# Empirical Evaluation of Privacy Efficiency in Blockchain Networks: Review and Open Challenges
## 3 Componential Classification of Blockchain Privacy Protecting Techniques
### Effect of Blockchain's Primary Cryptography on Privacy
#### 3.1.1 Public Key Cryptography/Encryption  
Blockchain uses asymmetric or public key cryptography to maintain reliability of the network. Public key cryptography uses a pair of keys, known as public and private keys, for data encryption and decryption. Public keys are distributed among the network participants for communication while private keys are kept private and protected from unauthorized access [48]. A study [49] exploited asymmetric cryptography for provision of privacy in eHealth-care system. The idea that the authors worked on was providing the medical data to the researchers for statistical analysis while ensuring that the privacy of the patients is  
Figure 2: Blockchain components for privacy protection. not breached. Incorporating asymmetric cryptography in blockchain networks ensures accomplishment of two out of three information security properties, i.e., authentication and confidentiality [50]. Blockchain Solution for User Authentication  
In most platforms, a user is authenticating by entering a password before he could utilize any of the services. This implies that if the main server of the platform is hacked, the hacker will get access to each user's password. Blockchain solves this problem by using asymmetric cryptography instead. For any user to participate in blockchain network, he must create his own pair of public and private keys. Public key is meant to be shared among the blockchain users to enable incoming transactions whereas private key must be kept secret. Any transaction that is initiated by the user must be digitally signed by the user. This signature is generated using the private key of the user [51]. This signature can be verified by other participants using the public key of the signer. A signature generated using the private key of a user cannot be forged by any other user as he does not have access to the private key that generated the signature. However, the ownership of the transaction can easily be verified by anyone knowing the public key of the user. This serves as a means of authenticating that a certain transaction originated from a particular user, which cannot be denied by the sender. This property of inability of denying the validity of something is known as non-repudiation [52] in information security. Blockchain Solution for Data Confidentiality  
Using asymmetric cryptography also ensures data confidentiality or information privacy in blockchain networks. Blockchain networks are public in nature since it is the participants that verify communication between two parties instead of intermediary party [53]. Hence, all the transactions from one end to the other end will be propagated to the entire network for anyone to see. However, public-private key cryptography in blockchain networks ensures that the data is concealed and can only be viewed by the intended receiver. If a transaction is meant to be received and seen by user A, it must be encrypted using public key of user A. This transaction can now only be decrypted by the private key of user A [54], which implies that even if an adversarial user is listening to the network, he will not be able to see the contents of the transaction. Hence the confidentiality of the transaction contents will be intact, and data will travel across the network very securely. Although, this emphasizes the fact that private keys should be kept safe and guarded. Besides maintaining information security properties, encryption has greater benefits to offer in the domain of blockchain privacy for various applications. A number of research articles, nowadays, are working on searching encrypted data stored in blockchain, while preserving the privacy of the data. This technique is known as searchable encryption. This kind of encryption is used to protect privacy and authenticity of data when enterprises store their sensitive records in external data centers [55]. Some studies [56] use single word searches while other advanced studies [57] present effective mechanisms to enable multi-keyword searches on the encrypted data in blockchains.