A blockchain is an ordered ledger built from blocks. Each block contains transactions and the hash of the previous block, linking the history back to the genesis block. In Bitcoin, proof of work lets one participant propose the next block by finding a qualifying hash. section 12.2.2
A simplified transaction path looks like this:
- A transaction is propagated through the network so miners can receive it. section 12.2.2
- A miner includes transactions in a proposed block and searches for a nonce producing a digest with the required leading zeros. section 12.2.2
- Later blocks refer to that block, making a change alter its digest and break the links that follow. section 12.2.2
- If competing branches appear, participants extend the longer chain; transactions on a discarded branch may be reversed or omitted. section 12.2.3
Why confirmations matter
Suppose a wine seller sees Alice’s payment in block 10 and waits six more blocks before shipping. A later, longer branch can replace that chain and omit the payment while spending Alice’s funds elsewhere. The payment was visible, but it was not absolutely final. section 12.2.3
Figure 12.4 On https://andersbrownworth.com/blockchain/blockchain, one can interactively play with a toy blockchain. Each block includes its parent’s digest, and each block contains a random nonce that allows its digest to start with four 0s. Notice that this is true for the top blockchain, but the bottom one contains a block (number 2) that has been modified (its data was initially empty). As the modification changed the block’s digest, it is no longer authenticated by subsequent blocks.
Interact with a toy chain to see how each block stores its parent’s digest and how modifying one block invalidates the links after it. section 12.2.2
The structure brings practical costs. A complete ledger can become too large for convenient use, although Mina demonstrates a different approach by representing history with a fixed-size zero-knowledge proof for lighter clients. section 12.3.3
