# 6G Cognitive Information Theory: A Mailbox Perspective
## 8 Key Technology for Information Cognitive
### Blockchain-Based Safe Transmission of Cognitive Information  
Blockchain adopts a distributed storage and accounting method that consists of multiple network nodes distributed at different geographical addresses. Moreover, data sharing and synchronization in blockchain are decentralized [71]. The nodes do not differentiate the server-client relation, and each node can demand or provide services. Nodes can directly exchange resources with each other without bridging by a server, and users can use and share resources with each other directly. That is, in the distributed network of blockchain, all nodes have equal status, which is similar to the mailbox theory. In the cognitive information theory, each piece of information is packed as a mailbox and stored in a distributed way.  
Blockchain is a dynamic network, where new nodes come all the time while the former nodes exit. The constantly incoming new nodes bring new resources to the system and thus, the entire network is built and developed, and abundance and diversity of resources expand accordingly, while the dispersity, robustness, availability, and overall performance of the point-to-point network increase with the increase in node numbers. In the cognitive information theory, new cognitions can be added continuously to each mailbox to cognize the information further.  
The privacy in blockchain relates to data transaction and identity privacy. The most common way used by transaction users to reinforce privacy is to hide transactions in a group where users exchange their funds, making the attacker unable to judge the user fund relation. Data transaction privacy protection includes all data-related processes in blockchain, i.e., data generation, validation, storage, and usage. Special data structures and consensus mechanisms are designed in blockchain to guarantee that data transaction is reliable, tamper-proof, and distributed uniformly to protect transaction records kept in the blockchain and information and value hidden behind such records.  
The tamper-proof technology in blockchain is essential to build trust in a value internet. Blockchain represents a tamper-proof database. For instance, a database built by blockchain is a CRW database having only basic operations; create, read, and write. Take BitCoin as an example. BitCoin stores transaction records generated since its very birth, i.e., whenever a miner digs up the record of a block, or a user out-transfers or in-transfers a transaction, the related information is kept in each block on the chain. If a single chain wants to modify the record, no record can be generated since such modifications cannot be approved by the other blockchain records. Therefore, false information generated by each mining node cannot be recorded in the blockchain.  
According to the characteristics of cognitive information, constant interaction with a user is required in the process of information cognition to continuously improve the value of information. However, in the process of this interaction, privacy information exists, so a safe, distributed learning-based network is required. In recent years, the blockchain has been widely studied, and it is distributed, tamper-proof, and capable of providing a new approach for the interaction between cognitive information. Moreover, the transformation process from information to value is similar to blockchain generation. Due to the collaboration between mailboxes, information value can be extracted and such extraction can go beyond the difference in culture and nationalities and finally realize an intelligent evolution of information cognition.