There are 2 security pillars that when combined give blockchain it’s infamous highly secure reputation i.e. the linked or chain aspect of a blockchain, and consensus algorithms.
As we’ve learned each block within a blockchain, outside of the genesis (or first block) contains a hash of the previous block, essentially forming a linked-chain of individual blocks, each of which also contains a set of records and a hash of the set of records.
Furthermore, we’ve also learned that if any data inside of a block is altered, even slightly, the hash of the block will change drastically.
This means that if a hacker attempted to alter a record within block 36, the hash of block 36 would change also. This, in turn, would mean that block 37, as well as all subsequent blocks, would become invalid, as the original hash of block 36 stored within block 37, would no longer match the newly tampered with hash of block 36.
As a result of this conflict, the system would reject or nullify the attempted change of record.
However, if someone had the computing power to recalculate and therefore reconnect all of the hashes between blocks 49 and 53, they could, in theory, make the blockchain valid again, and therefore force the system to accept the change of record - albeit this would take a considerable amount of computational resources.
A 51% Attack
In fact, one hacker working alone, let’s call him George, would never be able to achieve this, as George would have to have more computational power than all of the other miners on the network combined, to enable him to achieve this. However, theoretically, at least, it can be done with a large pool of miners working together, taking control of 51% or more of the network. This is known as a 51% attack.
In addition to the linked-chain aspects of a blockchain, blockchains implement one further complimentary security protocol, consensus algorithms, which we’ll cover in our next article.