Intro to Ethereum and Smart Contracts

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1.3 Example: a Perpetual Message System Using Webtasks and Bitcoin Although we have not delved into the specifics of how Bitcoin or Ethereum handle transactions, there is a certain programmability built into them. Bitcoin allows for certain conditions to be specified in each transaction. If these conditions are met, the transaction can be spent. Ethereum, on the other hand, goes much further: a Turing-complete programming language is built into the system. We will focus on Ethereum in the next chapter, but for now we will take a look at creative ways in which the concepts of the blockchain can be exploited for more than just sending money. For this, we will develop a simple perpetual message system on top of Bitcoin. How will it work? We have seen the blockchain stores transactions that can be verified. Each transaction is signed by the one who can perform it and then broadcast to the network. It is then stored inside a block after performing a proof-of-work. This means that any information embedded in the transaction is stored forever inside the blockchain. The timestamp of the block serves as proof of the message’s date, and the proof-of-work process serves as proof of its immutable nature. Bitcoin uses a scripting system that describes steps a user must perform to spend money. The most common script is simply “prove you are the owner of a certain private-key by signing this message with it”. This is known as the “pay to pubkey hash” script. In decompiled form it looks like: OP_DUP OP_HASH160 OP_EQUALVERIFY OP_CHECKSIG Where and are provided by the spender and the rest is specified by the original sender of the money. This is simply a sequence of mixed data and operations. The interpreter for this script is a stack-based virtual machine. The details of execution are out of scope for this handbook, but you can find a nice summary at the Bitcoin Wiki14. The important take from this is that transactions can have data embedded in them in the scripts. In fact, there exists a valid opcode for embedding data inside a transaction: the OP_RETURN opcode. Whatever data follows the OP_RETURN opcode is stored in the transaction. Of course, there is a limit for the amount of data allowed: 40-bytes. This is very little, but still certain interesting applications can be performed with such a tiny amount of storage. One of them is our perpetual message system. Another interesting use case is the “proof of existence” concept. By storing a hash of an asset in the blockchain, it serves as proof of its existence at the point it was added to a block. In fact, there already exists such a project15. There is nothing preventing you from using our perpetual message system for a similar use. Yet other uses allow the system to prepare transactions that can only be spent after conditions are met, or when the spender provides proof of having a certain digital asset, of when a certain minimum number of users agree to spend it. Programmability opens up many possibilities and makes for yet another great benefit of cryptocurrencies in contrast with traditional monetary systems. 14 https://en.bitcoin.it/wiki/Script#Standard_Transaction_to_Bitcoin_address_.28pay- to- pubkey- hash.29 15 https://proofofexistence.com 20

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