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116 lines
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116 lines
5.3 KiB
Markdown
# 7.2: Running a Bitcoin Script
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> **NOTE:** This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.
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Bitcoin Scripts may not initially seem that intuitive, but their execution is quite simple, using reverse Polish notation and a stack.
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## Understand the Scripting Language
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A Bitcoin Script has three parts: it has a line of input; it has a stack for storage; and it has specific commands for execution.
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### Understand the Ordering
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Bitcoin Scripts are run from left to right. That sounds easy enough, as it's the same way you read. However, this might be the most non-intuitive element of Bitcoin Script, because it means that functions don't look like you'd expect. Instead, _the operands go before the operator._
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For example, if you were adding together "1" and "2", your Bitcoin Script for that would be `1 2 OP_ADD`, _not_ "1 + 2". Since we know that OP_ADD operator takes two inputs, we know that the two inputs before it are its operands. At least that's true at the most basical level of Scripting.
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> **WARNING:** Technically, everything in Bitcoin Script is an opcode, thus it would be most appropriate to record the above example as `OP_1 OP_2 OP_ADD`. We leave the "OP" prefix off constants, but include it on all actual operators. Some writers prefer to also leave the "OP" prefix off operators, but we have opted not to.
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### Understand the Stack
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It's actually not quite correct to say that an operator applies to the inputs before it. Instead, an operator actually applies to the top inputs in Bitcoin's stack.
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_What is a stack?_ A stack is a LIFO (last-in-first-out) data structure. It has two access functions: push and pop. Push places a new object on top of the stack, pushing down everything below it. Pop removes the top object from the stack.
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Whenever Bitcoin Script encounters a constant, it's pushed onto the Stack. So the above example of `1 2 OP_ADD1` would actually look like this as it was processed:
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```
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Script: 1 2 OP_ADD
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Stack: [ ]
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Script: 2 OP_ADD
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Stack: [ 1 ]
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Script: OP_ADD
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Stack: [ 1 2 ]
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```
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_Note that in this and in following examples the top of the stack is to the right and the bottom is to the left._
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### Understand the Opcodes
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When a Bitcoin Script encounters an operator, it evaluates it. Each operator pops zero or more elements off the stack as inputs, usually one or two. It then processes them in a specific way before pushing zero or more elements back on the stack, usually one or two.
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OP_ADD pops two items off the stack (here: 2 then 1), adds then together, and pushes the result back on the stack (here: 3).
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```
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Script:
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Stack: [ 3 ]
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```
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## Run a Simple Script
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Here's an example of a more complex script. It shows how operators continue to interact with the stack, not just with the operands right before it:
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```
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Script: 3 2 OP_ADD 4 OP_SUB
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Stack: [ ]
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Script: 2 OP_ADD 4 OP_SUB
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Stack: [ 3 ]
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Script: OP_ADD 4 OP_SUB
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Stack: [ 3 2 ]
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Script: 4 OP_SUB
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Stack: [ 5 ]
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Script: OP_SUB
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Stack: [ 5 4 ]
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Script:
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Stack: [ 1 ]
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```
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## Understand the Usage of Bitcoin Script
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That's pretty much Bitcoin Scripting ... other than a few intricacies for how this Scripting language interacts with Bitcoin itself.
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### Understand scriptSig and scriptPubKey
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As we've seen, every input for a Bitcoin transaction contains a `scriptSig` which is used to unlock the `scriptPubKey` for the associated UTXO. The easy way to think of this is that `scriptSig` is run, then the `scriptPubKey` is run.
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So, presume that a UTXO were locked with a `scriptPubKey` that read `100 OP_EQUAL` and that the `scriptSig` `1 99 OP_ADD` were run to unlock it. The two scripts would effectively be run in order as `1 99 OP_ADD 100 OP_EQUAL` and the following were occur:
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```
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Script: 1 99 OP_ADD 100 OP_EQUAL
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Stack: []
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Script: 99 OP_ADD 100 OP_EQUAL
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Stack: [1]
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Script: OP_ADD 100 OP_EQUAL
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Stack: [1 99]
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Script: 100 OP_EQUAL
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Stack: [100]
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Script: OP_EQUAL
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Stack: [100 100]
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Script:
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Stack: [TRUE]
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```
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This abstraction isn't quite true: for security reasons, the `scriptSig` is run, then the contents of the stack are transferred for the `scriptPubKey` to run, but it's true enough for understanding how the key of `scriptSig` fits into the lock of `scriptPubKey`.
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> **WARNING** The above is a non-standard transaction type. It would not actually be accepted by nodes running Bitcoin Core with the standard settings. Chapter 8 discusses more about how you actually _could_ use a Bitcoin Script like this, thanks to the power of P2SH.
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### Get the Results
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Bitcoin will verify a transaction and allow the UTXO to be respent if two criteria are met when running `scriptSig` and `scriptPubKey`:
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1. The execution did not get marked as invalid at any point, for example with a failed OP_VERIFY or the usage of a disabled opcode.
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2. The top item in the stack at the end of execution is true (non-zero).
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In the above example, the transaction would succeed because the stack has a `TRUE` at its top.
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## Summary: Running a Bitcoin Script
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To process a Bitcoin Script, a `scriptSig` is run followed by the `scriptPubKey` that it's unlocking. These commands are run in order, from left to right, with constants being pushed onto a stack and operators popping elements off that stack, then pushing results back onto it. If the Script doesn't halt in the middle and if the item on top of the stack at the end is `TRUE`, then the UTXO is unlocked.
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