Cross-Chain Token (CCT) standard

The Cross-Chain Token (CCT) standard offers a streamlined and decentralized approach to enabling token transfers across blockchains using Chainlink's Cross-Chain Interoperability Protocol (CCIP). Traditionally, token developers had a friction-laden and manual process to enable their tokens for cross-chain operations. This process required manual deployment of token pools, making it time-consuming. With the introduction of CCTs, token developers now have the power to deploy, configure, and manage their own token pools in CCIP via a simple interface. This self-service model not only accelerates the deployment process but also empowers token developers with greater autonomy and control over their cross-chain token operations. This guide will explore the motivations behind Cross-Chain Token (CCT) and provide an overview of the architectural components in enabling an ERC20-compatible token in CCIP.

Motivations

The motivation for Cross-Chain Token (CCT) originates from two primary challenges: liquidity fragmentation in a multi-chain ecosystem and the need for greater autonomy for token developers.

• Liquidity Fragmentation: Hundreds of blockchains exist, each with their own rules, consensus mechanisms, and liquidity. This expansion has created fragmented liquidity, where assets are siloed on individual blockchains, making it difficult for users and developers to access liquidity across different ecosystems. Token developers face the dilemma of choosing which blockchain to deploy on, often deciding between the blockchain with the most liquidity or a new, emerging chain with future potential. Each decision comes with risks, such as competing in a crowded market or dealing with trust assumptions on new blockchains.

• Need for Greater Autonomy: Another critical motivation behind Cross-Chain Token is to empower projects to enable their tokens for cross-chain operations without the support of third-parties. By providing a self-service model, Cross-Chain Token allows projects to take control of their cross-chain applications. This enables rapid expansion to other blockchains supported by CCIP.

Cross-Chain Token (CCT) solves these challenges by allowing token developers to create assets that can move seamlessly across multiple blockchains without fragmenting liquidity. By deploying a token in multiple blockchain environments, token developers can maintain a consistent supply, manage liquidity efficiently, and reach users on different blockchains without introducing complex bridging processes. The CCT standard offers the following benefits:

• Self-service and Permissionless Deployment: With CCT, token developers can launch a cross-chain token or enable an existing token in a self-service manner within minutes. The fully audited token pool contracts handle the complexities of burning/minting or locking/minting tokens across blockchains, all without requiring liquidity pools. This enables a streamlined and permissionless deployment process that significantly reduces the barriers to cross-chain expansion.

• Developer Control and Flexibility: Token developers retain complete ownership of their token contracts, token pools, and implementation logic. This includes configuring rate limits across all blockchains, ensuring that token developers have full autonomy to manage their deployments. The CCT standard avoids vendor lock-in, hard-coded functions, and external dependencies, giving projects the flexibility they need to succeed.

• Defense-in-Depth Security: The CCT standard leverages Chainlink's industry-standard oracle networks, which secure over $16 trillion in Total Value Enabled (TVE). This strong foundation is further enhanced by additional layers of protection, such as the Risk Management Network and configurable transfer rate limits, providing comprehensive security for cross-chain operations.

• Programmable Token Transfers: Cross-Chain Token (CCT) supports the simultaneous transfer of tokens and messages in a single transaction. This programmability allows for complex use cases, such as executing on-chain actions alongside token transfers, improving the overall efficiency of cross-chain transactions. For instance, a programmable token transfer could execute a function to stake tokens on the destination blockchain.

• No Liquidity Pools Required: The fully audited token pool contracts provided by CCT eliminate the need for liquidity pools, simplifying cross-chain token management. By locking or burning tokens on the source chain and minting or unlocking them on the destination chain, token developers can maintain consistent liquidity without managing fragmented liquidity pools across multiple blockchains.

• Reduced Liquidity Fragmentation: The CCT standard addresses liquidity fragmentation by allowing tokens to move seamlessly across multiple blockchains rather than requiring liquidity to be present on both source and destination chains. This unified approach ensures that liquidity remains consistent and accessible across all supported blockchains, reducing inefficiencies and improving the user experience.

• Zero-Slippage Transfers: Token developers are provided access to pre-audited token pool contracts that enable zero-slippage cross-chain transfers of CCTs. The exact amount of tokens sent to the CCIP OnRamp on the source chain is always the exact amount of tokens received on the CCIP OffRamp on the destination chain.

• Ease of Cross-Chain Integration: With CCT, existing ERC20-compatible tokens can be easily extended to support cross-chain functionality. This makes it possible for token developers to extend their token's reach across multiple blockchains with minimal effort, avoiding the complexities associated with traditional bridging solutions.

Key Terms and Concepts

• Token Developer: The entity responsible for issuing and managing the token. Token developers decide which blockchains to deploy the token on, manage its lifecycle, and control its availability across different networks.

• Token Owner: The entity that owns the token contract. The token owner has the authority to manage the token contract, including upgrading the contract, changing the token administrator, or transferring ownership to another address.

• Token Administrator: A role assigned to manage cross-chain operations of a token. The token administrator is responsible for mapping a token to a token pool in the TokenAdminRegistry.. The token administrator can be the token owner or another designated entity assigned by the token owner. Technically, the token administrator can be an EOA or a smart account.

• EOA (Externally Owned Account): A standard blockchain account controlled by a private key, typically used by individuals. EOAs do not have associated code, unlike smart accounts, and are used for directly signing transactions.

• Smart Account: A blockchain account represented by a smart contract, offering advanced functionality like multi-signature authorization or custom transaction logic. Smart accounts provide better security and flexibility compared to EOAs. For example, smart accounts can use multi-signature mechanisms to add an extra layer of security.

• ERC20-Compatible Token: A token that adheres to the ERC20 token or any other token standard that improves upon it, such as ERC677 or ERC777. These contracts have the same interface as ERC20 but add extra functionalities. Cross-Chain tokens need to be ERC20-compatible to ensure interoperability across different blockchain ecosystems. The specific requirements for ERC20-compatible tokens in CCIP will be detailed later in this document, including requirements for Burn & Mint and Lock & Mint mechanisms.

• Token Pool: Each token is associated with its own token pool, which acts as an abstraction layer over ERC-20 tokens designed to facilitate token-related operations for OnRamping and OffRamping. Token pools provide rate limits, a security feature enabling token developers to set a maximum rate at which their token can be transferred per lane. Token pools are configured to lock or burn tokens on the source blockchain and unlock or mint tokens on the destination blockchain. This setup results in four primary mechanisms:

  • Burn and Mint: Tokens are burned on the source blockchain, and an equivalent amount of tokens are minted on the destination blockchain. This keeps the total supply of the token constant across blockchains.

    

  • Lock and Mint: This mechanism is suitable for projects with existing tokens on a single blockchain that have already been minted and lack functionality to control the supply via burn and mint functions on the source blockchain. In this approach, tokens are locked on the issuing blockchain, and fully collateralized "wrapped" tokens are minted on the destination blockchain. These wrapped tokens, which must support burn and mint functionality, can subsequently be transferred across other non-issuing blockchains using the Burn and Mint mechanism.

    

  • Burn and Unlock: Tokens are burned on the source blockchain (which is the non-issuing blockchain), and an equivalent amount of tokens are released on the destination blockchain (the issuing blockchain). This mechanism is the inverse of the Lock and Mint mechanism and applies when you send tokens back to their issuing source blockchain.

    

  • Lock and Unlock: Tokens are locked on the source blockchain, and an equivalent amount of tokens are released on the destination blockchain. This use case is not recommended, as it implies multiple issuing blockchains, which results in fragmented liquidity as well as the requirement of managing liquidity to ensure liveness of lanes. For additional guidance on Lock and Unlock configuration, please review best practices.

    

Requirements for Cross-Chain Tokens

Before enabling an ERC20-compatible token in CCIP, it's important to understand the requirements it must fulfill to integrate with CCIP.

• Recommended Permissionless Token Administrator address registration methods: A token can utilize either of these supported function signatures to register permissionlessly:

  • owner(): This function returns the token contract owner's address.

  • getCCIPAdmin(): This function returns the token administrator's address and is recommended for new tokens, as it allows for abstraction of the CCIP Token Administrator role from other common roles, like owner().

• Requirements for CCIP token transfers: The token's smart contract must meet minimum requirements to integrate with CCIP.

  • Burn & Mint Requirements:
    • The token smart contract must have the following functions:
      • mint(address account, uint256 amount): This function is used to mint the amount of tokens to a given account on the destination blockchain.
      • burn(uint256 amount): This function is used to burn the amount of tokens on the source blockchain.
      • decimals(): Returns the token's number of decimals.
      • balanceOf(address account): Returns the current token balance of the specified account.
      • burnFrom(address account, uint256 amount): This function burns a specified number of tokens from the provided account on the source blockchain. Note: This is an optional function. We generally recommend using the burn function, but if you use a tokenPool that calls burnFrom, your token contract will need to implement this function.
    • On the source and destination blockchains, the token contract must support granting mint and burn permissions. The token developers or another role (such as the token administrator) will grant these permissions to the token pool.
  • Lock & Mint Requirements:
    • The token smart contract must have the following functions:
      • decimals(): Returns the token's number of decimals.
      • balanceOf(address account): Returns the current token balance of the specified account.
    • On the destination blockchain, The token contract must support granting mint and burn permissions. The token developers or another role (such as the token administrator) will grant these permissions to the token pool.

If you don't have an existing token: For all blockchains where tokens need to be burned and minted (for example, the source or destination chain in the case of Burn and Mint, or the destination blockchain in the case of Lock and Mint), Chainlink provides a BurnMintERC677 contract that you can use to deploy your token in minutes. This token follows the ERC677 or ERC777, allowing you to use it as-is or extend it to meet your specific requirements.

Requirement for Token Pools

Common Requirements

All token pools, whether standard or custom, must adhere to the following guidelines:

1. Mandatory functions: When CCIP interacts with your token pools, it expects the presence of the following functions:

   1. Source Blockchain: This must include the following function:

      • lockOrBurn(Pool.LockOrBurnInV1 calldata lockOrBurnIn) external returns (Pool.LockOrBurnOutV1 memory): This function locks or burns tokens on the source blockchain. Refer to the Pool library API reference for details about the input and output parameters.

   2. Destination Blockchain: This must include the following function:

      • releaseOrMint(Pool.ReleaseOrMintInV1 calldata releaseOrMintIn) external returns (Pool.ReleaseOrMintOutV1 memory): This function releases or mints tokens on the destination blockchain. Refer to the Pool library API reference for details about the input and output parameters.

2. Gas Limit on the Destination Blockchain:

   1. On the destination blockchain, the CCIP OffRamp contract performs three key calls when releasing or minting tokens:

      1. balanceOf before minting/releasing tokens: To check the token balance of the receiver before the minting or releasing operation.
      2. releaseOrMint to mint or release tokens: To execute the minting or releasing of tokens on the destination blockchain. Note: The total execution of the releaseOrMint function includes both the token pool's logic and the token's execution logic. For example, if minting tokens is part of the process, the gas consumed by the minting operation is included in the total gas consumed by the releaseOrMint function.
      3. balanceOf after minting/releasing tokens: To verify the token balance of the receiver after the operation is complete.

   2. The combined execution of these three calls should not exceed the default gas limit of 90,000 gas. To verify this cost, it is recommended to perform a token transfer on testnet. If the transaction reverts on destination, manual execution can be utilized and the resulting transaction can be inspected for the amount of gas that was used.

   3. If the default gas limit of 90,000 gas is exceeded and a custom limit has not been configured, the CCIP execution on the destination blockchain will fail. In such cases, manual intervention by the user will be required to execute the transaction. For more information, see manual execution.

   4. If the combined execution requires more than 90,000 gas on the destination blockchain, you should contact Chainlink Labs to update an internal CCIP parameter to avoid execution failure. However, it is highly recommended to design your token pool to stay within the 90,000 gas limit whenever possible to ensure you enabled your tokens in CCIP without Chainlink Labs intervention.

Standard Token Pools

Depending on your use case (token handling mechanism), you need to deploy the appropriate token pool type for each blockchain you want to support. Chainlink provides a set of token pool contracts that you can use to deploy your token pools in minutes. These token pools are fully audited and ready for deployment on your blockchains. You can find the token pool contracts in the Chainlink GitHub repository. For most use cases, you should use either:

• BurnMintTokenPool: This token pool is used to burn or mint tokens. You can read the API reference here.
• BurnFromMintTokenPool: This is a variant of the BurnMintTokenPool that uses the burnFrom(from, amount) function to burn tokens from a specified account. You can read the API reference here. Note: If your token supports the standard burn function, you should typically use the BurnMintTokenPool instead of BurnFromMintTokenPool.
• LockReleaseTokenPool: This token pool is used to lock or release tokens. You can read the API reference here.

Note: Both token pools inherit from the same base TokenPool contract, which provides all the common functions necessary for a token pool. For example, it includes the applyChainUpdates function, which is used to configure the token pool. You can read the API reference here.

Token Handling Mechanisms and Token Pool Deployment

To facilitate cross-chain token transfers, you need to choose the appropriate token handling mechanism and deploy the correct combination of token pools for the source and destination blockchains. The table below summarizes the different token handling mechanisms and the recommended token pools to deploy for each scenario, ensuring a seamless token transfer process.

Custom Token Pools

If the standard token pools do not meet your requirements, you have the option to build a custom TokenPool. However, it is essential to adhere to the following guidelines:

• Your custom token pool must inherit from the appropriate base token pool contract depending on your token handling mechanism:

  • For Burn and Mint mechanisms: Your custom token pool should inherit from BurnMintTokenPoolAbstract. Use this base contract if your custom pool involves burning tokens on the source chain and minting them on the destination chain.
  • For Lock and Release mechanisms: Your custom token pool can either inherit from TokenPool and implement the ILiquidityContainer interface, or directly inherit from LockReleaseTokenPool and reimplement lockOrBurn and releaseOrMint functions as needed. This setup is appropriate when your pool involves locking tokens on the source blockchain and releasing them on the destination blockchain.

• Your custom TokenPool must implement the mandatory functions for both the source and destination blockchains. (Refer to the Common Requirements section for more details.)

Here are some examples of use cases that may require custom token pools:

• Tokens with rebasing or fee-on-transfer mechanisms:

  • Use Case: Rebasing tokens are a unique type of token that adjusts its supply in response to specific parameters (e.g., price). These tokens require custom logic to handle rebasing events during cross-chain transfers.
  • Solution:
    • Source Blockchain: When initiating a cross-chain transfer of rebasing tokens, the TokenPool on the source blockchain should lock or burn the underlying shares rather than a fixed amount of tokens. This ensures that the value is consistently represented, regardless of changes in supply. The number of shares being locked or burned is recorded in the destPoolData and passed to the destination blockchain.
    • Destination Blockchain: On the destination blockchain, the TokenPool should accurately convert the number of underlying shares to tokens using the current share-to-token ratio. The calculated token amount should then be minted on the destination blockchain and returned in the destinationAmount field of the ReleaseOrMintOutV1 struct, which is returned by the releaseOrMint function.

• Tokens with different decimals across blockchains:

  • Use Case: Some tokens have different decimal values across various blockchains.

  • Solution:

    • Source Blockchain: During the lock or burn process on the source blockchain, the TokenPool should include a shared denomination in the destPoolData field of the LockOrBurnOutV1 struct (returned by the lockOrBurn function) to represent the value in a standard format. This data is then passed to the destination blockchain via CCIP.
    • Destination Blockchain: On the destination blockchain, the TokenPool should use the information contained in the sourcePoolData of the ReleaseOrMintInV1 struct (used by the releaseOrMint function) to convert the value into the local denomination. The correct number of tokens should then be minted based on the destination blockchain's decimal format. The minted amount is returned in the destinationAmount field of the ReleaseOrMintOutV1 struct, which is returned by the releaseOrMint function.

CCT Architecture Overview

The Cross-Chain Token (CCT) architecture offers a streamlined, self-service approach to enabling cross-chain operations. This system integrates tightly with Chainlink's Cross-Chain Interoperability Protocol (CCIP), allowing token developers to configure, manage, and transfer tokens across multiple blockchains without requiring the support of third parties.

Architecture

The detailed architecture diagram below provides a comprehensive view of how CCT fits within the CCIP ecosystem, illustrating the interaction between key components such as token contracts, token pools, and registry modules. While you can explore the full details of these components in the CCIP Architecture document, the key takeaway for Cross-Chain Token (CCT) is understanding how the TokenAdminRegistry contract is used.

The TokenAdminRegistry contract is essential when transferring tokens across blockchains. CCIP's onRamp and offRamp contracts interact with it to fetch the token pool associated with a given token. For cross-chain transfers to work, token administrators need a way to set or configure the token pool linked to a token.

In the following sections, we will explore how token administrators can register their tokens in the TokenAdminRegistry, link them to the relevant token pools, and configure them for use in CCIP.

Key Contracts

In the Cross-Chain Token (CCT) standard, several key contracts work together to facilitate the secure transfer and management of tokens across multiple blockchains. These contracts can be categorized into three main groups:

Registry Contracts

The Registry contracts manage the registration tokens within the CCT system. They ensure that the correct entities have control over cross-chain operations.

• TokenAdminRegistry: This contract stores the token administrators and pools for all registered cross-chain tokens. It allows tokens to be registered in a self-service manner and handles administrator role changes via a two-step process (transfer request and acceptance).
• RegistryModuleOwnerCustom: This contract facilitates the registration of token administrators. It works with the TokenAdminRegistry to ensure that only authorized administrators are assigned to manage cross-chain operations.

Token Contract

The Token contract represents the actual token being managed and transferred across blockchains. This contract must be ERC20-compatible and have additional functionalities depending on the cross-chain handling mechanism used. For more details on the requirements for ERC20-compatible tokens, refer to the Requirements for ERC20-Compatible Tokens section.

Token Pool Contract

The Token Pool contract is responsible for executing the cross-chain token transfers. It manages how tokens are locked, burned, minted, or unlocked across blockchains.

• Token Pools: Each blockchain has its own token pool that interacts with the token contract. Depending on the cross-chain handling mechanism (e.g., Burn & Mint or Lock & Mint), different token pool contracts will be deployed. For example:
  • BurnMintTokenPool: Handles the burning, or minting of tokens depending whether it is the source or destination blockchain.
  • LockReleaseTokenPool: Handles the locking or releasing of tokens depending on whether it is the source or destination blockchain.

For more information on the token pool contracts and their functionalities, refer to the Requirement for Token Pools section.

Registration

The process of registering your token for Cross-Chain Token (CCT) involves a two-step process to ensure that the correct administrator is assigned. The token's administrator will initially be placed in a proposed state. The administrator must then explicitly accept the role to complete the registration.

Self-Service Registration Flow

If the token contract includes the necessary functions (getCCIPAdmin() or owner()), the registration can be done autonomously by the token administrator:

1. Admin Initiates Registration: The token administrator begins the process by calling either the registerAdminViaGetCCIPAdmin() or registerAdminViaOwner() function on the RegistryModuleOwnerCustom contract.

2. Determine Administrator: The RegistryModuleOwnerCustom contract retrieves the administrator from the token contract using the appropriate method (getCCIPAdmin() or owner()).

3. Propose Administrator: The retrieved administrator is then proposed to the TokenAdminRegistry using the proposeAdministrator() function, placing the administrator in a pending state.

4. Pending Administrator: At this point, the administrator is marked as "pending" in the TokenAdminRegistry and must complete the second step—accepting the role—before being officially assigned.

Non-Self-Service Registration Flow

If the token contract does not have the necessary functions (getCCIPAdmin() or owner()), the token developer must manually initiate the registration by submitting a request here.

Sequence Diagram: Proposing the Administrator

The following sequence diagram illustrates the process of proposing the administrator for both self-service and manual registration flows:

Accepting the Administrator Role

Once the administrator has been proposed and is in a pending state, they must accept the role to complete the registration process. This step finalizes the assignment of the administrator.

1. Administrator Accepts Role: The pending administrator must explicitly call the acceptAdminRole() function on the TokenAdminRegistry to complete the registration.
2. Finalize Registration: Once the administrator accepts the role, they are assigned as the token administrator, and the registration process is complete. At this point, the token administrator can set a token pool for the token in the TokenAdminRegistry.

Once the administrator has been proposed and is in a pending state, the final step in the registration process is for the pending administrator to accept the role. This sequence diagram illustrates how the pending administrator interacts with the TokenAdminRegistry to complete the registration. It also covers the scenario where an incorrect entity attempts to accept the role, leading to an error.

Transfer Administrator Role

The transferAdminRole function allows the current token administrator to initiate the transfer of their role to a new administrator. The transfer process is a secure two-step procedure, requiring the new administrator to explicitly accept the role before the transfer is finalized.

1. Initiate Role Transfer: The current administrator calls the transferAdminRole() function on the TokenAdminRegistry, specifying the token address and the new administrator’s address.
2. Set Pending Administrator: The TokenAdminRegistry contract verifies that the caller is the current administrator of the token and sets the new administrator as pending. The role will remain in this pending state until it is accepted.
3. Accept the Role: The new administrator must call the acceptAdminRole() function to finalize the transfer and assume the administrator role.

The following sequence diagram illustrates the process of transferring the administrator role and how the new administrator must accept the role to complete the transfer.

Setting the Token Pool

The setPool function allows the token administrator to assign or update the token pool for a specific token in the TokenAdminRegistry.

1. Set Token Pool: The current administrator calls the setPool() function on the TokenAdminRegistry, providing the token address and the new pool address.
2. Validate Pool: If the new pool address is not address(0), the contract validates that the provided pool supports the token by calling isSupportedToken() on the pool contract.
3. Update or Remove Pool: If validation succeeds, the token’s pool is updated in the registry. Setting the pool to address(0) effectively delists the token from cross-chain operations.

The sequence diagram below shows how the token administrator sets or updates the pool for a token. If the pool is set to address(0), the token is delisted from cross-chain operations.

Configuring the Token Pool

The configuration of token pools includes adding new blockchains, setting remote pool addresses, and applying rate limits for cross-chain transfers. The following functions from the TokenPool contract are used for configuring token pools:

1. applyChainUpdates:

   • Purpose: This function is the primary method for configuring which blockchains the token pool supports and defining rate limits for cross-chain transfers.
   • Details:
     • It allows the token pool owner to add new chains or remove existing ones.
     • Configures the pool and token addresses for remote blockchains.
     • Sets rate limits for both outbound and inbound transfers.
   • Usage:
     • To add a new blockchain, the pool owner provides the remote chain selector, pool address, token address, and rate limiter configurations.
     • To remove a blockchain, the allowed flag is set to false, and the chain is removed from the list of supported chains.

2. setRemotePool:

   • Purpose: Set the remote pool's address to link the current token pool to a corresponding pool on a remote blockchain.
   • Details:
     • This function is used to establish the connection between token pools on different blockchains.
     • It validates the provided chain selector and updates the remote pool address for the selected chain.
   • Usage: This function sets or updates the remote pool address for a specific blockchain.

3. setChainRateLimiterConfig:

   • Purpose: Configures the rate limits for outbound and inbound token transfers between blockchains.
   • Details:
     • The outbound rate limiter config controls how many tokens can be transferred out of the token pool per unit of time.
     • The inbound rate limiter config limits how many tokens can be transferred into the token pool per unit of time.
     • Only the pool owner or rate limit admin can call this function.
   • Notes on Rate Limit Admin: The rate limit admin is a designated address that is authorized to configure the rate limits for a token pool. This admin can be set by the pool owner using the setRateLimitAdmin function. If no rate limit admin is set, only the pool owner can modify the rate limits. You can retrieve the current rate limit admin address using the getRateLimitAdmin function.
   • Usage: This function adjusts rate limits to prevent excessive token transfers or overload of the token pool. Note: This function also supports disabling rate limits.

4. applyAllowListUpdates:

   • Purpose: Manages an allowlist of addresses permitted to interact with the token pool.
   • Details:
     • This function is only relevant if the token pool is access-controlled (i.e., an allowlist is enabled).
     • The allowlist ensures that only specific addresses, such as trusted addresses, can transfer tokens.
   • Usage: The pool owner uses this function to add or remove addresses from the allowlist, controlling who can transfer the tokens through CCIP.