lure timelocks.
  In order to prove our algorithm correct, we formalize the chilchain problem
as a Byzantine variant of the classic Atomic Commit problem, where closing a
childchain is equivalent to committing the whole set of payments previously
recorded on the childchain ``atomically'' on the main chain. Platypus is
resilience optimal and we explain how to generalize it to crosschain payments.
