## 2 Background on L2 Solutions  
This section provides a brief introduction and comparison of the available L2 solutions. The aim is to highlight their main differences and justify the choice of concentrating on the rollup-based ones. Table 1 compares the four main kinds of L2 solutions in terms of transaction throughput, latency, security, and trust assumption. _State channels_[8] is an offline solution that performs micro-payments outside the main chain through an established secure channel and stores them inside the main chain altogether through a single transaction at the end of the payments. The transactions with state channels are performed off-chain, which requires mutual trust, agreement, and secure communication among participants. The vulnerability arises if one party goes offline or fails to end the channel by broadcasting the final state to the blockchain within the specified time window. In this case, a malicious actor might take advantage by submitting an outdated state or withholding the final state, which results in an incorrect settlement of funds [9]. Therefore, this solution requires trust and the use of secure channels among participants. Conversely, _sidechains_[10] consist of the use of a secondary chain that is created as an attachment of the primary (main) one to allow the transfer of assets from the main chain to the secondary one at predetermined rates. The existence of the secondary chain is strictly related to the existence of the main chain, but the two chains remain separate and can use different consensus protocols. Thus, no security issues are transferred from one chain to the other. Therefore, the integrity and the security of a sidechain solution solely depend on the characteristics of the secondary chains, as the L1 protocol does not guarantee security. _Plasma_[11] is a particular sidechain for the Ethereum network which uses a smart contract as its core. In particular, transactions are performed on the sidechain to reduce the load of the Ethereum network. In contrast, the block headers of the Plasma sidechain are posted on the main chain periodically for verification. Plasma chains operate in an optimistic manner, where transactions are supposed to be correct until an agent, called a watcher, observes a fraudulent transaction and calls for a dispute resolution. The original data are not published on the main chain. Thus, there is a need for trust in the Plasma operator and the availability of at least one watcher at every moment to ensure that transactions published on L1 are not fraudulent. Rollups are similar to plasma since transaction execution is shifted to L2, but all the data derived from that execution are published back on the main chain [7], which allows the L1 blockchain to host the transactions processed by the L2. [PERSON] [12] introduced the concept of rollup in 2018; since then, rollups have emerged as a robust solution to overcome scalability challenges, allowing transactions to be processed off-chain while retaining the security and decentralization of the underlying L1 blockchain. _Zk-rollups_ and _optimistic rollups_ are the two primary types of rollup-based L2 solutions. In particular, zk-rollup is an off-chain solution that employs zk-proofs to bundle multiple transactions into a single light transaction and post them back to the main chain. Zk-rollup employs zero-knowledge proofs to prove the validity of transactions. Conversely, an optimistic rollup eliminates the requirement for zero-knowledge proof to reduce its computational intensity at the expense of additional trust assumptions. Finally, _validiums_[13] are novel systems similar to rollups, but differ in their data availability mechanism. In contrast to rollups, in validiums transaction data and a state