Data Structure

A blockchain shares some similarity to a linked list data structure. Both are linear data sequences. Linked lists contain pointers that link to successive nodes. This means when a new node is added to the linked list, the preexisting node gets a reference to the new node. A blockchain is a sequence of blocks that links to the previous block using a cryptographic hash. This means when a new block is added to the chain, it references the preexisting block.

The data that is stored in each block is hashed. The hash is a one way encryption of the transactions, a timestamp, a nonce (random number), and the previous block's hash. The hash is a fixed size and serves as an ID for the data in the block. When a new block is created it includes the previous block's hash. This creates data immutability because if any data is changed it will invalidate successive block.

In the graph below, the left most block labeled Block n represents the most recently created block. It points to the previous block. The arrow indicates a link based on the hash of the previous block. Each block going to the right represents the previous block in the chain. The original block is shown using a dotted line to indicate there may be many more blocks between Block n-2 and the start of a blockchain.