Access control patterns in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple owner check function in a smart contract.

Blockchain / Solidity

function isOwner(address user) public view returns (bool) {
    return user == [1];
}

Drag options to blanks, or click blank then click option'

Amsg.sender

Bowner

Caddress(this)

Dadmin

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.sender instead of the stored owner variable.

Comparing with address(this) which is the contract address.

The function checks if the given user address matches the stored owner address.

2fill in blank

medium

Complete the code to add a modifier that restricts function access to the owner only.

Blockchain / Solidity

modifier onlyOwner() {
    require([1] == owner, "Not owner");
    _;
}

Drag options to blanks, or click blank then click option'

Aaddress(this)

Btx.origin

Cmsg.sender

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Using tx.origin which can be unsafe for access control.

Comparing owner to itself.

The modifier checks that the caller (msg.sender) is the owner before running the function.

3fill in blank

hard

Fix the error in the access control check that uses a mapping for roles.

Blockchain / Solidity

mapping(address => bool) public admins;

function restrictedAction() public {
    require([1][msg.sender], "Not admin");
    // action code
}

Drag options to blanks, or click blank then click option'

Aadmins

BadminMapping

Cadmin

DadminList

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not exist.

Trying to access a mapping without brackets.

The mapping is named admins, so the require statement must check admins[msg.sender].

4fill in blank

hard

Fill both blanks to create a function that grants admin role only by the owner.

Blockchain / Solidity

function grantAdmin(address user) public [1] {
    admins[user] = true;
    emit AdminGranted(user);
}

modifier [2]() {
    require(msg.sender == owner, "Not owner");
    _;
}

Drag options to blanks, or click blank then click option'

AonlyOwner

Bpublic

ConlyAdmin

Dexternal

Attempts:

3 left

💡 Hint

Common Mistakes

Using public or external without access control.

Using a modifier name that does not exist.

The function uses the onlyOwner modifier to restrict access to the owner only.

5fill in blank

hard

Fill all three blanks to implement a role-based access control check inside a function.

Blockchain / Solidity

function performAction() public {
    require(roles[msg.sender] [1] [2], "Access denied");
    // action code
}

mapping(address => uint) public roles;
uint constant ADMIN_ROLE = [3];

Drag options to blanks, or click blank then click option'

A==

B>=

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using equality instead of greater or equal comparison.

Setting ADMIN_ROLE to 0 which usually means no role.

The function checks if the caller's role is at least ADMIN_ROLE (which is 1) to allow access.

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