What is a blockchain ?

If you think generally, a blockchain is a linked list. So what is a linked list? All of you know that a linked list is a set of blocks which are connected to each other by some kind of a link. In case of a data structure linked list, you have the nodes and nodes are connected by pointers and pointers are basically memory addresses. But, in case of a blockchain the blockchain is a distributed data structure, which is replicated at various nodes or various computers and therefore, the linking is not based on memory addresses. So we have a different notion of linking between nodes and each of these nodes are called blocks. So therefore, you can imagine a blockchain as a series of blocks and each block is connected to its previous block by some kind of a link.

So and it is replicated all over because replication gives you number of different advantages like if one of the replica gets corrupted, the other replicas are there to make sure that the integrity of the information contained in the data structure is maintained. And also replication gives you some kind of guarantee of integrity of data. And it is distributed in the sense that the the different computers involved in the blockchain platform actually are running distributed algorithms in order to maintain the data’s consistency and integrity. And the consistency of the data is maintained by a process called consensus. Consensus means that everybody agrees that the data that goes into the data structure is what they agree to put there.

And in traditional link list the linking is through memory addresses. But in this case, we cannot use memory addresses for linking. So there is a cryptographic technique called hashing. So we actually use something called a hash linking and the integrity of the data is maintained because of cryptography techniques and consensus and replication.

Therefore, blockchain is a data structure that is maintained distributed and that is replicated and main purpose of blockchain is to maintain the integrity of the data. And what is the integrity? Integrity means that the data once it has been agreed by all the relevant parties to put in the data structure, it has not been tampered with . Nobody has come and changed the data and and claim that this is the data that was put in. 

That is made virtually impossible in a blockchain and that is the main property of the blockchain that it maintains the integrity of the data and as we will see that most of the applications where blockchain is used be it cryptocurrency or be it some other application, the integrity of the data is the main thing in blockchain. So what is blockchain used for? So first of all, you know many times we keep logs of events, right. 

So when somebody accesses your computer, the computer keeps a log of the user names and and how they authenticated themselves. Microsoft Windows gives event logs every event that happens like you open a new program on your machine or something crashes or any any kind of event or you know it, you get connected to the internet. All these events are kept in event logs. So logs are very important.

When you do banking transaction bank keeps logs of when you interacted with its banking servers and what you did, what transactions you made, all this are logged. The main problem with keeping logs without any notion of protection of the integrity is that somebody can tamper with the logs and somebody can delete some of the accesses. And therefore, later on when you check the log, you would know some part of its history.

So therefore, the blockchain is designed in such a way, so that it is an immutable ledger of events, which means it is a log that cannot be changed by a malicious party or by mistake. And therefore, all the data that you put in there could be event logs, it could be transactions, it could be various kinds of accesses and and modifications you do to some other thing like a data or you do a property transaction.

All these things logs are has to be kept in an immutable ledger. And that is what blockchain provides. So it is and the tampering of this data is made virtually impossible. So we do not say it is impossible to tamper, as we will see, as we learn more that there if you have a very high computational power, which is almost impossible for individuals together.

But if you can gather that kind of computation power you can actually subvert and this all the protection and change but since it is virtually impossible, we would say that this is a tamper resistant log. And therefore, having these properties, we basically use blockchain as a platform to create and transact cryptocurrency. So cryptocurrency as we will see bitcoin Ethereum, these are cryptocurrencies.

And we will see that one of the first application of blockchain was bitcoin. So the whole idea of creating currency, that whose transactions whose creation whose use everything has to be put in a tamper proof log, and without a trusted third party or without a central agency, which keeps track of this logs and it will be clearer as we go into the course. And also you know many people confuse or conflate the idea of bitcoin and blockchain.


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