# eHealth Integrity Model Based on Permissioned Blockchain +
## 3 Discussion
### Blockchain Security  
The main purpose of the blockchain in the BEIM is to provide the same copy of the ledger to each node and ensure the integrity requirements described in Section 2.2. One of the main differences between the different blockchains is the different types of used consensus algorithms. The blockchain with Proof-of-Work consensus is not used because of a number of reasons related to long-term security aspects. To begin with, the PoW consensus in practice requires a blockchain that supports cryptocurrency [24]. Additionally, besides the risk related to the long-term validity of cryptographic algorithms, several current issues in PoW blockchains, like bitcoin or Ethereum, must be considered.  
The blockchain should be decentralized. However, recent observations about bitcoin show trends toward the centralization of mining pools. Centralization of mining power is a threat [28], as it increases the chances, e.g., for a 51% attack or for a selfish mining attack. Additionally, cryptocurrencies like bitcoin are not coordinated, i.e., there is no authority that might force changes (like increasing the number of transactions in a block) in the bitcoin protocol. In the long term, this might prevent a cryptocurrency from adapting to market requirements, and cause the cryptocurrency to be abandoned by users.  
On top of that, the implicit assumption that PoW blockchains are trust-free is not clear because blockchain users must have a certain amount of trust in blockchain providers and software developers [29]. For example, in 2010, a software bug caused problems with bitcoin when it was updated from version 0.7-0.8 [30]. Other problems, like legal restrictions imposed by governments concerning cryptocurrencies, and enormously high [31] energy consumption used by mining that equalled 0.22% of global power consumption (in March 2018) [32; 33], should also be considered.  
Taking into consideration the above issues, in practice, it is only possible to use blockchains that use the PBFT [25] or Proof-of-Authority consensus. However, according to analysis using the CAP theorem [34] carried out by [PERSON] et al. [25] in 2018, PoA does not provide adequate consistency guarantees in blockchains deployed over the Internet for scenarios where data integrity is important. This is mainly because current PoA algorithms can give up consistency for availability. To the contrary, PBFT keeps a blockchain consistent at the cost of availability. It is worth to mention that the two main PoA algorithms, Aura and Clique, lack appropriate documentation. Therefore, in large-scale eHealth systems deployed over the Internet (e.g., using Virtual Private Networks) for which the BEIM was designed, the PBFT should be chosen as the consensus mechanism.  
To sum up, all three integrity requirements, i.e., Agreement on Transaction Validity, Tamper Evidence, and Finality, are satisfied by the PBFT algorithm; PoW and PoS do not satisfy Finality [21], and PoA does not satisfy Tamper Evidence [25]. Additionally, cryptocurrency blockchains implementing PoW and PoS are not guaranteed by any means to be available in the long term.