In this article, we examine the contents of the block header in Bitcoin’s blockchain, including the previous block hash, Merkle Root, and other details.
Each block in Bitcoin’s blockchain public ledger has an 80 byte block header, which contains metadata about the block. The position of each block in the blockchain is identifiable by a hash of its block header and its number, known as its “height”. For the purpose of this example, we will look at Block 398765. The hash of our example is:
000000000000000004fb5c6a6285e983e49eec2b74078b5b0495a3cfe1bc7d25
Previous Block Hash
Every block other than the first genesis block contains a hash of the previous block in the block header’s 32-byte previous block hash field. The previous block in the chain is known as the parent, and a block can only have one parent.
In our example, the previous block hash is:
000000000000000006e070aceb085c3ffb1cfb714d86958c62f7cdbe98bf5132
As a block’s header contains a hash of the previous block, changes in a parent block cause subsequent changes in the more recent “child” block next along in the chain, which in turn changes the next block. It becomes more difficult for rewritten transaction history to be accepted by nodes on the network when such an action would require changes to multiple blocks in the chain. This acts as a significant obstacle to manipulation of transaction information in the blockchain, particularly for older blocks.
Merkle Root
The block header also contains a 32-byte hash of a summary of the Merkle Tree of the block’s transactions, known as the Merkle Root. The Merkle Tree contains summaries of all transactions recorded within a block, and can be used to verify records of transactions in a block.
The Merkle Root for block 398765 is:
866cfa8c0a49fc9c2f462a8a32630ebcec2fb0c446163afa135922af2e616acb
Other Details in the Block Header
The block header also contains 4 bytes each of information on the version of software used, a timestamp (measured in seconds from Unix time) the nonce and difficulty target.
The difficulty target is a representation of how difficult it is to mine a block, based on a periodic review of the network’s hashing power. The target is set according to the proof of work mechanism.
The header also contains the value of the nonce, a number which results in a block header hash under that expressed in the difficulty target.
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