Blockchain is a growing list of records, called blocks, that are linked using cryptography in a *linear sequence forming a chain. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data. By design, blockchain is resistant to any modification of the data.

Considering we’re in a pre-tech Pen and Paper-based era, how would the blockchain transaction work in the ‘crypto’ aspect?

Throwback to the early 1900s, knowing how to run the process of decentralized consensus — or in other words, operate a very simple blockchain by hand — can prove to be very useful. All you need is a few blockchain enthusiasts, a pen and a few pieces of paper.

Let’s go back to that original story and go through the process with our blockchain experts,  John, Dwayne, Daniel, and Dennis.

A short recap: the team is trying to implement CryptoCoin, a revolutionary new currency that will bring an end to the city’s crippled barter economy. The group has agreed it’s fair if each of them starts with 100 coins. Instead of minting actual coins, they will make it with a few pieces of paper. Riddled with trust issues, the gang hasn’t been able to agree on one person to keep track of balances. Their only option is to maintain the balances together.

What are we trying to achieve? It’s very simple actually — all we’re trying to do is maintain a simple table of balances on a piece of paper. This table will show how many coins each of our members has. The trick is because we can’t have one piece of paper that holds the only source of truth — we’re going to have to keep things equal and let each of the members maintain their own version — this is the decentralized part. And naturally, we’re also going to hope that all 4 pieces of paper eventually show the same thing — this is the consensus part.

So what would this piece of paper look like? This paper is the first one we agree on — that’s why it’s marked as day 1. Where did these balances come from? We’ve previously agreed it’s fair that each person starts with 100 coins. We’re also going to need one of them to write this paper. It doesn’t really matter who, so let’s take John. He will be the one to publish this paper to everybody and make sure they each save a copy (the city has an amazing photocopy machine).

Since coin balances are expected to change, we’re going to create a new updated piece of paper at the end of every day. It may not always be John, who publishes the update though . We want to keep things as fair as possible after all.

Another important part, due to the lack of mutual trust in the group, is having each of the gang confirm the status update individually. An easy way to achieve this is having each one of the members sign each of the papers — but only if they agree with what’s written on it.

How many people need to sign a paper for it to be considered final and approved? We need to reach consensus, so a majority will do. Since we have 4 people in total, the majority is at least 3 out of 4. The paper above was signed by all four, so it’s definitely final. Why don’t we want to require all four signatures on all papers? Because this will allow one individual to jeopardize the entire process. If Dwayne goes on a fishing trip for a few days, the gang can’t update balances until he gets back. This gives one person too much power. Why do we even need a majority? Why isn’t 2 out of 4 signatures enough? Because if we only require 2 out of 4, we may end up with 2 people (like John and Dennis) signing one version of balances, and the other 2 people ( Dwayne and Daniel) signing a different version that doesn’t match. We can’t have two conflicting versions of reality both considered final.

On the morning of the second day, Dwayne wants to buy a tomato. John, Dwayne, Daniel, and Dennis sell tomatoes for 2 coins each. He wants to transfer 2 coins to John. Daniel takes a new piece of paper and writes the transfer on it:It’s Daniel’s first action ever, so he labels it as such. In addition, Daniel signs this paper. We have to have his signature to make sure nobody else can forge a transfer request on his account.

The end of the second day is approaching and the gang wants to publish an updated set of balances. John published the paper for the first day and collected everybody’s signatures. It makes sense to take turns doing so. The gang agrees to use a simple rotating order: John, Dwayne, Daniel, Dennis, John, Dwayne, and so forth. This means that publishing the status paper for day 2 is Dwayne’s responsibility. The paper he publishes reflects Daniel’s transfer:

This paper that Dwayne made isn’t final yet because it’s only signed by Dwayne. He needs to collect more signatures. Dwayne goes through the gang and asks each one to verify and sign it. This paper is very easy to verify. First, the verifier needs to look in his own collection of papers and find the status paper that shows the balances for the previous day (day 1 in this case). Next, the verifier needs to go over the new list of transfers. In this case, we only have one transfer by Daniel. This transfer is easy to verify, we can make sure it’s indeed signed by Daniel and we can make sure Daniel indeed has enough coins in his balance to give this amount to John.

Once every island inhabitant completes his verification process and signs the paper, Dwayne now has a final status paper for day 2 to publish to everybody. Everybody makes a copy and goes to sleep happy and content. Day 3 is upon us. The system is working well and everybody is excited to spend their coins. John wants to buy some firewood from Dwayne for 10 coins. Dwayne wants to get some pills from Dennis for 25 coins, and Dennis is hungry for a tomato and wants to give John 2 coins to buy one. They each create a piece of paper detailing their transfer:

As evening approaches, the person publishing today’s balances is Daniel. To make sure Daniel includes these transfers in his paper proposal, they each need to give a copy of their transfer request. It actually makes sense to give copies to everybody because the person wanting to perform a transfer doesn’t necessarily remember whose turn it is to publish today’s update.

Dennis lingers with getting the copy of his transfer request to Daniel, and by the time he brings it to him, his balance status of the day is already written

Dennis is frustrated that this balance status doesn’t include his own transfer. This means John won’t receive Dennis’s payment for the tomato, and won’t give Dennis the tomato for dinner. Dennis is going to go to sleep hungry tonight. He storms out to look for something else to eat and doesn’t sign Daniel’s paper proposal for the day. Luckily, Daniel is able to get enough signatures from the rest of the gang:

Daniel managed to get 3 signatures on the paper, each verifying that the balances indeed match the transfers and yesterday’s balances. We have a majority, so this status sheet is considered final.

It’s day 4 and nobody knows where Dennis is. He went fishing the night before, got caught out in a storm and didn’t return to camp. It might be problematic because it’s Dennis’s turn today to publish the balance update.

Daniel wants to make a couple of transfers today, he writes them on a piece of paper and gives a copy to everybody:

Evening approaches and Dennis isn’t back yet. The gang had previously agreed that it’s his turn to publish the balances, but he’s nowhere to be found, so this day’s balance update is skipped.

It’s day 5, and John is the one responsible for publishing the daily update. It’s been raining for a few days and John wants a warmer place to sleep. He asks Dwayne to build him a small wooden hut. Dwayne wants 200 coins for the job. It’s a bit of a problem because John only has 94 coins. John has a crazy idea though. He is the one publishing the balance sheet today, so why not add a crazy transfer of this amount to Dwayne anyways?

There are no other transfers today. John has Daniel’s transfers from yesterday though, and Dennis’s transfer from the day before that didn’t make it in eventually. He adds all of those, together with his new crazy transfer:

John signs this update although his transfer doesn’t really make sense. To make this status update final, he needs to collect 2 more signatures. When he approaches Dwayne and Daniel, they notice that on the previous final balance sheet (from day 3), John had 92 coins. How can he send 200 to Dwayne? They refuse to sign this balance sheet until he fixes this error and removes this invalid transfer. He reluctantly agrees and publishes a new balance sheet which is correct. They eventually sign it:

This balance update is final because it has 3 signatures.

It’s the morning of day 6. Dennis finally comes back to camp. He had a rough couple of days with the storm and all. He wasn’t part of the discussions in the last two days and missed announcements of new transfers and publications of new balance updates. He’s actually unsure how many coins he has. He finds the first fellow survivor he sees and asks for the latest updates he missed. He is given the final update for day 5 and the approved update Daniel that published on day 3.

It’s easy for Dennis to synchronize back with the rest of the gang. He can see that these balance updates were indeed signed at least 3 times, so he can be relatively safe that they’re ok. He can also perform the calculations himself based on these updates and the latest update he has (from day 2). This will allow Dennis to participate in today’s transfers just like he didn’t miss anything.

The system seems to be working well. It’s true that it’s a bit simplistic, but it’s enough for what these island inhabitants need. Well, we can’t have a functioning blockchain without a white paper! The gang sits down and celebrates the accomplishment by publishing this magnificent one:

Why is this considered a blockchain? For starters, each piece of paper published daily represents a block. Each block is numbered and points to the previous one — forming a chain of blocks. In order to verify the current state of balances, any observer must start from the beginning of the chain (day 1 — the genesis block) and verify each of the blocks one after the other in succession. The balances are built incrementally.

Is this an ideal blockchain implementation? Probably not. It can be improved in many ways. For example, it only supports these 4 inhabitants and this protocol is also currently permissioned.