Access control patterns in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Access control patterns

User requests access

↓

Check access control pattern

↓

Role-based

↓

Verify role

↓

Grant or deny access

User requests access, system checks the chosen access control pattern (role, attribute, or capability), verifies permissions, then grants or denies access.

Execution Sample

Blockchain / Solidity

contract AccessControl {
  mapping(address => string) roles;
  function setRole(address user, string memory role) public {
    roles[user] = role;
  }
  function canAccess(address user) public view returns (bool) {
    return keccak256(bytes(roles[user])) == keccak256(bytes("admin"));
  }
}

A simple role-based access control contract that assigns roles and checks if a user is an admin.

Execution Table

Step	Action	Variable/State	Evaluation	Result
1	Assign role 'admin' to user 0xABC	roles[0xABC]	Set to 'admin'	Role set
2	Check access for user 0xABC	roles[0xABC]	Compare 'admin' == 'admin'	True
3	Check access for user 0xDEF	roles[0xDEF]	Compare '' == 'admin'	False
4	User 0xABC requests access	canAccess(0xABC)	Returns True	Access granted
5	User 0xDEF requests access	canAccess(0xDEF)	Returns False	Access denied

💡 Access granted only if user role matches 'admin'; otherwise denied.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
roles[0xABC]	''	'admin'	'admin'	'admin'	'admin'
roles[0xDEF]	''	''	''	''	''

Key Moments - 2 Insights

Why does user 0xDEF get denied access even though roles[0xDEF] exists?

Because roles[0xDEF] is empty string (''), which does not match 'admin' in the access check (see execution_table step 3).

What happens if we assign a different role than 'admin' to a user?

The access check compares the role to 'admin', so any other role will cause access denial (see execution_table step 3 logic).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of roles[0xABC] after step 1?

A 'user'

B 'admin'

C ''

D undefined

💡 Hint

Check the 'Variable/State' column in step 1 for roles[0xABC].

At which step does the access check return false for user 0xDEF?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Evaluation' and 'Result' columns for user 0xDEF in the execution table.

If we change the role check to 'user' instead of 'admin', what happens at step 4 for user 0xABC?

A Access denied

B Access granted

C Error occurs

D No change

💡 Hint

Since roles[0xABC] is 'admin', checking for 'user' will fail the comparison.

Concept Snapshot

Access control patterns manage who can do what.
Common types: Role-Based (RBAC), Attribute-Based (ABAC), Capability-Based.
RBAC assigns roles to users and checks roles.
ABAC checks user attributes like age or location.
Capability-based uses tokens granting specific rights.
Access is granted only if checks pass.

Full Transcript

This visual trace shows how access control patterns work in blockchain contracts. The user requests access, and the system checks the user's role. If the role matches the required one, access is granted; otherwise, it is denied. The example contract assigns roles to addresses and checks if a user is an admin. The execution table walks through assigning roles and checking access for two users, showing when access is granted or denied. Key moments clarify why some users are denied access and how role checks affect outcomes. The quiz tests understanding of variable states and access decisions at each step.

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