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Bitcoin · lesson 3 of 20

The blockchain

The blockchain is bitcoin's ledger. Transactions are gathered into blocks, and each block contains a cryptographic fingerprint of the previous block. The result is a chain.

Changing a past transaction would change the fingerprint of that block and of all the following ones. This is why the older a block is, the harder it is to alter.

Every full participant, called a node, keeps a copy of the entire ledger and checks on its own that the rules are respected. There is no need to trust anyone in particular.

Questions

1. What does each block contain besides transactions?

A cryptographic fingerprint of the previous block

This is the link that turns blocks into a chain.

2. Why is it hard to change an old transaction?

Because it would change the fingerprint of that block and all following ones

Altering the past would require redoing the whole subsequent chain.

3. What does a full node do?

It keeps a copy of the ledger and checks the rules on its own

Each node checks independently, without relying on an authority.

Continue from lesson 4

Bitcoin: an online course in 20 short lessons with quizzes and a final certificate, on Unicorn Academy.

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