Proof of Work & Difficulty

Why this module exists

Proof-of-work is the security backbone of Bitcoin. It turns hashing into a measurable cost and anchors consensus on most work.

1) Block headers

The header is the data being mined:

• Version, PrevHash, MerkleRoot, Timestamp, Bits, Nonce
• Header hash is double SHA-256

2) Target and difficulty

Bits encodes the target in a compact form. A block is valid if:

hash(header) <= target

This is a numeric comparison with the hash interpreted as a big-endian integer.

3) Mining loop

Mining is a brute-force search over the nonce (and extra fields if needed). The first valid nonce wins.

4) Subtleties

• Endianness mistakes invalidate work
• Off-by-one on target comparison breaks consensus
• Canonical compact encoding matters

What you will build

1. Compact target conversion
2. Header hashing
3. Nonce search (mining)

Key takeaways

• PoW = hard to find, easy to verify.
• Target math must be exact.
• Hash comparison is integer comparison, not string.