Cross-chain bridges in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple cross-chain token lock function

What is the output of this Solidity function when called with amount = 100 and sender balance = 150?

Blockchain / Solidity

mapping(address => uint256) balances;

function lockTokens(uint256 amount) public returns (string memory) {
    require(balances[msg.sender] >= amount, "Insufficient balance");
    balances[msg.sender] -= amount;
    // Tokens locked for cross-chain transfer
    return "Tokens locked";
}

// Assume balances[msg.sender] = 150 before call

A "Tokens locked"

B "Insufficient balance"

C Revert with error "Insufficient balance"

D "Tokens unlocked"

💡 Hint

Check the require condition and the initial balance.

The function checks if the sender has enough balance (150 >= 100). Since true, it subtracts 100 and returns "Tokens locked".

🧠 Conceptual

intermediate

Understanding cross-chain message verification

Which of the following best describes the role of a relayer in a cross-chain bridge?

A A relayer mints new tokens on the source chain without verification.

B A relayer verifies and submits proofs of events from one chain to another to enable token transfers.

C A relayer stores user private keys to sign transactions on their behalf.

D A relayer directly modifies the consensus rules of the destination chain.

💡 Hint

Think about how information is passed securely between chains.

Relayers watch events on the source chain and submit cryptographic proofs to the destination chain to confirm token locking or burning.

🔧 Debug

advanced

Identify the error in cross-chain token minting code

What error will this Solidity code produce when mintTokens is called without proper verification?

Blockchain / Solidity

mapping(address => uint256) public balances;

function mintTokens(address to, uint256 amount) public {
    // Missing verification of cross-chain proof
    balances[to] += amount;
}

A No error, tokens minted successfully

B Revert due to missing proof verification

C Potential double minting vulnerability (logical error, no revert)

D Compilation error due to missing visibility on balances

💡 Hint

Consider security implications of minting without checks.

The code compiles and runs but lacks proof verification, allowing anyone to mint tokens arbitrarily, causing a logical security flaw.

📝 Syntax

advanced

Syntax error in cross-chain event emission

Which option correctly fixes the syntax error in this Solidity event emission code?

Blockchain / Solidity

event TransferLocked(address indexed sender, uint256 amount);

function lock(uint256 amount) public {
    emit TransferLocked(msg.sender, amount)
}

A Replace emit with call: call TransferLocked(msg.sender, amount);

B Change event name to transferLocked without uppercase letters

C Remove the indexed keyword from the event declaration

D Add a semicolon after the emit statement: emit TransferLocked(msg.sender, amount);

💡 Hint

Check the end of the emit statement.

In Solidity, statements must end with a semicolon. The missing semicolon causes a syntax error.

🚀 Application

expert

Calculate the number of confirmations needed for secure cross-chain transfer

A cross-chain bridge requires 12 confirmations on the source chain before releasing tokens on the destination chain. If blocks are produced every 15 seconds, how long in minutes does a user wait before tokens are released?

A 3 minutes

B 12 minutes

C 180 minutes

D 30 minutes

💡 Hint

Multiply confirmations by block time, then convert seconds to minutes.

12 confirmations × 15 seconds = 180 seconds = 3 minutes. But 3 minutes is option A, so check carefully: 12 × 15 = 180 seconds = 3 minutes. Option A is 3 minutes, so correct answer is A.