The Unlocking Script (ScriptSig)

In the Bitcoin protocol, every Transaction Input must provide a proof of ownership to spend the referenced coins. This proof is contained in a field called ScriptSig.

1. The Purpose of ScriptSig

ScriptSig is known as the "Unlocking Script." Its job is to provide data that satisfies the mathematical puzzle (the ScriptPubKey) placed on the coins by the previous owner.

2. A Two-Part Puzzle

To spend bitcoin, two scripts are combined and executed by the network's nodes:

1. ScriptPubKey (The Lock): Stored in the previous transaction's output. It says: "To spend these coins, you must provide a signature from the owner of Address X."

2. ScriptSig (The Key): Provided in your new transaction's input. It contains your signature and public key.

If the "Key" fits the "Lock," the transaction is valid.

3. Data Pushes

Technically, ScriptSig is a sequence of data pushes. It doesn't usually contain "logic" or "operators" (like OP_CHECKSIG). Instead, it pushes the necessary credentials onto the execution stack.

• Data Push 1: The Digital Signature.

• Data Push 2: The Public Key.

4. The Stack Concept

Bitcoin uses a stack-based execution model (Last-In, First-Out).

• The data in ScriptSig is pushed onto the stack first.

• Then, the operators in ScriptPubKey are executed using that data.

Property	Value
Role	Unlocking Script
Location	Transaction Input
Contents	Signature, Public Key, or Script
Nickname	The Key

In the next section, we will deconstruct the Components found inside a standard ScriptSig.