Cambridge O Level Computer Science · Syllabus 2210 · The Internet and its Uses
Blockchain
What is Blockchain?
A blockchain is a digital ledger of transactions held as a time-stamped, chronologically ordered series of blocks, in which each block stores a value derived from the previous block so that altering an earlier record breaks the links and is detected.
This definition is part of the The Internet and its Uses chapter in Cambridge O Level Computer Science.
Blockchain in context
The internet is the worldwide infrastructure of interconnected networks. The world wide web is the collection of websites and web pages that is accessed using that infrastructure. When you type a URL, the browser separates out the domain name, asks the domain name server (DNS) to turn it into an IP address, sends an HTTP or HTTPS request to the web server at that address, receives HTML and other resources back, and renders the page on your screen. Along the way the browser may store cookies. The same infrastructure carries digital currency transactions, which a blockchain records as time-stamped, hash-linked blocks so that a change to an old record is detectable. And because everything above is reachable from anywhere, Topic 5 finishes with the cyber security threats that exploit it and the layered solutions that reduce them.
A blockchain, in its basic form, is a digital ledger: a time-stamped series of records that is designed so that the records cannot be altered. Transactions are checked, grouped into a block with a timestamp, and each block stores a value derived from the previous block — its hash — so the blocks form a chain in chronological order. Copies of the ledger are held by the participants. If someone changes the data in an old block, that block's own hash changes, the value stored in the next block no longer matches, and the chain becomes inconsistent from that point onwards — so the alteration is detected rather than hidden.
Common mistakes with Blockchain
- M11 — "All digital currency is cryptocurrency." Why it is wrong Cryptocurrency is one kind of digital currency — one that uses cryptography and typically a blockchain, and is often not centrally issued. Other digital currencies are issued and controlled centrally. Nor are they all anonymous: a public ledger shows every transaction, with identities behind identifiers rather than absent. Say instead "A digital currency exists only electronically; cryptocurrencies are one type of digital currency."
- M13 — "A blockchain cannot be hacked and it is impossible to change." Why it is wrong The design makes alteration detectable, not physically impossible. Changing data in a block changes its hash, so the value stored in the next block no longer matches and the chain is inconsistent from there on — and the altered copy disagrees with every other copy of the ledger. Whether a rewritten history could ever be accepted depends on the design of the system and on how it reaches agreement. Nor is every blockchain public. Say instead "Any change alters the hash and breaks the link to the next block, so the alteration is detected and the altered ledger is rejected."