**5. Possible problems with blockchains**

The data are on all computers on the blockchain. A trader who imports goods will not want his customer to know his supplier or how much his actual cost were. The solution is to use a permissioned blockchain rather than a public blockchain. In a public blockchain, all data are visible to any node in the network. However, there is a solution: a permissioned blockchain requires permission to see certain data. According to Wüst and Gervais [28], a permissioned blockchain is useful if there are multiple writers, it is not guaranteed that everybody is only all the time, all writers are known, not all writers are trusted, and public verifiability is required. If some of these conditions are not given, they suggest permissionless public blockchains or even not to use any blockchain.

However, if a permissioned blockchain does make sense due to the number of contributors and the complexity of modern blockchains, there are two issues to consider: 1.) the consensus mechanism used and 2.) data security and integrity.

There are different kinds of consensus mechanisms, Proof-of-Work (POW) and Proof-of-Stake (POS) being the most well-known ones. They are fundamentally different but come to the same question: Who can vote whether a block is added? And also, how many votes are needed as a minimum, and what percentage is the threshold to accept the new block?

Data security and integrity have to be addressed, as we earlier said that the whole blockchain—with all data—is on every node computer. It is encrypted, and in the permissioned blockchain, only those data are accessible to a user for which they have received permission. The security is only guaranteed as the login credentials are kept secure.

In the supply chain, the wholesaler would receive permission to view the country of origin and whether the manufacturer is confirmed to not use child labor, for example. This is the information they need. Customs will need information about the exporter and the importer and any licenses, but not about the exporter's supplier. With the new German Supply Chain Act [29], German authorities will want to know whether the supply chain conforms to the regulations with regard to ethical manufacturing. Therefore, it is crucial that only those entities on the blockchain that need certain data have access to it. Another issue is that blockchains are often outsourced to cloud computing services for various reasons. Limited trust in cloud computing can reduce the adoption of blockchain technology [30].

Different blockchains use different technologies and are often not compatible with each other. However, this problem is being solved by creating interfaces between the blockchains. This means that if a manufacturer opts for blockchain provider A and their supplier uses blockchain provider B, and both use different technologies, the data can ideally still be moved from one technology to the other without loss of data or functionality. However, this does not apply to all types of blockchains, and even if there is a "fork"—a splitting of a blockchain for example to update the technology—is applied, access to both sides of the fork may not be possible [31].

Using blockchain technology can be expensive [30]. Depending on the blockchain type, there are different charges that apply for every transaction.
