Blockchain
The block and its header, the chain of digests, the Merkle tree and the inclusion proof, proof of work and its difficulty, the longest-chain rule, double spending and proof of stake.
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Blockchain: what is it?
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A series of transaction blocks in which each block holds the digest of the previous one, so nothing can be changed without redoing every block after it.
The previous block's digest covers its whole content, including its own previous digest: a change propagates all the way to the last block.
In a chain of five blocks, one transaction in block 2 is modified. Which blocks stop being valid without new proof of work?
- Blocks 2, 3, 4 and 5
- Block 2 alone, the others keep their digest
- Blocks 3, 4 and 5, with block 2 staying valid
- Blocks 1 and 2, which are the closest to the change
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Blocks 2, 3, 4 and 5
The transaction changes the Merkle root, hence block 2's digest, which is no longer the one block 3 cites; that block must be redone, which changes its digest, and so on up to block 5. Block 1 is not touched.
A block header holds the digest of the … block, the … root, a timestamp and the ….
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A block header holds the digest of the previous block, the Merkle root, a timestamp and the nonce.
The Merkle root commits to the transactions without copying them into the header, and it is the header that gets hashed for proof of work.
Merkle tree: what is it?
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A binary tree of digests: each leaf is the digest of a transaction, each node the digest of the concatenation of its two children.
Its root summarises all transactions in one digest. To prove a transaction belongs to it, a path of log2(n) digests is enough.