A blockchain in 200 lines of code

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A blockchain in 200 lines of code

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simple blockchain python

Learn how Proof of Work is essential for securing blockchain in this article by Eric Traub, a software engineer in New York with an extensive experience working as a teacher and instructing people in a variety of different subjects. The proofOfWork method is very important and essential to the blockchain technology because it makes Bitcoin and many other blockchains secure.

Now, you must be getting curious about what a Proof of Work PoW actually is. Take a look at any blockchain — every blockchain is basically a list of blocks. Every single block has to be created and added to the chain. We want to make sure that every block that is added to the chain is legitimate, has the correct transactions, and has the correct data inside of it. Every time a new block is created, we first have to make sure that it is a legitimate block by mining it through PoW.

From this data, the proofOfWork method will try to generate a specific hash. This specific hash in our example is going to be a hash that starts with four zeros. So, with the given currentBlockData and the previousBlockHash , the method will somehow generate a resulting hash that begins with four zeros.

Suppose that the hash is random. So, if the resulting hash is effectively random, then how can we generate a hash from our current block that starts with four zeros?

The only way this can be done is by trial and error, or by guessing and checking. So, what we will have to do is run our hashBlock method many times until we end up getting lucky by generating a hash that has four zeros at the beginning. You might be wondering what the input to our hashBlock method is — they are previousBlockHash , currentBlockData , and nonce parameters. How can we alter this data in a way that does not change our currentBlockData or the previousBlockHash , but we still get a resulting hash that has four zeros at the beginning of it?

The answer to this question is that we are going to constantly change the nonce value. The first time that we run our hashBlock method; we are going to start with a nonce value of 0. Then, if that resulting hash does not have four zeros at the beginning of it, we are going to run our hashBlock method again, except this time we are going to increment our nonce value by 1. That is how our proofOfWork method will function. You might be wondering how this proofOfWork method actually secures the blockchain.

So, if somebody wanted to go back into the blockchain and try to change a block or the data in that block — perhaps to give themselves more Bitcoin — they would have to do numerous calculations and expend a great deal of energy to create the correct hash.

In most cases, going back and trying to recreate an already existing block or trying to re-mine an already existing block with your own fake data is not feasible. Not only does our hashBlock method take in the currentBlockData , but it also takes in the previous BlockHash. This means that all of the blocks in the blockchain are linked together by their data. If somebody tries to go back and re-mine or recreate a block that already exists, they would also have to re-mine and recreate every single block that comes after the first one that they recreated.

This would take an incredible amount of calculation and energy and is not feasible for a well-developed blockchain. A person would have to go in, recreate a block by using a proof of work and then recreate every block after that by doing a new proof of work for each block. This is not feasible for any well-produced blockchain, and this is the reason why blockchain technology is so secure. This method takes in two parameters: previousBlockHash and currentBlockData :.

The first thing that we want to do inside of our method is to define a nonce:. Next, we want to hash all our data for the first time, so type in the following highlighted line of code:. In the preceding code, you may notice that we used the term let because both our nonce and hash will be changing as we move through the method.

The next step that we want to do is constantly run the hashBlock method over and over again until we get a hash that starts with four zeros. Consequently, inside the while loop, add the following highlighted lines of code:.

Inside the while loop, we are running our hashBlock method again with all the same data, except this time our nonce is incremented and equal to 1 instead of 0. This will be the first iteration of our while loop. So, our while loop will run again, the nonce value will be incremented to 2 and a new hash will be created.

If that hash also does not start with four zeros then the while loop will run again, the nonce value will be incremented again and the hash will be generated again.

Our loop will continue doing this until it winds up with a hash that starts with four zeros. This might take many iterations. This could happen 10 times, 10, times, or , times. This loop is where all of the calculations will take place, and this is the reason why the proofOfWork method uses so much energy — there are a lot of calculations being made. When we finally have the correct hash, our while loop will stop running, and at the end of our proofOfWork , it will simply return the nonce value that gave us the valid hash:.

So, this is how our proofOfWork method will work and validate the hash. So, in our test case, get rid of the nonce value and add the following lines of code to our file:. Now, what we should get as a result from this proofOfWork method is a nonce value. What our proofOfWork method does is test to see that the correct nonce value is to hash with our block data and our previousBlockHash to generate a resulting block hash that starts with four zeros.

Here, proofOfWork finds the correct nonce for us. After the test is run, you will observe that a number pops up as an output on the screen:. What this number signifies is that it took 27, iterations for our proofOfWork method to find a hash that starts with four zeros. To understand this whole process in-depth, what we can do is, inside our while loop, log out of every hash that we try. We will have to make minor modifications to our while loop:.

None of these hashes will start with four zeros, except for the last one. Only the last hash that gets logged out should start with four zeros because after our method, this will terminate and return the nonce value for which the valid hash was obtained.

Save our test. You can now observe on your screen that we have a whole bunch of different hashes being logged out to the terminal:. Observe that for every single hash that has been logged, the beginning is never four zeros in a row until we obtain our final value. What is happening here is that we are generating the hash from our currentBlockData , previousBlockHash , and nonce of value 0. Then, for the next hash, we are incrementing the nonce by 1.

Finally, at 27,, with the value of a nonce, the valid hash is obtained. This value we obtained from our proofOfWork method. What we would observe as output is running the single hash with the correct nonce value that we obtained by running the proofOfWork method. By doing this, we should generate a hash that starts with four zeros on the first try. Once the test is run, you will get to observe the single hash that starts with four zeros, as shown in the following screenshot:.

The proofOfWork is a very important part of blockchain technology. It is very difficult to calculate, as you can observe from the test results — it took us more than 27, iterations to generate the correct hash.

Consequently, a proofOfWork takes in a lot of energy and many calculations and is very difficult to produce. Once we have the correct proof or the nonce value at which the desired hash is generated, it should be very easy for us to verify that we have the correct nonce value.

We can verify this by simply passing it into our hashBlock method — we would obtain the hash that starts with four zeros. It takes a lot of work to generate a proof of work, but it is very easy to verify that it is correct.

If this returns a valid hash to us that start with four zeros, then we already know that the block is valid. Thus, from our test, we can conclude that the proofOfWork method works as expected. If you found this article interesting, you can look at Learn Blockchain Programming with JavaScript to explore the essentials of blockchain technology with JavaScript to develop highly secure bitcoin-like applications. Learn Blockchain Programming with JavaScript can help you understand how decentralized blockchain network functions and why decentralization is such an important feature for securing a blockchain.

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a blockchain in 200 lines of code

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I recently attended an industry seminar where the concept of the Blockchain was explained. At the end of the session, walking out of the lecture room I heard one of the attendees say to a colleague "I'm still not sure what exactly Blockchain is Many of us know that Blockchain is a topic that is hot at the moment. It's a topic that is disruptive.

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