DEV Community

Cover image for How Bitcoin P2SH Scripts Work: From 2-of-3 Multisig to Timelocked Inheritance
aaron.recompile
aaron.recompile

Posted on • Originally published at Medium

How Bitcoin P2SH Scripts Work: From 2-of-3 Multisig to Timelocked Inheritance

#bitcoin

Excerpt from my book Mastering Taproot — full chapter.

HODLing is the beginning.
But Bitcoin was meant to be programmed.

"Not Just HODLing: Real Bitcoin Script Engineering" starts here.
Enter fullscreen mode Exit fullscreen mode

This article is an excerpt from my upcoming book Mastering Taproot, which explores the engineering principles behind Bitcoin’s most advanced scripting upgrade. You can follow the full book’s progress or purchase early-access chapters here:

👉 [https://leanpub.com/mastering-taproot]

# Chapter 3: P2SH Script Engineering — From Multi-signature to Time Locks

Chapter 3: P2SH Script Engineering — From Multi-signature to Time Locks

Bitcoin’s first real programmability emerges with Pay-to-Script-Hash (P2SH), where complex spending conditions are elegantly hidden behind a simple 20-byte hash. This chapter bridges the gap between basic P2PKH transactions and Taproot’s sophisticated script trees, demonstrating how Bitcoin’s script system enables real-world applications like corporate treasury management and time-locked inheritance planning.

3.1 P2SH Architecture: Scripts Behind the Hash

P2SH enables any script to be represented by a compact 20-byte hash, moving script complexity from the UTXO set to spending time.

The Two-Stage Verification Model

P2SH operates through two distinct phases:

Stage 1: Hash Verification

OP_HASH160 <script_hash> OP_EQUAL
Enter fullscreen mode Exit fullscreen mode

Stage 2: Script Execution

<revealed_script> → Execute as Bitcoin Script
Enter fullscreen mode Exit fullscreen mode

P2SH Address Generation Process

P2SH follows the same Hash160 → Base58Check pattern covered in Chapter 1, but hashes the script instead of a public key:

1. Script Serialization → hex_encoded_script
2. Hash160(script)     → 20_bytes_script_hash  
3. Version + Base58Check → 3...address (mainnet)
Enter fullscreen mode Exit fullscreen mode

All P2SH addresses begin with “3” on mainnet and “2” on testnet, immediately distinguishing them from P2PKH addresses.

ScriptSig Construction Pattern

The unlocking script (ScriptSig) for P2SH follows a specific pattern:

<script_data> <serialized_redeem_script>
Enter fullscreen mode Exit fullscreen mode

Where <script_data> contains the values needed to satisfy the redeem script's conditions, and <serialized_redeem_script> is the original script whose hash matches the locking script.

3.2 Multi-signature Treasury: 2-of-3 Corporate Security

Enterprise Bitcoin custody typically requires multi-party authorization to prevent single points of failure. A 2-of-3 multisignature scheme ensures that no single person can unilaterally access funds while maintaining operational flexibility.

Business Scenario: Startup Treasury Management

Consider a blockchain startup with three key stakeholders:

  • Alice: CEO with operational authority

  • Bob: CTO with technical oversight

  • Carol: CFO with financial controls

Their treasury policy requires any two signatures for fund movement, preventing both single-person risk and requiring unanimous consensus.

Multi-signature Script Construction

The redeem script implements the 2-of-3 logic using Bitcoin’s OP_CHECKMULTISIG opcode:

from bitcoinutils.setup import setup
from bitcoinutils.keys import PrivateKey
from bitcoinutils.script import Script
from bitcoinutils.keys import P2shAddress

def create_multisig_p2sh():
    setup('testnet')

    # Stakeholder public keys
    alice_pk = '02898711e6bf63f5cbe1b38c05e89d6c391c59e9f8f695da44bf3d20ca674c8519'
    bob_pk = '0284b5951609b76619a1ce7f48977b4312ebe226987166ef044bfb374ceef63af5'
    carol_pk = '0317aa89b43f46a0c0cdbd9a302f2508337ba6a06d123854481b52de9c20996011'

    # 2-of-3 multisig redeem script
    redeem_script = Script([
        'OP_2',           # Require 2 signatures
        alice_pk,         # Alice's public key
        bob_pk,           # Bob's public key  
        carol_pk,         # Carol's public key
        'OP_3',           # Total of 3 keys
        'OP_CHECKMULTISIG' # Multisig verification
    ])

    # Generate P2SH address
    p2sh_addr = P2shAddress.from_script(redeem_script)
    return p2sh_addr, redeem_script
Enter fullscreen mode Exit fullscreen mode

bitcoinutils Function Analysis

**Script([...])**** Constructor**: Creates a Script object from a list of opcodes and data. The library automatically handles the encoding of opcodes like 'OP_2' into their byte representations (0x52).

**P2shAddress.from_script(script)**: Generates a P2SH address by:

  1. Serializing the script to bytes

  2. Computing Hash160(script)

  3. Adding version byte (0x05 for mainnet, 0xc4 for testnet)

  4. Applying Base58Check encoding

Script Serialization: The redeem script serializes to:

522102898711e6bf63f5cbe1b38c05e89d6c391c59e9f8f695da44bf3d20ca674c8519210284b5951609b76619a1ce7f48977b4312ebe226987166ef044bfb374ceef63af5210317aa89b43f46a0c0cdbd9a302f2508337ba6a06d123854481b52de9c2099601153ae
Enter fullscreen mode Exit fullscreen mode

Breaking this down:

  • 52: OP_2

  • 21: Push 33 bytes (compressed public key)

  • 02898711...: Alice's public key

  • 21: Push 33 bytes

  • 0284b595...: Bob's public key

  • 21: Push 33 bytes

  • 0317aa89...: Carol's public key

  • 53: OP_3

  • ae: OP_CHECKMULTISIG

Spending the Multi-signature UTXO

When Alice and Bob decide to authorize a payment, they must provide their signatures in the correct order along with the redeem script:

def spend_multisig_p2sh():
    # Previous UTXO details
    utxo_txid = '4b869865bc4a156d7e0ba14590b5c8971e57b8198af64d88872558ca88a8ba5f'
    utxo_vout = 0
    utxo_amount = 0.00001600  # 1,600 satoshis

    # Create transaction
    txin = TxInput(utxo_txid, utxo_vout)
    txout = TxOutput(to_satoshis(0.00000888), recipient_address.to_script_pub_key())
    tx = Transaction([txin], [txout])

    # Sign with Alice and Bob's keys
    alice_sig = alice_sk.sign_input(tx, 0, redeem_script)
    bob_sig = bob_sk.sign_input(tx, 0, redeem_script)

    # Construct ScriptSig
    txin.script_sig = Script([
        'OP_0',                    # OP_CHECKMULTISIG bug workaround
        alice_sig,                 # First signature
        bob_sig,                   # Second signature  
        redeem_script.to_hex()     # Reveal the redeem script
    ])
Enter fullscreen mode Exit fullscreen mode

bitcoinutils Signature Functions

**private_key.sign_input(tx, input_index, script)**: Creates an ECDSA signature for a specific transaction input using the provided script for the signature hash calculation. The script parameter should be the redeem script for P2SH inputs.

**script.to_hex()**: Serializes the Script object into its hexadecimal byte representation, which is pushed as data onto the stack during script execution.

Multi-signature Stack Execution Analysis

Let’s trace through the complete script execution using our real transaction data, understanding Bitcoin Core’s two-phase P2SH execution mechanism:

Transaction ID: e68bef534c7536300c3ae5ccd0f79e031cab29d262380a37269151e8ba0fd4e0

Phase 1: ScriptSig + ScriptPubKey Execution

Initial State:

│ (empty)                                │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

1. PUSH OP_0: Multisig bug workaround

Bitcoin’s OP_CHECKMULTISIG has a known off-by-one bug that pops an extra item from the stack. An OP_0 is pushed to compensate.

│ 00 (op_zero)                          │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

2. PUSH Alice’s Signature: First authorization 

│ 30440220694f...7a6501 (alice_sig)     │
│ 00 (op_zero)                          │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

3. PUSH Bob’s Signature: Second authorization 

│ 3044022065f8...fd9e01 (bob_sig)       │
│ 30440220694f...7a6501 (alice_sig)     │
│ 00 (op_zero)                          │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

4. PUSH Redeem Script: Reveal the spending conditions 

│ 522102898711...601153ae (redeem_script) │
│ 3044022065f8...fd9e01 (bob_sig)          │
│ 30440220694f...7a6501 (alice_sig)        │
│ 00 (op_zero)                             │
└──────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

5. OP_HASH160: Verify script hash matches

The P2SH locking script OP_HASH160 <script_hash> OP_EQUAL is executed:

│ dd81b5beb3d8...5cb0ca (computed_hash)    │
│ 3044022065f8...fd9e01 (bob_sig)          │
│ 30440220694f...7a6501 (alice_sig)        │
│ 00 (op_zero)                             │
└──────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

6. PUSH Expected Hash: From locking script 

│ dd81b5beb3d8...5cb0ca (expected_hash)    │
│ dd81b5beb3d8...5cb0ca (computed_hash)    │
│ 3044022065f8...fd9e01 (bob_sig)          │
│ 30440220694f...7a6501 (alice_sig)        │
│ 00 (op_zero)                             │
└──────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

7. OP_EQUAL: Confirm hash match 

│ 1 (true)                               │
│ 3044022065f8...fd9e01 (bob_sig)        │
│ 30440220694f...7a6501 (alice_sig)      │
│ 00 (op_zero)                           │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

(Phase 1 complete: Hash verification successful)

P2SH Transition: Stack Reset Mechanism

Critical Point: Bitcoin Core recognizes the P2SH pattern and transitions to a second validation phase by:

  1. Detects P2SH pattern: OP_HASH160 <hash> OP_EQUAL

  2. Resets stack: Back to post-scriptSig state (discards TRUE)

  3. Extracts redeem script: From original scriptSig data

  4. Prepares clean execution: For redeem script with signature data

Stack Reset to Post-ScriptSig State:

│ 3044022065f8...fd9e01 (bob_sig)        │
│ 30440220694f...7a6501 (alice_sig)      │
│ 00 (op_zero)                           │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

(TRUE is discarded — redeem script begins with clean stack)

Phase 2: Redeem Script Execution

Bitcoin Core now executes the redeem script: OP_2 alice_pk bob_pk carol_pk OP_3 OP_CHECKMULTISIG

8. OP_2: Push required signature count 

│ 2 (required_sigs)                      │
│ 3044022065f8...fd9e01 (bob_sig)        │
│ 30440220694f...7a6501 (alice_sig)      │
│ 00 (op_zero)                           │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

9–11. PUSH Public Keys: Load verification keys 

│ 0317aa89b43f...996011 (carol_pk)       │
│ 0284b5951609...eef63af5 (bob_pk)       │
│ 02898711e6bf...674c8519 (alice_pk)     │
│ 2 (required_sigs)                      │
│ 3044022065f8...fd9e01 (bob_sig)        │
│ 30440220694f...7a6501 (alice_sig)      │
│ 00 (op_zero)                           │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

12. OP_3: Push total key count 

│ 3 (total_keys)                         │
│ 0317aa89b43f...996011 (carol_pk)       │
│ 0284b5951609...eef63af5 (bob_pk)       │
│ 02898711e6bf...674c8519 (alice_pk)     │
│ 2 (required_sigs)                      │
│ 3044022065f8...fd9e01 (bob_sig)        │
│ 30440220694f...7a6501 (alice_sig)      │
│ 00 (op_zero)                           │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

13. OP_CHECKMULTISIG: Verify signatures

The opcode consumes:

  • Key count (3)

  • Public keys (Alice, Bob, Carol)

  • Signature count (2)

  • Signatures (Alice’s, Bob’s)

  • Extra item (OP_0, due to bug)

Verification process:

  1. Alice’s signature verified against Alice’s public key ✓

  2. Bob’s signature verified against Bob’s public key ✓

  3. Required threshold (2-of-3) satisfied ✓

Final State: Multisig Verification Complete 

│ 1 (true)                               │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

(P2SH execution successful: Clean two-phase verification)

3.3 Time-Locked Inheritance: CSV-Enhanced P2SH

CheckSequenceVerify (CSV) enables relative time locks, where spending is delayed relative to when the UTXO was created. Let’s examine a real implementation using actual testnet data.

Real-World Implementation: 3-Block Time Lock

Transaction ID: 34f5bf0cf328d77059b5674e71442ded8cdcfc723d0136733e0dbf180861906f

This transaction demonstrates a P2SH script that combines CSV time lock with P2PKH signature verification — a common pattern for inheritance and escrow applications.

CSV Script Construction

Unlike complex conditional branching, the time lock uses a simple linear script:

from bitcoinutils.setup import setup
from bitcoinutils.transactions import Sequence
from bitcoinutils.constants import TYPE_RELATIVE_TIMELOCK

def create_csv_script():
    setup('testnet')

    # 3-block relative time lock
    relative_blocks = 3
    seq = Sequence(TYPE_RELATIVE_TIMELOCK, relative_blocks)

    # Combined CSV + P2PKH script
    redeem_script = Script([
        seq.for_script(),           # Push 3
        'OP_CHECKSEQUENCEVERIFY',   # Verify time lock
        'OP_DROP',                  # Remove delay value
        'OP_DUP',                   # Standard P2PKH starts here
        'OP_HASH160',
        p2pkh_addr.to_hash160(),
        'OP_EQUALVERIFY', 
        'OP_CHECKSIG'
    ])

    return redeem_script
Enter fullscreen mode Exit fullscreen mode

bitcoinutils CSV Functions

**Sequence(TYPE_RELATIVE_TIMELOCK, blocks)**: Creates a sequence object for relative block-based delays. The sequence value encodes the time constraint that will be enforced by OP_CHECKSEQUENCEVERIFY.

**seq.for_script()**: Returns the sequence value formatted for use in script opcodes (pushes the delay value onto the stack).

**seq.for_input_sequence()**: Returns the sequence value for the transaction input's sequence field, which CSV validates against.

Spending the Time-Locked UTXO 

def spend_csv_script():
    # Must wait 3 blocks after UTXO creation
    seq = Sequence(TYPE_RELATIVE_TIMELOCK, 3)

    # Set sequence in transaction input
    txin = TxInput(utxo_txid, utxo_vout, sequence=seq.for_input_sequence())

    # Provide signature and redeem script
    sig = private_key.sign_input(tx, 0, redeem_script)
    txin.script_sig = Script([
        sig,                        # Signature for P2PKH
        public_key.to_hex(),        # Public key for P2PKH  
        redeem_script.to_hex()      # Reveal the script
    ])
Enter fullscreen mode Exit fullscreen mode

CSV Stack Execution Analysis

Let’s trace through the execution using real transaction data from our testnet example:

ScriptSig Data:

  • Signature: 30440220... (71 bytes)

  • Public Key: 0250be5fc44ec580c387bf45df275aaa8b27e2d7716af31f10eeed357d126bb4d3 (33 bytes)

  • Redeem Script: 53b27576a9145cdc...88ac (28 bytes)

Phase 1: P2SH Hash Verification

(Stack reset mechanism applies — see multisig section for details)

Phase 2: CSV + P2PKH Execution

Initial State (after P2SH reset):

│ 0250be5fc44ec...4d3 (pubkey)           │
│ 30440220a1b2...c3d401 (signature)      │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

1. PUSH 3: Time delay requirement 

│ 3 (delay_blocks)                       │
│ 0250be5fc44ec...4d3 (pubkey)           │
│ 30440220a1b2...c3d401 (signature)      │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

2. OP_CHECKSEQUENCEVERIFY: Verify time lock

CSV validates that the transaction input’s sequence number ≥ 3:

│ 3 (delay_blocks)                       │
│ 0250be5fc44ec...4d3 (pubkey)           │
│ 30440220a1b2...c3d401 (signature)      │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

(Verification: nSequence ≥ 3 blocks since UTXO creation)

3. OP_DROP: Remove delay value 

│ 0250be5fc44ec...4d3 (pubkey)           │
│ 30440220a1b2...c3d401 (signature)      │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

4. OP_DUP: Begin P2PKH verification 

│ 0250be5fc44ec...4d3 (pubkey)           │
│ 0250be5fc44ec...4d3 (pubkey)           │
│ 30440220a1b2...c3d401 (signature)      │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

5. OP_HASH160: Hash public key 

│ 5cdc28d6b1876...cabaadcc (pubkey_hash) │
│ 0250be5fc44ec...4d3 (pubkey)           │
│ 30440220a1b2...c3d401 (signature)      │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

6. PUSH Expected Hash: From redeem script 

│ 5cdc28d6b1876...cabaadcc (expected_hash) │
│ 5cdc28d6b1876...cabaadcc (computed_hash) │
│ 0250be5fc44ec...4d3 (pubkey)             │
│ 30440220a1b2...c3d401 (signature)        │
└──────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

7. OP_EQUALVERIFY: Confirm hash match 

│ 0250be5fc44ec...4d3 (pubkey)           │
│ 30440220a1b2...c3d401 (signature)      │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

8. OP_CHECKSIG: Final signature verification 

│ 1 (true)                               │
└────────────────────────────────────────┘
Enter fullscreen mode Exit fullscreen mode

(Time lock satisfied and signature verified — CSV spending successful)

Time Lock Error Handling

Common Error: **`non-BIP68-final**`

If you attempt to spend before the time lock expires:

# This will fail if fewer than 3 blocks have passed
response = requests.post(mempool_api, data=signed_tx)
# Returns: "non-BIP68-final"
Enter fullscreen mode Exit fullscreen mode

The transaction is rejected because nSequence < required_delay, violating the CSV constraint.

CSV Applications

Digital Inheritance: Assets automatically become accessible to heirs after a specified period of owner inactivity.

Business Continuity: Corporate funds can include automatic release mechanisms for operational emergencies.

Payment Channels: Lightning Network uses CSV to enforce settlement delays, enabling dispute resolution periods.

3.4 P2SH vs P2PKH: Evolution of Script Complexity

P2SH extends Bitcoin Script from simple single-signature authorization to complex multi-party and temporal conditions, while maintaining the same compact address format.

P2SH’s Inherent Limitations

However, while P2SH improves efficiency by replacing explicit scripts with compact hashes, it still exposes the full redeem script during spending. This means every possible condition — regardless of which branch is taken — must be revealed when funds are spent. There’s no way to selectively reveal only the relevant branch.

This design makes P2SH inherently linear and opaque in structure. Unlike Taproot, it cannot express script trees or use Merkle branches to hide unused logic. Every signature path, time lock clause, or fallback condition is fully exposed on-chain.

In addition, since redeem scripts must be included in the scriptSig, P2SH transactions carry significant overhead in input size. This leads to higher fees and lower scalability, especially for multisig or inheritance setups.

Taproot addresses these limitations directly by allowing complex scripts to remain entirely hidden until needed, and by embedding them in a tree structure where only the executed path is revealed.

Chapter Summary

This chapter bridged the gap between basic P2PKH transactions and Taproot’s advanced capabilities by exploring P2SH’s two fundamental patterns: multi-signature authorization and time-locked conditions.

Key Concepts Mastered:

Two-Stage Verification: P2SH’s hash-then-execute model provides the conceptual foundation for Taproot’s commitment schemes, where complex scripts remain private until spending.

Multi-party Authorization: The 2-of-3 multisignature pattern demonstrates how Bitcoin Script handles conditional logic and multiple verification requirements — skills essential for understanding Taproot’s script tree execution.

Temporal Constraints: CSV-based time locks introduce relative time concepts that underpin Lightning Network and other Layer 2 protocols built on Taproot foundations.

Stack-Based Programming: Detailed stack execution traces for both multisig and time lock scenarios provide the analytical skills needed to debug and optimize Taproot script paths.

bitcoinutils Proficiency: Practical experience with Script construction, P2shAddress generation, and signature creation prepares developers for Taproot’s more sophisticated primitives.

Real Transaction Analysis: Working with actual testnet transactions and mempool data builds the empirical skills needed for production Taproot development.

The progression from P2PKH’s simple signature verification through P2SH’s complex conditional logic establishes the foundation for Taproot’s revolutionary approach: making sophisticated smart contracts indistinguishable from simple payments while providing unprecedented script flexibility and privacy.

In the next chapter, we’ll examine how SegWit’s witness structure revolutionized transaction malleability and fee calculation — concepts that directly enable Taproot’s efficiency improvements and form the basis for understanding P2TR’s witness-based spending paths.

— -

If you found this chapter useful, you’ll love what comes next: SegWit witness structures, Taproot trees, Merkle path construction, and real testnet path spending with control block analysis.

📘 Keep reading on Leanpub: Mastering Taproot

📩 You can also follow my updates on Twitter: [@aaron_recompile](https://twitter.com/aaron_recompile)

By Aaron Recompile on July 14, 2025.

Canonical link

Exported from Medium on July 3, 2026.

Top comments (0)