Cross-chain bridges in Blockchain / Solidity - Mini Project: Build & Apply

Cross-chain Bridges Basics

📖 Scenario: You are building a simple program to track token transfers between two blockchains using a cross-chain bridge. This helps users move tokens safely from one chain to another.

🎯 Goal: Create a program that stores token balances on two chains, sets a minimum transfer amount, processes transfers only if they meet the minimum, and shows the updated balances.

📋 What You'll Learn

Create a dictionary called chain_balances with exact keys 'ChainA' and 'ChainB' and initial balances 1000 and 500 respectively

Create a variable called min_transfer and set it to 100

Write a function called transfer_tokens that takes amount and transfers tokens from 'ChainA' to 'ChainB' only if amount is greater than or equal to min_transfer

Print the updated chain_balances after a transfer of 150 tokens

💡 Why This Matters

🌍 Real World

Cross-chain bridges let users move tokens between different blockchains, enabling more flexible and powerful blockchain applications.

💼 Career

Understanding how to track and control token transfers across chains is important for blockchain developers working on decentralized finance (DeFi) and interoperability solutions.

1

Set up initial chain balances

Create a dictionary called chain_balances with keys 'ChainA' and 'ChainB' having values 1000 and 500 respectively.

Blockchain / Solidity

# Create the chain_balances dictionary with initial values
# Your code here

Hint

Use curly braces {} to create a dictionary with the given keys and values.

2

Set minimum transfer amount

Create a variable called min_transfer and set it to 100.

Blockchain / Solidity

chain_balances = {'ChainA': 1000, 'ChainB': 500}
# Set the minimum transfer amount
# Your code here

Hint

Just assign the number 100 to the variable min_transfer.

3

Write transfer function

Write a function called transfer_tokens that takes a parameter amount. Inside the function, check if amount is greater than or equal to min_transfer. If yes, subtract amount from chain_balances['ChainA'] and add it to chain_balances['ChainB']. If not, do nothing.

Blockchain / Solidity

chain_balances = {'ChainA': 1000, 'ChainB': 500}
min_transfer = 100
# Define the transfer_tokens function below
# Your code here

Hint

Use def to create the function and an if statement to check the amount.

4

Perform transfer and show balances

Call the function transfer_tokens with 150 as the argument. Then print the chain_balances dictionary to show updated balances.

Blockchain / Solidity

chain_balances = {'ChainA': 1000, 'ChainB': 500}
min_transfer = 100

def transfer_tokens(amount):
    if amount >= min_transfer:
        chain_balances['ChainA'] -= amount
        chain_balances['ChainB'] += amount

# Call transfer_tokens with 150 and print chain_balances
# Your code here

Hint

Call the function with 150 and then print the dictionary to see the new balances.

Practice

1. What is the main purpose of a cross-chain bridge in blockchain technology?

easy

A. To mine new blocks faster on a single blockchain

B. To increase the block size limit on a blockchain

C. To create new cryptocurrencies from scratch

D. To connect different blockchains and allow asset transfers between them

Solution

1. Step 1: Understand the role of cross-chain bridges

   Cross-chain bridges enable communication and asset transfers between different blockchains.

2. Step 2: Compare options with this role

   Only To connect different blockchains and allow asset transfers between them describes connecting blockchains and transferring assets, which matches the bridge's purpose.

3. Final Answer:

   To connect different blockchains and allow asset transfers between them -> Option D

4. Quick Check:

   Cross-chain bridge = connect blockchains [OK]

Hint: Bridges connect blockchains to move tokens or data [OK]

Common Mistakes:

• Confusing bridges with mining or block creation
• Thinking bridges create new cryptocurrencies
• Assuming bridges only increase block size

2. Which of the following is the correct basic step in a cross-chain bridge operation?

easy

A. Lock tokens on the source chain and mint equivalent tokens on the destination chain

B. Mint tokens on the source chain and burn on the destination chain

C. Burn tokens on the source chain and mine new tokens on the same chain

D. Transfer tokens directly without locking or minting

Solution

1. Step 1: Recall how cross-chain bridges work

   They lock tokens on the source chain to prevent double spending and mint equivalent tokens on the destination chain.

2. Step 2: Match this with the options

   Lock tokens on the source chain and mint equivalent tokens on the destination chain correctly describes locking on source and minting on destination, which is the standard process.

3. Final Answer:

   Lock tokens on the source chain and mint equivalent tokens on the destination chain -> Option A

4. Quick Check:

   Lock then mint = bridge step [OK]

Hint: Tokens are locked first, then minted on another chain [OK]

Common Mistakes:

• Mixing up minting and burning order
• Thinking tokens transfer directly without locking
• Assuming minting happens on source chain

3. Consider this simplified pseudocode for a cross-chain bridge function:

function bridgeTransfer(amount, sourceChain, destChain) {
  lockTokens(sourceChain, amount);
  mintTokens(destChain, amount);
  return 'Transfer complete';
}

What will be the output when calling bridgeTransfer(100, 'ChainA', 'ChainB')?

medium

A. 'Transfer complete'

B. Error: lockTokens undefined

C. 'Tokens locked on ChainA'

D. 100

Solution

1. Step 1: Analyze the function steps

   The function calls lockTokens and mintTokens, then returns the string 'Transfer complete'.

2. Step 2: Determine the output of the function call

   Assuming lockTokens and mintTokens work correctly, the function returns 'Transfer complete'.

3. Final Answer:

   'Transfer complete' -> Option A

4. Quick Check:

   Function returns 'Transfer complete' [OK]

Hint: Look at the return statement for output [OK]

Common Mistakes:

• Confusing function calls with return value
• Assuming intermediate functions print output
• Ignoring the return statement

4. The following code snippet is intended to lock tokens on the source chain and mint on the destination chain, but it has a bug:

function bridgeTransfer(amount, sourceChain, destChain) {
  lockTokens(destChain, amount);
  mintTokens(sourceChain, amount);
  return 'Transfer complete';
}

What is the bug in this code?

medium

A. The function uses incorrect function names

B. The lockTokens and mintTokens calls have swapped chain arguments

C. The function does not return any value

D. The amount parameter is missing

Solution

1. Step 1: Check the order of locking and minting

   Tokens should be locked on the source chain and minted on the destination chain.

2. Step 2: Identify the argument mismatch

   The code locks tokens on destChain and mints on sourceChain, which is reversed.

3. Final Answer:

   The lockTokens and mintTokens calls have swapped chain arguments -> Option B

4. Quick Check:

   Lock on source, mint on destination [OK]

Hint: Lock on source chain, mint on destination chain [OK]

Common Mistakes:

• Swapping source and destination chains
• Forgetting to return a value
• Using wrong function names

5. You want to design a cross-chain bridge that prevents double spending by ensuring tokens are locked before minting on the destination chain. Which approach best achieves this in a smart contract environment?

hard

A. Allow users to mint tokens on destination chain without locking on source chain

B. Mint tokens on destination chain first, then lock tokens on source chain

C. Use an event listener to confirm tokens are locked on source chain before minting on destination chain

D. Transfer tokens directly between chains without any locking or minting

Solution

1. Step 1: Understand double spending prevention

   Tokens must be locked on the source chain before minting on the destination chain to avoid duplicates.

2. Step 2: Evaluate approaches for enforcing this

   Using an event listener to confirm locking before minting ensures the correct order and security.

3. Final Answer:

   Use an event listener to confirm tokens are locked on source chain before minting on destination chain -> Option C

4. Quick Check:

   Confirm lock event before minting [OK]

Hint: Confirm lock event before minting tokens [OK]

Common Mistakes:

• Minting before locking causes double spending
• Allowing mint without lock breaks security
• Skipping locking or minting steps