Access control with OpenZeppelin in Blockchain / Solidity - Mini Project: Build & Apply

Access control with OpenZeppelin

📖 Scenario: You are creating a simple smart contract for a club membership system on the Ethereum blockchain. Only the club owner should be able to add new members. To manage this, you will use OpenZeppelin's Access Control features.

🎯 Goal: Build a smart contract that uses OpenZeppelin's Ownable contract to restrict the ability to add members only to the owner of the contract.

📋 What You'll Learn

Use OpenZeppelin's Ownable contract for access control

Create a mapping to store members' addresses and their membership status

Write a function addMember that only the owner can call to add a member

Write a function isMember to check if an address is a member

💡 Why This Matters

🌍 Real World

Access control is essential in blockchain contracts to restrict sensitive actions to authorized users, such as owners or admins.

💼 Career

Understanding OpenZeppelin's access control contracts is a key skill for blockchain developers working on secure smart contracts.

1

Set up the contract and member storage

Create a Solidity contract named Club. Import OpenZeppelin's Ownable contract. Inside Club, create a public mapping called members that maps address to bool to store membership status.

Blockchain / Solidity

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/access/Ownable.sol";

contract Club is Ownable {
    // Your code here
}

Hint

Use mapping(address => bool) public members; inside the contract to store membership.

2

Add a function to add members with owner restriction

Add a public function named addMember that takes an address parameter called newMember. Use the onlyOwner modifier to restrict access. Inside the function, set members[newMember] to true.

Blockchain / Solidity

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/access/Ownable.sol";

contract Club is Ownable {
    mapping(address => bool) public members;

    // Your code here
}

Hint

Use the onlyOwner modifier to restrict the addMember function.

3

Add a function to check membership status

Add a public view function named isMember that takes an address parameter called user and returns a bool. The function should return the membership status from the members mapping.

Blockchain / Solidity

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/access/Ownable.sol";

contract Club is Ownable {
    mapping(address => bool) public members;

    function addMember(address newMember) public onlyOwner {
        members[newMember] = true;
    }

    // Your code here
}

Hint

Return the membership status from the members mapping inside isMember.

4

Test the membership addition and check

Add a public function named testMembership that calls addMember with your own address msg.sender and then returns the result of isMember(msg.sender). Print the result using return.

Blockchain / Solidity

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/access/Ownable.sol";

contract Club is Ownable {
    mapping(address => bool) public members;

    function addMember(address newMember) public onlyOwner {
        members[newMember] = true;
    }

    function isMember(address user) public view returns (bool) {
        return members[user];
    }

    // Your code here
}

Hint

Call addMember(msg.sender) and then return isMember(msg.sender) inside testMembership.

Practice

1. What is the main purpose of using onlyRole modifier in OpenZeppelin's Access Control?

easy

A. To restrict function access to accounts with a specific role

B. To automatically assign roles to all users

C. To allow anyone to call the function without restrictions

D. To log all function calls for auditing

Solution

1. Step 1: Understand the purpose of onlyRole

   The onlyRole modifier is used to limit access to functions so only users with a certain role can execute them.

2. Step 2: Analyze the options

   To restrict function access to accounts with a specific role correctly states that it restricts function access to accounts with a specific role. Other options describe unrelated behaviors.

3. Final Answer:

   To restrict function access to accounts with a specific role -> Option A

4. Quick Check:

   Access control = restrict by role [OK]

Hint: Remember: onlyRole means only users with that role can call [OK]

Common Mistakes:

• Thinking onlyRole assigns roles automatically
• Believing onlyRole allows open access
• Confusing onlyRole with event logging

2. Which of the following is the correct way to declare a role constant in OpenZeppelin Access Control?

easy

A. address constant ADMIN_ROLE = 0x123;

B. string public ADMIN_ROLE = "ADMIN_ROLE";

C. uint256 constant ADMIN_ROLE = 1;

D. bytes32 public constant ADMIN_ROLE = keccak256("ADMIN_ROLE");

Solution

1. Step 1: Recall role declaration syntax

   OpenZeppelin uses bytes32 constants with keccak256 hash of a string to define roles.

2. Step 2: Check each option

   bytes32 public constant ADMIN_ROLE = keccak256("ADMIN_ROLE"); matches the correct pattern. Options B, C, and D use wrong types or formats.

3. Final Answer:

   bytes32 public constant ADMIN_ROLE = keccak256("ADMIN_ROLE"); -> Option D

4. Quick Check:

   Role constants use bytes32 + keccak256 [OK]

Hint: Roles are bytes32 constants hashed with keccak256 [OK]

Common Mistakes:

• Using string instead of bytes32 for roles
• Assigning numeric or address types to roles
• Forgetting to use keccak256 hash

3. Given the following Solidity code snippet, what will happen if an account without the ADMIN_ROLE calls secureFunction()?

contract MyContract is AccessControl {
    bytes32 public constant ADMIN_ROLE = keccak256("ADMIN_ROLE");

    constructor() {
        _grantRole(ADMIN_ROLE, msg.sender);
    }

    function secureFunction() public onlyRole(ADMIN_ROLE) {
        // critical logic
    }
}

medium

A. The function executes normally

B. The call reverts with an access control error

C. The function executes but emits a warning

D. The function executes only if the caller is the contract owner

Solution

1. Step 1: Understand the onlyRole modifier behavior

   The onlyRole(ADMIN_ROLE) modifier restricts access to accounts with ADMIN_ROLE. If the caller lacks this role, the call reverts.

2. Step 2: Analyze the scenario

   The constructor grants ADMIN_ROLE only to msg.sender at deployment. Any other account calling secureFunction will trigger a revert due to missing role.

3. Final Answer:

   The call reverts with an access control error -> Option B

4. Quick Check:

   Missing role causes revert [OK]

Hint: Only accounts with role can call; others revert [OK]

Common Mistakes:

• Assuming function runs without role
• Thinking warnings are emitted instead of revert
• Confusing role with ownership

4. Identify the error in this OpenZeppelin Access Control code snippet:

contract MyContract is AccessControl {
    bytes32 public constant ADMIN_ROLE = keccak256("ADMIN_ROLE");

    constructor() {
        _grantRole(ADMIN_ROLE, msg.sender);
    }

    function restricted() public onlyRole(ADMIN_ROLE) {
        // restricted logic
    }

    function grantAdmin(address user) public {
        _grantRole(ADMIN_ROLE, user);
    }
}

medium

A. The grantAdmin function lacks access control and can be called by anyone

B. The _setupRole function is deprecated and should not be used

C. The onlyRole modifier is missing from grantAdmin

D. The ADMIN_ROLE constant is incorrectly declared

Solution

1. Step 1: Review access control on grantAdmin

   The grantAdmin function calls _grantRole but has no modifier restricting who can call it.

2. Step 2: Identify security risk

   Without access control, anyone can call grantAdmin and assign ADMIN_ROLE to themselves or others, breaking security.

3. Final Answer:

   The grantAdmin function lacks access control and can be called by anyone -> Option A

4. Quick Check:

   Grant functions need access control [OK]

Hint: Always protect grantRole functions with onlyRole [OK]

Common Mistakes:

• Ignoring missing access control on grant functions
• Thinking _grantRole is protected like grantRole
• Confusing role declaration syntax

5. You want to create a smart contract where only users with the MINTER_ROLE can mint tokens, and only the contract owner can assign the MINTER_ROLE. Which OpenZeppelin pattern correctly enforces this?

hard

A. Use Ownable and allow only the owner to mint tokens directly without roles

B. Use AccessControl but allow anyone to assign MINTER_ROLE to themselves

C. Use AccessControl with MINTER_ROLE and add onlyRole(DEFAULT_ADMIN_ROLE) modifier to the role assignment function, granting DEFAULT_ADMIN_ROLE to the owner

D. Use AccessControl and assign MINTER_ROLE to everyone by default

Solution

1. Step 1: Understand role assignment control

   To restrict who can assign MINTER_ROLE, use AccessControl's DEFAULT_ADMIN_ROLE for admin rights and protect assignment functions with onlyRole(DEFAULT_ADMIN_ROLE).

2. Step 2: Connect owner with DEFAULT_ADMIN_ROLE

   Grant DEFAULT_ADMIN_ROLE to the contract owner so only they can assign MINTER_ROLE to others.

3. Final Answer:

   Use AccessControl with MINTER_ROLE and add onlyRole(DEFAULT_ADMIN_ROLE) modifier to the role assignment function, granting DEFAULT_ADMIN_ROLE to the owner -> Option C

4. Quick Check:

   Admin role controls role assignment [OK]

Hint: Use DEFAULT_ADMIN_ROLE for owner to control role assignments [OK]

Common Mistakes:

• Allowing anyone to assign roles
• Using Ownable without roles for minting
• Assigning roles to everyone by default