CMTA Token

To use the CMTAT, we recommend the latest audited version, from the Releases page. Currently, it is the version v2.3.0

Introduction

The CMTA token (CMTAT) is a framework enabling the tokenization of securities in compliance with Swiss law. This repository provides CMTA's reference implementation of CMTAT for Ethereum, as an ERC-20 compatible token.

The CMTAT is an open standard from the Capital Markets and Technology Association (CMTA), which gathers Swiss finance, legal, and technology organizations. The CMTAT was developed by a working group of CMTA's Technical Committee that includes members from Atpar, Bitcoin Suisse, Blockchain Innovation Group, Hypothekarbank Lenzburg, Lenz & Staehelin, Metaco, Mt Pelerin, SEBA, Swissquote, Sygnum, Taurus and Tezos Foundation. The design and security of the CMTAT was supported by ABDK, a leading team in smart contract security.

The preferred way to receive comments is through the GitHub issue tracker. Private comments and questions can be sent to the CMTA secretariat at admin@cmta.ch. For security matters, please see SECURITY.md.

Note that CMTAT may be used in other jurisdictions than Switzerland, and for tokenizing various asset types, beyond equity and debt products.

Functionality

Overview

The CMTAT supports the following core features:

• Basic mint, burn, and transfer operations
• Pause of the contract and freeze of specific accounts

Furthermore, the present implementation uses standard mechanisms in order to support:

• Upgradeability, via deployment of the token with a proxy
• "Gasless" transactions
• Conditional transfers, via a rule engine

This reference implementation allows the issuance and management of tokens representing equity securities. It can however also be used for other forms of financial instruments such as debt securities.

You may modify the token code by adding, removing, or modifying features. However, the core modules must remain in place for compliance with Swiss law.

Kill switch

CMTAT initially supported a kill() function relying on the SELFDESTRUCT opcode (which effectively destroyed the contract's storage and code). However, Ethereum's Cancun upgrate (rolled out in Q1 of 2024) has removed support for SELFDESTRUCT (see EIP-6780).

The kill() function will therefore not behave as it was used

We have replaced this function by a new function deactivateContract, introduced in the version v2.3.1 inside the PauseModule, to deactivate the contract. This function set a boolean state variable isDeactivated to true and puts the contract in the pause state. The function unpauseis updated to revert if the previous variable is set to true, thus the contract is in the pause state "forever".

The consequences are the following:

• In standalone mode, this operation is irreversible, it is not possible to rollback.
• With a proxy, it is still possible to rollback by deploying a new implementation.

Gasless support

The CMTAT supports client-side gasless transactions using the Gas Station Network (GSN) pattern, the main open standard for transfering fee payment to another account than that of the transaction issuer. The contract uses the OpenZeppelin contract ERC2771ContextUpgradeable, which allows a contract to get the original client with _msgSender() instead of the fee payer given by msg.sender while allowing upgrades on the main contract (see Deployment via a proxy above).

At deployment, the parameter forwarder inside the CMTAT contract constructor has to be set with the defined address of the forwarder. Please note that the forwarder can not be changed after deployment, and with a proxy architecture, its value is stored inside the implementation contract bytecode instead of the storage of the proxy.

Please see the OpenGSN documentation for more details on what is done to support GSN in the contract.

Architecture

CMTAT architecture is divided in two main componentes: module and engines

Module

Modules describe a logical code separation inside CMTAT. They are defined as abstract contracts. Their code and functionalities are part of the CMTAT and therefore are also included in the calculation of the contract size and the maximum size limit of 24 kB.

It is always possible to delete a module but this requires modifying the code and compiling it again, which require to perform a security audit on these modifications.

Modules are also separated in different categories.

• Internal modules: implementation for a module when OpenZeppelin does not provide a library to use. For example, this is the case for the SnapshotModule.

• Wrapper modules: abstract contract around OpenZeppelin contracts or internal module. For example, the wrapper PauseModule provides public functions to call the internal functions from OpenZeppelin.

  • Core (Wrapper sub-category): Contains the modules required to be CMTA compliant
  • Extension (Wrapper sub-category): not required to be CMTA compliant, "bonus features" (snapshotModule, debtModule)

List

Here the list of the different modules with the links towards the documentation and the main file.

Controller

Name	Documentation	Main File
ValidationModule	validation.md	ValidationModule.sol

Core

Generally, these modules are required to be compliant with the CMTA specification.

Name	Documentation	Main File
BaseModule	base.md	BaseModule.sol
BurnModule	ERC20Burn.md	ERC20BurnModule.sol
EnforcementModule	enforcement.md	EnforcementModule.sol
ERC20BaseModule	erc20base.md	ERC20BaseModule.sol
MintModule	ERC20Mint.md	ERC20MintModule.sol
PauseModule	pause.md	PauseModule.sol

Extensions

Generally, these modules are not required to be compliant with the CMTA specification.

Name	Documentation	Main File
MetaTxModule	metatx.md	MetaTxModule.sol
SnapshotModule	ERC20Snapshot.md	ERC20SnapshotModule.sol
DebtModule	debt.md	DebtModule.sol
DocumentModue	document.md	Document.sol

Security

Name	Documentation	Main File
AuthorizationModule	authorization.md	AuthorizationModule.sol

Engines

Engines are external smart contracts called by CMTAT modules.

These engines are optional and their addresses can be left to zero.

More details are available in ./doc/general/Engine.md

RuleEngine (IERC-1404)

The RuleEngine is an external contract used to apply transfer restriction to the CMTAT through whitelisting, blacklisting,...

This contract is defined in the ValidationModule.

An example of RuleEngine is also available on GitHub.

Here is the list of the different version available for each CMTAT version.

Name	RuleEngine
CMTAT 2.5.0 (unaudited)	RuleEngine >= v2.0.3 (unaudited)
CMTAT 2.4.0 (unaudited)	RuleEngine >=v2.0.0 Last version: v2.0.2(unaudited)
CMTAT 2.3.0	RuleEngine v1.0.2
CMTAT 2.0 (unaudited)	RuleEngine 1.0 (unaudited)
CMTAT 1.0	No ruleEngine available

This contract acts as a controller and can call different contract rule to apply rule on each transfer.

A possible rule is a whitelist rule where only the address inside the whitelist can perform a transfer

Since the version 2.4.0, the requirement to use a RuleEngine are the following:

The RuleEngine has to import an implement the interface IRuleEngine which declares the function operateOnTransfer.

This interface can be found in ./contracts/interfaces/engine/IRuleEngine.sol.

Before each transfer, the CMTAT calls the function operateOnTransfer which is the entrypoint for the RuleEngine.

AuthorizationEngine

Warning: this engine may be deleted in the future

The AuthorizationEngine is an external contract to add supplementary check on the functions grantRole and revokeRolefrom the CMTAT.

This contract is managed in the AuthorizationModule.

The AuthorizationEngine has to import an implement the interface IAuthorizationEngine.sol which declares the functions operateOnGrantRole and operateOnRevokeRole

Currently, there is only a prototype available: CMTA/AuthorizationEngine

DebtEngine

This engine replaces the modules Debt and Credit included since CMTAT version.

CMTAT only implements two functions , available in the interface IDebtEngine to get information from the debtEngine.

interface IDebtEngine is IDebtGlobal {
    function debt() external view returns(IDebtGlobal.DebtBase memory);
    function creditEvents() external view returns(IDebtGlobal.CreditEvents memory);
}

Use an external contract provides two advantages:

• Reduce code size of CMTAT, which is near of the maximal size limit
• Allow to manage this information for several different tokens (CMTAT or not).

Here is the list of the different version available for each CMTAT version.

Name	DebtEngine
CMTAT v2.5.0 (unaudited)	DebtEngine v0.2.0 (unaudited)

DocumentEngine (IERC-1643)

The DocumentEngine is an external contract to support ERC-1643 inside CMTAT, a standard proposition to manage document on-chain. This standard is notably used by ERC-1400 from Polymath.

This EIP defines a document with three attributes:

• A short name (represented as a bytes32)
• A generic URI (represented as a string) that could point to a website or other document portal.
• The hash of the document contents associated with it on-chain.

CMTAT only implements two functions from this standard, available in the interface IERC1643 to get the documents from the documentEngine.

interface IERC1643 {
function getDocument(bytes32 _name) external view returns (string memory , bytes32, uint256);
function getAllDocuments() external view returns (bytes32[] memory);
}

The DocumentEngine has to import an implement this interface. To manage the documents, the engine is completely free on how to do it.

Use an external contract provides two advantages:

• Reduce code size of CMTAT, which is near of the maximal size limit
• Allow to manage documents for several different tokens (CMTAT or not).

Here is the list of the different version available for each CMTAT version.

Name	DocumentEngine
CMTAT v2.5.0 (unaudited)	DocumentEngine v0.3.0 (unaudited)

Deployment model

Model	Contract
Standalone	CMTAT_STANDALONE
Transparent or Beacon Proxy	CMTAT_PROXY
UUPS Proxy	CMTAT_PROXY_UUPS

Standalone

To deploy CMTAT without a proxy, in standalone mode, you need to use the contract version CMTAT_STANDALONE.

With a proxy

The CMTAT supports deployment via a proxy contract. Furthermore, using a proxy permits to upgrade the contract, using a standard proxy upgrade pattern.

• The implementation contract to use with a TransparentProxy is the CMTAT_PROXY.
• The implementation contract to use with a UUPSProxy is the CMTAT_PROXY_UUPS.

Please see the OpenZeppelin upgradeable contracts documentation for more information about the proxy requirements applied to the contract.

Please see the OpenZeppelin Upgrades plugins for more information about plugin upgrades in general.

CMTAT also implements the ERC-7201 to manage the storage location.

Note that deployment via a proxy is not mandatory, but is recommended by CMTA.

Factory

Factory contracts are available to deploy the CMTAT with a beacon proxy, a transparent proxy or an UUPS proxy.

• CMTAT_BEACON_FACTORY.sol

• CMTAT_TRANSPARENT_FACTORY.sol

• CMTAT_UUPS_FACTORY.sol

Beacon Proxy Factory

The factory will use the same beacon for each beacon proxy. This beacon provides the address of the implementation contract, a CMTAT_PROXY contract. If you upgrade the beacon to point to a new implementation, it will change the implementation contract for all beacon proxy.

Transparent Proxy Factory

The factory will use the same implementation for each transparent proxy deployed. Each transparent proxy has its owned proxy admin, deployed inside the constructor of the transparent proxy. Each transparent proxy can upgrade their implementation to a new one independently and without impact on other proxies.

UUPS ProxyFactory

The factory will use the same implementation for each UUPS proxy deployed. Each UUPS proxy can upgrade their implementation to a new one independently and without impact on other proxies.

Contrary to the Transparent Proxy, the logic to upgrade the proxy is situated in the implementation and not in the proxy.

This is the reason whey there is a specific CMTAT contract which includes this logic to use: CMTAT_PROXY_UUPS.sol

Security

Vulnerability disclosure

Please see SECURITY.md.

Module

See the code in modules/security.

Access control is managed thanks to the module AuthorizationModule.

Audit

The contracts have been audited by ABDKConsulting, a globally recognized firm specialized in smart contracts security.

First audit - September 2021

Fixed version : 1.0

Fixes of security issues discovered by the initial audit were reviewed by ABDK and confirmed to be effective, as certified by the report released on September 10, 2021, covering version c3afd7b of the contracts. Version 1.0 includes additional fixes of minor issues, compared to the version retested.

A summary of all fixes and decisions taken is available in the file CMTAT-Audit-20210910-summary.pdf

Second audit - March 2023

Fixed version : v2.3.0

The second audit covered version 2.2.

Version v2.3.0 contains the different fixes and improvements related to this audit.

The report is available in ABDK_CMTA_CMTATRuleEngine_v_1_0.pdf.

Tools

Slither

You will find the report produced by Slither in

Version	File
Last version	slither-report.md
v2.3.0	v2.3.0-slither-report.md
v2.3.1	v2.3.1-slither-report.md
v2.4.0	v2.4.0-slither-report.md
v2.5.0	v2.5.0-slither-report.md

Mythril

Version	File
Last version	mythril-report-standalone.md mythril-report-proxy.md

Test

A code coverage is available in index.html.

Remarks

As with any token contract, access to the owner key must be adequately restricted. Likewise, access to the proxy contract must be restricted and seggregated from the token contract.

Documentation

Here a summary of the main documents:

Document	Link/Files
Documentation of the modules API.	doc/modules
Documentation on the toolchain	doc/TOOLCHAIN.md
How to use the project	doc/USAGE.md
Project architecture	doc/general/ARCHITECTURE.md

CMTA providers further documentation describing the CMTAT framework in a platform-agnostic way, and covering legal aspects, see

• CMTA Token (CMTAT)
• Standard for the tokenization of shares of Swiss corporations using the distributed ledger technology

Further reading

• CMTA - A comparison of different security token standards
• Taurus - Security Token Standards: A Closer Look at CMTAT
• Taurus - Equity Tokenization: How to Pay Dividend On-Chain Using CMTAT
• Taurus - Token Transfer Management: How to Apply Restrictions with CMTAT and ERC-1404
• Taurus - Addressing the Privacy and Compliance Challenge in Public Blockchain Token Transactions

Others implementations

Two versions are available for the blockchain Tezos

• CMTAT FA2 Official version written in SmartPy
• @ligo/cmtat Unofficial version written in Ligo
  • See also Tokenization of securities on Tezos by Frank Hillard

Contract size

Intellectual property

The code is copyright (c) Capital Market and Technology Association, 2018-2024, and is released under Mozilla Public License 2.0.