How does the blockchain prevent just theft of data?

Question

I have been reading about blockchain for some time. The basic premise of security of the blockchain, as far as I can tell, is that in a very large network, change in the hash value of one block renders every next block as invalid. Moreover, one also has to outperform other peers. Since the hash value is sensitive to the data inside, it is very difficult to hack.

However, what prevents me from just stealing the data. Let me reemphasize this: I am just copying the data from the blockchain. As far as my understanding goes, the hash value shouldn't change because the data is still the same.

I am not able to understand how does the blockchain prevents this. I am sure there is something I am missing because otherwise there is no way Bitcoin (or any other crypto-currency in that regard) provides 'safe' and 'anonymous' transactions when the full chain is available to everybody and hash value doesn't change on copying the data. I mean this is practically the best place to steal data.

So, I will repeat the question: how does the blockchain prevent just theft of data?

Answer 1

A blockchain guarantees the integrity of the data that's on it. It doesn't aim to keep anything confidential. If there's any mechanism to prevent “stealing” some data, it isn't a blockchain.

Any data that's on the blockchain is public (assuming that the blockchain itself is public). Accessing that data is not “stealing” since the data is meant to be public.

It is, however, possible for a blockchain to contain only a hash of certain information, but not the information itself. In this case, if think you have the information, you can verify that it is correct by validating its hash on the blockchain; but if you don't have the information then you can't find it except by guessing and validating your guess (which is impossible if the secret information includes a sufficiently large random value).

With respect to Bitcoin and other similar cryptocurrencies, they are not anonymous! All transactions are public, and the public information includes the identity of the sender and the recipient. They are however customarily pseudonymous: the identity (“address” in Bitcoin terminology) of a participant is their public key, and there is nothing that ties this public key to anything but the history of the transactions made with that key. On the blockchain, you aren't “John Smith”, you're 14835649238 when you buy and spend this particular coin, and 18365173562 when you buy and send this other coin (and there's no information to say that 14835649238 and 18365173562 are the same person), etc. Analyzing transactions may reveal spending patterns and may hint (but does not prove) that certain transactions were made by the same person (e.g. when a transaction has multiple spenders, it's typically multiple coins that were owned by the same person).