Bitcoin blockchain
The Ethereum blockchain, which powers the Ethereum cryptocurrency, is currently the most open to experimentation. But this openness does not always play into the hands. New blockchain schemes are created every day, including by the largest tech corporations. Microsoft offers its customers tools to experiment with cryptocurrencies in its Azure cloud.
Changing any information in a block will change its hash sum. To comply with the rules for building a chain, changes in the hash sum will need to be written to the next block, which will cause changes to its own hash sum. In this case, the previous blocks are not affected.
I'm far from the hype around bitcoin fee and exchanges, for me blockchain is a technology. New, strange, incomprehensible, but it seems to be changing the world, contrary to those stories of yours. Apparently, she's been with us for a long time. I wrote this post as if I was talking about blockchain to my parents. I can even send it to my friends in the humanities and I'm sure they'll understand.
Blockchain was described in Satoshi Nakamoto's article "Bitcoin: A Peer-to-Peer Electronic Cash System". There, in just eight pages, the author described the fundamentals of Bitcoin cryptocurrency, which was based on the Blockchain algorithm. Blockchain appeared along with bitcoin, but it can be used independently of it and even modified. Anyone can create their own blockchain, even on their laptop.
Trust Centralization
Our friends liked the idea of keeping an unforgeable list of "who borrowed whom." They also do not want to remember who paid for whom at the bar and how much more they owe - everything is written on the wall. You discussed the idea and decided that now you need a single list for everyone (like explained here Bitcoin Fortress website).
But who is to be entrusted with such important bookkeeping? When it comes to money, trust comes first. We will not trust the custody of our money to an unknown person. Our ancestors came up with
banks for this, which over time they began to trust, because they are backed by a license, laws and insurance from the Central Bank.
In a circle of friends, everyone trusts each other and you can simply choose the most responsible person for this role. But what if the question is about strangers? An entire city, country or the whole world, as is the case with bitcoin? Nobody can be trusted there.
Decentralization: no one trusts anyone
So they came up with an alternative approach: keep a copy of the list for everyone. Therefore, the attacker will not only have to rewrite a list, but also sneak into each house and rewrite the lists there. And then it turns out that someone kept several lists at home, which no one knew about. This is called decentralization - see for a detailed description.
The downside to this approach is that in order to make new entries, you will have to call all the other participants and inform each of them of the latest changes. But if these participants are soulless machines, that ceases to be at least some kind of problem.
In such a system, there is no single point of trust, and therefore the possibility of corruption and fraud. All participants in the system act according to one rule: nobody trusts anyone. Everyone just believes the information he has. This is the main law of any decentralized network.
Transactions
The blockchain uses the mechanism of public and private keys for this, computer scientists have long used them for authorization in the same SSH. Briefly about how this complex but beautiful math works: you generate a pair of long primes on your computer - a public key and a private key. The private key is considered super secret because it can decrypt what is encrypted with the public key. But the reverse also works. If you share the public key with all your friends, they will be able to encrypt any message with it so that only you can read it, since you have the private key. But in addition to this, the public key has a useful effect - with its help, you can verify that the data was encrypted with your private key, without decrypting the data itself.