A blockchain is a special type of data structure (ie a database), in which the data is set out and built up in successive blocks. Each of the blocks of data includes a small piece of data that verifies the content of the previous block. As a result, if an attempt is made to modify an earlier block in the chain, all of the later blocks cease to match up. Imagine that the database looks like a tower of Lego pieces which follow a particular sequence red-green-green-blue-yellow-red. If a change is made to the second block, the rest of the sequence upwards from the second block will change and become, say, red-black-brown-orange-purple-pink. The system that maintains the blockchain will be able to detect and reject the attempted modification, and this is what makes the blockchain tamper-proof.
Public key cryptography
The use of public key cryptography ensures that each participant in the system is uniquely identified and can validate any change to the blockchain using a cryptographically secure private key. While public key cryptography is not unique to blockchain, it is one of the essential underlying technologies which ensure that blockchains are secure and that only authorised participants can make changes to a blockchain. It can also be used to encrypt data stored on the blockchain so that the data can only be accessed by those with the key to decrypt it.
Distributed ledgers
Traditional ledger systems either require each participant to maintain its own decentralised ledger or they require the participants to trust a centralised ledger. The problem with decentralised ledgers is that they can be costly to maintain and to keep secure, and it may not become immediately apparent when they diverge – until a transaction down the line reveals that each ledger in fact records a different version of the facts. A centralised ledger, on the other hand, requires all the parties to trust the holder of the authoritative central ledger and creates a critical vulnerability – what happens if the central ledger is hacked or a disgruntled employee deletes it? The key to a distributed ledger is that each authorised participant (a node) maintains a complete version of the ledger and each transaction, ie each proposal to modify the ledger, is sent out to all of the nodes and is only approved if a sufficient number of nodes agree that it is a valid transaction.
Consensus mechanisms
This validation of proposed changes to the blockchain is performed by the nodes in accordance with certain pre-set rules whereby the nodes will reach a consensus as to whether the new data entry will be permitted (eg, the nodes might conduct a check to confirm that according to the records on the blockchain, the participant purporting to conduct a particular transaction owns the relevant asset which is the subject of that transaction). This is the consensus mechanism and only if there is agreement between the nodes as to the validity of the transaction represented by that data entry will that data entry be permitted to be appended to the blockchain (ie another Lego block will be added to the tower). Once that transaction has been approved, however, the updated version of the blockchain with the newly‑appended entry will rapidly spread throughout the system, so that that all of the nodes end up with an identical version of the ledger.
This consensus mechanism means that there is a rigorous means, applied uniformly by all participants, that ensures that only valid data can be appended to the blockchain. It is the consensus mechanism that enables the gate‑keeping function to be entrusted to a network of participants, rather than a single central authority.
Why is blockchain technology relevant?
In the early days blockchain technologies first captured the imagination of Bitcoin and cryptocurrency enthusiasts – often of a techno-utopian and libertarian persuasion – the versatility of the technology means that it is now being embraced, at least experimentally, by more established sectors of the economy.
Compared with traditional database technologies and centralised systems, blockchain implementations can be relatively cheap and require considerably less IT investment to maintain. However, as the technology is still relatively immature, for the time being these savings on the ongoing operational costs may be offset by significant upfront development costs.
Because of its application to ledger technologies, blockchain has generated particular interest from the financial sector. Initiatives have included bank‑specific cryptocurrencies modelled on Bitcoin and self-executing smart contracts that can automatically implement certain types of simple financial contracts. One of the most high‑profile initiatives in this space has been R3, a consortium of over 70 financial institutions launched in September 2015 and dedicated to developing blockchain technologies for use in the financial sector.
Because of its application in creating resilient, tamper-proof distributed records, a number of initiatives have been proposed in the public sector for government-maintained registries to be implemented as blockchains, eg real estate title registries in Honduras and Sweden and the aid/public interest sector, eg blockchain-based tracing of donations from donor to recipient to ensure the money goes where it is needed.
Enthusiasm for blockchain has not abated among technologists, who have continued to push the boundaries of blockchain technology. The same self‑executing smart contracts technology (mentioned above) that financial institutions have been cautiously exploring, has been taken much further by blockchain pioneers. May 2016 saw the launch of the DAO (Decentralized Autonomous Organization) which was effectively an autonomous crowd‑sourced venture capital fund implemented by way of smart contracts, without recourse to traditional legal structures. An exploit that enabled one of the participants to extract a large part of the funds resulted in the DAO’s prompt demise, but the ambitious project demonstrated the potential of blockchain technologies.