Proof of Work Bitcoin's consensus security: miners compete to find a such that . The first to find one broadcasts the block, claiming the subsidy + fees. Block header (80 bytes) Hash function: (SHA256d). Target & difficulty encodes the target in compact form: - where MM is exponent (3 bytes) and NNNNNN is mantissa. - . . 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 s…