zkCRO

On this page, you can find the contracts and their interactions involved when bridging $CRO and $zkCRO to and from Ethereum (L1) and Cronos zkEVM (L2) and unbonding $zkCRO, it includes:

[L1 -> L2] Minting $zkCRO with $CRO

[L2 -> L1] Withdrawing $zkCRO from L2

Unstaking $zkCRO for $CRO

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[L1 -> L2] Minting $zkCRO with $CRO

Before we begin, please note that the decimals of $CRO on Ethereum is 8

Developers can refer to this repository for an example script on converting $CRO (on L1 - Ethereum mainnet) to $zkCRO (on L2 - Cronos zkEVM mainnet).

To bridge $CRO Ethereum mainnet (L1) -> Cronos zkEVM mainnet(L2), and obtain $zkCRO on L2, we would need to interact with:

• $CRO token contract

  • Ethereum: 0xa0b73e1ff0b80914ab6fe0444e65848c4c34450b

• $zkCRO token contract

  • Ethereum: 0x28ff2e4dd1b58efeb0fc138602a28d5ae81e44e2

• ZkCroMintAndBridge

  • Ethereum: 0xe69a535730858fd8dc386b448972a9f801ab4e12

Step 1) Calculating the $CRO amount to bridge

On Ethereum - Using the $zkCRO contract, call theconvertToShare function to calculating the $CRO amount to bridge:

zkCro.convertToShare(_tokenAmount)

Parameters	Comments
_tokenAmount	The amount of $CRO to bridge, in 8 decimals. Note that additional $CRO to pay for gas on L2, We suggest to put at least 1 $CRO (i.e. adding 10^8 base unit as a buffer)

Keep this output amount for use in later steps.

Step 2) Approving the zkCROBridge to spend $CRO

On Ethereum - Using the $CRO contract to approve zkCROBridge contract for the amount you want to bridge to zkEVM. Note that the decimals of $CRO on Ethereum is 8.

Step 3) Minting and Bridging $CRO to L2

On Ethereum - Using the ZkCroMintAndBridge contract, call the mintAndBridge function:

function mintAndBridge(
   address _l2Receiver,
   uint256 _zkCroBridgeAmount,
   uint256 _l2GasLimit,
   uint256 _totalCro
)

Parameters	Comments
_l2Receiver	The address on Cronos zkEVM to receive $zkCRO
_zkCroBridgeAmount	Amount of $zkCRO to bridge (as calculated in Step 1)
_l2GasLimit	The maximum amount of gas to be used on L2. Set the value high enough or the transaction will fail
_totalCro	This value is the sum of $CRO the user wants to bridge, plus the gas fee in $zkCRO. Please include a small buffer. The contract throws an error if the buffer is zero. We suggest to put at least 1 $CRO (i.e. adding 10^8 base unit as a buffer)

Once done, it mints $zkCRO and bridges it to Cronos zkEVM, some of the $CRO on the Ethereum side will be consumed in the buffer range set to pay for the gas on Cronos zkEVM.

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[L2 -> L1] Withdrawing $zkCRO from L2

From Cronos zkEVM mainnet (L2) -> Ethereum mainnet (L1)

To withdraw $zkCRO fromCronos zkEVM mainnet -> Ethereum mainnet, we would need to interact with the following contract:

• $zkCRO token contract:

  • Cronos zkEVM: 0x000000000000000000000000000000000000800a
• zkChain Shared Bridge:

  • Ethereum : 0xD7f9f54194C633F36CCD5F3da84ad4a1c38cB2cB

Step 1) Initiating a withdrawal transaction

On Cronos zkEvm, call the withdraw function of zkCRO token contract:

function withdraw(address _l1Receiver) external payable;

Parameters	Comments
_l1Receiver	The address to recieve zkCRO on Ethereum mainnet
payable	The amount of $zkCRO to be transfer.

It will take at least 24 hours before the withdrawal can be finalized on Ethereum. Store the transaction hash from the withdrawal transaction for later steps.

Step 2) Gathering information for $zkCRO withdrawal on L1

Extra information for the L2 -> L1 transaction is required for the next step in claiming the $zkCRO on L1. This information can be gathered by finalizeWithdrawalParams, using zksync-ethers package

finalizeWithdrawalParams(withdrawalHash)

This takes the L2 withdrawal hash as parameters and returns the parameters required for finalizing a withdrawal in the next step.

Example outputs:

{
  l1BatchNumber: 18,
  l2MessageIndex: 59,
  l2TxNumberInBlock: 2919,
  message: '0x6c0960f935f448644c86dbda241ecb57f18a15c80f35a066000000000000000000000000000000000000000000000a2a15d09519be000000',
  sender: '0x000000000000000000000000000000000000800a',
  proof: [
    '0x23ff0fb003a6a6e111f87cff02afc8680944c711abbe792f07ab4206e9efb9a3',
    '0x09fece9c46307f2a0976a3a9b57873e453cf0f4773c2377dcd19f054166aef22',
    '0x33c106ae5601983b53c751129d1921c17e40b69aa3fdbfaf07b7a0e0750d35e4',
    '0x952ffa5de22d99815d9513dd7454949edb2a1cbcf86008f6fa8b169ffe9b4aee',
    '0x8f10560a0b5412ca2e0578fb9266f55411f357d52c8fc3b2cad43b9877a559a6',
    '0x8e70b93d3c90798df26473a3ad32a86170e8d52023a4044cf1d4c2d645471b14',
    '0xdfaacb39891bbbf4509af408239b97a6f5ef715a5c299ba1f3625c20aec4ef8f',
    '0xb04e5ee349086985f74b73971ce9dfe76bbed95c84906c5dffd96504e1e5396c',
    '0xac506ecb5465659b3a927143f6d724f91d8d9c4bdb2463aee111d9aa869874db',
    '0x124b05ec272cecd7538fdafe53b6628d31188ffb6f345139aac3c3c1fd2e470f',
    '0xc3be9cbd19304d84cca3d045e06b8db3acd68c304fc9cd4cbffe6d18036cb13f',
    '0xfef7bd9f889811e59e4076a0174087135f080177302763019adaf531257e3a87',
    '0xa707d1c62d8be699d34cb74804fdd7b4c568b6c1a821066f126c680d4b83e00b',
    '0xf6e093070e0389d2e529d60fadb855fdded54976ec50ac709e3a36ceaa64c291'
  ]
}

Step 3) Claiming the $zkCRO on L1

Once the transaction in Step 1 has been finalized on L1, we can claim the withdrawn $zkCRO from the zk Chain Shared Bridge contract by calling its finalizeWithdrawal function. The inputs can be found in Step 2,

    function finalizeWithdrawal(
            uint256   _chainId,
            uint256   _l2BatchNumber,
            uint256   _l2MessageIndex,
            uint16    _l2TxNumberInBatch,
            bytes calldata _message,
            bytes32 calldata _merkleProof
        )

Parameters	Comments
_chainId	Chain id for Cronos zkEVM, which is 388
_l2BatchNumber	The L2 batch number where the withdrawal was processed, obtained in Step 2.
_l2MessageIndex	The position in the L2 logs Merkle tree of the l2Log that was sent with the message, obtained in Step 2.
_l2TxNumberInBatch	The L2 transaction number in the batch, in which the log was sent, obtained in Step 2.
_message	The L2 withdraw data, stored in an L2 -> L1 message, obtained in Step 2.
_merkleProof	The Merkle Proof, obtained in Step 2.

Once finalizeWithdrawal is completed, the withdrawn amount of $zkCRO will go into the receiver's wallet on Ethereum.

We can use zksync-ethers package isWithdrawalFinalized call to confirm whether the withdrawal transaction is finalized on the L1 network.

isWithdrawalFinalized(withdrawalHash)

Parameters	Comments
withdrawalHash	The transaction hash of the L2 transaction includes the withdrawal transaction from Step 1.

Note - If you would like to convert $zkCRO on Ethereum to $CRO, you can do so by unstaking it to obtain the underlying $CRO staked and receive the $CRO on "Cronos EVM Mainnet" . Please refer to the following guide Unstaking $zkCRO for $CRO to complete the process.

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Unstaking $zkCRO for $CRO

To redeem $zkCRO fromEthereum mainnet and receive the underlying unstaked $CRO ( on "Cronos EVM Mainnet"), we would need to interact with the following contracts:

• $zkCRO token contract

  • Ethereum: 0x28ff2e4dd1b58efeb0fc138602a28d5ae81e44e2
• $LCRO token contract

  • Cronos EVM Mainnet: 0x9fae23a2700feecd5b93e43fdbc03c76aa7c08a6
• vnoNFT

  • Cronos EVM Mainnet: 0xb15533a0bc7c530d692a9660785226dfd3633965

Step 1) Approving $zkCRO to be spend by zkCRO token contract

On Ethereum, approve $zkCRO to be spent by zkCRO contract with appropriate amount.

Step 2) Unstaking $zkCRO to recieve $LCRO

On Ethereum, call the unstake function on the zkCRO token contract:

function unstake(
        address _receiver, 
        uint256 _shareAmount
        )

Parameters	Comments
_receiver	The Cronos EVM mainnet address to receive $LCRO.
_shareAmount	The amount of $zkCRO to unstake.

Once done, $LCRO token will be automatically released within 24 hours and transfer to the _receiver address of "Cronos EVM mainnet".

Step 3) Approving $LCRO spent

On Cronos EVM mainnet, approve $LCRO to be spent by $LCRO contract with appropriate amount

Step 4) Unbonding $LCRO

On Cronos EVM mainnet, call requestUnbond on the LCRO token contract,

function requestUnbond(
        uint256 _shareAmount, 
        address _receiver
        )

Parameters	Comments
_shareAmount	The amount of $LCRO that is to be burned to unbond
_receiver	The wallet address on ethereum to receive the receipt NFT. This NFT is needed to withdraw $CRO once the unbond period has ended

Once done you will receive an NFT (vnoNFT) in return. Wait for batch of unbonding to be processed which takes 28-32 days.

Step 5) Approving NFT to be spend by LCRO contract

On Cronos EVM mainnet, call setApprovalForAll on the vnoNFT contract, to approve to transfer of the vnoNFT obtained in Step 4)

setApprovalForAll(
        address operator, 
        bool _approved
        )

Parameters	Comments
operator	$LCRO contract address on ethereum
_approved	Set to "True" to grant approval.

Step 6) Withdrawing $CRO

On Cronos EVM mainnet, call unbond function on the LCRO token contract to burn the receipt NFT and receive the underlying unstaked $CRO.

 function unbond(
        uint256 _tokenId, 
        address _receiver
        )

Parameters	Comments
_tokenId	The tokenId of the receipt NFT, recieved in Step 4
_receiver	The address on Cronos EVM mainnet to receive $CRO

This transaction burns the receipt NFT and sends the unstaked $CRO to the specified receiver.