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Blockchain technology structure was first described in the Bitcoin white paper as a peer-to-peer distributed time-stamp server. The author, Satoshi Nakamoto (possibly a fictitious name), wanted to create a peer-to-peer electronic cash system that did not need a network of banks to operate. Satoshi described “blocks” and “chains” as a way of organizing and securing records, such that once entries had been made into a shared database, they could be proved mathematically correct and to have remained unchanged.

Satoshi’s description of blocks are groups of transactions that have occurred over a period of time. A transaction, in the case of Bitcoin, represents the transfer of some cryptocurrency, known as bitcoin, from one user to another.

For example, Sally sends you a bitcoin, you receive it, and the transfer of the bitcoin between the two of you is recorded as a “transaction”. Bob, Joe, Mark, and Tammy send each other bitcoins at the same time. All of these transactions are bundled into a block and are recorded in the Bitcoin blockchain.

Blockchains have a special way of recording the transfer of bitcoins from one party to another. The transactions are time-stamped and signed by the sender of the bitcoin. So, in the example above, Sally signs the transfer of bitcoin to you. Sally’s signature for the transfer of bitcoin is not an ink and paper kind. Sally signs electronically or rather cryptographically, with what is called a private key. What this means is that the blockchain software can tell she and no one else has the authority to transfer that bitcoin.

Once Sally’s transaction with you has been recorded in the block with all the other bitcoin transfers, the block is sealed and linked to the other blocks of transactions. Blocks are sealed and linked by hashes. Hashes are created through a cryptographic hash function.

How hash functions are used in blockchains is very clever but simple. All the data that make up a block of transactions are processed. The output of this mathematical process is a string of numbers and letters of a fixed-size, for Bitcoin it is 32 bytes. If the input does not change, the hash function will always result in the same output string. Hash functions are a covenant way in computer science to prove data has not changed.

Once a hash has been generated from a block, the fixed string of numbers and letters is recorded in the next new block of transactions. Recording the hash of the previous block of transactions links one block to another chronologically. Removing a block, or even a single transaction, from within a block would break the record and would instantly be noticeable to everyone, as your fixed string of 32 characters would not match their fixed string. See figure 2.

Figure 1 What is a blockchain?

Figure 2 Hash function in blocks of transactions.

Satoshi’s goal was to prevent Sally from sending the same bitcoin to you and someone else and thus defrauding the network. The “block” and “chain” of blockchain technology is a clever way of structuring and recording transaction data chronologically. It keeps track of “who” owns “what” and “when”.

The Bitcoin white paper incorporated an incentive program for participants to process new transactions and to keep an unaltered record of every past transaction. In Bitcoin, this incentive system is called mining, and the incentive given to the miners is the cryptocurrency bitcoin, see figure 3.

Figure 3 The concept of mining.

Satoshi understood that if a single person or entity had master editing power over the records, then the transaction could be altered, defeating the purpose. If the record was broken, then it may be possible for Sally to send you and Bob the same bitcoin.

Satoshi, possibly inspired by the financial crisis of 2008, wanted to stop fraudulent transactions without needing a third party to aggregate records and provide trust that everyone would operate in good faith. Satoshi proposed that the aggregation of records could be done with software via a peer-to-peer distributed time-stamp server and trust could be established through cryptographically-provable mathematics. This system of record keeping is what you now known as a blockchain.