What Is a Cryptocurrency
Edit one old block. Every block after it stops fitting.
A blockchain is a shared register: an ordered list of entries that many participants store and check, instead of one institution keeping it. The blocks are chained — and that is exactly why quiet tampering doesn't work.
The problem
Rewrite a transaction that's already buried.
Block 3 of seven, four blocks below the tip. It's just data on a disk — nothing physically stops you changing it. Press edit and watch how far the damage travels.
Your copy of the chain
INVALIDATED 0 / 7The four-character fingerprints are illustrative stand-ins so the mechanism is readable — they are not real hashes.
Four other copies of the same register
REJECTED 0 / 4Seven blocks, each one pointing back at the one before it. Press Edit block 3.
A block bundles data — with Bitcoin, mostly transactions. The chain exists because every new block is appended at the end and carries a cryptographic reference to the block before it: older blocks behind, newer ones in front, in one chronological, linked order. Change something inside an old block and its fingerprint changes with it, so the reference stored in the next block no longer fits — and nothing built on top of that block fits either. Tampering doesn't stay quiet. It becomes visible.
Visible is still not the same as prevented — and this is the half of the answer that chaining alone doesn't give you. The other participants each hold their own copy and check every incoming entry against the shared consensus rules, so a copy that doesn't line up is simply not accepted. Security comes out of the combination: chaining, checking by many participants, and shared rules, with economic incentives making honest behaviour the profitable option. It does not come from one strong participant policing everyone else — that would put a central place back in the middle, which is the thing the whole design is trying to avoid.
The definition
Three things "shared ledger" leaves out.
Tap each card for what the phrase is actually describing.
Hands on
Six claims about where the security sits.
Pick one. Four of them build real security. Two are myths that sound reasonable.
No single item on this list is the answer on its own — that's the point of the list.
The bridge
Four answers to carry forward.
What it is, why it resists forgery, where that resistance comes from — and when you shouldn't use one at all.
Chain it, distribute it, check it — that combination is what makes a register tamper-resistant without anyone sitting at the centre of it. What has been left deliberately vague so far is the content: what exactly goes into a block, and what it means for a transaction to be confirmed. That's the next lesson.
Check yourself
Five questions.
Answers come straight from this lesson. Submitting completes it.