Access control patterns in Blockchain / Solidity

Introduction

Access control patterns help decide who can do what in a blockchain system. They keep things safe by letting only the right people make changes.

When you want only certain users to add or change data on the blockchain.

When you need to protect sensitive information from everyone except trusted parties.

When you want to track who did what and prevent unauthorized actions.

When building a blockchain app that requires roles like admin, user, or guest.

When you want to limit access to smart contract functions based on user identity.

Syntax

Blockchain / Solidity

contract AccessControl {
    mapping(address => bool) private admins;

    modifier onlyAdmin() {
        require(admins[msg.sender], "Not an admin");
        _;
    }

    function addAdmin(address user) public onlyAdmin {
        admins[user] = true;
    }

    function removeAdmin(address user) public onlyAdmin {
        admins[user] = false;
    }

    function restrictedAction() public onlyAdmin {
        // action only admins can do
    }
}

modifier is a way to check permissions before running a function.

mapping(address => bool) stores which addresses have access.

Examples

This example uses a whitelist to allow only certain addresses access.

Blockchain / Solidity

mapping(address => bool) private whitelist;

function addToWhitelist(address user) public {
    whitelist[user] = true;
}

function checkAccess() public view returns (bool) {
    return whitelist[msg.sender];
}

This example uses roles to control access, allowing more than one type of permission.

Blockchain / Solidity

enum Role { None, User, Admin }
mapping(address => Role) roles;

function setRole(address user, Role role) public {
    roles[user] = role;
}

modifier onlyAdmin() {
    require(roles[msg.sender] == Role.Admin, "Not admin");
    _;
}

Sample Program

This smart contract lets the creator be the first admin. Admins can add or remove other admins. Only admins can call these functions.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleAccessControl {
    mapping(address => bool) private admins;

    constructor() {
        admins[msg.sender] = true; // creator is admin
    }

    modifier onlyAdmin() {
        require(admins[msg.sender], "Not an admin");
        _;
    }

    function addAdmin(address user) public onlyAdmin {
        admins[user] = true;
    }

    function removeAdmin(address user) public onlyAdmin {
        admins[user] = false;
    }

    function isAdmin(address user) public view returns (bool) {
        return admins[user];
    }
}

Important Notes

Always test access control carefully to avoid security holes.

Use modifiers to keep your code clean and avoid repeating permission checks.

Remember that blockchain data is public, so access control protects actions, not data visibility.

Summary

Access control patterns decide who can use blockchain functions.

Use mappings or enums to track permissions.

Modifiers help check permissions before running code.

Practice

1. What is the main purpose of access control patterns in blockchain smart contracts?

easy

A. To restrict who can perform certain actions on the blockchain

B. To increase the speed of blockchain transactions

C. To store large amounts of data efficiently

D. To create new cryptocurrencies automatically

Solution

1. Step 1: Understand access control concept

   Access control limits permissions, deciding who can do what on the blockchain.

2. Step 2: Identify the main goal

   The goal is to protect blockchain data and functions by restricting unauthorized actions.

3. Final Answer:

   To restrict who can perform certain actions on the blockchain -> Option A

4. Quick Check:

   Access control = restrict actions [OK]

Hint: Access control means who can do what, not speed or storage [OK]

Common Mistakes:

• Confusing access control with transaction speed
• Thinking access control stores data
• Assuming access control creates cryptocurrencies

2. Which of the following is the correct Solidity syntax to define a modifier named onlyOwner for access control?

easy

A. modifier onlyOwner() => { require(msg.sender == owner); _; }

B. function onlyOwner() { require(msg.sender == owner); }

C. modifier onlyOwner { if(msg.sender == owner) _; }

D. modifier onlyOwner() { require(msg.sender == owner); _; }

Solution

1. Step 1: Recall modifier syntax in Solidity

   A modifier uses the keyword 'modifier', parentheses for parameters, and the '_;' placeholder.

2. Step 2: Check each option

   modifier onlyOwner() { require(msg.sender == owner); _; } correctly uses 'modifier onlyOwner() { require(msg.sender == owner); _; }'. Others have syntax errors or missing parts.

3. Final Answer:

   modifier onlyOwner() { require(msg.sender == owner); _; } -> Option D

4. Quick Check:

   Modifier syntax = modifier onlyOwner() { require(msg.sender == owner); _; } [OK]

Hint: Modifiers need 'modifier', parentheses, and '_;' inside [OK]

Common Mistakes:

• Using 'function' instead of 'modifier'
• Omitting parentheses after modifier name
• Using arrow syntax which is invalid here

3. Consider this Solidity snippet:

mapping(address => bool) public admins;

modifier onlyAdmin() {
  require(admins[msg.sender], "Not admin");
  _;
}

function addAdmin(address newAdmin) public onlyAdmin {
  admins[newAdmin] = true;
}

constructor() {
  admins[msg.sender] = true;
}

What happens if a non-admin calls addAdmin?

medium

A. The contract crashes and becomes unusable

B. The transaction reverts with error 'Not admin'

C. The function runs but does nothing

D. The new admin is added successfully

Solution

1. Step 1: Understand the onlyAdmin modifier

   The modifier requires that the caller's address is in the admins mapping as true, else it reverts with "Not admin".

2. Step 2: Analyze calling addAdmin by non-admin

   If the caller is not an admin, require fails, causing the transaction to revert with the error message.

3. Final Answer:

   The transaction reverts with error 'Not admin' -> Option B

4. Quick Check:

   Non-admin call triggers require revert [OK]

Hint: Require false causes revert with error message [OK]

Common Mistakes:

• Assuming function silently fails
• Thinking non-admin can add admins
• Believing contract crashes permanently

4. Identify the bug in this Solidity access control code:

address public owner;

modifier onlyOwner() {
  require(msg.sender = owner, "Not owner");
  _;
}

constructor() {
  owner = msg.sender;
}

medium

A. Missing semicolon after owner declaration

B. Constructor should be named differently

C. Using assignment '=' instead of comparison '==' in require

D. Modifier missing parentheses

Solution

1. Step 1: Check require statement syntax

   The require uses '=' which assigns, not '==' which compares. This causes a compile error or unexpected behavior.

2. Step 2: Verify other parts

   Semicolon is present, constructor name is correct, and modifier has parentheses, so no issues there.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in require -> Option C

4. Quick Check:

   Use '==' for comparison in require [OK]

Hint: Use '==' to compare, '=' is assignment [OK]

Common Mistakes:

• Confusing '=' and '==' in conditions
• Thinking constructor name must differ
• Ignoring parentheses in modifier

5. You want to implement a multi-level access control in Solidity where: - The contract owner can add or remove admins. - Admins can add or remove users. - Users can only call a restricted function. Which pattern best fits this scenario?

hard

A. Use nested mappings with modifiers for owner, admin, and user roles

B. Use a single boolean mapping for all roles and check in functions

C. Use public variables for roles without modifiers

D. Use events only to track roles without restrictions

Solution

1. Step 1: Understand multi-level access control needs

   Different roles require different permissions; owner controls admins, admins control users, users have limited access.

2. Step 2: Evaluate patterns

   Nested mappings with modifiers (Use nested mappings with modifiers for owner, admin, and user roles) is complex but correct. Single boolean mapping (Use a single boolean mapping for all roles and check in functions) cannot distinguish roles well. Public variables without modifiers (Use public variables for roles without modifiers) is insecure and insufficient. Events only (Use events only to track roles without restrictions) do not enforce control.

3. Step 3: Choose best practical pattern

   Nested mappings with modifiers provide clear, secure role checks and control flow.

4. Final Answer:

   Use nested mappings with modifiers for owner, admin, and user roles -> Option A

5. Quick Check:

   Multi-level roles need nested mappings + modifiers [OK]

Hint: Use nested mappings and modifiers for clear role control [OK]

Common Mistakes:

• Using single mapping for all roles
• Skipping modifiers for access checks
• Relying only on events without restrictions