Written by Hankyung Ko and Chanyang Ju, Researcher at Radius. Radius is at the forefront of enhancing rollup composability through the development of ‘Shared Sequencer’. Enhancing composability between rollups allows users to access more opportunities, such as arbitrage, enhancing the blockchain ecosystem's fluidity and potential for innovation. Service providers benefit as well, as they can manage multiple app-specific rollups and support seamless operations, scaling their services effectively. Our research indicates that the 'Shared Sequencer' is foundational to achieving the vision of inter-rollup composability. The 'Shared Sequencer' ensures atomic inclusion of transactions across multiple rollups or between Layer 1 (L1) and Layer 2 (L2) networks, representing a critical infrastructure component in our system. The 'Shared Sequencer' must be a neutral entity, more so than any other within the ecosystem. As the entity responsible for constructing blocks across multiple rollups, it must operate without bias, adhering strictly to predefined rules. The essence of being the most neutral sequencer means that the sequencing of blocks must be free from any hidden agendas, creating a trustable system where blocks are produced based solely on these established rules. This neutrality is paramount to prevent any potential for manipulation or unfair advantage within the network. Many of the security concerns in blockchain are addressed through cryptography or crypto-economic solutions, with the latter being effective when actions can be retrospectively verified for legitimacy. However, the role of the sequencer presents unique challenges; if a sequencer acts maliciously, such as censoring transactions or engaging in front-running attacks, these actions are not easily identifiable after the fact. Therefore, the development of a neutral shared sequencer necessitates a cryptographic approach. We see potential in an 'encrypted mempool,' where the encryption of transaction details hinders the ability of malicious actors to censor, reorder, or manipulate transactions for their benefit, fostering a more secure and trustworthy blockchain environment. 1.1. Delay Encryption within an Encrypted Mempool We have concentrated on designing an encrypted mempool using 'Delay encryption,' a cryptographic algorithm that requires a predetermined amount of computation time to obtain the decryption key, utilizing timelock puzzles. This design ensures that the sequencer can commit to the transaction order before reaching the timelock parameter, thus enforcing an unbiased ordering process. The use of delay encryption introduces a mechanism where transactions are secured and ordered without the possibility of tampering or bias, establishing a fair and transparent sequencing process.