@btc-vision/btc-runtime/docs/advanced/bitcoin-scripts.md

Bitcoin L1 Smart Contract Runtime for OP_NET. Build decentralized applications on Bitcoin using AssemblyScript and WebAssembly. Fully audited.

# Bitcoin Scripts

OP_NET provides utilities for working with Bitcoin scripts, addresses, and timelocks. This enables contracts to interact with Bitcoin's native scripting capabilities.

## Overview

```typescript
import {
    Blockchain,
    BitcoinOpcodes,
    BitcoinScript,
    BitcoinAddresses,
    ScriptNumber,
    ScriptIO,
    Segwit,
    Networks,
    Network,
} from '@btc-vision/btc-runtime/runtime';
```

## Networks

OP_NET supports three Bitcoin networks:

```typescript
import { Networks, Network } from '@btc-vision/btc-runtime/runtime';

// Network enum values
Networks.Unknown  // -1 (uninitialized)
Networks.Mainnet  // 0
Networks.Testnet  // 1
Networks.Regtest  // 2

// Get human-readable prefix for addresses
const hrp = Network.hrp(Networks.Mainnet);  // "bc"
const hrpTestnet = Network.hrp(Networks.Testnet);  // "tb"
const hrpRegtest = Network.hrp(Networks.Regtest);  // "bcrt"

// Get chain ID (32-byte identifier)
const chainId = Network.getChainId(Networks.Mainnet);

// Convert chain ID back to network enum
const network = Network.fromChainId(chainId);  // Networks.Mainnet
```

## Address Types

### P2PKH (Pay to Public Key Hash)

```mermaid
---
config:
  theme: dark
---
flowchart LR
    PK["Public Key<br/>33 bytes compressed"] --> H1["SHA256"]
    H1 --> H2["RIPEMD160"]
    H2 --> PKH["20-byte hash"]
    PKH --> B58["Base58Check Encoding"]
    B58 --> ADDR["Address: 1..."]
```

### P2SH (Pay to Script Hash)

```mermaid
---
config:
  theme: dark
---
flowchart LR
    SC["Redeem Script"] --> SH1["SHA256"]
    SH1 --> SH2["RIPEMD160"]
    SH2 --> SHH["20-byte hash"]
    SHH --> B58["Base58Check Encoding"]
    B58 --> ADDR["Address: 3..."]
```

### P2WPKH (Pay to Witness Public Key Hash)

```mermaid
---
config:
  theme: dark
---
flowchart LR
    PK["Public Key<br/>33 bytes"] --> H1["SHA256"]
    H1 --> H2["RIPEMD160"]
    H2 --> WPH["20-byte hash"]
    WPH --> SEG["Bech32 Encoding"]
    SEG --> ADDR["Address: bc1q..."]
```

```typescript
import { BitcoinAddresses, Segwit } from '@btc-vision/btc-runtime/runtime';

// P2WPKH from compressed public key
public createP2WPKHAddress(pubkey: Uint8Array, hrp: string): string {
    return BitcoinAddresses.p2wpkh(pubkey, hrp);
}
```

### P2TR (Pay to Taproot)

```mermaid
---
config:
  theme: dark
---
flowchart LR
    PK["x-only Public Key<br/>32 bytes"] --> TR["Tweak key"]
    TR --> TRK["Tweaked key<br/>32 bytes"]
    TRK --> SEG["Bech32m Encoding"]
    SEG --> ADDR["Address: bc1p..."]
```

```typescript
import { BitcoinAddresses, Networks, Network } from '@btc-vision/btc-runtime/runtime';

// P2TR address from 32-byte x-only public key
public createP2TRAddress(outputKeyX32: Uint8Array): string {
    const hrp = Network.hrp(Networks.Mainnet);
    return BitcoinAddresses.p2trKeyPathAddress(outputKeyX32, hrp);
}

// Verify a P2TR address
public verifyP2TRAddress(outputKeyX32: Uint8Array, address: string): bool {
    const hrp = Network.hrp(Networks.Mainnet);
    return BitcoinAddresses.verifyP2trAddress(outputKeyX32, address, hrp);
}
```

### P2WSH (Pay to Witness Script Hash)

For complex scripts requiring witness script hash:

```typescript
import { BitcoinAddresses, Segwit, sha256 } from '@btc-vision/btc-runtime/runtime';

// Create P2WSH from script
public createP2WSHAddress(script: Uint8Array, hrp: string): string {
    return Segwit.p2wsh(hrp, script);
}

// Create multisig P2WSH address
public createMultisigP2WSH(
    m: i32,
    pubkeys: Array<Uint8Array>,
    hrp: string
): MultisigP2wshResult {
    return BitcoinAddresses.multisigP2wshAddress(m, pubkeys, hrp);
}
```

## Bitcoin Opcodes

The `BitcoinOpcodes` class provides all standard Bitcoin opcodes:

```typescript
import { BitcoinOpcodes } from '@btc-vision/btc-runtime/runtime';

// Stack operations
BitcoinOpcodes.OP_DUP         // 118 - Duplicate top stack item
BitcoinOpcodes.OP_DROP        // 117 - Remove top stack item
BitcoinOpcodes.OP_SWAP        // 124 - Swap top two items

// Crypto operations
BitcoinOpcodes.OP_HASH160     // 169 - RIPEMD160(SHA256(x))
BitcoinOpcodes.OP_SHA256      // 168 - SHA256(x)
BitcoinOpcodes.OP_CHECKSIG    // 172 - Verify signature
BitcoinOpcodes.OP_CHECKMULTISIG // 174 - Verify multiple signatures

// Control flow
BitcoinOpcodes.OP_IF          // 99 - Conditional execution
BitcoinOpcodes.OP_ELSE        // 103
BitcoinOpcodes.OP_ENDIF       // 104
BitcoinOpcodes.OP_VERIFY      // 105 - Verify top is truthy
BitcoinOpcodes.OP_RETURN      // 106 - Mark output as unspendable

// Timelocks
BitcoinOpcodes.OP_CHECKLOCKTIMEVERIFY  // 177 - nLockTime check
BitcoinOpcodes.OP_CHECKSEQUENCEVERIFY  // 178 - Relative timelock

// Number opcodes (OP_0 through OP_16)
BitcoinOpcodes.OP_0           // 0 - Push empty/zero
BitcoinOpcodes.OP_1           // 81 - Push 1
BitcoinOpcodes.OP_2           // 82 - Push 2
BitcoinOpcodes.OP_3           // 83 - Push 3
// ... up to OP_16

// Get OP_N dynamically
BitcoinOpcodes.opN(5);        // Returns OP_5 (85)
```

## Script Building

### Building Scripts with BytesWriter

```typescript
import { BytesWriter, BitcoinOpcodes, ScriptIO, ScriptNumber } from '@btc-vision/btc-runtime/runtime';

// Build a simple P2PKH-style script
public buildP2PKHScript(pubkeyHash: Uint8Array): Uint8Array {
    const script = new BytesWriter(25);
    script.writeU8(BitcoinOpcodes.OP_DUP);
    script.writeU8(BitcoinOpcodes.OP_HASH160);
    script.writeU8(20);  // Push 20 bytes
    script.writeBytes(pubkeyHash);
    script.writeU8(BitcoinOpcodes.OP_EQUALVERIFY);
    script.writeU8(BitcoinOpcodes.OP_CHECKSIG);
    return script.getBuffer();
}
```

### Building Multisig Scripts

Use the built-in `BitcoinScript.multisig()` method:

```typescript
import { BitcoinScript } from '@btc-vision/btc-runtime/runtime';

// Build a 2-of-3 multisig script
public buildMultisigScript(
    pubkey1: Uint8Array,
    pubkey2: Uint8Array,
    pubkey3: Uint8Array
): Uint8Array {
    const pubkeys = new Array<Uint8Array>(3);
    pubkeys[0] = pubkey1;
    pubkeys[1] = pubkey2;
    pubkeys[2] = pubkey3;

    // m = 2 (required signatures), n = 3 (total keys)
    return BitcoinScript.multisig(2, pubkeys);
}
```

Manual multisig script building:

```typescript
public buildMultisigManual(
    pubkey1: Uint8Array,
    pubkey2: Uint8Array,
    pubkey3: Uint8Array
): Uint8Array {
    const script = new BytesWriter(105);

    // OP_2 - require 2 signatures
    script.writeU8(BitcoinOpcodes.OP_2);

    // Push pubkey1 (33 bytes compressed)
    script.writeU8(33);
    script.writeBytes(pubkey1);

    // Push pubkey2
    script.writeU8(33);
    script.writeBytes(pubkey2);

    // Push pubkey3
    script.writeU8(33);
    script.writeBytes(pubkey3);

    // OP_3 - 3 total pubkeys
    script.writeU8(BitcoinOpcodes.OP_3);

    // OP_CHECKMULTISIG
    script.writeU8(BitcoinOpcodes.OP_CHECKMULTISIG);

    return script.getBuffer();
}
```

## Timelock Scripts

### CSV (CheckSequenceVerify) - Relative Timelock

CSV enables relative timelocks based on block count or time since the UTXO was confirmed:

```mermaid
---
config:
  theme: dark
---
flowchart LR
    CSV1["Transaction Input<br/>nSequence field"] --> CSV2{"Timelock Type?"}
    CSV2 -->|"Block-based"| CSV3["Blocks since confirmation"]
    CSV2 -->|"Time-based"| CSV4["512-second units + FLAG"]
    CSV3 --> CSV5["OP_CSV verifies:<br/>nSequence >= CSV_VALUE"]
    CSV4 --> CSV5
    CSV5 --> CSV6{"Valid?"}
    CSV6 -->|"Yes"| CSV7["Continue execution"]
    CSV6 -->|"No"| CSV8["Transaction invalid"]
```

OP_NET provides a built-in method for CSV scripts:

```typescript
import { BitcoinScript, BitcoinAddresses, Network, Networks } from '@btc-vision/btc-runtime/runtime';

// Create a CSV timelock script (144 blocks = ~1 day)
public createCSVScript(pubkey: Uint8Array, lockBlocks: i32): Uint8Array {
    // lockBlocks must be 0-65535
    return BitcoinScript.csvTimelock(pubkey, lockBlocks);
}

// Create a P2WSH address with CSV timelock
public createCSVAddress(pubkey: Uint8Array, lockBlocks: i32): CsvP2wshResult {
    const hrp = Network.hrp(Networks.Mainnet);
    return BitcoinAddresses.csvP2wshAddress(pubkey, lockBlocks, hrp);
}

// Verify a CSV P2WSH address
public verifyCSVAddress(
    pubkey: Uint8Array,
    lockBlocks: i32,
    address: string
): bool {
    const hrp = Network.hrp(Networks.Mainnet);
    return BitcoinAddresses.verifyCsvP2wshAddress(pubkey, lockBlocks, address, hrp);
}
```

### Manual CSV Script Building

```typescript
import { BytesWriter, BitcoinOpcodes, ScriptNumber, ScriptIO } from '@btc-vision/btc-runtime/runtime';

public buildCSVScriptManual(pubkey: Uint8Array, lockBlocks: i32): Uint8Array {
    const script = new BytesWriter(50);

    // Push lock value using proper encoding
    if (lockBlocks == 0) {
        script.writeU8(BitcoinOpcodes.OP_0);
    } else if (lockBlocks <= 16) {
        // Use OP_1 through OP_16
        script.writeU8(BitcoinOpcodes.opN(lockBlocks));
    } else {
        // Encode as script number and push
        const encoded = ScriptNumber.encode(lockBlocks);
        ScriptIO.writePush(script, encoded);
    }

    // CSV check
    script.writeU8(BitcoinOpcodes.OP_CHECKSEQUENCEVERIFY);
    script.writeU8(BitcoinOpcodes.OP_DROP);

    // Then normal sig check
    ScriptIO.writePush(script, pubkey);
    script.writeU8(BitcoinOpcodes.OP_CHECKSIG);

    return script.getBuffer();
}
```

### Time-Based Timelock

```typescript
// Lock for ~1 week (in 512-second units)
// Bit 22 set indicates time-based
const TIME_FLAG: u32 = 0x400000;
const SECONDS_PER_UNIT: u32 = 512;

public buildTimeCSV(pubkey: Uint8Array, seconds: u32): Uint8Array {
    const units = seconds / SECONDS_PER_UNIT;
    const csvValue = TIME_FLAG | units;

    const script = new BytesWriter(50);

    // Encode the time-based CSV value
    const encoded = ScriptNumber.encode(csvValue);
    ScriptIO.writePush(script, encoded);

    script.writeU8(BitcoinOpcodes.OP_CHECKSEQUENCEVERIFY);
    script.writeU8(BitcoinOpcodes.OP_DROP);
    ScriptIO.writePush(script, pubkey);
    script.writeU8(BitcoinOpcodes.OP_CHECKSIG);

    return script.getBuffer();
}
```

### CLTV (CheckLockTimeVerify) - Absolute Timelock

CLTV enables absolute timelocks based on block height or Unix timestamp:

```mermaid
---
config:
  theme: dark
---
flowchart LR
    CLTV1["Transaction<br/>nLockTime field"] --> CLTV2{"Timelock Type?"}
    CLTV2 -->|"Block-based"| CLTV3["Block height<br/>Value < 500000000"]
    CLTV2 -->|"Time-based"| CLTV4["Unix timestamp<br/>Value >= 500000000"]
    CLTV3 --> CLTV5["OP_CLTV verifies:<br/>nLockTime >= CLTV_VALUE"]
    CLTV4 --> CLTV5
    CLTV5 --> CLTV6{"Valid?"}
    CLTV6 -->|"Yes"| CLTV7["Continue execution"]
    CLTV6 -->|"No"| CLTV8["Transaction invalid"]
```

## Script Number Encoding

Bitcoin Script uses a unique number encoding format:

```typescript
import { ScriptNumber } from '@btc-vision/btc-runtime/runtime';

// Encode a number for script
const encoded: Uint8Array = ScriptNumber.encode(144);

// Get encoded length without encoding
const len: i32 = ScriptNumber.encodedLen(144);

// Decode a script number
const decoded: i64 = ScriptNumber.decode(encoded);

// Safe decoding with result type
const result = ScriptNumber.decodeResult(encoded, true);
if (result.success) {
    const value = result.value;
} else {
    const error = result.error;
}
```

## Script Recognition

OP_NET can parse and recognize common script patterns:

### Recognize CSV Timelock

```typescript
import { BitcoinScript } from '@btc-vision/btc-runtime/runtime';

public parseCSVScript(script: Uint8Array): void {
    const result = BitcoinScript.recognizeCsvTimelock(script);

    if (result.ok) {
        const csvBlocks: i64 = result.csvBlocks;
        const pubkey: Uint8Array | null = result.pubkey;
        // Process recognized script
    }
}
```

### Recognize Multisig

```typescript
public parseMultisigScript(script: Uint8Array): void {
    const result = BitcoinScript.recognizeMultisig(script);

    if (result.ok) {
        const m: i32 = result.m;           // Required signatures
        const n: i32 = result.n;           // Total keys
        const keys: Array<Uint8Array> | null = result.pubkeys;
    }
}
```

## Transaction Parsing

### Reading Transaction Outputs

```typescript
// Access current transaction outputs
public analyzeOutputs(): void {
    const outputs = Blockchain.tx.outputs;

    for (let i = 0; i < outputs.length; i++) {
        const output = outputs[i];

        // Output value in satoshis
        const value: u64 = output.value;

        // Output script (may be null)
        const script: Uint8Array | null = output.scriptPublicKey;

        // Check if output has script before parsing
        if (script !== null) {
            // Parse script type
            if (this.isP2TR(script)) {
                // Taproot output
            } else if (this.isP2WSH(script)) {
                // Witness script hash
            }
        }
    }
}

private isP2TR(script: Uint8Array): bool {
    // P2TR: OP_1 <32-byte key>
    return script.length == 34 && script[0] == 0x51 && script[1] == 0x20;
}

private isP2WSH(script: Uint8Array): bool {
    // P2WSH: OP_0 <32-byte hash>
    return script.length == 34 && script[0] == 0x00 && script[1] == 0x20;
}

private isP2WPKH(script: Uint8Array): bool {
    // P2WPKH: OP_0 <20-byte hash>
    return script.length == 22 && script[0] == 0x00 && script[1] == 0x14;
}
```

### Parsing Inputs

```typescript
// Access transaction inputs
public analyzeInputs(): void {
    const inputs = Blockchain.tx.inputs;

    for (let i = 0; i < inputs.length; i++) {
        const input = inputs[i];

        // Previous transaction hash
        const txId: Uint8Array = input.txId;

        // Output index being spent (u16)
        const outputIndex: u16 = input.outputIndex;

        // Script signature
        const scriptSig: Uint8Array = input.scriptSig;

        // Witness data (may be null)
        const witnesses: Uint8Array[] | null = input.witnesses;

        // Check if input has witnesses
        if (input.hasWitnesses && witnesses !== null) {
            // Process witness data
        }

        // Check if this is a coinbase input
        if (input.isCoinbase) {
            // This is a coinbase transaction
        }
    }
}
```

## Common Patterns

### Escrow with CSV Timeout

```typescript
// Build escrow script: Either both parties agree, or timeout to sender
public buildEscrowScript(
    senderPubkey: Uint8Array,
    recipientPubkey: Uint8Array,
    timeoutBlocks: i32
): Uint8Array {
    const script = new BytesWriter(150);

    // Path 1: Both signatures
    script.writeU8(BitcoinOpcodes.OP_IF);
    script.writeU8(BitcoinOpcodes.OP_2);
    script.writeU8(33);
    script.writeBytes(senderPubkey);
    script.writeU8(33);
    script.writeBytes(recipientPubkey);
    script.writeU8(BitcoinOpcodes.OP_2);
    script.writeU8(BitcoinOpcodes.OP_CHECKMULTISIG);

    // Path 2: Timeout to sender
    script.writeU8(BitcoinOpcodes.OP_ELSE);

    // Push CSV blocks
    if (timeoutBlocks <= 16) {
        script.writeU8(BitcoinOpcodes.opN(timeoutBlocks));
    } else {
        const encoded = ScriptNumber.encode(timeoutBlocks);
        ScriptIO.writePush(script, encoded);
    }

    script.writeU8(BitcoinOpcodes.OP_CHECKSEQUENCEVERIFY);
    script.writeU8(BitcoinOpcodes.OP_DROP);
    script.writeU8(33);
    script.writeBytes(senderPubkey);
    script.writeU8(BitcoinOpcodes.OP_CHECKSIG);
    script.writeU8(BitcoinOpcodes.OP_ENDIF);

    return script.getBuffer();
}
```

### Verifying Output to Contract

```typescript
// Verify transaction sends to this contract
public verifyPaymentToContract(requiredAmount: u64): bool {
    const contractAddress = Blockchain.contract.address;
    const outputs = Blockchain.tx.outputs;

    for (let i = 0; i < outputs.length; i++) {
        const output = outputs[i];

        if (output.value >= requiredAmount) {
            // Check if output is to contract address
            if (this.outputMatchesAddress(output.script, contractAddress)) {
                return true;
            }
        }
    }

    return false;
}

private outputMatchesAddress(script: Uint8Array, address: Address): bool {
    // Implementation depends on script type
    // For P2TR, compare the 32-byte key in script[2..34] with address bytes
    if (script.length == 34 && script[0] == 0x51 && script[1] == 0x20) {
        for (let i: i32 = 0; i < 32; i++) {
            if (script[i + 2] != address[i]) return false;
        }
        return true;
    }
    return false;
}
```

### Create OP_RETURN Data

```typescript
// Embed data in an OP_RETURN output
public buildOpReturnScript(data: Uint8Array): Uint8Array {
    if (data.length > 80) {
        throw new Revert('OP_RETURN data too large');
    }

    const script = new BytesWriter(data.length + 2);
    script.writeU8(BitcoinOpcodes.OP_RETURN);
    script.writeU8(u8(data.length));
    script.writeBytes(data);

    return script.getBuffer();
}
```

## Solidity vs OP_NET: Bitcoin Scripts Comparison

Bitcoin scripting is fundamentally different from Solidity, as it operates on UTXOs rather than account balances. OP_NET uniquely bridges smart contract functionality with native Bitcoin scripting capabilities.

### Feature Comparison Table

| Feature | Solidity/EVM | OP_NET | OP_NET Advantage |
|---------|--------------|-------|-----------------|
| **Bitcoin Script Support** | Not supported | Full support via `BitcoinOpcodes` | Native Bitcoin integration |
| **Address Types** | Single type (20 bytes) | P2PKH, P2SH, P2WPKH, P2WSH, P2TR | Full Bitcoin address compatibility |
| **Native Timelocks** | Custom implementation required | OP_CLTV, OP_CSV built-in | Consensus-enforced timelocks |
| **Multi-signature** | Custom contract logic | Native OP_CHECKMULTISIG | Bitcoin-native security |
| **Script Execution Model** | Turing-complete EVM | Stack-based Bitcoin Script | Predictable, secure execution |
| **Data Embedding** | Events, storage (expensive) | OP_RETURN (80 bytes) | Immutable on-chain data |
| **Taproot Support** | Not applicable | Full P2TR support | Schnorr-based privacy |
| **Witness Scripts** | Not applicable | P2WSH, SegWit support | Lower transaction fees |
| **Network Awareness** | Chain ID only | Mainnet/Testnet/Regtest support | Full Bitcoin network support |
| **UTXO Introspection** | Not possible | Full transaction input/output access | Bitcoin transaction analysis |

### Capability Matrix

| Capability | Solidity | OP_NET |
|------------|:--------:|:-----:|
| Create P2PKH addresses | No | Yes |
| Create P2SH addresses | No | Yes |
| Create P2WPKH (SegWit) addresses | No | Yes |
| Create P2WSH (Witness Script) addresses | No | Yes |
| Create P2TR (Taproot) addresses | No | Yes |
| Build multisig scripts | No | Yes |
| Build timelock scripts (CSV) | No | Yes |
| Build absolute timelock scripts (CLTV) | No | Yes |
| Parse/recognize Bitcoin scripts | No | Yes |
| Access transaction inputs | No | Yes |
| Access transaction outputs | No | Yes |
| Verify Bitcoin script patterns | No | Yes |
| Create OP_RETURN data | No | Yes |

### Timelock Comparison

#### Solidity Approach (Custom Implementation)

```solidity
// Solidity - time-based lock (requires custom implementation)
contract TimeLock {
    uint256 public unlockTime;
    mapping(address => uint256) public deposits;

    constructor(uint256 _lockDuration) {
        unlockTime = block.timestamp + _lockDuration;
    }

    function deposit() external payable {
        deposits[msg.sender] += msg.value;
    }

    function withdraw() external {
        require(block.timestamp >= unlockTime, "Still locked");
        uint256 amount = deposits[msg.sender];
        deposits[msg.sender] = 0;
        payable(msg.sender).transfer(amount);
    }

    // Limitations:
    // - Relies on block.timestamp (can be manipulated by miners)
    // - No relative timelocks
    // - Must implement custom logic
    // - No Bitcoin script compatibility
}
```

#### OP_NET Approach (Consensus-Enforced)

```typescript
// OP_NET - CSV relative timelock (consensus-enforced)
import { BitcoinScript, BitcoinAddresses, Network, Networks } from '@btc-vision/btc-runtime/runtime';

// Create a timelock script - 144 blocks = ~1 day
const csvScript = BitcoinScript.csvTimelock(pubkey, 144);

// Create P2WSH address with CSV timelock
const result = BitcoinAddresses.csvP2wshAddress(pubkey, 144, Network.hrp(Networks.Mainnet));
const address = result.address;
const witnessScript = result.witnessScript;

// Advantages:
// - Enforced by Bitcoin consensus (not contract logic)
// - Cannot be manipulated by miners
// - Supports both relative (CSV) and absolute (CLTV) timelocks
// - Native to Bitcoin - no custom implementation needed
// - Works with any Bitcoin wallet
```

### Multisig Comparison

#### Solidity Approach (Custom Contract)

```solidity
// Solidity - custom multisig (complex, resource-intensive)
contract MultiSig {
    mapping(address => bool) public owners;
    uint256 public required;
    uint256 public transactionCount;

    struct Transaction {
        address to;
        uint256 value;
        bytes data;
        bool executed;
        uint256 confirmations;
    }

    mapping(uint256 => Transaction) public transactions;
    mapping(uint256 => mapping(address => bool)) public confirmations;

    function submitTransaction(address to, uint256 value, bytes memory data) public returns (uint256) {
        // ... complex submission logic
    }

    function confirmTransaction(uint256 transactionId) public {
        require(owners[msg.sender], "Not owner");
        confirmations[transactionId][msg.sender] = true;
        // ... confirmation logic
    }

    function executeTransaction(uint256 transactionId) public {
        Transaction storage txn = transactions[transactionId];
        require(txn.confirmations >= required, "Not enough confirmations");
        // ... execution logic
    }

    // Limitations:
    // - High storage costs
    // - Complex signature verification
    // - Must manually count confirmations
    // - No native Bitcoin integration
}
```

#### OP_NET Approach (Native Bitcoin Multisig)

```typescript
// OP_NET - native Bitcoin multisig (simple, consensus-enforced)
import { BitcoinScript, BitcoinAddresses, Network, Networks } from '@btc-vision/btc-runtime/runtime';

// Build a 2-of-3 multisig script - one line of code!
const multisigScript = BitcoinScript.multisig(2, [pubkey1, pubkey2, pubkey3]);

// Create P2WSH address for the multisig
const result = BitcoinAddresses.multisigP2wshAddress(2, [pubkey1, pubkey2, pubkey3], Network.hrp(Networks.Mainnet));
const address = result.address;

// Verify and parse existing multisig scripts
const recognized = BitcoinScript.recognizeMultisig(someScript);
if (recognized.ok) {
    const requiredSigs = recognized.m;  // e.g., 2
    const totalKeys = recognized.n;      // e.g., 3
    const publicKeys = recognized.pubkeys;
}

// Advantages:
// - No custom signature verification needed
// - Bitcoin consensus handles signature counting
// - Works with any Bitcoin wallet supporting multisig
// - No storage costs for signature storage
// - Native OP_CHECKMULTISIG opcode
```

### Script Building Comparison

| Task | Solidity | OP_NET |
|------|----------|-------|
| Build P2PKH script | Not possible | `buildP2PKHScript(pubkeyHash)` |
| Build multisig script | Custom contract | `BitcoinScript.multisig(m, pubkeys)` |
| Build CSV timelock | Custom logic | `BitcoinScript.csvTimelock(pubkey, blocks)` |
| Create witness address | Not possible | `Segwit.p2wsh(hrp, script)` |
| Parse script patterns | Not possible | `BitcoinScript.recognizeCsvTimelock(script)` |
| Encode script numbers | Not needed | `ScriptNumber.encode(value)` |
| Access Bitcoin opcodes | Not available | All opcodes via `BitcoinOpcodes` |

### Transaction Introspection

| Feature | Solidity | OP_NET |
|---------|----------|-------|
| Access tx outputs | `msg.value` only | `Blockchain.tx.outputs` (full array) |
| Access tx inputs | Not possible | `Blockchain.tx.inputs` (full array) |
| Get output value | Limited | `output.value` (satoshis) |
| Get output script | Not possible | `output.scriptPublicKey` (full script bytes, nullable) |
| Get input txid | Not possible | `input.txId` |
| Get input output index | Not possible | `input.outputIndex` (u16) |
| Get witness data | Not possible | `input.witnesses` (array, nullable) |
| Get script signature | Not possible | `input.scriptSig` |
| Check coinbase input | Not possible | `input.isCoinbase` |
| Parse script type | Not possible | Pattern matching on script bytes |

### Data Embedding Comparison

```solidity
// Solidity - Events (expensive, not part of state)
contract DataEmbed {
    event DataStored(bytes32 indexed hash, bytes data);

    function storeData(bytes memory data) external {
        emit DataStored(keccak256(data), data);
        // Cost scales with data size
        // Data is not part of UTXO set
    }
}
```

```typescript
// OP_NET - OP_RETURN (native Bitcoin, permanent)
import { BytesWriter, BitcoinOpcodes } from '@btc-vision/btc-runtime/runtime';

function buildOpReturnScript(data: Uint8Array): Uint8Array {
    if (data.length > 80) {
        throw new Revert('OP_RETURN data too large');
    }

    const script = new BytesWriter(data.length + 2);
    script.writeU8(BitcoinOpcodes.OP_RETURN);
    script.writeU8(u8(data.length));
    script.writeBytes(data);

    return script.getBuffer();
}

// Advantages:
// - Standard Bitcoin OP_RETURN
// - Permanent, immutable storage
// - Recognized by all Bitcoin explorers
// - No complex event parsing needed
```

### Why OP_NET for Bitcoin Integration?

| Solidity Limitation | OP_NET Solution |
|---------------------|----------------|
| Cannot interact with Bitcoin | Full Bitcoin script support |
| No UTXO awareness | Complete transaction introspection |
| Single address format | All Bitcoin address types |
| No native timelocks | CSV and CLTV support |
| Custom multisig required | Native OP_CHECKMULTISIG |
| No Taproot support | Full P2TR integration |
| EVM-only execution | Bitcoin consensus enforcement |

## Best Practices

### 1. Validate All Script Inputs

```typescript
public processScript(script: Uint8Array): void {
    // Check minimum length
    if (script.length < 1) {
        throw new Revert('Empty script');
    }

    // Check maximum length
    if (script.length > 10000) {
        throw new Revert('Script too large');
    }

    // Validate script structure
    // ...
}
```

### 2. Use Named Opcode Constants

```typescript
// Good: Named constants from BitcoinOpcodes
if (script[0] == BitcoinOpcodes.OP_RETURN) { }

// Bad: Magic numbers
if (script[0] == 0x6a) { }
```

### 3. Use Networks Enum for Network Selection

```typescript
// Good: Use Networks enum
const hrp = Network.hrp(Networks.Mainnet);

// Bad: Hardcoded strings
const hrp = 'bc';
```

### 4. Use Built-in Script Builders

```typescript
// Good: Use BitcoinScript methods
const csvScript = BitcoinScript.csvTimelock(pubkey, 144);
const multisigScript = BitcoinScript.multisig(2, pubkeys);

// Only build manually when needed for custom scripts
```

---

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