Inside a block

Let's now discuss the contents of a block of bitcoin blockchain. This will help us in understanding and comparing Ethereum, which we will take up in Chapter 2, Grokking Ethereum. A block is a very interesting concept, because a block gives blockchain its properties. It is the basic element of a blockchain.

Therefore, it is important to know what a block consists of. Figure 1.8 lists the basic parts of a bitcoin block. First of all, we have a magic number, also called bitcoin network ID.

It is four bytes long and is an arbitrary number that signals that this is a bitcoin block. The magic number is not something specific to bitcoin. All nodes communicate using transmission control protocol.

In TCP, different types of data packets use different magic numbers to identify themselves. It is like declaring our gender while filling out a passport to identify ourselves as male, female, or transgender. In our context, a block is actually a sequence of 0 and 1. When any machine reads such a data sequence and it encounters the binary version of 0xD9B4BEF9, it will identify the data as a bitcoin block. So, this number is the same for all bitcoin blocks. Now, why was 0xD9B4BEF9 chosen for bitcoin blocks?

As per the comments found in the bitcoin main.cpp files (bitcoin implementations were written in C++ language), the magic number was chosen in such a way that the characters are rarely used in upper ASCII, not valid as UTF-8, and produce a large four-byte integer at any alignment.

Next, we have the block size which is also four bytes and tells us how long this block is with all of its transactions. As of July 2017, the size of an entire bitcoin block can be a maximum of 1 MB. We then have the block header, which is 80 bytes and it is the most interesting part; we will talk about its contents in a short while. The next one is the transaction counter, which is an integer that tells us how many transactions this block has and next we have a list of transactions, which simply contains all the transactions that are in this block.

For example, if we have the transaction counter as 20, we have 20 transactions in the block. The transaction counter is one to nine bytes. So, these were the basic parts of the block.

So, let's go ahead and look up what the bitcoin block header consists of. Here, in Figure 1.9, we have the header parts. The header has a version that simply tells us which version of the format we are on currently.

For example, the current bitcoin version is 2.

Now, if for some reason a bitcoin block from the version 1 client comes, it will be ignored by blockchain. In the future, there might be a change in the size and format of a bitcoin block. Then, we would have to change this version in each block to keep them relevant in blockchain. The version number is four bytes. Next, we have 32 bytes of hash of the previous block. This represents the hash of the previous block header. This field is very powerful because, in case of any unforeseen catastrophic hack that breaks away blockchain, we can generate the entire blockchain if even one single block remains preserved. Next, we have the hash of the Merkle root. These 32 bytes store the hash of the Merkle root of all the transactions associated with the present block as depicted earlier in Figure 1.5 as Tx_Root. We saw that the body of the block contains transactions. These are indirectly hashed through the Merkle root. So, hashing a block with one transaction takes exactly the same amount of effort as hashing a block with 10,000 transactions. Next, we have a four-byte timestamp field, which updates every few seconds to keep the current timestamp. The next field of four bytes, called the target, is another interesting field. This field tells us the level of difficulty of this current block. To understand how the target field works, we need to understand how the four-byte nonce field works, where the word nonce stands for nonsensical incrementer, which leads us to our next section on mining and forking.