Multi-signature wallet concept in Blockchain / Solidity - Mini Project: Build & Apply

Multi-signature Wallet Concept

📖 Scenario: You are building a simple model of a multi-signature wallet. This wallet requires multiple owners to approve a transaction before it can be executed. This is like a shared bank account where several people must agree before money is spent.

🎯 Goal: Create a program that stores wallet owners, sets a minimum number of approvals needed, collects approvals for a transaction, and checks if the transaction can be executed.

📋 What You'll Learn

Create a list called owners with exactly these names: 'Alice', 'Bob', 'Charlie'

Create an integer variable called required_approvals and set it to 2

Create a list called approvals that will store the names of owners who approve the transaction

Write a loop to add approvals from owners to approvals until the number of approvals equals required_approvals

Print Transaction approved if the number of approvals is enough, otherwise print Transaction pending

💡 Why This Matters

🌍 Real World

Multi-signature wallets are used in blockchain to increase security by requiring multiple people to approve transactions.

💼 Career

Understanding multi-signature wallets is important for blockchain developers and security engineers working with cryptocurrencies.

1

Create the wallet owners list

Create a list called owners with these exact names: 'Alice', 'Bob', 'Charlie'

Blockchain / Solidity

# Create the list of wallet owners
# Your code here

Hint

Use square brackets [] to create a list and separate names with commas.

2

Set the required number of approvals

Create an integer variable called required_approvals and set it to 2

Blockchain / Solidity

owners = ['Alice', 'Bob', 'Charlie']
# Set the number of required approvals
# Your code here

Hint

Use = to assign the value 2 to the variable required_approvals.

3

Collect approvals from owners

Create an empty list called approvals. Then use a for loop with variable owner to iterate over owners. Inside the loop, add owner to approvals until the length of approvals equals required_approvals. Use break to stop the loop when enough approvals are collected.

Blockchain / Solidity

owners = ['Alice', 'Bob', 'Charlie']
required_approvals = 2
# Create an empty list for approvals
# Use a for loop to add owners to approvals until required_approvals is reached
# Your code here

Hint

Start with an empty list. Add each owner to it. Stop when you have enough approvals.

4

Check and print transaction status

Write an if statement to check if the length of approvals is greater than or equal to required_approvals. If yes, print Transaction approved. Otherwise, print Transaction pending.

Blockchain / Solidity

owners = ['Alice', 'Bob', 'Charlie']
required_approvals = 2
approvals = []
for owner in owners:
    approvals.append(owner)
    if len(approvals) == required_approvals:
        break
# Check if approvals are enough and print the status
# Your code here

Hint

Use print() to show the result. Compare the number of approvals with the required number.

Practice

1. What is the main purpose of a multi-signature wallet in blockchain?

easy

A. To require multiple approvals before spending funds

B. To speed up transaction processing

C. To store private keys on a single device

D. To allow unlimited spending by one user

Solution

1. Step 1: Understand the multi-signature wallet concept

   A multi-signature wallet requires more than one person to approve a transaction before it can be executed.

2. Step 2: Identify the main purpose

   This setup protects funds by preventing a single user from spending money alone, increasing security.

3. Final Answer:

   To require multiple approvals before spending funds -> Option A

4. Quick Check:

   Multi-signature = multiple approvals [OK]

Hint: Multi-signature means multiple people must approve [OK]

Common Mistakes:

• Thinking it speeds up transactions
• Believing one user controls all funds
• Confusing it with single-key wallets

2. Which of the following is the correct way to define a multi-signature wallet threshold in Solidity?

easy

A. bool threshold = true;

B. uint8 threshold = '2';

C. string threshold = 2;

D. uint8 threshold = 1;

Solution

1. Step 1: Identify correct data type for threshold

   The threshold is a number representing how many signatures are needed, so an unsigned integer like uint8 is appropriate.

2. Step 2: Check syntax correctness

   Assigning a number directly to uint8 is correct. Using quotes or wrong types causes errors.

3. Final Answer:

   uint8 threshold = 1; -> Option D

4. Quick Check:

   Threshold is a number, use uint8 [OK]

Hint: Threshold is a number, use uint type without quotes [OK]

Common Mistakes:

• Using quotes around numbers
• Assigning string type to threshold
• Using boolean type for threshold

3. Given this Solidity snippet for a multi-signature wallet, what will be the value of isApproved after calling approveTransaction(1, msg.sender) if the threshold is 2 and only one approval is made?

mapping(uint => mapping(address => bool)) approvals;
uint8 threshold = 2;

function approveTransaction(uint txId, address approver) public {
  approvals[txId][approver] = true;
}

function isApproved(uint txId) public view returns (bool) {
  uint count = 0;
  for (uint i = 0; i < owners.length; i++) {
    if (approvals[txId][owners[i]]) {
      count++;
    }
  }
  return count >= threshold;
}

medium

A. true

B. false

C. Compilation error

D. Undefined behavior

Solution

1. Step 1: Understand approval counting logic

   The function counts how many owners approved the transaction and compares it to the threshold.

2. Step 2: Analyze given scenario

   Only one approval is made but threshold is 2, so count is 1 which is less than 2.

3. Final Answer:

   false -> Option B

4. Quick Check:

   Approvals < threshold = false [OK]

Hint: Approval count must meet threshold to be true [OK]

Common Mistakes:

• Assuming one approval is enough
• Ignoring threshold comparison
• Confusing approval mapping structure

4. Identify the bug in this Solidity function for approving transactions in a multi-signature wallet:

function approveTransaction(uint txId) public {
  approvals[txId][msg.sender] = true;
  if (isApproved(txId)) {
    executeTransaction(txId);
  }
}

function isApproved(uint txId) public view returns (bool) {
  uint count = 0;
  for (uint i = 0; i <= owners.length; i++) {
    if (approvals[txId][owners[i]]) {
      count++;
    }
  }
  return count >= threshold;
}

medium

A. Missing event emission after approval

B. approveTransaction should be external, not public

C. Loop condition should be < instead of <=

D. executeTransaction should not be called inside approveTransaction

Solution

1. Step 1: Check the for loop boundary

   The loop uses i <= owners.length, which causes out-of-bounds access because array indices go from 0 to length-1.

2. Step 2: Correct the loop condition

   Changing to i < owners.length prevents accessing invalid index and runtime errors.

3. Final Answer:

   Loop condition should be < instead of <= -> Option C

4. Quick Check:

   Array index out of bounds fixed by < [OK]

Hint: Array loops use < length, not <= length [OK]

Common Mistakes:

• Using <= in loops causing errors
• Ignoring array index limits
• Thinking event emission fixes logic bugs

5. You want to create a multi-signature wallet that requires 3 out of 5 owners to approve a transaction. Which approach correctly enforces this rule in Solidity?

mapping(uint => mapping(address => bool)) approvals;
address[5] owners;
uint8 threshold = 3;

function executeTransaction(uint txId) public {
  uint count = 0;
  for (uint i = 0; i < owners.length; i++) {
    if (approvals[txId][owners[i]]) {
      count++;
    }
  }
  if (count >= threshold) {
    // execute the transaction
  } else {
    revert("Not enough approvals");
  }
}

hard

A. This code correctly enforces the 3-of-5 approval rule

B. The threshold should be set to 5 to require all owners

C. The loop should iterate over approvals, not owners

D. Revert should be replaced with a simple return statement

Solution

1. Step 1: Analyze the approval counting logic

   The code counts how many owners approved the transaction by checking the approvals mapping for each owner.

2. Step 2: Check threshold enforcement

   If the count is at least the threshold (3), the transaction executes; otherwise, it reverts with an error.

3. Final Answer:

   This code correctly enforces the 3-of-5 approval rule -> Option A

4. Quick Check:

   Count approvals >= threshold = enforce rule [OK]

Hint: Count approvals, compare with threshold, revert if not met [OK]

Common Mistakes:

• Setting threshold incorrectly
• Looping over wrong data structure
• Using return instead of revert for errors