Blockchain Smart Contracts with Solidity · レッスン

SafeMathによるセキュアコーディング

SafeMathなどのライブラリを使い、算術演算における整数のオーバーフローやアンダーフロー攻撃を防ぐ方法を学びます。

レッスン 3/412 ステップ

「SafeMathによるセキュアコーディング」はCoddyKit上の無料Blockchain Smart Contracts with Solidityレッスンです。これはレッスン3/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはBlockchain Smart Contracts with Solidity学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Blockchain Smart Contracts with Solidityコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

The Integer Problem

In Solidity, integer types like uint256 have a fixed size. This means they can only store numbers up to a certain maximum value and down to a minimum (usually 0 for unsigned integers).

When an arithmetic operation exceeds these limits, it can lead to critical vulnerabilities called integer overflows and underflows.

Unchecked Math Dangers

Solidity's default arithmetic operations (+, -, *, /) do not automatically check for overflows or underflows. Instead, the number 'wraps around'.

This behavior can be exploited by attackers, leading to incorrect token balances, unexpected contract state, and financial losses.

Overflow in Action

Consider a uint8 variable, which can hold values from 0 to 255. What happens if we try to add 1 to 255? Run this code and call incrementUnsafely(). You'll see the value reset to 0!

/*
  This contract demonstrates an integer overflow.
  A uint8 can only hold values from 0 to 255.
  Adding 1 to 255 will cause it to wrap around to 0.
*/
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract UnsafeCounter {
    uint8 public count = 255; // Max value for uint8

    // Function to increment the counter unsafely
    function incrementUnsafely() public {
        count = count + 1;
    }
}

Underflow Example

Similarly, an underflow occurs when a number goes below its minimum value. For a uint (unsigned integer), the minimum is 0.

If you subtract 1 from 0, it wraps around to the maximum value (255 for uint8, or 2^256 - 1 for uint256).

/*
  This contract demonstrates an integer underflow.
  A uint8 can only hold values from 0 to 255.
  Subtracting 1 from 0 will cause it to wrap around to 255.
*/
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract UnderflowDemo {
    uint8 public value = 0; // Min value for uint8

    // Function to decrement the value unsafely
    function decrementUnsafely() public {
        value = value - 1;
    }
}

Introducing SafeMath

To prevent these critical errors, we use libraries like SafeMath. SafeMath provides functions for arithmetic operations (addition, subtraction, multiplication, division) that revert the transaction if an overflow or underflow would occur.

This ensures your contract's state remains consistent and secure, preventing malicious exploits.

Solidity Libraries Explained

A Solidity Library is a special type of contract that contains reusable code. Unlike regular contracts, libraries are stateless (they don't store data directly) and cannot hold Ether.

They are deployed once and their functions are called via DELEGATECALL.
This means the library's code runs in the context of the calling contract.
Libraries are perfect for shared utility functions like SafeMath.

Integrating SafeMath

To use SafeMath, you typically import it from a trusted source like OpenZeppelin. Then, you tell Solidity to apply SafeMath's functions to a specific integer type using the using A for B; directive.

This makes SafeMath's functions available as member functions on type B.

/*
  This contract demonstrates how to integrate and use SafeMath.
  We're including a simplified mock SafeMath library for demonstration.
*/
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

// A simplified mock SafeMath library for demonstration
library SafeMath {
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");
        return c;
    }
}

contract MySafeContract {
    // Use SafeMath functions for all uint256 variables
    using SafeMath for uint256;

    uint256 public balance = 100;

    function deposit(uint256 amount) public {
        // Now you can call .add() directly on balance
        balance = balance.add(amount);
    }
    
    function getBalance() public view returns (uint256) {
        return balance;
    }
}

Safe Addition in Action

With SafeMath integrated, you use .add() instead of the standard + operator. If the addition would overflow, the transaction will revert, preventing incorrect state changes.

Call safeAdd() with a value like 10. Try calling it with a value that would cause an overflow (e.g., if total was max uint8 and you added 1).

/*
  This contract uses SafeMath for secure addition.
  If the addition causes an overflow, the transaction will revert.
*/
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

library SafeMath {
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");
        return c;
    }
}

contract SafeAdder {
    using SafeMath for uint256;
    uint256 public total = 0;

    function safeAdd(uint256 _value) public {
        total = total.add(_value); // Uses SafeMath.add
    }
}

Safe Subtraction in Action

Similarly, use .sub() for subtraction. This prevents underflows, ensuring that a subtraction operation will revert if the result would be negative (below zero for unsigned integers).

Call safeSubtract() with a value like 10. Try calling it with a value larger than balance (e.g., 101) to see it revert.

/*
  This contract uses SafeMath for secure subtraction.
  If the subtraction causes an underflow, the transaction will revert.
*/
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

library SafeMath {
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a, "SafeMath: subtraction underflow");
        uint256 c = a - b;
        return c;
    }
}

contract SafeSubtractor {
    using SafeMath for uint256;
    uint256 public balance = 100;

    function safeSubtract(uint256 _value) public {
        balance = balance.sub(_value); // Uses SafeMath.sub
    }
}

Multiply, Divide, Modulo

SafeMath also provides .mul(), .div(), and .mod() for multiplication, division, and modulo operations, respectively.

.mul() checks for overflow.
.div() checks for division by zero and overflow.
.mod() checks for division by zero.

Always use these safe versions for critical arithmetic in your contracts.

Quick Check on SafeMath

You've learned about the importance of SafeMath. Let's test your understanding.

対話的なレッスンで答える

Recap: Secure Math

You've learned about the critical vulnerabilities of integer overflows and underflows in Solidity and how SafeMath provides a robust solution.

Always use SafeMath (or similar audited libraries) for arithmetic operations on unsigned integers in your smart contracts.
This prevents unexpected behavior and protects your contract's integrity.

Keep practicing secure coding! The next lessons will dive deeper into advanced security patterns.

無料で開始

AI チューターと学ぶ Blockchain Smart Contracts with Solidity — 無料

ブラウザでリアルコードを書いて実行し、24/7 の AI チューターから瞬時にサポートを受け、ウェブまたはアプリで続きから学習できます。

コース: 12
レッスン: 48

無料で学習を始めるコース全体を表示

よくある質問

「SafeMathによるセキュアコーディング」レッスンは無料ですか？

はい。「SafeMathによるセキュアコーディング」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Blockchain Smart Contracts with Solidityコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Blockchain Smart Contracts with Solidityコースには全4レッスンが含まれています。

「SafeMathによるセキュアコーディング」で何を学びますか？

SafeMathなどのライブラリを使い、算術演算における整数のオーバーフローやアンダーフロー攻撃を防ぐ方法を学びます。ブラウザで直接実行するハンズオンコードでBlockchain Smart Contracts with Solidityを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Blockchain Smart Contracts with Solidityを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのBlockchain Smart Contracts with Solidityは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン3/4です。

「SafeMathによるセキュアコーディング」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このBlockchain Smart Contracts with Solidityレッスンでコードを書いて実行できますか？

はい。すべてのBlockchain Smart Contracts with Solidityレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

← Blockchain Smart Contracts with Solidityに戻る