Multi-signature wallet concept in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a multi-signature wallet in blockchain?

A multi-signature wallet is a digital wallet that requires multiple private keys to approve a transaction, adding extra security by needing more than one person to agree.

beginner

How does a multi-signature wallet improve security?

It improves security by requiring multiple approvals before spending funds, so no single person can move money alone, reducing risks of theft or mistakes.

intermediate

What does 'm-of-n' mean in a multi-signature wallet?

'm-of-n' means that out of 'n' total owners, at least 'm' must sign to approve a transaction. For example, 2-of-3 means 2 out of 3 owners must agree.

beginner

Give a real-life example of when a multi-signature wallet is useful.

It is useful for a company where multiple managers must approve spending company funds, preventing one person from using money without others' consent.

beginner

What happens if not enough signatures are collected in a multi-signature wallet?

The transaction will not be approved or executed, keeping the funds safe until the required number of signatures is reached.

What does a multi-signature wallet require to approve a transaction?

A Multiple private keys' signatures

B Only one private key signature

C No signatures needed

D A password only

A multi-signature wallet requires multiple private keys to sign a transaction for approval.

In a 3-of-5 multi-signature wallet, how many signatures are needed?

A 2

B 5

C 1

D 3

3-of-5 means at least 3 signatures out of 5 owners are needed to approve a transaction.

Why might a business use a multi-signature wallet?

A To allow anyone to spend funds freely

B To require multiple approvals for spending funds

C To avoid using private keys

D To speed up transactions without approval

Businesses use multi-signature wallets to require multiple approvals, increasing security.

What happens if a transaction does not get enough signatures in a multi-signature wallet?

A The wallet sends funds to a random address

B The transaction executes anyway

C The transaction is not executed

D The wallet deletes funds

Without enough signatures, the transaction is blocked and not executed.

Which of these is NOT a benefit of multi-signature wallets?

A Faster transaction approval with fewer signatures

B Shared control over funds

C Increased security

D Protection against single key loss

Multi-signature wallets require multiple signatures, so they do not speed up approval with fewer signatures.

Explain how a multi-signature wallet works and why it is more secure than a single-signature wallet.

Think about how needing more than one key protects money.

Describe a situation where using a multi-signature wallet would be important and how it helps.

Imagine a group managing money together.

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