  Digital payments traditionally rely on online communications with several
intermediaries such as banks, payment networks, and payment processors in order
to authorize and process payment transactions. While these communication
networks are designed to be highly available with continuous uptime, there may
be times when an end-user experiences little or no access to network
connectivity.
  The growing interest in digital forms of payments has led central banks
around the world to explore the possibility of issuing a new type of
central-bank money, known as central bank digital currency (CBDC). To
facilitate the secure issuance and transfer of CBDC, we envision a CBDC design
under a two-tier hierarchical trust infrastructure, which is implemented using
public-key cryptography with the central bank as the root certificate authority
for generating digital signatures, and other financial institutions as
intermediate certificate authorities. One important design feature for CBDC
that can be developed under this hierarchical trust infrastructure is an
offline capability to create secure point-to-point offline payments through the
use of authorized hardware. An offline capability for CBDC as digital cash can
create a resilient payment system for consumers and businesses to transact in
any situation.
  We propose an offline payment system (OPS) protocol for CBDC that allows a
user to make digital payments to another user while both users are temporarily
offline and unable to connect to payment intermediaries (or even the Internet).
OPS can be used to instantly complete a transaction involving any form of
digital currency over a point-to-point channel without communicating with any
payment intermediary, achieving virtually unbounded throughput and real-time
transaction latency.
