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judymcconville@roadrunner.com 2017-05-01 15:07:47 -07:00
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@ -31,7 +31,7 @@ One way to think about the blockchain is like layers in a geological formation,
[[block_header]]
=== Block Header
((("blocks", "headers")))((("blockchain technology", "block headers")))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 <<bitcoin_network_ch08>>. The third piece of metadata is the merkle tree root, a data structure used to efficiently summarize all the transactions in the block. <<block_header_structure_ch09>> describes the structure of a block header.
((("blocks", "headers")))((("blockchain technology", "block headers")))((("headers")))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 <<bitcoin_network_ch08>>. The third piece of metadata is the merkle tree root, a data structure used to efficiently summarize all the transactions in the block. <<block_header_structure_ch09>> describes the structure of a block header.
[[block_header_structure_ch09]]
@ -52,7 +52,7 @@ The nonce, difficulty target, and timestamp are used in the mining process and w
[[block_hash]]
=== Block Identifiers: Block Header Hash and Block Height
The primary identifier of a block is its cryptographic hash, a digital fingerprint, made by hashing the block header twice through the SHA256 algorithm. The resulting 32-byte hash is called the _block hash_ but is more accurately the _block header hash_, pass:[<span role="keep-together">because only the block header is used to compute it. For example,</span>] +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+ is the block hash of the first bitcoin block ever created. The block hash identifies a block uniquely and unambiguously and can be independently derived by any node by simply hashing the block header.
((("blockchain technology", "block identifiers")))((("blocks", "block height")))((("blocks", "block hash")))The primary identifier of a block is its cryptographic hash, a digital fingerprint, made by hashing the block header twice through the SHA256 algorithm. The resulting 32-byte hash is called the _block hash_ but is more accurately the _block header hash_, pass:[<span role="keep-together">because only the block header is used to compute it. For example,</span>] +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+ is the block hash of the first bitcoin block ever created. The block hash identifies a block uniquely and unambiguously and can be independently derived by any node by simply hashing the block header.
Note that the block hash is not actually included inside the block's data structure, neither when the block is transmitted on the network, nor when it is stored on a node's persistence storage as part of the blockchain. Instead, the block's hash is computed by each node as the block is received from the network. The block hash might be stored in a separate database table as part of the block's metadata, to facilitate indexing and faster retrieval of blocks from disk.