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Reword description of transaction execution

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I believe that the current description of transaction execution is not
accurately describing how the stack machine works. (It's close but uses
slightly misleading wording.) While data values are pushed when executed
in the script, other opcodes are never pushed; they are simply executed.

Wording improved per apoelstra's comments.

Rebased-from: 54f01b2d0b2b2383fd0d9085a829b55d83f7cd67
Rebased-by: David A. Harding
Closes #816
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gwillen 2015-04-10 09:59:03 -04:00 • committed by David A. Harding
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@ -199,8 +199,8 @@ sections about stacks. These are programming terms. Also "above",
"below", "top", and "bottom" are commonly used relative directions or "below", "top", and "bottom" are commonly used relative directions or
locations in stack descriptions. -harding --> locations in stack descriptions. -harding -->
To test whether the transaction is valid, signature script and pubkey script arguments To test whether the transaction is valid, signature script and pubkey script operations
are pushed to the stack one item at a time, starting with Bob's signature script are executed one item at a time, starting with Bob's signature script
and continuing to the end of Alice's pubkey script. The figure below shows the and continuing to the end of Alice's pubkey script. The figure below shows the
evaluation of a standard P2PKH pubkey script; below the figure is a description evaluation of a standard P2PKH pubkey script; below the figure is a description
of the process. of the process.
@ -211,38 +211,37 @@ of the process.
Because it's just data, nothing is done except adding it to the stack. Because it's just data, nothing is done except adding it to the stack.
The public key (also from the signature script) is pushed on top of the signature. The public key (also from the signature script) is pushed on top of the signature.
* From Alice's pubkey script, the `OP_DUP` operation is pushed. `OP_DUP` replaces * From Alice's pubkey script, the `OP_DUP` operation is executed. `OP_DUP` pushes onto the stack
itself with a copy of the data from one level below it---in this a copy of the data currently at the top of it---in this
case creating a copy of the public key Bob provided. case creating a copy of the public key Bob provided.
* The operation pushed next, `OP_HASH160`, replaces itself with a hash * The operation executed next, `OP_HASH160`, pushes onto the stack a hash
of the data from one level below it---in this case, Bob's public key. of the data currently on top of it---in this case, Bob's public key.
This creates a hash of Bob's public key. This creates a hash of Bob's public key.
* Alice's pubkey script then pushes the pubkey hash that Bob gave her for the * Alice's pubkey script then pushes the pubkey hash that Bob gave her for the
first transaction. At this point, there should be two copies of Bob's first transaction. At this point, there should be two copies of Bob's
pubkey hash at the top of the stack. pubkey hash at the top of the stack.
* Now it gets interesting: Alice's pubkey script adds `OP_EQUALVERIFY` to the * Now it gets interesting: Alice's pubkey script executes `OP_EQUALVERIFY`.
stack. `OP_EQUALVERIFY` expands to `OP_EQUAL` and `OP_VERIFY` (not shown). `OP_EQUALVERIFY` is equivalent to executing `OP_EQUAL` followed by `OP_VERIFY` (not shown).
`OP_EQUAL` (not shown) checks the two values below it; in this `OP_EQUAL` (not shown) checks the two values at the top of the stack; in this
case, it checks whether the pubkey hash generated from the full case, it checks whether the pubkey hash generated from the full
public key Bob provided equals the pubkey hash Alice provided when public key Bob provided equals the pubkey hash Alice provided when
she created transaction #1. `OP_EQUAL` then replaces itself and she created transaction #1. `OP_EQUAL` pops (removes from the top of the stack)
the two values it compared with the result of that comparison: the two values it compared, and replaces them with the result of that comparison:
zero (*false*) or one (*true*). zero (*false*) or one (*true*).
`OP_VERIFY` (not shown) checks the value immediately below it. If `OP_VERIFY` (not shown) checks the value at the top of the stack. If
the value is *false* it immediately terminates stack evaluation and the value is *false* it immediately terminates evaluation and
the transaction validation fails. Otherwise it pops both itself and the transaction validation fails. Otherwise it pops the *true* value off the stack.
the *true* value off the stack.
* Finally, Alice's pubkey script pushes `OP_CHECKSIG`, which checks the * Finally, Alice's pubkey script executes `OP_CHECKSIG`, which checks the
signature Bob provided against the now-authenticated public key he signature Bob provided against the now-authenticated public key he
also provided. If the signature matches the public key and was also provided. If the signature matches the public key and was
generated using all of the data required to be signed, `OP_CHECKSIG` generated using all of the data required to be signed, `OP_CHECKSIG`
replaces itself with *true.* pushes the value *true* onto the top of the stack.
If *false* is not at the top of the stack after the pubkey script has been If *false* is not at the top of the stack after the pubkey script has been
evaluated, the transaction is valid (provided there are no other evaluated, the transaction is valid (provided there are no other