How I Built a Time-Locked Bitcoin Script with CSV and P2SH

HODLing is the beginning.

But Bitcoin was meant to be programmed.



“Not Just HODLing: Real Bitcoin Script Engineering” starts here.

Why Read This Article?

In the Bitcoin development world, most tutorials only teach you concepts, not how to actually write code, and even fewer guide you to thoroughly dissect a real Bitcoin transaction from the perspective of on-chain data and script principles.

This article uses a real case and complete code to help you master, in one go:

1. P2SH script validation mechanism




2. CSV (CheckSequenceVerify) timelock mechanism




3. P2PKH signature validation mechanism

You’ll see not just code, but also step-by-step on-chain data animation, as well as debug ideas and common error analysis at every stage.

Article Structure Preview

• Motivation and Case Overview




• Building and Spending the P2SH Script (Full Code)




• Three-Layer Script Mechanism Breakdown




• Animated Stack Execution Walkthrough




• Debugging and On-Chain Data Analysis




• Summary and Takeaways

1. Motivation and Case Overview

The goal of this case:

• Use Python to build a P2SH address that can only be spent after 3 blocks (a P2PKH script with a CSV timelock)




• Actually send and spend coins, broadcasting on-chain for real




• Through this single case, help you fully understand P2SH, CSV, and P2PKH script mechanisms, and be able to reverse every step in a block explorer

The real on-chain transaction for this case:

Step 2: Compare ScriptSig and scriptPubKey

• ScriptSig: See OP_PUSHBYTES_71 (signature), OP_PUSHBYTES_33 (pubkey), OP_PUSHBYTES_28 (redeem script)




• Previous output script (scriptPubKey): OP_HASH160 <20-byte hash> OP_EQUAL

Step 3: Manually step through script execution

• Use ScriptSig data to execute P2SH validation (OP_HASH160 + OP_EQUAL)




• If valid, execute redeem script: first CSV timelock (OP_CHECKSEQUENCEVERIFY), then P2PKH signature check




• Use the stack animation above to follow each step

Step 4: Compare HEX and ASM

• In the block explorer, find the HEX and ASM for ScriptSig and scriptPubKey




• Match each byte and opcode to your code and the execution steps

3. Debugging Tips

• When you see errors, check mempool.space or :

  
  
  
  
  

  • Each ScriptSig item (signature, pubkey, redeem script)

  
  

  • The scriptPubKey hash

  
  

  • The actual execution order and stack changes

  
  

  • If you spend too early, you’ll see non-BIP68-final

  
  
  
  
  
  
  
  

  With this code-onchain-debug workflow, you can not only write scripts, but also understand and debug every on-chain transaction.

  
  
  
  
  
  

  
  
  
  6. Summary and Takeaways
  

  
  
  

  Through a real case and code, you have mastered:

  
  
  
  
  

  • P2SH script validation

  
  

  • CSV timelock mechanism

  
  

  • P2PKH signature validation

  
  

  • Animated stack-based script execution

  
  

  • Debugging and on-chain data analysis

  
  
  
  
  

  This is the essence of Real Bitcoin Script Engineering.

  
  
  
  
  
  

  
  
  
  Source Code
  

  
  
  

  All code and on-chain data analysis are available at my

By .

on July 3, 2026.