What Is a Blockchain? Hashes, Links and the Cost of Rewriting
A blockchain is a shared ledger stored as a chain of blocks, where each block carries a fingerprint, called a hash, of the block before it. Changing an old entry changes its hash and breaks every link after it, so rewriting history means redoing all the later blocks.
The word blockchain gets attached to almost anything in crypto, which makes it sound more complicated than it is. Underneath, it is a list of records with one clever rule about how each entry points back to the one before. This page shows that rule with real hashes and the arithmetic from the original paper.
How it works
Start with the hash. A hash function turns any input into a fixed-length fingerprint. The US standard for the SHA-256 function, NIST’s FIPS 180-4 (its summary page read on 25 Sep 2026), says the digests exist “to detect whether messages have been changed.” SHA-256 always gives 64 hexadecimal characters, and a one-character change to the input gives an unrelated result.
A block is a batch of records plus the hash of the previous block. The first block points back to a fixed starting value. The second stores the first block’s hash, the third stores the second’s, and so on. Because each hash covers the stored link as well as the records, every block depends on everything before it.
The Bitcoin whitepaper, published under the name Satoshi Nakamoto and read on bitcoin.org on 25 Sep 2026, describes the network as timestamping transactions “by hashing them into an ongoing chain of hash-based proof-of-work.” That is the blockchain in one phrase: records, hashed in order, with work attached to each block.
The work is what makes an edit costly. On Bitcoin, a new block only counts if its hash begins with enough zero bits, which takes enormous numbers of guesses to find.
Once the work is done, the whitepaper says, a block cannot be changed without redoing it, and the blocks after it must be redone as well. The mining page covers who does that work and why.
Who keeps the copies
The ledger is not stored in one place. Every full participant keeps a copy and checks each new block against the rules. The whitepaper’s answer to disagreement is that the longest chain, the one with the most work in it, is the one that counts.
Not every blockchain uses proof-of-work. Ethereum dropped mining on 15 Sep 2022, according to ethereum.org, and now has validators with staked ETH propose blocks; some ledgers let a known set of validators agree instead. The hash links are common to all of them; who is allowed to add the next block is where they differ.
That is the question that matters for a trader. A ledger that only a few named parties can extend behaves more like a shared database than like Bitcoin. Neither is wrong, but they are not the same kind of promise.
A worked example
Take three toy blocks, hashed with SHA-256 by this site’s script. Block 1 holds “Ann pays Ben 5 coins” and points back to a starting value of 64 zeros. Its hash begins dacc671b34b9.
Block 2 holds “Ben pays Cal 2 coins”, stores dacc671b34b9 as its link, and hashes to a value beginning 0f7a43d7a27a. Block 3 holds “Cal pays Dee 1 coin”, stores 0f7a43d7a27a, and hashes to a0149b640280. Each hash covers the block’s text and its stored link, joined by a vertical bar.
Now change one character in block 1, so Ann pays Ben 50 coins instead of 5. Block 1’s hash becomes 68e533a5e1a4. Block 2 still says it follows dacc671b34b9, which no longer exists, so the link is broken. To repair it, block 2 must store the new hash, which changes block 2’s own hash to 981ead8a15f0, and block 3’s to bf551b48c7bd.
That is the whole mechanism. Anyone holding the original chain can see at a glance that the edited version does not match. On a proof-of-work chain, the forger also has to redo the work for every rewritten block, while the honest network keeps adding new ones.
The original data
The whitepaper puts a number on the cost of rewriting. Its section 11 gives a short formula for the chance that an attacker who controls a share q of the computing power ever catches up from z blocks behind.
Transcribed and run by this site’s script, it matches every value the paper prints. The paper lists the 30% case only at every fifth block, so the 1-to-4 values for it below come from the formula.
With q at 10%, the chance is 0.2045873 from 1 block behind, 0.0509779 from 2, 0.0131722 from 3, 0.0034552 from 4 and 0.0009137 from 5. Each extra block cuts the odds by roughly four times. With q at 30%, the same five distances give 0.6277491, 0.4457171, 0.3245841, 0.2391269 and 0.1773523, a much slower fall.
This is where the habit of waiting for confirmations comes from. A payment buried under several blocks is far harder to reverse than one in the newest block, but only while no one holds a large share of the power. The protection is a probability, not a wall.
The subject draws a large audience. In the 24,971-video corpus this site studies, 17 titles from 13 channels contain the word blockchain, at a median of 69,306 views, and 11 of the 17 passed 20,000 views. Each video is counted once, after removing duplicates by video id.
Explainers dominate the top of that list. The six most watched are explainers or full courses, led by one at 10,505,717 views. Two of the 17 titles pitch ways to make money from blockchains, which is a different subject from how one works.
What a blockchain does not do
It does not make the records true. A blockchain proves that an entry has not been changed since it was added. If the entry was false when it went in, the chain preserves a false entry very reliably.
It does not make a coin valuable. The ledger and the token that runs on it are separate things. A well-built chain can carry a coin that falls 90%, and the chain keeps working throughout.
And it does not protect a holder from their own mistakes. A payment sent to the wrong address is recorded as faithfully as a correct one. The wallet page covers the keys that control what a holder can send.
When it fails
The first failure is a majority of the power. The whitepaper’s security rests on honest nodes controlling more computing power than any attacker. A small chain with little work behind it can be rewritten by a group that rents enough machines.
The second is a small validator set. Where a handful of parties approve blocks, the hash links still work, but the trust has moved to those parties. If they collude or are compromised, the chain records whatever they approve.
A third is everything around the chain. Exchanges, bridges and apps are built on top of a ledger, and the hash links do nothing to protect them. A secure chain does not make a lending app or an exchange account secure.
A fourth is the newest block. A transaction with no blocks on top of it can still be replaced, and the whitepaper’s own numbers show the first confirmations do most of the protecting.
Related
The Bitcoin page covers the first blockchain and how its coin trades. The Ethereum page explains a chain that also runs code, and what changed when it dropped mining. And mining explains the proof-of-work that makes rewriting a Bitcoin block so costly.
Before buying any coin, find out what its blockchain actually records and who is allowed to add blocks. If the answer is a small group that can be named, the ledger is only as trustworthy as that group, however the marketing describes it.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.