http://whirtorrgetftvz4g466sjqkegtyi35bjl4bvotfkfossunf5my4x6ad.onion/bitcoin-white-paper
Proof-of-Work To implement a distributed timestamp server on a peer-to-peer basis, we will need to use a proof- of-work system similar to Adam Back's Hashcash [6], rather than newspaper or Usenet posts. The proof-of-work involves scanning for a value that when hashed, such as with SHA-256, the hash begins with a number of zero bits.