The P2PKH Unlocking Script

To spend a P2PKH output, the spender must provide two pieces of data in their input: a cryptographic signature and the original public key that hashes to the target.

1. The ScriptSig Template

The unlocking script looks like this: [Signature] [Public Key Bytes]

Notice that unlike P2PK, the spender must provide the public key here because it is not stored in the locking script.

2. Combined Execution Flow

When a node verifies the spend, it runs the scripts together on the stack: [Signature] [PubKey] OP_DUP OP_HASH160 [TargetHash] OP_EQUALVERIFY OP_CHECKSIG

The Step-by-Step Logic:

1. Push Data: Signature and PubKey are pushed onto the stack.

2. OP_DUP: PubKey is duplicated. Stack: [Sig, PubKey, PubKey].

3. OP_HASH160: The top PubKey is hashed. Stack: [Sig, PubKey, CalculatedHash].

4. Push Target: TargetHash (from the sender) is pushed. Stack: [Sig, PubKey, CalculatedHash, TargetHash].

5. OP_EQUALVERIFY: Compares the two hashes. If equal, they are popped. Stack: [Sig, PubKey].

6. OP_CHECKSIG: Verifies the Sig against the PubKey. If valid, pushes 1 (Success).

3. The Security Barrier

If an attacker tries to spend your coins with their own signature, they have two problems:

1. If they use your Public Key, their signature won't match.

2. If they use their Public Key, the OP_EQUALVERIFY step will fail because their key's hash doesn't match the TargetHash.

4. Size Considerations

Because a P2PKH spend requires both a signature (~71 bytes) and a public key (33-65 bytes), it is physically larger than a P2PK spend. However, because the outputs are smaller (20 bytes vs 65 bytes), P2PKH is considered more efficient for the overall health of the UTXO set.

Component	Provided By	Role in Execution
Signature	Spender	Proves ownership of private key
Public Key	Spender	Bridges the hash to the signature
Hash Target	Sender	Defines the owner

In the next section, we will discuss Base58Check Encoding.