Blocks and the Chain
Suppose Ravi opens his own copy of the co-op chain and changes one old entry from "Ravi pays Meera 3" to "Ravi pays Meera 1". On a blockchain that edit cannot hide. It breaks something visible, and every other node can see the break. The reason is the way blocks are joined together.
The rule
Each block stores the previous block's hash, so one edit breaks every later link.
A hash is a short fingerprint computed from data. The same data always gives the same fingerprint, and changing even one character gives a completely different one. Section 2 shows how hash functions work; for now, "fingerprint" is enough.
A block has two parts:
- Body: the batch of transactions the block adds.
- Header: a small summary. It holds the hash of the previous block, a timestamp, and one hash that summarises every transaction in the body, called the Merkle root (section 2). Proof-of-work chains add a nonce, a number miners adjust (section 3).
A block's own hash is computed from its header. Because the header contains the Merkle root, that hash depends on every transaction in the block as well.
Block 0 (genesis) Block 1 Block 2 Block 3
prev: none prev: hash(Block 0) prev: hash(Block 1) prev: hash(Block 2)
co-op opens Asha pays Ravi 5 Ravi pays Meera 3 Kabir pays Asha 1The first block, the genesis block, has no parent. A block's position is its height, counted from zero, so the genesis block has height 0 and the newest block is the tip.
The shortcut
Picture the chain as dominoes of fingerprints. Change a block and its hash changes. The next block's "prev" field no longer matches. To hide that, you must rebuild the next block too, and the one after it, all the way to the tip.
The edit
Ravi changes "Ravi pays Meera 3" to "Ravi pays Meera 1". Block 2's transactions are now different.
To make the lie consistent, Ravi would have to rebuild every block from the one he changed up to the tip, and then persuade the whole network to adopt his version. Section 3 shows why consensus rules make that extremely expensive. Hash links make tampering evident; consensus makes it impractical.
The trap is to think blocks are "locked" or encrypted. They are not hidden at all: anyone can read them. They are linked, which is a different thing.
Drills
Answer fast — trust the rule.
Which header field joins a block to the chain?
What is the height of the genesis block?
A chain has blocks at heights 0 to 9. Ravi edits Block 2. To make his copy consistent again, which blocks must he rebuild?
Why does a block's hash change when one of its transactions changes?
What do hash links on their own give a blockchain?
Remember
- Each block stores the previous block's hash, so one edit breaks every later link.
- Shortcut: dominoes of fingerprints. Edit one block and you must rebuild everything after it.
- Trap: linked is not locked. Anyone can read the blocks; hashes make tampering evident, and consensus makes it impractical.