Programming Bitcoin

Chapter 6. Script

The ability to lock and unlock coins is the mechanism by which we transfer bitcoin. Locking is giving some bitcoins to some entity. Unlocking is spending some bitcoins that you have received.

In this chapter we examine this locking/unlocking mechanism, which is often called a smart contract. Elliptic curve cryptography (Chapter 3) is used by Script to validate that a transaction was properly authorized (Chapter 5). Script essentially allows people to prove that they have the right to spend certain UTXOs. We’re getting a little ahead of ourselves, though, so let’s start with how Script works and go from there.

Mechanics of Script

If you are confused about what a smart contract is, don’t worry. “Smart contract” is a fancy way of saying “programmable,” and the “smart contract language” is simply a programming language. In Bitcoin, Script is the smart contract language, or the programming language used to express the conditions under which bitcoins are spendable.

Bitcoin has the digital equivalent of a contract in Script. Script is a stack-based language similar to Forth. It’s intentionally limited in the sense that it avoids certain features. Specifically, Script avoids any mechanism for loops and is therefore not Turing complete.

Why Bitcoin Isn’t Turing Complete

Turing completeness in a programming language essentially means that the program has the ability to loop. Loops are a useful construct in programming, so you may be wondering at this point why Script ...