Proof of Work

Bitcoin's consensus security: miners compete to find a nonce such that SHA256d(block_header) ≤ target. The first to find one broadcasts the block, claiming the subsidy + fees.

Block header (80 bytes)

[4 bytes]   version
[32 bytes]  prev_block hash
[32 bytes]  merkle_root (of all txs)
[4 bytes]   timestamp (unix seconds)
[4 bytes]   bits (compact target encoding)
[4 bytes]   nonce

Hash function: SHA256(SHA256(header)) (SHA256d).

Target & difficulty

bits encodes the target in compact form:

bits = 0xMM_NNNNNN where MM is exponent (3 bytes) and NNNNNN is mantissa.
target = mantissa × 2^(8*(MM-3)).

difficulty = max_target / current_target. Higher difficulty = harder.

Difficulty adjusts every 2016 blocks (~2 weeks):

If actual time < target time (~2 weeks), difficulty increases.
If actual time > target time, difficulty decreases.
Adjustment factor capped at 4x per epoch.

Hashrate

Number of header hashes per second. Per-block expected time: 10 minutes (protocol target, held by the retarget).

The figures below are a dated snapshot - they age fast, re-check a live source before quoting them.

As of 15 September 2026 (mempool.space mining API):

Difficulty: ~127.45T (127,450,789,715,843), set at the retarget of 5 September 2026 (block 965,664, +1.31%).
Network hashrate: ~0.9-1.0 ZH/s (10^21 H/s). Monthly means stayed in the 900-940 EH/s band through Q3 2026; the spot reading is a rolling estimate and wandered from 967 to 981 EH/s within 15 September 2026.
All-time highs, both still unbeaten: daily-average hashrate ~1.31 ZH/s on 25 October 2025; difficulty 155.97T on 29 October 2025 (block 921,312).

Live sources: getdifficulty / getnetworkhashps RPC, or https://mempool.space/api/v1/mining/hashrate/1m.

Mining algorithm

def mine_block(header, target):
    while True:
        for nonce in range(2**32):
            header[76:80] = pack('<I', nonce)
            h = sha256(sha256(header))
            if h < target:
                return header
        # Exhausted nonce space; bump extra-nonce in coinbase, redo merkle
        bump_extra_nonce()
        recompute_merkle_root()

extranonce lives in the coinbase tx scriptSig, allowing many 2^32 nonce-space sweeps per block template.

Header search space (nonce rolling)

2^32 is only ~4.3 billion hashes; a 200 TH/s ASIC sweeps the whole nonce field in ~21 microseconds. Miners therefore roll four fields, cheapest first:

nonce - 32 bits, header-only.
nVersion reserved bits - 16 bits (BIP 320) or 24 bits (BIP 323), header-only.
timestamp - one increment per second, and must stay inside the consensus bounds (> median-time-past of the last 11 blocks, <= network-adjusted time + 2h).
extranonce in the coinbase scriptSig - unbounded, but every bump rebuilds the coinbase and the merkle root, so the controller and not the hash core does the work.

Only the first three are headers-only mining: the device mutates the 80-byte header without touching transactions or merkle trees.

Standardization of the nVersion slice:

BIP 310 (Braiins, Informational, Draft since 2018) defines the Stratum mining.configure negotiation and with it the version-rolling extension, through which a pool advertises the version mask a miner may roll.
BIP 320 reserves nVersion bits 13-28 (mask 0xe0001fff, 16 bits) for general use and removes them from BIP8/BIP9 signaling.
BIP 323 (Matt Corallo, Draft, merged into the BIPs repo 12 May 2026) replaces BIP 320 and reserves bits 5-28 (mask 0xe000001f, 24 bits). 16 bits proved too few; some devices had started taking ~7 bits out of nTime instead, which distorts block timestamps. Future soft forks SHOULD NOT signal on bits 5-28.

Core support, as of September 2026:

Core never shipped BIP 320's 16-bit mask.
PR #34779 ignores bits 5-28 for BIP9 signaling and for unknown-soft-fork warnings. Merged 3 June 2026, milestoned v32.0.
Newest release line is 31.1 (8 July 2026); 32.0 has not shipped, so released nodes still warn on blocks that set those bits.

Subsidy / Halving

Block reward = subsidy + fees. Subsidy halves every 210,000 blocks (~4 years):

2009-2012: 50 BTC.
2012-2016: 25 BTC.
2016-2020: 12.5 BTC.
2020-2024: 6.25 BTC.
2024-2028: 3.125 BTC.
2028-2032: 1.5625 BTC.
...

Total supply asymptote: 21 million BTC.

Mining hardware

CPU: 2009-2010, megahash range.
GPU: 2010-2013, gigahash range.
FPGA: 2011-2013, low gigahash range.
ASIC: 2013-present, terahash to petahash range per device.

ASIC manufacturers: Bitmain (Antminer), MicroBT (Whatsminer), Canaan (AvalonMiner), Bitfury, others.

Selfish mining attack

Theoretical attack where a miner with > 25% hashrate withholds blocks to gain disproportionate share. Mitigated by random network delays in practice.

bitcoin-mining-pow