What is Blockchain?

Blockchain literally means a chain made of blocks.

It is a shared and immutable digital ledger, such as – for example – a record of business transactions where dates, signatures, money transfers are listed in chronological order.

In a blockchain the data is stored in blocks of fixed size. This means that each line in the record is like a block containing data.

Each block in the blockchain is stored by ascending order of date, including hours, minutes, and seconds, thus providing a chronological definition of the data recorded in the blockchain itself.


Blockchain is a shared ledger because all its nodes in the blockchain network share the same data, each node holds a copy of the ledger.

It is a characteristic chosen by design: it means that every network member can check that the ledger has not been tampered with or changed in any way.

How is it possible to get this copy onto each and every node? Thanks to a peer-to-peer network, where each node, and its members have the same properties, without the need for a central server, a central entity that guarantees or authorizes transactions, because each node can communicate with the others.  


Blockchain is an immutable ledger because each time a data block is recorded, it cannot be changed.

Whenever I write data inside the block, this information is authorized by all nodes through a consensus algorithm that validates data writing in blockchain. Once this information has been recorded in the blockchain, it cannot be changed any more, the data is immutable.

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How can I get a shared and immutable ledger?

To create the blockchain, meaning to ensure that the blocks are linked to one another, hash functions are needed. These are mathematical functions that from an initial data block create a synthesis of the block.  

The hash function is designed in such a way that the slightest change in the input results – with an avalanche effect – a substantial change in the output. Consequently, any change in the initial block will result in a completely different output.  

The hash function is a sort of digital fingerprint of the block, which identifies the block univocally.

Moreover, the hash function is not invertible: starting from the output, from a computer point-of-view, it is very difficult – virtually impossible – to identify the block that generated the output.

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How is this blockchain built?

  • The first block is called Genesis: it contains the data that you wish to write and the hash of the preceding block. Since this is the first block, the preceding block hash is zero/null.
  • The Genesis block hash is calculated to get the output.
  • Block no. 2: it contains all its specific data, plus the data from the preceding block.
  • Block no. 2 hash is calculated using its data, plus the hash of the preceding block to get an output.
  • Block no. 3: it contains all its specific data, plus the data from block no. 2, etc.

And so on for each block.

The blockchain created following these steps is very difficult to be tampered with, because to do so it is necessary to modify not only the information written in each block, but also the information in all preceding blocks. In fact, each block calculates its hash based on the hash of the preceding block.  

Another step towards security: there are cryptography techniques to sign, for example, the data inside the block.  

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