Multi-signature wallet concept in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the minimum number of signatures required.

Blockchain / Solidity

uint constant MIN_SIGNATURES = [1];

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A2

B4

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the minimum signatures to 1 defeats the purpose of multi-signature.

Using a number higher than the total number of owners.

The minimum number of signatures required is set to 3 to ensure multi-signature security.

2fill in blank

medium

Complete the code to check if the number of collected signatures meets the minimum required.

Blockchain / Solidity

require(signaturesCount [1] MIN_SIGNATURES, "Not enough signatures");

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A<

B>=

C!=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' would reject valid transactions.

Using '!=' would allow invalid counts.

The code checks if the collected signatures are greater than or equal to the minimum required.

3fill in blank

hard

Fix the error in the function that verifies if a signer is authorized.

Blockchain / Solidity

function isAuthorized(address signer) public view returns (bool) {
    return owners[[1]];
}

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Asigner

Bmsg.sender

Caddress

Downer

Attempts:

3 left

💡 Hint

Common Mistakes

Using msg.sender instead of signer checks the wrong address.

Using 'address' or 'owner' are not valid variables here.

The function should check if the given signer address is in the owners list.

4fill in blank

hard

Fill both blanks to create a mapping that tracks signatures for each transaction.

Blockchain / Solidity

mapping(uint => mapping(address => [1])) public signatures;

function signTransaction(uint txId) public {
    signatures[txId][[2]] = true;
}

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Abool

Bmsg.sender

Cuint

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using uint instead of bool for signature status.

Using address instead of msg.sender to mark the signer.

The mapping stores a boolean to track if an owner signed a transaction. The signer is msg.sender.

5fill in blank

hard

Fill all three blanks to create a function that executes a transaction after enough signatures.

Blockchain / Solidity

function executeTransaction(uint txId) public {
    require(signatures[txId][[1]], "Not signed by owner");
    require(signaturesCount [2] MIN_SIGNATURES, "Insufficient signatures");
    transactions[txId].[3]();
}

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Amsg.sender

B>=

Ccall

Dtransfer

Attempts:

3 left

💡 Hint

Common Mistakes

Using transfer instead of call may not work for all transaction types.

Using '<' instead of '>=' causes logic errors.

The function checks if the caller signed, verifies enough signatures, then calls the transaction.

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