A blockchain is a distributed ledger in which transactions are incorporated into cryptographically secure blocks. This allows a trustless, peer-to-peer method for achieving consensus, and was a solution to a long standing unsolved problem in computer science known as the Byzantine Generals Problem. A valid chain is one which at least 51% of the network agrees is the longest. Computations supporting the network are performed by 'miners', programs which maintain a copy of the longest chain and use brute force computation to discover the next block, which will be some cryptographic hash function that satisfies the encrypted header of the previous block, for bitcoin this function is SHA-2, other cryptocurrencies use different functions, for example Litecoin uses Scrypt. Blockchain was first described by Satoshi Nakamoto in 2009 and applied to the bitcoin protocol. Distributed currency grew from the cypherpunk movement in the 1980s and 1990s and is a successor to the earlier hashcach system.
Users are incentivised to host and secure the database through a process known as 'mining', whereby a node is rewarded for a proof of work with a portion of uniquely signed cryptographic data. This may be performed by solving a cryptographic puzzle, showing proof of bandwidth or proof of storage. The proof of bandwidth model is similar to that of the popular BitTorrent protocol, which forces users to upload sufficient data to be eligible to download data.
Ownership of arbitrary 'blocks' of the chain can be cryptographically transferred to different addresses across the network, requiring ultimately the entire network reaches a consensus over who-owns-what within the network.
Bitcoin itself has enjoyed success as a censorship-resistant digital cash-like currency. Due to the potential level of anonymity offered, it has become the currency of choice of cyber criminals involved with carding and darknet markets and has various associations with the dark web. However it has allowed websites and individuals to accept anonymous payments and donations without creating a digital footprint uses of PayPal or credit cards would otherwise leave, protecting the anonymity of both parties involved from traditional payment system compromise and mass surveillance.
Artists have suggested automatic digital registration of creations could provide digitally signed proofs of creation.
Transparent voting systems has been proposed with this technology.
Blockchain technology is potentially one of the most powerful methods for achieving trustless, distributed consensus. Its ability to transparently show cryptographically secure decisions has applications to contracts, law, and government. Original peer-reviewed research articles on technical and social aspects of blockchain technology can be found in the open-access journal Ledger.
Bitcoin suffers from being both overly hyped and overly criticised. Criticisms include:
- Pump and dump investors caused by lack of traditional regulation
- Price volatility, especially notorious bubbles and crashes
- Use as an investment commodity
- A series of embarrassing security failures (notably Mt. Gox)
- Use by cyber criminals
- Technophilia by its advocates
- Criticism of the anarchist or libertarian politics advocated by many of its users
- Dysfunction of its centralised components, notably core development team and increasingly centralised mining pools