Umbra Smart Contracts

1 Executive Summary

This report presents the results of our engagement with ScopeLift to review the Umbra Protocol smart contracts.

The review was conducted by Nicholas Ward between March 22^nd and March 26^th, 2021.

2 Scope

The review focused on the commit hash fa2e17367d66a85f20c77299ded5942d9ab64fe0. A cursory review of the ENS Resolver contract for stealth keys was also performed at commit hash 2d7795082308d303eb23c66490579a5b21a1bac9. The list of files in scope can be found in the Appendix.

Notably, all off-chain libraries and cryptography used in the Umbra Protocol were explicitly excluded from the scope, as were all Gas Station Network integrations.

3 System Overview

This section should not serve as an alternative for documentation, which ScopeLift provides in the project README and in the code itself. In brief:

Umbra allows semi-private payments between two parties using “stealth addresses”. The Umbra contract forwards ETH payments to these stealth addresses, emitting an encrypted message provided by the sender. This encrypted message can be used by the recipient to identify payments intended for them and to recover the forwarded funds.

For token payments, the Umbra contract accepts tokens from the sender and allows the recipient to withdraw the tokens. Optionally, the recipient of a token payment can withdraw their tokens via a signed message submitted to the contract by a “sponsor”. Token payment withdrawals can also include a call to a hook receiver contract, which can be used forward deposits into another privacy solution, interact with other on-chain protocols, or take some arbitrary action upon receipt of a payment. Crucially, nonce-based signature replay protection is NOT provided by the Umbra contract under the expectation that stealth addresses are used only once.

Users should note that the privacy gained from use of the Umbra Protocol can be directly impacted by their own privacy hygience as well as that of their counterparties.

4 Findings

Each issue has an assigned severity:

• Minor issues are subjective in nature. They are typically suggestions around best practices or readability. Code maintainers should use their own judgment as to whether to address such issues.
• Medium issues are objective in nature but are not security vulnerabilities. These should be addressed unless there is a clear reason not to.
• Major issues are security vulnerabilities that may not be directly exploitable or may require certain conditions in order to be exploited. All major issues should be addressed.
• Critical issues are directly exploitable security vulnerabilities that need to be fixed.

function _validateWithdrawSignature(
  address _stealthAddr,
  address _acceptor,
  address _tokenAddr,
  address _sponsor,
  uint256 _sponsorFee,
  IUmbraHookReceiver _hook,
  bytes memory _data,
  uint8 _v,
  bytes32 _r,
  bytes32 _s
) internal view {
  bytes32 _digest =
    keccak256(
      abi.encodePacked(
        "\x19Ethereum Signed Message:\n32",
        keccak256(abi.encode(chainId, version, _acceptor, _tokenAddr, _sponsor, _sponsorFee, address(_hook), _data))
      )
    );

  address _recoveredAddress = ecrecover(_digest, _v, _r, _s);
  require(_recoveredAddress != address(0) && _recoveredAddress == _stealthAddr, "Umbra: Invalid Signature");
}

uint256 _chainId;

assembly {
  _chainId := chainid()
}

5 Recommendations

function setStealthKeys(bytes32 node, uint256 spendingPubKeyPrefix, uint256 spendingPubKey, uint256 viewingPubKeyPrefix, uint256 viewingPubKey) external authorised(node) {
    require(
        (spendingPubKeyPrefix == 2 || spendingPubKeyPrefix == 3) &&
        (viewingPubKeyPrefix == 2 || viewingPubKeyPrefix == 3),
        "StealthKeyResolver: Invalid Prefix"
    );

    emit StealthKeyChanged(node, spendingPubKeyPrefix, spendingPubKey, viewingPubKeyPrefix, viewingPubKey);

    // Shift the spending key prefix down by 2, making it the appropriate index of 0 or 1
    spendingPubKeyPrefix -= 2;

    // Ensure the opposite prefix indices are empty
    delete _stealthKeys[node][1 - spendingPubKeyPrefix];
    delete _stealthKeys[node][5 - viewingPubKeyPrefix];

    // Set the appropriate indices to the new key values
    _stealthKeys[node][spendingPubKeyPrefix] = spendingPubKey;
    _stealthKeys[node][viewingPubKeyPrefix] = viewingPubKey;
}

if (address(_hook) != address(0)) {
  _hook.tokensWithdrawn(_withdrawalAmount, _stealthAddr, _acceptor, _tokenAddr, _sponsor, _sponsorFee, _data);
}

abi.encodePacked(
    "\x19Ethereum Signed Message:\n32",
    keccak256(abi.encode(chainId, version, _acceptor, _tokenAddr, _sponsor, _sponsorFee, address(_hook), _data))
)

Appendix 1 - Files in Scope

This audit covered the following files:

Appendix 2 - Disclosure

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