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In the previous section, the basic introductory part of blockchain is discussed. In this part the requirement of privacy including different tools and techniques are going to be discussed.

As shown in Figure 1.4 in this section, a blockchain is a highly secure mechanism or technology which allows the user to make secure and safe transactions [1, 3]. It is a statement said by many engineers, scientist and researchers but how......?

To know about this said statement in detail, one should back track a blockchain and need to collect the required information about how this technique is too strong and safe for transactions. Here we also need to know if this technology is too tremendous then why was it being garbaged in 1991 and then again re-launched in 2004 [1].

The working paradigm of a blockchain:

Let’s have a closer and a deeper look at Figure 1.4 that illustrates what a block is in a blockchain.

Each block in a blockchain consists of:

 certain data

 the hash of the block

 the hash from the previous block (HOPB).

The data stored in each block completely depends on the type of blockchain. For instance, in the Bitcoin blockchain structure, the block maintains data about the receiver, sender, and the amount of coins.

A hash is a unique set of alphanumeric codes which is auto generated by the algorithms and the functions of the blockchain (in general terms it can be treated as the fingerprint, long record consisting of some digits and letters). Each block hash is generated with the help of a cryptographic hash algorithm (SHA 256) [1]. As a result, this helps to identify each and every block in a blockchain structure easily. The moment a block (of blockchain) is created, it automatically gets connected or attaches a hash, while any changes made in a block either in the ledger field or anywhere it straightened affects the change of a hash too which makes the technology precisely highly safe and secure. In simple terms, the hashes help to detect any changes in blocks of a blockchain.

Figure 1.4 The figure states the internal working paradigm of a blockchain.

The final element within the block is the hash which is from a previous block (refer to Figure 1.4). This creates a chain of blocks and is the main element behind blockchain architecture’s security [4]. As an example, assume a bock range from 1 till 46; block 45 points to block 46. The very first block in a chain is a bit special—all confirmed and validated blocks are derived from the genesis/creator block.

Any corrupt attempt provokes/results the blocks to change [4, 5]. All the assumed block’s hashes get changed resulting to the mismatch of hashes which then carry incorrect or invalid information and render the whole blockchain system invalid.

On the other hand, in theory, it could be possible enough to adjust or alter all the blocks with the help of strong computer processors (processors here means highly configured computers). However, there is a solution that eliminates this possibility called proof-of-work [1]. This allows a user to slow down the process of creation of new blocks. In the architecture of Bitcoin blockchain, it majorly takes around 10 min to determine or collects the necessary information of proof-of-work and adds a new block to the chain, but as it was discussed the block can only be added by the person which have the best computational system, here logical ability works rare rather than the system power, hence this work is done by miners—special nodes within the Bitcoin blockchain structure. The miners who win in the race get to keep the transaction fees from the block that they verified as a reward [1, 4].

Each new user (node) joining the peer-to-peer network (a network which have internet connection to share files and folders) of blockchain receives a full copy of the system. Once a new block is created, the detail of it is sent to each node within the blockchain system [4]. Then, each node verifies the information of the block and checks whether the information/data stated there in the block is correct. If everything is alright, the block is added to the local blockchain in each node.

All the nodes inside blockchain architecture create a consensus protocol. A consensus system is a set of network rules, and if everyone abides by them, they become self-enforced inside the blockchain.

To manage and protect the term privacy under blockchain technology, one must satisfy the subsequent requirements:

1 The links between the transactions must not be visible or discoverable.

2 The data of the transactions is merely and only known to their members.

The private or open blockchain must have an entrance control strategy or approval plan to fulfil the security prerequisites of blockchain, which fulfils the total straightforwardness of the blockchain information. Be that as it may, if the case is of an open setting, everybody can have an access to the blockchain with no limitations, the protection issues must be handled on the following factors:

1 Identity Privacy: Which alludes the intractability and unmanageability in the middle of the transaction contents and thus the original identities of their partakers stay sheltered, safe and secure about.

2 Transaction Privacy: In the following the transactional contents (e.g., amount or transacting patterns) can only be accessed and captured by the specified user(s), and kept secret, unknown and safe to the common or general public blockchain network [1, 4].

As it is referenced over, a transaction or a block of a blockchain contains the identity of the previous transaction, the addresses of its members or participants, values (trade), timestamp and unique mark of its sender. Due to its natural behavior or characteristic, it is possible to trace back and follow the flow of transactions to extract and collect the users’ physical identities or other common and additional private information through the tools techniques and also of data mining. In this section, it is referred towards the Bitcoin system as a typical instance to analyze the privacy threats for the blockchain network.