A Blockchain (or block chain) is a method of storing a list of entries, which cannot be changed easily after they are created. This also applies to the list. This is done by using several concepts from cryptography, including digital signatures and hash functions. In very basic terms, a blockchain combines the following two ideas:
- Given some data, it is easy to calculate a checksum over the data. Special hash functions can be designed to calculate this checksum. These functions can be designed to return a value that always has the same length, which is not dependent on the length of the input. This value is called hash value, or message digest. The functions also have another property: Given the same input, they must return the same output (hash value/message digest).
- In addition to the hash values, a block typically also contains a timestamp, and some payload. Each block uses a digital signature, which allows detecting any change in the data since the signature was made. When new blocks of data are created, the newly created block will also contain the hash value of the previous block.
In most cases, a blockchain is managed by a peer-to-peer network. All peers use a common protocol that specifies how they should communicate with each other, how a new block is created and validated. Once recorded, the data in any given block cannot be changed easily any more. Changing the block means all the blocks after it need to be changed as well. Depending on the protocol, this will require a majority of the peers, or even all the peers, to agree.
Blockchains are secure by design. Blockchain technology is used where keeping a correct record is important. Use cases include medical records, identity management,, food traceability, and voting.
Blockchain was invented by Stuart Haber and Scott Stornetta in 1991 as a means to assure the integrity of digital records. Haber and Stornetta launched the world's first commercial blockchain; Surety in 1995.
In 2008, Satoshi Nakamoto included as references 3 and 4 of Bitcoin: A Peer to Peer Electronic Cash System,  the two papers by Haber and Stornetta   to serve as the public transaction ledger of the cryptocurrency bitcoin. Because of its blockchain, bitcoin became the first digital currency to solve the double-spending problem without the need of a trusted authority or central server. The bitcoin design has inspired other applications.
- Narayanan, Arvind; Bonneau, Joseph; Felten, Edward; Miller, Andrew; Goldfeder, Steven (2016). Bitcoin and cryptocurrency technologies: a comprehensive introduction. Princeton: Princeton University Press. ISBN 978-0-691-17169-2.
- Yuan, Ben; Lin, Wendy; McDonnell, Colin. "Blockchains and electronic health records" (PDF). mcdonnell.mit.edu. Archived (PDF) from the original on 25 December 2016. Cite uses deprecated parameter
- Ekblaw, Ariel; Azaria, Asaf (19 September 2016). "MedRec: Medical Data Management on the Blockchain". PubPub. Archived from the original on 19 January 2017. Cite uses deprecated parameter
- Yurcan, Bryan (8 April 2016). "How Blockchain Fits into the Future of Digital Identity". American Banker. SourceMedia. Archived from the original on 19 January 2017. Cite uses deprecated parameter
- Prisco, Giulio (3 June 2016). "Microsoft Building Open Blockchain-Based Identity System With Blockstack, ConsenSys". Bitcoin Magazine. BTC Media LLC. Archived from the original on 31 January 2017. Cite uses deprecated parameter
- Prisco, Giulio (18 August 2016). "Department of Homeland Security Awards Blockchain Tech Development Grants for Identity Management and Privacy Protection". Bitcoin Magazine. BTC Media LLC. Archived from the original on 19 January 2017. Cite uses deprecated parameter
- Browne, Ryan (22 August 2017). "IBM partners with Nestle, Unilever and other food giants to trace food contamination with blockchain". CNBC. Archived from the original on 26 August 2017. Cite uses deprecated parameter
- "What is Blockchain Technology?". Follow My Vote. Archived from the original on 15 January 2018. Retrieved 18 December 2017. Cite uses deprecated parameter
- Nakamoto, Satoshi (Oct 31, 2008). "Bitcoin: A Peer to Peer Electronic Cash System" (PDF). bitcoin.org.
- Stuart Haber, Scott Stornetta (1991). "How to time stamp a digital document" (PDF). www.anf.es.
- Stuart Haber, Scott Stornetta, Dave Bayer (1993). "Improving the efficiency and reliability of digital time stamping". citeseerx.ist.psu.edu. doi:10.1007/978-1-4613-9323-8_24.CS1 maint: uses authors parameter (link)
- "Blockchains: The great chain of being sure about things". The Economist. 31 October 2015. Archived from the original on 3 July 2016. https://web.archive.org/web/20160703000844/http://www.economist.com/news/briefing/21677228-technology-behind-bitcoin-lets-people-who-do-not-know-or-trust-each-other-build-dependable. Retrieved 18 June 2016. "The technology behind bitcoin lets people who do not know or trust each other build a dependable ledger. This has implications far beyond the crypto currency.".
- Popper, Nathan (21 May 2016). "A Venture Fund With Plenty of Virtual Capital, but No Capitalist". The New York Times. Archived from the original on 22 May 2016. Retrieved 23 May 2016. Cite uses deprecated parameter