What is a blockchain, and what makes rewriting history hard?
A block is a batch of transactions carrying the hash of the one before it. That hash is what makes the chain: changing one block invalidates every block after it. We set out what the structure does and what it does not guarantee.
In short
The definition fits in one sentence. Ethereum's documentation says:
"Blocks are batches of transactions with a hash of the previous block in the chain."
Two parts: transactions are gathered together, and each batch carries a hash of the one before it.
Why batches rather than one at a time
If transactions were processed individually it would be far harder for every participant to see the same order. The documentation states the reason plainly:
"To ensure that all participants on the Ethereum network maintain a synchronized state and agree on the precise history of transactions, we batch transactions into blocks."
Batching is not an efficiency trick but a means of agreement. Dozens or hundreds of transactions are committed at once, and everyone arrives at the same point together.
The hash is what makes the chain
What links blocks is not a list or a number:
"This links blocks together (in a chain) because hashes are cryptographically derived from the block data."
The hash is derived from the block's data. Change the data and the hash changes. Because the next block carries that hash inside it, a single change breaks everything after it:
"one change in any block in history would invalidate all the following blocks as all subsequent hashes would change and everyone running the blockchain would notice"
The point worth holding onto: the structure does not make change impossible, it makes change visible. The assurance comes from everyone keeping the same ledger, not from secrecy.
Order is part of the structure
The documentation also defines ordering: blocks are strictly ordered and every new block contains a reference to its parent, and transactions within a block are strictly ordered as well.
Strict ordering matters because it gives the question "which happened first" a single answer.
Risks
Biggest uncertainty Differences in consensus and block production between networks were not examined; this piece rests on the Ethereum documentation.
- Immutability is not absolute, it is expensive. Rewriting history is not impossible; it requires convincing a majority of those running the network, which is what makes it costly.
- Being on chain is not being true. A blockchain records that a transaction happened; it does not record that the claim behind it was correct.
- Confirmed and finalised differ. A transaction may be in a block while whether it counts as irreversible depends on the network's rules (proof of stake).
- This piece rests on the Ethereum documentation. Other networks produce blocks and reach consensus differently.
What was not examined is listed in the data status above.
Sources
This article rests on the sources below. Whether each has a permanent copy is stated next to it.
- #1
Primary source for the block definition, the reason for batching and the chain being built on hashes: the ethereum.org documentation.
ethereum.org· no permanent copy· no screenshot
Evidence chain: 1 sources
Tweets get deleted, sites go down. Below is whether a durable copy exists for each source.
Sources (1)
- #1
Primary source for the block definition, the reason for batching and the chain being built on hashes: the ethereum.org documentation.
“one change in any block in history would invalidate all the following blocks as all subsequent hashes would change and everyone running the blockchain would notice”
ethereum.org/developers/docs/blocks/- no durable copy: if the source disappears, this claim loses its basis
- no screenshot captured
Correction history
Articles are never deleted, only corrected. Every change appears here.
- Version 1 ·
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