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

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Send proof to Chain B

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Verify proof on Chain B

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Mint or release tokens on Chain B

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User receives tokens on Chain B

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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.