UCEF Contracts

Confidential token contracts built with the Unopinionated Confidential ERC-20 Framework (UCEF), for two token standards:

• Confidential ERC-20 (UCEF): fungible tokens
• Confidential ERC-3643 (UCEF3643): regulated tokens, such as tokenised securities

Introduction

The rapid growth of decentralised finance highlights a critical tension between transparency and privacy. ERC-20 tokens expose user balances and transaction amounts, which can deter mainstream adoption and compromise sensitive financial data. ERC-3643 tokens have the same problem: they verify identities and compliance rules on chain, but balances and transfers are still public.

Numerous proposals for confidential tokens have emerged, all of which are firmly tied to a particular cryptographic approach, be it TEE, FHE, MPC or zero-knowledge proofs. This creates issues for the interoperability of confidential tokens. It also ties a contract developer to a specific cryptographic implementation.

An unopinionated implementation

To achieve a truly unopinionated implementation, cryptographic details should be abstracted away from the Solidity interface. This ensures that different cryptographic approaches can be swapped without altering the contract's external behaviour. Furthermore, logic around permissions and visibility controls should be programmable using standard Solidity constructs, avoiding proprietary libraries or custom extensions. The UCEF approach preserves the simplicity and composability of the token standards while enabling flexible privacy rules that can adapt to evolving security requirements and compliance needs.

Features

• The same ERC-20 and ERC-3643 interfaces
• No custom libraries
• Flexible implementation expressible in Solidity
• Private events: events are visible only to the addresses involved

UCEF enforces privacy using standard Solidity authorisation checks. This eliminates the need for custom libraries or cryptographic proofs, preserving compatibility with existing token interfaces and maintaining gas efficiency. By leveraging native Solidity constructs, this approach simplifies auditing, enhances composability with decentralised applications, and keeps cryptographic complexity outside the smart contract. This minimalistic design ensures adaptability and maintainability while allowing developers to focus on enforcing data access rules through their preferred cryptographic technologies.

Confidential ERC-20 (UCEF)

UCEF.sol is a confidential version of the OpenZeppelin ERC-20 implementation. It keeps the ERC-20 interface and adds internal functions that control balance visibility, event visibility and minting.

Balance visibility

Every token defines who can read a balance by implementing _authorizeBalance. For example, UCEFOwned only allows the account owner:

    function _authorizeBalance(address account) internal view virtual override returns (bool) {
        if (msg.sender != account) {
            revert UCEFUnauthorizedBalanceAccess(msg.sender, account);
        }
        return true;
    }

The package includes three privacy models:

Contract	Who can read a balance
UCEFOwned	The account owner
UCEFRegulated	The account owner and a regulator
UCEFSharable	The account owner, the addresses the owner grants access to, and an optional supervisor

Private events

UCEF replaces the standard Transfer and Approval events with a PrivateEvent. Each PrivateEvent contains:

• allowedViewers: the addresses that can view the event
• eventType: the hash of the original event signature, for example Transfer(address,address,uint256)
• payload: the ABI-encoded event arguments

By default, only the sender and the receiver can view a transfer, and only the owner and the spender can view an approval. A token can change the viewers by overriding _getTransferEventViewers and _getApprovalEventViewers.

Minting policy

Every token must implement _authorizeMint, which runs on every mint. This forces each token to define who can mint. A token without it does not compile.

Extensions

packages/ucef/contracts/extensions contains UCEF versions of the OpenZeppelin ERC-20 extensions: ERC1363, ERC4626, UCEFBurnable, UCEFCapped, UCEFFlashMint, UCEFPausable, UCEFPermit, UCEFVotes and UCEFWrapper.

XUCEF is a UCEF version of xERC20 (ERC-7281), for tokens that bridges mint and burn within rate limits.

Comparison with existing confidential ERC-20 implementations

	UCEF by Applied Blockchain	fhEVM ERC-20	Confidential ERC-20 Framework using FHE	Private ERC-20
Confidential Balances	✅	✅	✅	✅
Fully Anonymous Accounts	✅	❌	❌	❌
Programmable Confidentiality	✅	🟠 Partial Support	🟠 Partial Support	❌
Unmodified ERC-20 Interface	✅	❌	❌	❌
Cryptography Agnostic	✅	❌	❌	❌

Confidential ERC-3643 (UCEF3643)

ERC-3643 is the token standard for regulated assets, such as tokenised securities. It provides regulatory compliance through identity verification and transfer restrictions.

UCEF3643.sol extends the T-REX Token, the reference implementation of ERC-3643, with the UCEF approach. It keeps all ERC-3643 compliance mechanisms and adds programmable confidentiality, so regulated entities can protect sensitive financial data and still meet their compliance requirements.

• Regulatory compliance: identity verification, compliance checks, transfer restrictions, token freezing and agent management work as in ERC-3643
• Confidential balances: only the account holder, or the holder's identity contract, can read a balance
• Confidential allowances: only the owner and the spender can read an allowance
• Private events: Transfer, Approval, TokensFrozen and TokensUnfrozen are replaced by a PrivateEvent that only the involved addresses and the auditors can view
• Auditors: agents can add and remove auditors, who can view all private events for compliance and monitoring

See UCEF3643.md for the private events system, auditor management, testing and deployment.

Packages

Package	Path	Description
@appliedblockchain/ucef	packages/ucef	UCEF base contracts and extensions for confidential ERC-20 tokens
@appliedblockchain/ucef-3643	packages/ucef-3643	Confidential ERC-3643 token with private events and auditors

This repository merges the former confidential-erc-20 and confidential-erc-3643 repositories.

Testing and Deployment

• Confidential ERC-20 tokens: see DEPLOY.md
• Confidential ERC-3643 token (UCEF3643): see UCEF3643.md

Examples

• ERC-20: a range of examples are available at contracts/examples/
• ERC-3643: the Ignition modules in ignition/modules/ deploy UCEF3643 with or without a proxy. See UCEF3643.md

Adopted By

License

This project is licensed under the MIT License - see the LICENSE file for details.