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T. Paul and S. Rakshit

2.2

Structure of Blockchain

A blockchain is a timestamped collection of immutable digital records maintained

by a cluster of computers not owned by any single individual. Each of these blocks

of data is secured and bound to each other using cryptographic principles. Therefore,

a blockchain is a chain of blocks that contains information (Fig. 2).

In the above example, the Blockchain has three blocks; each block has a hash and

the previous block hash. Thus, block 3 points to block 2, while block 2 points to

block 1. The first block is a little unique in that it cannot refer to the preceding block

due to the fact that it is the first. Therefore, the first block of a blockchain is called

the genesis block [24].

Now let us say that somebody tampers with the second block. That causes the

hash of the block to change as well (for example, 7AW2 to H65Y). As a result, block

3 and all subsequent blocks become invalid, as they no longer include a valid hash

of the previous block. Thus, altering a single block renders all subsequent blocks

invalid [15].

However, using hashes alone is insufficient to avoid manipulation; because

computer these days are very fast and can calculate hundreds of thousands of hashes

per second. Thus, there is a possibility that someone could successfully tamper

with a block and recalculate all the hashes of subsequent blocks to restore the

validity of your Blockchain. So to mitigate this, Blockchain has a technique called

proof-of-work [26].

Fig. 2 Example of a blockchain