## 3 Design of the Proposed System
### Used Components  
This section introduces components and technologies used to create a new solution for TCG and achieve the defined objectives. First of all, blockchain technology was chosen. Being a distributed ledger, blockchain will resolve issues of a single point of failure that come from using a single server and database to store ownership data. The transparency of the system will allow verification of the current owner, but also the full chain of custody, which in the case of collectible assets might be an added value. The design of the system and its example implementation will be kept chain agnostic. To achieve that, Ethereum Virtual Machine will be used as the platform of choice [24]. The reason for this choice is that all major blockchain platforms support it. Ethereum, Binance Smart Chain, Arbitrum, Polygon, Avalanche, and Optimism are only some of the examples of blockchains that provide EVM natively. Other chains, like Solana, achieve this compatibility through third-party services, in this case, NeonEVM [25]. That means the system will function the same way, regardless of which chain is used for deployment. This also ensures that in the future, bridges can be built to allow cross-chain interaction between different deployments of the system. To deal with this issue, layer 2 solutions--optimistic rollups--were created. They increase the throughput of the blockchain and decrease the price of transaction [26]. To achieve this, calculations and the execution of smart contracts are done off-chain. Then, the off-chain computations and transactions are periodically rolled up into a single batch and submitted to the main blockchain (layer 1). Optimistic rollups post the minimal necessary data on-chain to reconstruct the state, such as transaction summaries and state roots, rather than the full transaction data. This significantly reduces the amount of data that needs to be stored and processed on-chain, which in turn lowers the fees and increases the throughput. The term \"optimistic\" comes from the assumption that all off-chain transactions are valid and honest by default. Instead of verifying every transaction on-chain, the network optimistically accepts the batched transactions as correct. However, to maintain security and trustlessness, optimistic rollups implement a fraud-proof mechanism. After the block is proposed, it can be challenged for a predefined amount of time. If it is found to be malicious the challenger earns a reward, and malicious actor loses their stake. This creates an economic incentive to guard the correctness of introduced blocks for both parties. Digital assets are described on the blockchain using two main standards. For the purposes of this system, specifically, Ethereum standards were used, as they are EVM compatible. Fungible tokens are the simpler type, representing interchangeable asset [27]. For a given token, any number of identical ones might exist, each of them holding the same value. The majority of cryptocurrencies are represented using fungible tokens. The second, more advanced type is a non-fungbile token (NFT) [28]. Those tokens carry additional data, making each token distinct from the others. Even though two tokens can be similar, they are never interchangeable. NFTs found wide use in the world of collectibles [29]. The system presented in this paper is focused on the second type, but with slight modifications, as while trading cards are close to regular collectibles, in many cases, two copies of the same card will represent the same exact value and will be interchangeable, slightly breaking the regular usage of NFTs. Aside from transaction fees, storage of data is another key feature of blockchain that might generate costs. While for fungible tokens limited amount of data needs to be stored, as each token is identical, so the data can be stored once and shared across all balances, NFTs require much more space, as each token will need to have its metadata stored.