What does immutable mean? 📖
Immutable can be referred to as the ability of something to not be changed and altered. It’s derived from the Latin word immutabillis.
The term is used in several areas, like functional and object-oriented programming where an object is considered immutable when its state can not be changed after it was created.
Similarly, it also exists in blockchain technology. In fact, it is one of the most important features of the technology where a blockchain ledger is unable to be manipulated.
Immutability in the blockchain is achieved by cryptographic hashes which cannot be reverse-engineered. The most famous hash function is Secure Hash Algorithm 256 (SHA-256).
In public blockchains, immutability doesn’t just enhance the trust and audit system but also makes it much more effective and simpler by reducing the cost and time required in auditing. Immutability is also able to provide clarity to different stakeholders as it provides a shared source of truth that can be verified.
However there are concerns around immutability as well in blockchains. It’s because if a miner is able to get control of the majority of the blockchain network and alter the immutable data, it can suffer from a 51% attack.
The cost of a 51% attack on a blockchain would be around cost $752,000 for 1 hour. The Ethereum Proof of Stake consensus mechanism is considered to be the solution to this attack.
The founder of the Ethereum Blockchain, when discussing that immutability can not depend on anyone, said:
If a blockchain’s immutability depends on me (or core devs in general) just deciding out of the kindness of our hearts to not do things that are eminently practical, then that’s not a very immutable blockchain
Vitalik Buterin
A blockchain ledger and its function is the best practical example around to understand the term.