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@ -381,7 +381,7 @@ In this section, we will demonstrate the basic components of the bitcoin transac
((("locking scripts")))((("unlocking scripts")))((("scripting", "locking scripts")))A locking script is a spending condition placed on an output: it specifies the conditions that must be met to spend the output in the future. ((("scriptPubKey")))Historically, the locking script was called a _scriptPubKey_, because it usually contained a public key or bitcoin address (public key hash). In this book we refer to it as a "locking script" to acknowledge the much broader range of possibilities of this scripting technology. In most bitcoin applications, what we refer to as a locking script will appear in the source code as +scriptPubKey+. ((("witnesses")))((("cryptographic puzzles")))You will also see the locking script referred to as a _witness script_ (see <<segwit>>) or more generally as a _cryptographic puzzle_. These terms all mean the same thing, at different levels of abstraction.
An unlocking script is a script that "solves," or satisfies, the conditions placed on an output by a locking script and allows the output to be spent. Unlocking scripts are part of every transaction input. Most of the time they contain a digital signature produced by the user's wallet from his or her private key. Historically, the unlocking script is called _scriptSig_, because it usually contained a digital signature. In most bitcoin applications, the source code refers to the unlocking script as +scriptSig+. You will also see the unlocking script referred to as a _witness_ (see <<segwit>>). In this book, we refer to it as an "unlocking script" to acknowledge the much broader range of locking script requirements, because not all unlocking scripts must contain signatures.
An unlocking script is a script that "solves," or satisfies, the conditions placed on an output by a locking script and allows the output to be spent. Unlocking scripts are part of every transaction input. Most of the time they contain a digital signature produced by the user's wallet from his or her private key. ((("scriptSig")))Historically, the unlocking script is called _scriptSig_, because it usually contained a digital signature. In most bitcoin applications, the source code refers to the unlocking script as +scriptSig+. You will also see the unlocking script referred to as a _witness_ (see <<segwit>>). In this book, we refer to it as an "unlocking script" to acknowledge the much broader range of locking script requirements, because not all unlocking scripts must contain signatures.
Every bitcoin validating node will validate transactions by executing the locking and unlocking scripts together. Each input contains an unlocking script and refers to a previously existing UTXO. The validation software will copy the unlocking script, retrieve the UTXO referenced by the input, and copy the locking script from that UTXO. The unlocking and locking script are then executed in sequence. The input is valid if the unlocking script satisfies the locking script conditions (see <<script_exec>>). All the inputs are validated independently, as part of the overall validation of the transaction.
@ -436,7 +436,7 @@ image::images/mbc2_0604.png["TxScriptSimpleMathExample"]
[TIP]
====
Transactions are valid if the top result on the stack is +TRUE+ (noted as ++&#x7b;0x01&#x7d;++), any other nonzero value, or if the stack is empty after script execution. Transactions are invalid if the top value on the stack is +FALSE+ (a zero-length empty value, noted as ++&#x7b;&#x7d;++) or if script execution is halted explicitly by an operator, such as +OP_VERIFY+, +OP_RETURN+, or a conditional terminator such as +OP_ENDIF+. See <<tx_script_ops>> for details.
((("transactions", "valid and invalid")))Transactions are valid if the top result on the stack is +TRUE+ (noted as ++&#x7b;0x01&#x7d;++), any other nonzero value, or if the stack is empty after script execution. Transactions are invalid if the top value on the stack is +FALSE+ (a zero-length empty value, noted as ++&#x7b;&#x7d;++) or if script execution is halted explicitly by an operator, such as +OP_VERIFY+, +OP_RETURN+, or a conditional terminator such as +OP_ENDIF+. See <<tx_script_ops>> for details.
====
The following is a slightly more complex script, which calculates ++2 + 7 -- 3 + 1++. Notice that when the script contains several operators in a row, the stack allows the results of one operator to be acted upon by the next operator:
@ -450,7 +450,7 @@ Try validating the preceding script yourself using pencil and paper. When the sc
[[script_exec]]
===== Separate execution of unlocking and locking 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 next.
((("security", "locking and unlocking 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 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 left over), 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.