@ -275,11 +275,11 @@ An unlocking script is a script that "solves," or satisfies, the conditions plac
Every bitcoin client will validate transactions by executing the locking and unlocking scripts together. For each input in the transaction, the validation software will first retrieve the UTXO referenced by the input. That UTXO contains a locking script defining the conditions required to spend it. The validation software will then take the unlocking script contained in the input that is attempting to spend this UTXO and execute the two scripts.
In the original bitcoin client, the unlocking and locking scripts were concatenated and executed in sequence. For security reasons, this was changed in 2010, because of a vulnerability that allowed a malformed unlocking script to push data onto the stack and corrupt the locking script. In the current implementation the scripts are executed separately with the stack transferred between the two executions, as described below.
In the original bitcoin client, the unlocking and locking scripts were concatenated and executed in sequence. For security reasons, this was changed in 2010, because of a vulnerability that allowed a malformed unlocking script to push data onto the stack and corrupt the locking script. In the current implementation, the scripts are executed separately with the stack transferred between the two executions, as described next.
First, the unlocking script is executed, using the stack execution engine. If the unlocking script executed without errors (e.g it has no "dangling" operators leftover), the main stack (not the alternate stack) is copied and the locking script is executed. If the result of executing the locking script with the stack data copied from the unlocking script is "TRUE", the unlocking script has succeeded in resolving the conditions imposed by the locking script and therefore the input is a valid authorization to spend the UTXO. If any result other than "TRUE" remains after execution of the combined script, the input is invalid as it has failed to satisfy the spending conditions placed on the UTXO. Note that the UTXO is permanently recorded in the blockchain, and therefore is invariable and is unaffected by failed attempts to spend it by reference in a new transaction. Only a valid transaction that correctly satisfies the conditions of the UTXO results in the UTXO being marked as "spent" and removed from the set of available (unspent) UTXO.
First, the unlocking script is executed, using the stack execution engine. If the unlocking script executed without errors (e.g., it has no "dangling" operators leftover), the main stack (not the alternate stack) is copied and the locking script is executed. If the result of executing the locking script with the stack data copied from the unlocking script is "TRUE," the unlocking script has succeeded in resolving the conditions imposed by the locking script and, therefore, the input is a valid authorization to spend the UTXO. If any result other than "TRUE" remains after execution of the combined script, the input is invalid because it has failed to satisfy the spending conditions placed on the UTXO. Note that the UTXO is permanently recorded in the blockchain, and therefore is invariable and is unaffected by failed attempts to spend it by reference in a new transaction. Only a valid transaction that correctly satisfies the conditions of the UTXO results in the UTXO being marked as "spent" and removed from the set of available (unspent) UTXO.
Below is an example of the unlocking and locking scripts for the most common type of bitcoin transaction (a payment to a public key hash), showing the combined script resulting from the concatenation of the unlocking and locking scripts prior to script validation:
<<scriptSig and scriptPubKey>> is an example of the unlocking and locking scripts for the most common type of bitcoin transaction (a payment to a public key hash), showing the combined script resulting from the concatenation of the unlocking and locking scripts prior to script validation.
[[scriptSig and scriptPubKey]]
.Combining scriptSig and scriptPubKey to evaluate a transaction script
The bitcoin transaction script language, also named confusingly _Script_, is a Forth-like reverse-polish notation stack-based execution language. If that sounds like gibberish, you probably haven't studied 1960's programming languages. Script is a very simple, lightweight language that was designed to be limited in scope and executable on a range of hardware, perhaps as simple as an embedded device, like a handheld calculator. It requires minimal processing and cannot do many of the fancy things modern programming languages can do. In the case of programmable money, that is a deliberate security feature.
The bitcoin transaction script language, also confusingly, _Script_, is a Forth-like reverse-polish notation stack-based execution language. If that sounds like gibberish, you probably haven't studied 1960's programming languages. Script is a very simple, lightweight language that was designed to be limited in scope and executable on a range of hardware, perhaps as simple as an embedded device, like a handheld calculator. It requires minimal processing and cannot do many of the fancy things modern programming languages can do. In the case of programmable money, that is a deliberate security feature.
Bitcoin's scripting language is called a stack-based language because it uses a data structure called a _stack_. A stack is a very simple data structure, which can be visualized as a stack of cards. A stack allows two operations: push and pop. Push adds an item on top of the stack. Pop removes the top item from the stack.
The scripting language executes the script by processing each item from left to right. Numbers (data constants) are pushed onto the stack. Operators push or pop one or more parameters from the stack, act on them, and may push a result onto the stack. For example, OP_ADD will pop two items from the stack, add them and push the resulting sum onto the stack.
The scripting language executes the script by processing each item from left to right. Numbers (data constants) are pushed onto the stack. Operators push or pop one or more parameters from the stack, act on them, and may push a result onto the stack. For example, +OP_ADD+ will pop two items from the stack, add them, and push the resulting sum onto the stack.
Conditional operators evaluate a condition producing a boolean result of TRUE or FALSE. For example, OP_EQUAL pops two items from the stack and pushes TRUE (TRUE is represented by the number 1) if they are equal or FALSE (represented by zero) if they are not equal. Bitcoin transaction scripts usually contain a conditional operator, so that they can produce the result TRUE that signifies a valid transaction.
Conditional operators evaluate a condition producing a boolean result of TRUE or FALSE. For example, +OP_EQUAL+ pops two items from the stack and pushes TRUE (TRUE is represented by the number 1) if they are equal or FALSE (represented by zero) if they are not equal. Bitcoin transaction scripts usually contain a conditional operator, so that they can produce the TRUE result that signifies a valid transaction.
In the following example, the script +2 3 OP_ADD 5 OP_EQUAL+ demonstrates the arithmetic addition operator _OP_ADD_, adding two numbers and putting the result on the stack, followed by the conditional operator OP_EQUAL which checks the resulting sum is equal to +5+. For brevity, the OP_ prefix is omitted in the step-by-step example.
In <<simplemath_script>>, the script +2 3 OP_ADD 5 OP_EQUAL+ demonstrates the arithmetic addition operator +OP_ADD+, adding two numbers and putting the result on the stack, followed by the conditional operator +OP_EQUAL+, which checks that the resulting sum is equal to +5+. For brevity, the +OP_+ prefix is omitted in the step-by-step example.
drusselloctal@gmail.com
10 years ago