Security Best Practices for Python Smart Contracts on Xian

Introduction

Security is a fundamental aspect of smart contract development. Unlike traditional applications, blockchain-based contracts are immutable once deployed, making it crucial to write secure code from the outset.

Xian allows developers to write smart contracts in Python, but this ease of development also comes with potential security risks. In this guide, we’ll cover essential security best practices to help you write robust and attack-resistant Xian smart contracts.

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1. Input Validation & Sanitization

User input is a major attack vector in smart contracts. Always validate inputs to prevent unexpected behaviors and security vulnerabilities.

Example:

balances = Hash(default_value=0)

@export
def transfer(receiver: str, amount: int):
    assert amount > 0, "Invalid amount"
    assert balances[ctx.caller] >= amount, 'Not enough coins to send!'

Why?

• Ensures only positive values are processed in financial transactions.
• Requires the user to have enough balance.

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2. Use Access Control & Permissions

Unauthorized access to critical contract functions can lead to catastrophic failures. Implement access control mechanisms to restrict who can call sensitive functions.

Example:

owner = Variable()

@construct
def initialize():
    owner.set(ctx.caller)

@export
def privileged_function():
    assert ctx.caller == owner.get(), "Unauthorized access"
    # Perform sensitive action

Why?

• Prevents non-owners from calling privileged functions.
• Uses ctx.caller to verify the identity of the function caller.

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3. Prevent Integer Overflows & Underflows

In Python, integers are arbitrary-precision, but it’s still good practice to set explicit boundaries to prevent unexpected behaviors.

Example:

MAX_SUPPLY = 1_000_000

@export
def mint(to: str, amount: int):
    assert amount > 0, "Amount must be positive"
    assert state['supply'] + amount <= MAX_SUPPLY, "Exceeds maximum supply"
    state['supply'] += amount
    state['balances'][to] += amount

Why?

• Prevents minting more tokens than allowed.
• Ensures values remain within the intended range.

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4. Safe Handling of Decimal Operations in Xian Smart Contracts

In the Contracting language, floating-point numbers are automatically converted to ContractingDecimal. This ensures safe calculations within predefined precision limits, avoiding rounding errors and excessive values. The maximum limit for the decimal type is 1e29, and values exceeding this limit are truncated.

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5. Avoid Reentrancy Attacks

Reentrancy occurs when a contract makes an external call before updating its state, allowing an attacker to re-enter and manipulate the contract’s logic. This can lead to unintended execution and fund theft.

Example:

reentrancy_lock = Variable()

@construct
def seed():
    reentrancy_lock.set(False)

@export
def withdraw(amount: float, to: str):
    assert not reentrancy_lock.get(), "Reentrancy detected"
    reentrancy_lock.set(True)

    # Checks - Ensure user has enough balance
    assert state['balances'][ctx.caller] >= amount, "Insufficient balance"

    # Effects - Deduct the amount first before making any external call
    state['balances'][ctx.caller] -= amount
    state['balances'][to] += amount

    # Interactions - External function call happens *last* to avoid reentrancy risk
    external_contract.call_some_function()

    # Reset the lock
    reentrancy_lock.set(False)

Why?

• Prevents an attacker from re-entering the function before the previous execution completes.
• Follows the Checks-Effects-Interactions pattern to update state before making external calls.
• Ensures safe execution even if an external contract attempts to re-enter.

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6. Regularly Audit Your Smart Contracts

Before deploying a contract, conduct thorough testing and audits to catch vulnerabilities early.

Steps to Audit:

1. Static Analysis: Use linting tools to detect code issues.
2. Automated Tests: Write unit tests covering all possible contract behaviors.
3. Peer Reviews: Have other developers review your contract code.
4. Bug Bounties: Encourage security researchers to find vulnerabilities.

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Conclusion

Writing secure smart contracts on Xian requires careful consideration of access controls, input validation, and attack prevention. By following these best practices, you can protect your contracts from common vulnerabilities and ensure a safer blockchain ecosystem.

🚀 Next Steps:

• Implement these practices in your own smart contracts.
• Join the Xian Developer Community for security discussions.
• Read the Xian Docs for in-depth guidance.

💡 Have security tips to share? Drop a comment below and help make the Xian blockchain even safer! 🔐