## 2 6G Key Enabling Technologies
### Blockchain Technology  
Blockchain technology is well known from previous generation networks. In this context, each sensitive message is divided into a finite number of blocks, which are then connected via cryptographic hashes. Hence, message interception can be extremely difficult due to the multitude of blocks, since the identity of the next block as well as the previous block in the chain is encrypted. Previous records of blockchain as well as transactions cannot be deleted or altered. A key novelty of blockchain is that the participating nodes do not have to trust each other, since they all have the capability to store local records of the  
Figure 4: O-RAN architecture for white-box 6G networks and the three cases of intelligence loops through rApps, xApps, and dApps.  
blocks and transactions. Moreover, decentralization is leveraged as well, since there is no need to involve third parties and authorities. All nodes that participate in the transaction are treated as equal [45].  
Within the context of 6G networks, not all lightweight IoT devices will have the capability to execute advanced security protocols or to create and decode blockchains. To this end, novel architectural approaches have been proposed over the last few years in an effort to equally divide network load among the participating entities. In [46], for example, the concept of IoT-edge-cloud (IEC) systems is introduced, where IoT devices with limited processing capacity can offload certain computationally demanding tasks either to the closest MEC servers or to the cloud. Hence, blockchain creation as well as decoding can be offloaded to the MEC server closest to the IoT device. In non-latency-critical applications, cloud processing may be performed as well. In all cases of task offloading, various factors are considered, such as the complexity of the task, the offloading time, the computational burden either in the IoT devices or in the servers, the energy consumption, etc.  
Blockchain can also be combined with AI/ML. As mentioned in Section 2.1, AI/ML deployments over 6G networks should follow a decentralized/distributed approach. Hence, with the help of blockchain, secure data exchange among the participating nodes in ML model training can take place (blockchain in AI) [47]. This strategy can be applied, for example, to FL scenarios to enhance secure data exchange and avoid data replication from model parameters. Blockchain can securely manage and authenticate device identities, which is crucial for the large-scale deployment and interoperability of devices in 6G networks. In 6G, devices or services can be authenticated through the blockchain, with each device's status updates or service requests recorded and verified on the blockchain, thus achieving true end-to-end security [48]. To this end, a challenging research field that has attracted scientific interest over the last few years is the concept of zero trust. In this context, this security model requires strict identity verification for every person and device trying to access resources on a private network, regardless of whether they were granted similar access in a previous state. In [49], for example, a novel blockchain-based zero-trust supply chain security framework integrated with DRL is proposed and evaluated. To this end, the blockchain architecture ensures secure, transparent, and traceable record-keeping and automated execution of supply chain transactions. The integration of the zero-trust security model enables continuous verification and validation of all supply chain entities and transactions, thus mitigating the risk of security breaches and unauthorized access. In the same framework, the usage of smart contracts minimizes human errors that can jeopardize the system's integrity.