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How does blockchain achieve immutability and permanence of its records?Dec. 30, 2017, 5:49 a.m. GMT
In plain English, immutability is used to denote something which can never be modified or deleted. In a Blockchain, it refers to the logs of transactions, which is created by consensus among the chain’s participants. The basic notion is this: once a blockchain transaction has received a sufficient level of validation it can never be replaced or reversed or edited.
Immutability, the concept itself, is somewhat relative. For example, if I send an email to a large list of friends, that data is pretty immutable from my perspective. To change it, I’d have to persuade my friends each to delete the email. Or, I would have to persuade the email provider, say Gmail and the companies running all the mail servers of my friends. From my perspective, and with the control I have, that email is immutable – I can’t unsend or revoke it without collaboration and risk of detection. The same relativity holds for Blockchain too, though suffice it to say transactions on Blockchain are in fact pretty immutable.
Now let us see how Blockchain attains immutability:
If a miner tries to change a transaction from history, he will have to re-mine all the blocks from that block till the current block and this will have to be reflected in every copy of the ledger in the network. Miners will have to rebuild the merkle tree of the block in which the transaction is present and redo all the proof of work for that block.
Now, since the next block stores the hash of this block, the next block will also have to be re-mined. This is because the next block will have to be edited with the new “previous block hash”. This change will result in a different block hash. The new block hash might result in a hash that does not match the set difficulty level. Thus, this block will also have to be re-mined.
The same process will have to be propagated to the latest block in the chain. While this miner is busy remining old blocks, there will be new blocks getting added to the chain. Thus to edit a historical record, the miner will have to remine the old blocks and keep up with the pace of newly generated blocks too. This is extremely difficult, close to impossible. The computing power required to achieve this is enormous and probably only theoretical.
To learn more about immutability and different characteristics of Blockchain like Decentralization, Privacy, Security, and Trust network, pls visit Blockchain and Bitcoin Certification by Blockchain Semantics.