Bitcoin’s hash puzzle is its proof-of-work: miners repeatedly hash a candidate block header with double SHA-256 until the resulting number is at or below the network’s target. It is a threshold search, not a riddle with a single answer that can be worked out in advance.
What the Bitcoin hash puzzle asks miners to do
A miner assembles a candidate block and searches for header data whose hash meets Bitcoin’s current target. Because the result of hashing is difficult to predict, miners cannot know which candidate will qualify; they test candidates repeatedly.
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The rule is numerical: the 256-bit hash must be less than or equal to the target. A lower target makes qualifying hashes rarer, so miners generally need more attempts to find one. The familiar description that a hash must “begin with a number of zero bits” is a useful illustration, but the implemented validity check is the target comparison.
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The proof-of-work calculation uses Bitcoin’s serialized 80-byte block header, hashed twice with SHA-256. The header includes the previous block’s hash, the Merkle root summarizing the block’s transactions, a timestamp, a difficulty-target representation, and a 32-bit nonce. The previous-block hash and Merkle root connect the proof to the chain and the candidate block’s contents.
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How miners search for a qualifying hash
Vary the nonce
Mining hardware tests different values in the header’s 32-bit nonce and hashes the resulting header. A mining ASIC is specialized hardware used to perform this repeated hashing; the work is trying candidates, not solving for a hash by algebra.
Change other header inputs when needed
The nonce has a finite range. When miners have exhausted the values they are searching, mining software can change extra-nonce data in the coinbase transaction. That changes the transaction’s Merkle root, giving the miner new header inputs to hash. Other header fields can also vary as the candidate block is updated.
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How difficulty and block timing relate
Bitcoin adjusts its target every 2,016 blocks. The adjustment uses timestamps to aim for 1,209,600 seconds—two weeks—for that block interval. If blocks have arrived faster or slower than the target pace, the target changes so that qualifying hashes become more or less rare.
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The protocol aims for an average interval of 10 minutes per block, not a fixed timer. Since each attempt is probabilistic, a block may be found soon after the previous one or take longer; the average target does not guarantee the timing of any particular block.
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Why proof of work protects the chain’s history
Every block header commits to the hash of its predecessor. If someone changes an earlier block, its transaction summary and proof-of-work no longer match; that changed hash also breaks the link in the next block. To make the altered history acceptable, the attacker would have to redo the proof-of-work for that block and subsequent blocks, then catch up with the work being added to the chain.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why explanations mention leading zeroes
In the 2008 white paper, Satoshi Nakamoto explained that proof-of-work scans for a value whose hash “begins with a number of zero bits.” That description conveys why a lower threshold is harder to meet. For Bitcoin’s precise rule, however, a node checks whether the numerical double-SHA-256 hash of the header is no greater than the target.
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Sources
- Bitcoin Developer Reference: Block Chain — block-header structure and size.
- Bitcoin Developer Guide: Block Chain — mining, proof-of-work, and difficulty adjustment.
- Bitcoin: A Peer-to-Peer Electronic Cash System — section 4, “Proof-of-Work.”
- Bitcoin.org FAQ: How does mining help secure Bitcoin? — mining and average block timing.
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