Blockchain, Cryptography, and Key Actors

A blockchain is a shared public record made of linked blocks. Blocks link together using hashes, which work like tamper-evident seals. Because each block points to the previous one, changing history breaks the links. A Bitcoin block contains a list of transactions, a reference to the previous block, a fingerprint (hash) of the block's data, and Proof of Work evidence. If someone tries to change an old block, it changes the fingerprint, which breaks the links to later blocks, and fixing those links would require redoing massive amounts of Proof of Work. Bitcoin blocks are added about every 10 minutes on average. Cryptography is the use of mathematical rules to protect information and prove something is true without needing to trust a person. In Bitcoin, cryptography answers two questions: who is allowed to spend these coins, and how can everyone verify that spending is allowed without seeing the secret?