OP_DUP - Duplicating the Top Item

3. OP_DUP — Duplicating the Top Item

Overview

OP_DUP is arguably the most widely used opcode in all of Bitcoin Script. It duplicates the topmost item on the stack, placing a copy of it on top. The original item remains in place. OP_DUP forms the cornerstone of the Pay-to-Public-Key-Hash (P2PKH) script type, which is the most common Bitcoin transaction output format used historically.

Opcode Reference

Opcode:     OP_DUP
Hex:        0x76
Word:       DUP
Input:      x
Output:     x x
Stack Before: [ ..., x ]
Stack After:  [ ..., x, x ]

How It Works

OP_DUP peeks at the top of the stack, creates a duplicate of the top item, and pushes that duplicate onto the stack. The original item is not consumed. After OP_DUP, there are two identical copies of the top item on the stack.

Execution Trace

Script: OP_5 OP_DUP

Step 1 — OP_5: Stack: [ 5 ]
Step 2 — OP_DUP: Stack: [ 5, 5 ]   ← a copy of 5 is pushed

The P2PKH Pattern

The canonical use of OP_DUP is in the P2PKH locking script:

Locking Script (scriptPubKey):
OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG

Unlocking Script (scriptSig):
<signature> <pubKey>

Combined script execution:

Step 1: Push

Stack: [ ]

Step 2: Push

Stack: [ , ]

Step 3: OP_DUP

Stack: [ , , ] ← pubKey is duplicated

Step 4: OP_HASH160

Stack: [ , , ]

Step 5: Push (from locking script)

Stack: [ , , , ]

Step 6: OP_EQUALVERIFY

Stack: [ , ] ← verified equal; both popped

Step 7: OP_CHECKSIG

Stack: [ TRUE ] ← signature verified against pubKey

This pattern ensures:

1. The spender provides a public key that hashes to the expected value.

2. The same public key is used to verify the signature. OP_DUP makes both checks possible from a single provided public key.

Why OP_DUP is Critical

Without OP_DUP, the P2PKH pattern would require the spender to push the public key twice, increasing transaction size and wasting block space. OP_DUP enables a single-push public key to serve double duty in the script.

Deep Copy vs Shallow Copy

In Bitcoin Script, all stack items are byte arrays. When OP_DUP creates a copy, it creates a byte-for-byte copy of the data. There is no concept of reference types or shared memory in Script — every duplication is a full copy.

Failure Conditions

• If the stack is empty, OP_DUP causes the script to fail.

Summary

OP_DUP is the foundation of P2PKH and many other Bitcoin script patterns. Its ability to duplicate the top stack item enables elegant single-pass verification patterns that would otherwise require redundant data in transaction inputs.