What is scriptSig? (Unlocking Scripts and Execution)

If the scriptPubKey is the lock, the scriptSig is the key. Inside every transaction input is a scriptSig field containing the cryptographic proof—signatures and public keys—required to unlock and spend the referenced UTXO.

️ How Nodes Execute Validation Scripts

To confirm that Alice is authorized to spend her coins, a validating full node must run Alice's unlocking script against the output's locking script.

The Historical Vulnerability: Concatenation

In early versions of Bitcoin, nodes validated transactions by physically concatenating the two scripts together:

$$\text{Combined Script} = \text{scriptSig} + \text{scriptPubKey}$$

The node then ran this single combined string from left to right. However, this introduced a severe security flaw: a malicious spender could craft a scriptSig containing custom opcodes (such as a premature execution bypass) that manipulated the trailing scriptPubKey behavior.

️ The Modern Fix: Isolated Execution

To close this security hole, modern nodes validate scripts in two isolated phases:

┌────────────────────────────────────────────────────────┐
│ PHASE 1: scriptSig EXECUTION │
├────────────────────────────────────────────────────────┤
│ • Node executes scriptSig on an empty Stack. │
│ • Pushes Signatures and Public Keys onto the Stack. │
└───────────────────────────┬────────────────────────────┘
 │
 ▼ (Passes State)
┌────────────────────────────────────────────────────────┐
│ PHASE 2: scriptPubKey EXECUTION │
├────────────────────────────────────────────────────────┤
│ • Node executes scriptPubKey using the Phase 1 Stack. │
│ • Runs cryptographic operations on the pushed keys. │
└───────────────────────────┬────────────────────────────┘
 │
 ▼ (Evaluates Final State)
 Is Top Item = 1?
 [YES] ──► Spend Approved (Valid)
 [NO] ──► Transaction Rejected (Invalid)

Step-by-Step Stack Trace of P2PKH Validation

Let's watch a node evaluate a standard legacy payment (P2PKH).

• Alice's scriptSig (Unlocking Script): <Alice_Sig> <Alice_PubKey>

• Previous scriptPubKey (Locking Script): OP_DUP OP_HASH160 <PubKeyHash> OP_EQUALVERIFY OP_CHECKSIG

Below is the cycle-by-cycle trace of the virtual machine's RAM Stack during validation:

The Final Result:

The execution terminates with exactly 1 (TRUE) on top of the stack. Because the top item is non-zero, the validation checks out, and Alice's transaction is verified as mathematically authentic!