A block chain is a transaction database owned by all participating nodes in the system based on the Bitcoin protocol. Complete copies of the currency block chain contain every transaction ever made in the currency. With this information, one can know how much value each address has at any point in history.
Each block contains a hash of the previous block. This has the effect of creating a block chain from the genesis block to the current block. Each block is guaranteed to come after the previous block in chronological order since the previous block hash will not be known. Each block is also practically impractical to modify after being in the chain for a while because each block thereafter must also be regenerated. It is this nature that makes doubling of the bitcoin becomes very difficult. The block chain is a major innovation of Bitcoin.
The honest generator builds only one block (by referencing it in the block they created) if it is the last block in the valid longest chain. "Length" is calculated as the total combined difficulty of the chain, not the number of blocks, although this distinction is only important in the context of some possible attacks. The chain applies if all blocks and transactions in it are valid, and only if it starts with the genesis block ...
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