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34 lines
2.8 KiB
Markdown
# Block Header
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Block headers serve an important intermediary role in the creation and transmission of blocks. They are a fixed-width (80-byte) representation of the entire block. With a block header, you can:
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1. Calculate the hash of the block
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- The double SHA-256 hash of the block header
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2. Confirm the proof of work was executed correctly
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- See [Difficulty Adjustment Algorithm](/protocol/blockchain/proof-of-work/difficulty-adjustment-algorithm) for more details
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3. Determine the relative location of the block in the blockchain
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- Using the previous block hash contained in the header
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Since validation of all the transactions in the block can be expensive, the ability to perform these checks on the block before downloading and validating its transactions helps make denial-of-service attacks on the network significantly more expensive for attackers.
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## Block Header Format
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| Field | Length | Format | Description |
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|--|--|--|--|
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| version | 4 bytes | unsigned integer<sup>[(LE)](/protocol/misc/endian/little)</sup> | The block format version. |
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| previous block hash | 32 bytes | [block hash](/protocol/blockchain/hash)<sup>[(LE)](/protocol/misc/endian/little)</sup> | The hash of the block immediately preceding this block in the blockchain. |
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| merkle root | 32 bytes | [merkle root](/protocol/blockchain/merkle-tree)<sup>[(LE)](/protocol/misc/endian/little)</sup> | The merkle tree root of the transactions in the block. |
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| timestamp | 4 bytes | unix timestamp<sup>[(LE)](/protocol/misc/endian/little)</sup> | The epoch timestamp of the block in seconds. |
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| difficulty target | 4 bytes | [difficulty target](#difficulty-target-encoding)<sup>[(LE)](/protocol/misc/endian/little)</sup> | The difficulty that the block must achieve to be valid. This value is determined by the timestamps of previously mined blocks. |
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| nonce | 4 bytes | bytes<sup>[(LE)](/protocol/misc/endian/little)</sup> | A random value that is repeatedly changes during block mining in order to achieve the block hash requirements. |
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### Difficulty Target Encoding
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Within the block header, the difficulty target uses a special floating-point representation that helps keep the size of the block header small. While the [Difficulty Adjustment Algorithm](/protocol/blockchain/proof-of-work/difficulty-adjustment-algorithm) attempts to calculate the ideal target (i.e. the value the block hash must be "less than"), it undergoes a lossy conversion when put in the block header:
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| Field | Length | Format | Description |
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|--|--|--|--|
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| exponent | 1 byte | byte | Used to calculate the offset for the signficand. The actual exponent is <code>8 * (exponent - 3)</code>. |
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| significand | 3 byte | bytes | The significand, or mantissa, of the value. |
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Ultimately, the difficulty target is equal to: <code>significand * 2<sup>(8 * (exponent - 3))</sup></code> |