Bitcoin block


Bitcoin block is the building block of the Bitcoin Blockchain

Each Bitcoin block within the blockchain is identified by a hash, generated using the SHA256 Cryptographic Hash Function on the header of the Bitcoin block. Each Bitcoin block also references a previous Bitcoin block, known as the parent Bitcoin block, through the "previous block hash" field in the block header. In other words, each Bitcoin block contains the hash of its parent inside its own header. The sequence of hashes linking each block to its parent creates a chain going back all the way to the first block ever created, known as the genesis block.

Although a Bitcoin block has just one parent, it can temporarily have multiple children. Each of the children refers to the same Bitcoin block as its parent and contains the same (parent) hash in the "previous block hash" field. Multiple children arise during a blockchain "fork," a temporary situation that occurs when different Bitcoin block are discovered almost simultaneously by different miners (see “Blockchain Forks”). Eventually, only one child Bitcoin block becomes part of the blockchain and the "fork" is resolved. Even though a block may have more than one child, each block can have only one parent. This is because a Bitcoin block has one single "previous block hash" field referencing its single parent.

The "previous block hash" field is inside the block header and thereby affects the current Bitcoin block’s hash. The child’s own identity changes if the parent’s identity changes. When the parent is modified in any way, the parent’s hash changes. The parent’s changed hash 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 grandchild, 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 Bitcoin block. Because such a recalculation would require enormous computation, the existence of a long chain of blocks makes the blockchain’s deep history immutable, which is 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 might 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 millions of years. In the blockchain, the most recent few blocks might 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 more deeply into the blockchain, beyond six blocks, Bitcoin block 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, for all practical purposes. While the protocol always allows a chain to be undone by a longer chain and while the possibility of any block being reversed always exists, the probability of such an event decreases as time passes until it becomes infinitesimal.

Structure of a Bitcoin 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, followed 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 therefore 1,000 times larger than the block header. Table 7-1 describes the structure of a block.
4 bytesBlock SizeThe size of the block, in bytes, following this field
80 bytesBlock HeaderSeveral fields form the block header
1-9 bytes (VarInt)Transaction CounterHow many transactions follow
VariableTransactionsThe 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. The second set of metadata, namely the difficulty, timestamp, and nonce, relate to the mining competition, as detailed in Chapter 8. The third piece of metadata is the merkle tree root, a data structure used to efficiently summarize all the transactions in the block. Table 7-2 describes the structure of a block header.
4 bytesVersionA version number to track software/protocol upgrades
32 bytesPrevious Block HashA reference to the hash of the previous (parent) block in the chain
32 bytesMerkle RootA hash of the root of the Merkle Tree of this Bitcoin block’s transactions
4 bytesTimestampThe approximate creation time of this block (seconds from Unix Epoch)
4 bytesDifficulty TargetThe proof-of-work algorithm difficulty target for this block
4 bytesNonceA counter used for the proof-of-work algorithm

More Information#

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