OP_2DUP - Duplicating Top Two Items

4. OP_2DUP — Duplicating Top Two Items

Overview

OP_2DUP duplicates the top two items on the stack, preserving their order. After execution, the stack contains the original two items followed by two copies of those items on top. This opcode is useful when a pair of values — such as a signature and public key — need to be used in two separate verification steps.

Opcode Reference

Opcode:     OP_2DUP
Hex:        0x6e
Word:       2DUP
Input:      x1 x2
Output:     x1 x2 x1 x2
Stack Before: [ ..., x1, x2 ]   ← x2 is top
Stack After:  [ ..., x1, x2, x1, x2 ]

Execution Trace

Script: OP_3 OP_7 OP_2DUP

Step 1 — OP_3: Stack: [ 3 ]
Step 2 — OP_7: Stack: [ 3, 7 ]
Step 3 — OP_2DUP: Stack: [ 3, 7, 3, 7 ]

The pair (3, 7) is duplicated and placed on top in the same order.

Use Case: Dual Verification

OP_2DUP is used when a pair of values must be passed through two different verification routines:

<sig> <pubkey>
OP_2DUP
OP_CHECKSIG   ← first check
OP_SWAP OP_HASH160 <expected_hash> OP_EQUALVERIFY   ← second check

In this pattern:

• The pair <sig> <pubkey> is duplicated.

• One copy is used in OP_CHECKSIG.

• The other copy is used to verify the public key hash.

Relationship to OP_DUP

OP_DUP   → Duplicates 1 item: [ x ]       → [ x, x ]
OP_2DUP  → Duplicates 2 items: [ x1, x2 ] → [ x1, x2, x1, x2 ]

OP_2DUP is NOT equivalent to executing OP_DUP twice:

OP_DUP twice on [ x1, x2 ]:
  After 1st OP_DUP: [ x1, x2, x2 ]
  After 2nd OP_DUP: [ x1, x2, x2, x2 ]   ← x2 duplicated twice!

OP_2DUP on [ x1, x2 ]:

Result: [ x1, x2, x1, x2 ] ← both items duplicated once

Failure Conditions

• Requires at least 2 items on the stack; fails with underflow otherwise.

Summary

OP_2DUP provides efficient two-item duplication, preserving order and enabling paired-value operations like dual-channel verification in a single script execution pass.