Cross-chain bridges in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Cross-chain bridges

User initiates transfer on Chain A

↓

Lock tokens on Chain A

↓

Generate proof of lock

↓

Send proof to Chain B

↓

Verify proof on Chain B

↓

Mint or release tokens on Chain B

↓

User receives tokens on Chain B

↓

Process complete

This flow shows how tokens move from one blockchain to another by locking on the first chain and minting or releasing on the second after proof verification.

Execution Sample

Blockchain / Solidity

function bridgeTransfer(amount, fromChain, toChain) {
  lockTokens(fromChain, amount);
  const proof = generateProof(fromChain, amount);
  if (verifyProof(toChain, proof)) {
    mintTokens(toChain, amount);
  }
}

This code simulates locking tokens on one chain, generating a proof, verifying it on another chain, and minting tokens there.

Execution Table

Step	Action	Chain	Tokens	Proof Generated	Proof Verified	Tokens Minted/Released
1	User initiates transfer	Chain A	100 locked	No	No	No
2	Tokens locked	Chain A	100 locked	No	No	No
3	Proof generated	Chain A	100 locked	Yes	No	No
4	Proof sent to Chain B	Chain B	0	Yes	No	No
5	Proof verified	Chain B	0	Yes	Yes	No
6	Tokens minted	Chain B	100 minted	Yes	Yes	Yes
7	User receives tokens	Chain B	100 minted	Yes	Yes	Yes
8	Process complete	Both	Chain A: 0 locked, Chain B: 100 minted	Yes	Yes	Yes

💡 Process stops after tokens are minted on Chain B and user receives them.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 6	Final
lockedTokens	0	100	100	100	0	0
proof	None	None	Generated	Generated	Generated	Generated
proofVerified	False	False	False	True	True	True
mintedTokens	0	0	0	0	100	100

Key Moments - 3 Insights

Why do tokens get locked on Chain A instead of just sending them?

Tokens are locked on Chain A to ensure they are not spent twice; this is shown in execution_table rows 1 and 2 where tokens become locked before proof generation.

What is the purpose of the proof in the bridge process?

The proof confirms tokens are locked on Chain A and is verified on Chain B before minting tokens, as seen in rows 3 to 5 of the execution_table.

Why are tokens minted on Chain B instead of transferring the original tokens?

Tokens are minted on Chain B to represent the locked tokens on Chain A without moving the original tokens, ensuring security and trust, shown in rows 6 and 7.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the proof verified on Chain B?

A Step 5

B Step 3

C Step 2

D Step 6

💡 Hint

Check the 'Proof Verified' column in the execution_table to see when it changes to 'Yes'.

According to variable_tracker, what is the value of mintedTokens after Step 6?

A None

B 0

C 100

D 50

💡 Hint

Look at the 'mintedTokens' row under 'After Step 6' in variable_tracker.

If tokens were not locked on Chain A, what would likely happen in the process?

A Proof would still be generated correctly

B Tokens could be spent twice causing security issues

C Tokens would be minted twice on Chain B

D Process would complete faster

💡 Hint

Refer to key_moments about why locking tokens is important to prevent double spending.

Concept Snapshot

Cross-chain bridges move tokens between blockchains by locking tokens on the source chain,
generating a proof of lock, verifying it on the destination chain, and minting tokens there.
This ensures tokens are not duplicated or lost.
Key steps: lock -> proof -> verify -> mint.
This process maintains trust and security across chains.

Full Transcript

Cross-chain bridges allow tokens to move safely between different blockchains. The process starts when a user locks tokens on the first chain. Then, a proof is created to show these tokens are locked. This proof is sent to the second chain, where it is checked carefully. Once verified, the second chain mints new tokens representing the locked ones. The user then receives these tokens on the second chain. This method prevents tokens from being spent twice and keeps the system secure. The key steps are locking tokens, generating and verifying proof, and minting tokens on the new chain.

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