# Blockchain-Based Model for Incentivized Cyber Threat Intelligence Sharing
## 3 Proposed Solution
### Sharing of Sensitive Information  
A channel support is required to ensure the sharing of sensitive CTI data according to the TLP [30]. A channel in the Hyperledger Fabric network is a private subnet of communication between two or more specific network members. A channel replicates the structure of the whole blockchain: peers, the shared ledger, chaincode applications, and the ordering service node. Each transaction on the network is executed on a channel. Only authenticated and authorized parties to the specific channel are allowed to transact on that channel. Each party joining a channel has their own identity assigned by a membership service provider that is responsible for the authentication of parties to the specific channel and providing the services designed on that channel. Although any party can belong to and take part in multiple channels, the data from one ledger on one channel cannot pass to another. The separation of ledgers, according to the channels, is defined and implemented by the configuration chaincode and membership service provider. This isolation of parties and ledger data ensures the privacy and confidentiality of the transactions.  
The TLP [30] has four colors: red, amber, green, and clear (previously known as white). However, the color amber comes in two varieties, TLP:AMBER+STRICT and TLP:AMBER. TLP:AMBER+STRICT restricts the disclosure of information to the participant's organization, while TLP:AMBER restricts the disclosure of information to the participant's organization and its clients. The TLP is implemented at its full scale (Figure 2).  
As shown in Figure 2, five channels are organized within the Hyperledger Fabric blockchain. The channels are named according to the sensitivity of the information shared on these channels. To obtain access to a specific channel, clients pay a subscription fee, as explained in the previous subsection, except for access to the Clear channel, which has no subscription fee. The fee depends on the sensitivity of information--for more sensitive information, the fee is higher. Clients with access to the channel containing highly sensitive information also have access to those with less sensitive information. Clients with access to the Clear channel have the same possibilities as the clients of other channels, except they do not participate in the distribution of subscription funds.  
As shown in Figure 2, Org1 only has access to the Clear channel. This means that Org1 did not pay a subscription fee. Org2 has access to the Clear, Green, and Amber channels. This means that Org2 has paid the subscription fee required for access to the Amber channel. Org3 has access to all of the available channels, meaning that they paid the highest subscription fee.  
For off-chain storage, our design uses the Interplanetary File System (IPFS), which is open-source and provides a distributed file storage system. The IPFS supplies a unique hash for stored files, which has addressed content. The hash, which is 46 bytes long, is updated every time the content of the file is updated [31]. Only this hash is stored in the blockchain; this way, a significant reduction in the amount of storage needed for the blockchain is obtained. In addition, the IPFS enhances scalability, as it enables concurrent access to the stored files.