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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 certi cate authority for generating digital signatures, and other nancial institutions as intermediate certi cate authorities. One important design feature for CBDC that can be developed under this hierarchical trust infrastructure is an "o ine" capability to create secure pointto-point o ine payments through the use of authorized hardware. An o ine capability for CBDC as digital cash can create a resilient payment system for consumers and businesses to transact in any situation.In this paper, we propose an o ine payment system (OPS) protocol for CBDC that allows a user to make digital payments to another user while both users are temporarily o ine 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. One needs to ensure funds cannot be double-spent during o ine payments as no trusted intermediary is present in the payment loop to protect against replay of payment transactions. Our OPS protocol prevents double-spending by relying on digital signatures generated by trusted execution environments (TEEs) which are already available on most computer devices, including smartphones and tablets. While a TEE brings the primary point of trust to an o ine device, an OPS system requires several cryptographic protocols to enable the secure exchange of funds between multiple TEE-enabled devices, and hence a reliable nancial ecosystem that can securely support CBDC at scale.

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Stablecoins are designed to address the volatility of crypto assets by maintaining a peg to a non-volatile currency such as the US Dollar. Decentralized Stablecoins that maintain their collateral on-chain need a pricing oracle to determine the current market value of the collateral. They also employ a decentralized governance system to make policy changes. In this paper, we analyze the inner-workings of the pricing oracle and the decentralized governance mechanism employed in the MakerDAO stablecoin, one of the largest and fully developed on-chain stablecoins. We study the accuracy of the pricing oracle over time, as well as disagreements between pricing reports received by MakerDAO. We also study the robustness of the decentralized governance system. This work sheds detailed light on the practical operation of a pricing oracle and a decentralized governance mechanism in a large deployed system. We make a number of recommendations for improvements based on our findings.