Cryptography ensures that users can only edit the parts of the blockchain that they “own” by possessing the private keys necessary to write to the file. It also ensures that everyone’s copy of the distributed blockchain is kept in synch.
Imagine a digital medical record: each entry is a block. It has a timestamp, the date and time when the record was created. And by design, that entry cannot be changed retroactively, because we want the record of diagnosis, treatment, etc. to be clear and unmodified. Only the doctor, who has one private key, and the patient, who has the other, can access the information, and then information is only shared when one of those users shares his or her private key with a third party — say, a hospital or specialist. This describes a blockchain for that medical database.
Blockchains are secure databases by design.
The concept was introduced in 2008 by Satoshi Nakamoto, and then implemented for the first time in 2009 as part of the digital bitcoin currency; the blockchain serves as the public ledger for all bitcoin transactions. By using a blockchain system, bitcoin was the first digital currency to solve the double spending problem (unlike physical coins or tokens, electronic files can be duplicated and spent twice) without the use of an authoritative body or central server.
IBM’s Blockchain for Dummies guide explains how the Blockchain works, and why it is important for our future.