Blockchain: A Definition

This post is the first one of a series dedicated to the blockchain. In the coming weeks, I will discuss many aspects of the blockchain. As some of my views may be perceived as pessimistic, a cautionary note is mandatory: I am a skeptical blockchain enthusiast. Blockchain has great potential but also many pitfalls. I hope that these posts will shed some lights on the blockchain.

The first step is to propose a definition for blockchain.

A blockchain is a secure distributed ledger.

Let us examine the four elements of this definition.

  • A blockchain is a ledger. It stores the complete chronological records of transactions. The transactions are combined in a data structure called a block. Each block is cryptographically bound to its predecessor, thus creating a chain of blocks. The blockchain is well suited for transactions and time series. For instance, Bitcoin records the exchange of bitcoins. Other types of information, for instance, graphs, are not necessarily well suited for blockchain. Nevertheless, many
    information can be transcribed in a set of transactions.
  • A blockchain is shared. Many entities use the same ledger. They may not all have the same access rights: Some entities may be allowed to submit transactions to the blockchain whereas other entities may only read these transactions. The use case defines the rules for access control. If the ledger is not to be shared, then probably a traditional database is more suitable than a blockchain.
  • A blockchain is distributed. No central server holds all the blockchain. Every node has the same complete copy of the ledger. And the nodes are connected through a peer-to-peer network. Therefore, the blockchain offers high availability and resilience. There is not one point of failure in the system.
  • A blockchain is secure. Each issuer signs its transactions. Each node validates every transaction according to validation rules that are defined by the blockchain governance. For instance, for a cryptocurrency, the validation of a transaction verifies that Alice owned the coins she transfers to Bob. In the case of a land registry or a supply chain tracking, the validation will be most probably more complicated.
    Once all the transactions of the block validated, the nodes engage in a consensus protocol to decide whether the block is to be appended to the blockchain. Once the consensus reached, all nodes add the new block to their copy of the blockchain. The consensus is the most complex element of the blockchain. Many consensus protocols are available. The most famous one is the Proof of Work (PoW) designed by Nakamoto Sato for Bitcoin. Mining is establishing the consensus for Bitcoin. In a next post, we will study in detail the PoW and other types of consensus. The consensus ensures that every node has the same ledger.
    Transactions are immutable. To alter an already recorded transaction, the attacker must modify the block containing the forged transaction, and also adjust all the subsequent blocks to maintain the cryptographic link. Furthermore, the attacker must trick the consensus protocol to vote the forged fork to be the valid one. Thus, it is reasonable to assume that the transactions are carved in stone. As a side note, immutability may become an issue in case of an error or if the “right to forget” is needed. What is in the blockchain stays in the blockchain.

This first post provides a broad definition of the blockchain. Next posts will explore technical elements of a blockchain.


Nakamoto, Satoshi. “Bitcoin: A Peer-to-Peer Electronic Cash System,” 2008.

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