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necessitates a change in the “previous block hash” pointer of the child. This in turn causes the child’s hash to change, which requires a change in the pointer of the grand‐ child, which in turn changes the grandchild and so on. This cascade effect ensures that once a block has many generations following it, it cannot be changed without forcing a recalculation of all subsequent blocks. Because such a recalculation would require enormous computation, the existence of a long chain of blocks makes the blockchain’s deep history immutable, a key feature of bitcoin’s security. One way to think about the blockchain is like layers in a geological formation, or glacier core sample. The surface layers may change with the seasons, or even be blown away before they have time to settle. But once you go a few inches deep, geological layers become more and more stable. By the time you look a few hundred feet down, you are looking at a snapshot of the past that has remained undisturbed for millennia or millions of years. In the blockchain, the most recent few blocks may be revised if there is a chain recalculation due to a fork. The top six blocks are like a few inches of topsoil. But once you go deeper into the blockchain, beyond 6 blocks, blocks are less and less likely to change. After 100 blocks back there is so much stability that the “coinbase” transaction, the transaction containing newly-mined bitcoins, can be spent. A few thousand blocks back (a month) and the blockchain is settled history. It will never change. Structure of a Block A block is a container data structure that aggregates transactions for inclusion in the public ledger, the blockchain. The block is made of a header, containing metadata, fol‐ lowed by a long list of transactions that make up the bulk of its size. The block header is 80 bytes, whereas the average transaction is at least 250 bytes and the average block contains more than 500 transactions. A complete block, with all transactions, is there‐ fore 1000 times larger than the block header. Table 7-1. The structure of a block Size 4 bytes 80 bytes 1-9 bytes (VarInt) Variable Field Block Size Block Header Transaction Counter Transactions Description The size of the block, in bytes, following this field Several fields form the block header (see below) How many transactions follow The transactions recorded in this block Block Header The block header consists of three sets of block metadata. First, there is a reference to a previous block hash, which connects this block to the previous block in the blockchain. We will examine this in more detail in Chapter 7. The second set of metadata, namely the difficulty, timestamp and nonce, relate to the mining competition, as detailed in 164 | Chapter 7: The BlockchainPDF Image | Mastering Bitcoin
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