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Blockchain

The word blockchain, originally called block chain, is derived from the link between blocks. Because the hash of one block is based on the hash of the previous block, it is like a chain linking all the way back to the first, or genesis, block. This process confirms the integrity of the previous blocks in the chain. The following diagram shows how blocks are linked together using the previous block hash:

Every block is linked to its previous block through the block hash. The generated hash of a new block uses the hash of the previous block

Because the hash of each new block is based on the hash of the previous block, manipulation of transactions in one of the existing blocks is easily detectable. For example, when a transaction is manipulated, the hash of the block to which it belongs will be different. As with the previously-calculated nonce, the hash would not start with the expected number of zeros. In the following screenshots, you can see the four blocks that are currently part of our blockchain. All the blocks are finalized, or "solved," and have the correct nonce to create a hash that follows the blockchain rules:

Notice that the nonce is random and not always higher than the nonce of the previous block

When the data in one of these blocks is manipulated, all of the blocks that are linked from that point to the last block in the chain will become invalid. The chain can only become valid when all of the blocks are resolved again.

In the following screenshot, you can see that when the data within the second block is manipulated, it immediately becomes invalid:

Beyond that, you can see that it immediately invalidates all of the blocks that were created after the one that was manipulated, since the hash and the nonce no longer match, as it can be seen in the following screenshot:

To make the blockchain valid again, all of the blocks need to be revalidated—the longer the chain of blocks, the longer the process to solve the correct hashes takes. Some of the blockchains create a new block every few seconds, which makes this nearly impossible to do. You can see this revalidation (or "mining") process in the following screenshot, where the nonce of the original second block had the value 77863, while it changed to 28460 after resolving the block: