FIGURE 1-1: Each block’s hash is stored in the next block of data. The hashes chain the blocks together in an orderly fashion.

      Blockchain is “immutable”

      You may have heard that the blockchain is virtually immutable, which simply means that it can’t easily be changed. If the Bitcoin blockchain says you own x Bitcoin, then you do own x Bitcoin, and there can be no disagreement … and nobody can go into the blockchain and hack it or somehow change or mutate it.

      Imagine what would happen if someone went into a block (we’ll call it Block A) and changed a little bit of data — for example, they go in and show that instead of sending someone 1 Bitcoin you sent 9 Bitcoin.

      Well, the hash in Block A would no longer match its data. Remember, a hash is a fingerprint that identifies the data, so if you change the data, the hash no longer matches.

Okay, so the hacker could rehash Block A’s data and then save the “corrected” hash. But wait, now the next block (Block B) would not match because Block B is carrying Block A’s hash. So now say the hacker changes the Block A hash stored in Block B.

      But now Block B’s hash doesn’t match Block B’s data, because that hash was created from a combination of Block B’s transaction data and Block A’s hash!

      So, Block B would have to be re-hashed, and the hash updated. But wait! That means Block B’s hash stored in Block C now doesn’t match!

      See where we’re going? This would ripple through the entire blockchain. The entire blockchain is now broken, by just modifying one single character in a block lower down. In order to fix the problem, the entire blockchain has to be recalculated. From the hacked block onwards, it must be “re-mined.” What may look like a simple hack and database edit now turns into a major computational headache that cannot be easily completed.

      So, this hashing function, combined with the fact that thousands of other nodes must be in sync with identical copies of the blockchain, makes the blockchain virtually immutable; it simply can’t be easily hacked.

      Nobody can change it or destroy it. Hackers can’t get into the peer-to-peer node network and create transactions in order to steal crypto, governments can’t close it down (China, for example, could attempt to shut down Bitcoin within its borders, but the blockchain would continue to exist in many other countries), a terrorist group can’t destroy it, one nation can’t attack another and destroy its blockchain, and so on. Because there are so many copies of the blockchain, and as long as enough people want to continue working with the blockchain, it’s practically immutable and indestructible.

Where’s the Money?

      You may be wondering, “So where is the cryptocurrency? Where’s the money?” Or perhaps you’ve heard of cryptocurrency wallets and think that’s where the money is stored. Wrong. There’s no money in a cryptocurrency wallet. In fact, there is no cryptocurrency.

      Cryptocurrency blockchains are often described as ledgers. A ledger is described by Google Dictionary as “a book or other collection of financial accounts of a particular type.” Ledgers have been around for hundreds of years, used to record transactions for individuals, businesses, government departments, and so on. The statement you get from your bank account or credit card is a form of ledger, showing you your individual transactions; money you pay to others, and money you receive from others.

      FINDING THE BALANCE IN THE BLOCKCHAIN

      Well, okay, the blockchain doesn’t actually store a balance for each address. Nowhere in the blockchain does it state how much of the cryptocurrency any particular owner owns or how much any particular address has associated with it. Rather, you can use a blockchain explorer to follow all your transactions, incoming and outgoing, and the blockchain explorer can figure out your balance based on those transactions.

In the context of cryptocurrency, the blockchain is a digital ledger recording cryptocurrency you send to others, and cryptocurrency you receive from others.

      Think of it this way. Say that you’re a little compulsive and like to keep a record of the cash in your pocket. You carry a notepad, to record every time you put money into your pocket and every time you spend it, and you calculate the current balance. That notepad is a kind of transaction ledger, right?

      Cryptocurrency is very similar to this ledger of cash transactions … except there’s no pocket. The blockchain is the ledger; it stores a record of every transaction (when you first purchased or were sent the cryptocurrency, when you spent it or sold it, and the balance you own).

      But there’s no pocket and no cryptocurrency sitting in storage somewhere. The blockchain is simply a series of “mythical” (or virtual) transactions stored in the ledger. No currency is being physically transferred; we simply update the record to state that currency has been transferred.

      The ledger says you own cryptocurrency, so everyone can validate and accept that you own it. And remember, that ledger can’t be edited after being solidified into the chain — it can’t be hacked. (See the preceding section for more on this topic.) So if the ledger says you own, say, half a Bitcoin, then you absolutely