# Leveraging Blockchain Technology for Secure 5G Offloading Processes +
## 2 State-of-the-Art
### Blockchain-Based Marketplaces  
Blockchain is a decentralized and distributed digital ledger that records transactions in a secure, transparent, and immutable way [34]. A transaction is a record of an operation within a Blockchain network which can refer to an exchange of cryptocurrencies, the transfer of digital assets or the execution of smart contracts, among others. Transactions are gathered inside blocks which contain a set of transactions and are linked to the previous block by a cryptographic hash, forming a continuous and secure chain. This structure ensures that once a block is recorded, it cannot be altered without modifying all subsequent blocks, providing high resistance to manipulation and fraud. Each block is verified by the nodes in the network through a consensus process. Once validated, it cannot be modified or deleted. Blockchain technology has revolutionized several industries by providing a decentralized, secure, and transparent method of conducting transactions [35; 36; 37]. One of the most promising applications of Blockchain is the creation of secure marketplaces [38; 39; 40]. These Blockchain-based marketplaces take advantage of Blockchain's unique features to offer greater security, transparency, and efficiency compared to traditional marketplaces:  
* Blockchain removes the need for intermediaries. Unlike traditional marketplaces that rely on a central authority, Blockchain-based marketplaces operate on a decentralized network. This way, costs are usually reduced. * Transparency is increased as all transactions on a Blockchain are visible to all participants in the network, increasing trust and fairness among users, as they can verify the authenticity and history of transactions. * Security based on cryptographic techniques used for the link between blocks to provide an immutable ledger. This ensures that all transactions are tamper-proof and verifiable. * Smart Contracts automatically execute transactions when specific conditions are met, reducing the need for manual interaction and minimizing the risk of fraud [41]. All these features make Blockchain technology well matched to marketplaces where P2P trading between marketplace customers can take place. Inside Blockchain, a secure, fair, and transparent catalog of products can be created allowing search requests execution faster and fairer [42]. In addition, Blockchain allows the definition and automatic application of data access policies, increasing security [43]. Finally, the usage of Blockchain in the bidding process makes it more trusted and tamper-proof--decisions are taken in a transparent and traceable way [44]. Regarding marketplaces considered necessary for offloading processes, there are already several related solutions in the literature. In [45], a Blockchain-based marketplace is designed for secure trading of excess or idle traffic between private individuals. In [46], a marketplace for enhancing vehicular communications is presented. Moreover, in [47; 48], marketplaces for secure Federated Learning resources or general telecommunications resources are designed and analyzed, respectively. Although they are not directly related to 5G offloading resources, they can be considered as a reference. Finally, ref. [49] presents a Blockchain-based marketplace especially designed for 5G offloading. Although all these solutions increase the security of previous marketplaces, there are still some security aspects that need to be addressed. Most of them are usually based on public Blockchain networks, which present some disadvantages for the 5G offloading marketplace [50]:  
* Public Blockchain networks usually face scalability issues due to the large number of transactions that need to be processed. This often results in longer processing times and higher transaction costs. * Although transactions on public networks are transparent, the identity of the participants is not always known, which could be an issue within bidding processes.