claude-dev-suite 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_NNNNNNwhere 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. As of 2025:
- Network hashrate: ~700-900 EH/s (10^18 H/s).
- Difficulty: ~100T (100,000,000,000,000).
- Per-block expected time: 10 minutes.
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.
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.