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Tornado Cash

85%

Process Quality Review (0.8)

Tornado Cash

Final score:85%
Date:01 Mar 2022
Audit Process:version 0.8
Author:David J. Desjardins
PQR Score:85%

PASS

Protocol Website:

Scoring Appendix

The final review score is indicated as a percentage. The percentage is calculated as Achieved Points due to MAX Possible Points. For each element the answer can be either Yes/No or a percentage. For a detailed breakdown of the individual weights of each question, please consult this document.

The blockchain used by this protocol
Arbitrum
Avalanche
BnB Smart Chain
Ethereum
Polygon
xDai / Gnosis
Optimism
#QuestionAnswer
100%
1.100%
2.100%
3.Yes
4.100%
5.100
43%
6.Yes
7.Yes
8.0%
9.0%
75%
10.100%
11.50%
12.Yes
13.50%
14.No
15.Yes
83%
16.80%
17.100%
99%
18.100%
19.100%
20.100%
21.100%
22.100%
23.90%
24.100%
25.100%
100%
26.100%
27.Yes
28.Yes
Total:85%

Very simply, the audit looks for the following declarations from the developer's site. With these declarations, it is reasonable to trust the smart contracts.

  • Here is my smart contract on the blockchain
  • You can see it matches a software repository used to develop the code
  • Here is the documentation that explains what my smart contract does
  • Here are the tests I ran to verify my smart contract
  • Here are the audit(s) performed to review my code by third party experts

This report is for informational purposes only and does not constitute investment advice of any kind, nor does it constitute an offer to provide investment advisory or other services. Nothing in this report shall be considered a solicitation or offer to buy or sell any security, token, future, option or other financial instrument or to offer or provide any investment advice or service to any person in any jurisdiction. Nothing contained in this report constitutes investment advice or offers any opinion with respect to the suitability of any security, and the views expressed in this report should not be taken as advice to buy, sell or hold any security. The information in this report should not be relied upon for the purpose of investing. In preparing the information contained in this report, we have not taken into account the investment needs, objectives and financial circumstances of any particular investor. This information has no regard to the specific investment objectives, financial situation and particular needs of any specific recipient of this information and investments discussed may not be suitable for all investors.

Any views expressed in this report by us were prepared based upon the information available to us at the time such views were written. The views expressed within this report are limited to DeFiSafety and the author and do not reflect those of any additional or third party and are strictly based upon DeFiSafety, its authors, interpretations and evaluation of relevant data. Changed or additional information could cause such views to change. All information is subject to possible correction. Information may quickly become unreliable for various reasons, including changes in market conditions or economic circumstances.

This completed report is copyright (c) DeFiSafety 2021. Permission is given to copy in whole, retaining this copyright label.

Smart Contracts & Team

100%

This section looks at the code deployed on the relevant chain that gets reviewed and its corresponding software repository. The document explaining these questions is here.

1. Are the smart contract addresses easy to find? (%)

Answer: 100%

All Tornado Cash smart contracts are listed here.

Percentage Score Guidance:
100%
Clearly labelled and on website, documents or repository, quick to find
70%
Clearly labelled and on website, docs or repo but takes a bit of looking
40%
Addresses in mainnet.json, in discord or sub graph, etc
20%
Address found but labeling not clear or easy to find
0%
Executing addresses could not be found

2. How active is the primary contract? (%)

Answer: 100%

The 10 ethereum deposit/withdraw contract was used at its very least 65 times a day during the past month ((270 ETH + 380 ETH)/10 ETH), as indicated in the Appendix.

Percentage Score Guidance:
100%
More than 10 transactions a day
70%
More than 10 transactions a week
40%
More than 10 transactions a month
10%
Less than 10 transactions a month
0%
No activity

3. Does the protocol have a public software repository? (Y/N)

Answer: Yes
Score Guidance:
Yes
There is a public software repository with the code at a minimum, but also normally test and scripts. Even if the repository was created just to hold the files and has just 1 transaction.
No
For teams with private repositories.

4. Is there a development history visible? (%)

Answer: 100%

With 340 commits across the 6 branches in the tornado-core repository, this is a healthy repository.

Percentage Score Guidance:
100%
Any one of 100+ commits, 10+branches
70%
Any one of 70+ commits, 7+branches
50%
Any one of 50+ commits, 5+branches
30%
Any one of 30+ commits, 3+branches
0%
Less than 2 branches or less than 30 commits

5. Is the team public (not anonymous)? (Y/N)

Answer: 100

Team members/contributors were found at https://github.com/tornadocash/tornado-core/graphs/contributors.

Score Guidance:
100%
At least two names can be easily found in the protocol's website, documentation or medium. These are then confirmed by the personal websites of the individuals / their linkedin / twitter.
50%
At least one public name can be found to be working on the protocol.
0%
No public team members could be found.

Documentation

43%

The difference between this and the old link is solely the link.    This section looks at the software documentation. The document explaining these questions is here.

6. Is there a whitepaper? (Y/N)

Answer: Yes

Location: https://github.com/tornadocash/tornado-core#whitepaper

7. Is the protocol's software architecture documented? (Y/N)

Answer: Yes

This protocol's private transaction software architecture is documented at https://github.com/tornadocash/tornado-core#specs.

Score Guidance:
Yes
The documents identify software architecture and contract interaction through any of the following: diagrams, arrows, specific reference to software functions or a written explanation on how smart contracts interact.
No
Protocols receive a "no" if none of these are included.

8. Does the software documentation fully cover the deployed contracts' source code? (%)

Answer: 0%

There is no software function documentation.

Percentage Score Guidance:
100%
All contracts and functions documented
80%
Only the major functions documented
79 - 1%
Estimate of the level of software documentation
0%
No software documentation

9. Is it possible to trace the documented software to its implementation in the protocol's source code? (%)

Answer: 0%

There is no software function documentation.

Percentage Score Guidance:
100%
Clear explicit traceability between code and documentation at a requirement level for all code
60%
Clear association between code and documents via non explicit traceability
40%
Documentation lists all the functions and describes their functions
0%
No connection between documentation and code

Testing

75%

10. Has the protocol tested their deployed code? (%)

Answer: 100%

Code examples are in the Appendix. As per the SLOC, there is 949/461=206% testing to code (TtC).    This score is guided by the Test to Code ratio (TtC). Generally a good test to code ratio is over 100%. However the reviewers best judgement is the final deciding factor.

Percentage Score Guidance:
100%
TtC > 120% Both unit and system test visible
80%
TtC > 80% Both unit and system test visible
40%
TtC < 80% Some tests visible
0%
No tests obvious

11. How covered is the protocol's code? (%)

Answer: 50%

No test for coverage seen. However, Tornado Cash certainly has rigorous testing practices.

Percentage Score Guidance:
100%
Documented full coverage
99 - 51%
Value of test coverage from documented results
50%
No indication of code coverage but clearly there is a complete set of tests
30%
Some tests evident but not complete
0%
No test for coverage seen

12. Does the protocol provide scripts and instructions to run their tests? (Y/N)

Answer: Yes

Scripts/Instructions location: https://github.com/tornadocash/tornado-core#initialization

Score Guidance:
Yes
Scripts and/or instructions to run tests are available in the testing suite
No
Scripts and/or instructions to run tests are not available in the testing suite

13. Is there a detailed report of the protocol's test results?(%)

Answer: 50%

Although there is in depth CI in the github, numerous tests have encountered failed checks, making results inconclusive. CI is published here.

Percentage Score Guidance:
100%
Detailed test report as described below
70%
GitHub code coverage report visible
0%
No test report evident

14. Has the protocol undergone Formal Verification? (Y/N)

Answer: No

There is no evidence to show Tornado Cash has undergone formal verification.

Score Guidance:
Yes
Formal Verification was performed and the report is readily available
No
Formal Verification was not performed and/or the report is not readily available.

15. Were the smart contracts deployed to a testnet? (Y/N)

Answer: Yes

The smart contracts were deployed to the Etherium Goerli network. The contracts' respective addresses can then be located using the corresponding inputs you would find on any other networks or as shown in the Appendix.

Score Guidance:
Yes
Protocol has proved their tesnet usage by providing the addresses
No
Protocol has not proved their testnet usage by providing the addresses

Security

83%

This section looks at the 3rd party software audits done. It is explained in this document.

16. Is the protocol sufficiently audited? (%)

Answer: 80%
  1. Tornado Cash's Contracts were submitted for etherscan verification on the 16th of December 2019.  2. Tornado Cash's Smart Contracts repository were audited by on November 19th 2019.  3. There are a sizeable number of unresolved minor issues that are either commented on (to refute the issue) or scheduled to be fixed in future releases, despite no releases in over 2 years.  4. Tornado Cash Nova is has a rudimentary audit here, but is clearly stated as experimental.    Due to the details mentioned in the points 1 and 2, this would earn Tornado Cash a 90% for a single smart contract audit performed before deployment. However, due to the large number of currently unresolved issues, I will be deducting 10% from their score for this question, bringing the question 16 score down to 80%.  
Percentage Score Guidance:
100%
Multiple Audits performed before deployment and the audit findings are public and implemented or not required
90%
Single audit performed before deployment and audit findings are public and implemented or not required
70%
Audit(s) performed after deployment and no changes required. The Audit report is public.
65%
Code is forked from an already audited protocol and a changelog is provided explaining why forked code was used and what changes were made. This changelog must justify why the changes made do not affect the audit.
50%
Audit(s) performed after deployment and changes are needed but not implemented.
30%
Audit(s) performed are low-quality and do not indicate proper due diligence.
20%
No audit performed
0%
Audit Performed after deployment, existence is public, report is not public OR smart contract address' not found.
Deduct 25% if the audited code is not available for comparison.

17. Is the bounty value acceptably high (%)

Answer: 100%

Tornado cash has no documentation of their active bug bounty program of 1.3M, it is in effect nonetheless.

Percentage Score Guidance:
100%
Bounty is 10% TVL or at least $1M AND active program (see below)
90%
Bounty is 5% TVL or at least 500k AND active program
80%
Bounty is 5% TVL or at least 500k
70%
Bounty is 100k or over AND active program
60%
Bounty is 100k or over
50%
Bounty is 50k or over AND active program
40%
Bounty is 50k or over
20%
Bug bounty program bounty is less than 50k
0%
No bug bounty program offered / the bug bounty program is dead
An active program means that a third party (such as Immunefi) is actively driving hackers to the site. An inactive program would be static mentions on the docs.

Admin Controls

99%

This section covers the documentation of special access controls for a DeFi protocol. The admin access controls are the contracts that allow updating contracts or coefficients in the protocol. Since these contracts can allow the protocol admins to "change the rules", complete disclosure of capabilities is vital for user's transparency. It is explained in this document.

18. Is the protocol's admin control information easy to find?

Answer: 100%

The protocol's admin control information is well labelled and well explained. You can find a brief overview in their home page under the "Our strapiucts" section, or in depth information in their governance Medium article.

Percentage Score Guidance:
100%
Admin Controls are clearly labelled and on website, docs or repo, quick to find
70%
Admin Controls are clearly labelled and on website, docs or repo but takes a bit of looking
40%
Admin Control docs are in multiple places and not well labelled
20%
Admin Control docs are in multiple places and not labelled
0%
Admin Control information could not be found

19. Are relevant contracts clearly labelled as upgradeable or immutable? (%)

Answer: 100%

In the home page's FAQ section, under the "Is it possible to compromise the protocol and find out information about depositors?" question, it is stated that all smart contracts are immutable and have no admins. The code also reflects the immutability of the contracts.

Percentage Score Guidance:
100%
Both the contract documentation and the smart contract code state that the code is not upgradeable or immutable.
80%
All Contracts are clearly labelled as upgradeable (or not)
50%
Code is immutable but not mentioned anywhere in the documentation
0%
Admin control information could not be found

20. Is the type of smart contract ownership clearly indicated? (%)

Answer: 100%

The smart contracts are under the ownership of their DAO as outlined in their Medium article. However, it is documented that community funds are governed through a MultiSignature wallet on Gnosis Safe.

Percentage Score Guidance:
100%
The type of ownership is clearly indicated in their documentation. (OnlyOwner / MultiSig / etc)
50%
The type of ownership is indicated, but only in the code. (OnlyOwner / MultiSig / etc)
0%
Admin Control information could not be found

21. Are the protocol's smart contract change capabilities described? (%)

Answer: 100%

The documentation covers the capabilities for change for all smart contracts. More information can also be found in the Medium governance article and at https://viewer.diagrams.net/?highlight=0000ff&edit=_blank&layers=1&nav=1&title=tornado-cash-contract-overview.drawio#Uhttps%3A%2F%2Fraw.githubusercontent.com%2FRezan-vm%2Ftornado-cash-edu%2Fmain%2Ftornado-cash-contract-overview.drawio.

Percentage Score Guidance:
100%
The documentation covers the capabilities for change for all smart contracts
50%
The documentation covers the capabilities for change in some, but not all contracts
0%
The documentation does not cover the capabilities for change in any contract

22. Is the protocol's admin control information easy to understand? (%)

Answer: 100%

All the contracts are immutable, and admin control information is well explained and documented.

Percentage Score Guidance:
100%
All the contracts are immutable
90%
Description relates to investments safety in clear non-software language
30%
Description all in software-specific language
0%
No admin control information could be found

23. Is there sufficient Pause Control documentation? (%)

Answer: 90%

There is a mention of the DAO being able to pause/unpause the TORN token in a proposal. In the event of a security incident, this proposal to pause the TORN token would have to undergo a 2 day timelock, where in the case of a critical bug, could result in a significant loss of funds while all TORN holders' hands are tied. However, the main contracts are clearly stated as immutable, which therefor means no party would have the admin ability to pause the contracts.

Percentage Score Guidance:
100%
Pause control(s) are clearly documented and there is records of at least one test within 3 months
80%
Pause control(s) explained clearly but no evidence of regular tests
40%
Pause controls mentioned with no detail on capability or tests
0%
Pause control not documented or explained

24. Is there sufficient Timelock documentation? (%)

Answer: 100%

The contracts are immutable, any and all new deployments through proposals are well documented to undergo a 2 day timelock.

Percentage Score Guidance:
100%
Documentation identifies and explains why the protocol does not need a Timelock OR Timelock documentation identifies its duration, which contracts it applies to and justifies this time period.
60%
A Timelock is identified and its duration is specified
30%
A Timelock is identified
0%
No Timelock information was documented

25. Is the Timelock of an adequate length? (Y/N)

Answer: 100%

The timelock is exactly 48 hours.

Percentage Score Guidance:
100%
Timelock is between 48 hours to 1 week OR justification as to why no Timelock is needed / is outside this length.
50%
Timelock is less than 48 hours or greater than 1 week.
0%
No Timelock information was documented OR no timelock length was identified.

Oracles

100%

This section goes over the documentation that a protocol may or may not supply about their Oracle usage. Oracles are a fundamental part of DeFi as they are responsible for relaying tons of price data information to thousands of protocols using blockchain technology. Not only are they important for price feeds, but they are also an essential component of transaction verification and security. This is explained in this document.

26. Is the protocol's Oracle sufficiently documented? (%)

Answer: 100%

Tornado Cash does not make use of oracles as you are not trading one asset against another asset. You simply deposit and withdraw the same amount of the same cryptocurrency.

Percentage Score Guidance:
100%
If it uses one, the Oracle is specified. The contracts dependent on the oracle are identified. Basic software functions are identified (if the protocol provides its own price feed data). Timeframe of price feeds are identified. OR The reason as to why the protocol does not use an Oracle is identified and explained.
75%
The Oracle documentation identifies both source and timeframe, but does not provide additional context regarding smart contracts.
50%
Only the Oracle source is identified.
0%
No oracle is named / no oracle information is documented.

27. Is front running mitigated by this protocol? (Y/N)

Answer: Yes

Due to the nature of the Tornado Cash protocol, it cannot be front run.

Score Guidance:
Yes
The protocol cannot be front run and there is an explanation as to why OR documented front running countermeasures are implemented.
No
The Oracle documentation identifies both source and timeframe, but does not provide additional context regarding smart contracts.

28. Can flashloan attacks be applied to the protocol, and if so, are those flashloan attack risks mitigated? (Y/N)

Answer: Yes

Flashloan attacks cannot be applied to Tornado Cash due to the fact that there are no native profit generating functions.

Score Guidance:
Yes
The protocol's documentation includes information on how they mitigate the possibilities and extents of flash loan attacks.
No
The protocol's documentation does not include any information regarding the mitigation of flash loan attacks.

Appendices

1pragma solidity ^0.5.8;
2
3
4
5contract IVerifier {
6  function verifyProof(bytes memory _proof, uint256[6] memory _input) public returns(bool);
7}
8
9contract Tornado is MerkleTreeWithHistory, ReentrancyGuard {
10  uint256 public denomination;
11  mapping(bytes32 => bool) public nullifierHashes;
12  // we store all commitments just to prevent accidental deposits with the same commitment
13  mapping(bytes32 => bool) public commitments;
14  IVerifier public verifier;
15
16  // operator can update snark verification key
17  // after the final trusted setup ceremony operator rights are supposed to be transferred to zero address
18  address public operator;
19  modifier onlyOperator {
20    require(msg.sender == operator, "Only operator can call this function.");
21    _;
22  }
23
24  event Deposit(bytes32 indexed commitment, uint32 leafIndex, uint256 timestamp);
25  event Withdrawal(address to, bytes32 nullifierHash, address indexed relayer, uint256 fee);
26
27  /**
28    @dev The constructor
29    @param _verifier the address of SNARK verifier for this contract
30    @param _denomination transfer amount for each deposit
31    @param _merkleTreeHeight the height of deposits' Merkle Tree
32    @param _operator operator address (see operator comment above)
33  */
34  constructor(
35    IVerifier _verifier,
36    uint256 _denomination,
37    uint32 _merkleTreeHeight,
38    address _operator
39  ) MerkleTreeWithHistory(_merkleTreeHeight) public {
40    require(_denomination > 0, "denomination should be greater than 0");
41    verifier = _verifier;
42    operator = _operator;
43    denomination = _denomination;
44  }
45
46  /**
47    @dev Deposit funds into the contract. The caller must send (for ETH) or approve (for ERC20) value equal to or `denomination` of this instance.
48    @param _commitment the note commitment, which is PedersenHash(nullifier + secret)
49  */
50  function deposit(bytes32 _commitment) external payable nonReentrant {
51    require(!commitments[_commitment], "The commitment has been submitted");
52
53    uint32 insertedIndex = _insert(_commitment);
54    commitments[_commitment] = true;
55    _processDeposit();
56
57    emit Deposit(_commitment, insertedIndex, block.timestamp);
58  }
59
60  /** @dev this function is defined in a child contract */
61  function _processDeposit() internal;
62
63  /**
64    @dev Withdraw a deposit from the contract. `proof` is a zkSNARK proof data, and input is an array of circuit public inputs
65    `input` array consists of:
66      - merkle root of all deposits in the contract
67      - hash of unique deposit nullifier to prevent double spends
68      - the recipient of funds
69      - optional fee that goes to the transaction sender (usually a relay)
70  */
71  function withdraw(bytes calldata _proof, bytes32 _root, bytes32 _nullifierHash, address payable _recipient, address payable _relayer, uint256 _fee, uint256 _refund) external payable nonReentrant {
72    require(_fee <= denomination, "Fee exceeds transfer value");
73    require(!nullifierHashes[_nullifierHash], "The note has been already spent");
74    require(isKnownRoot(_root), "Cannot find your merkle root"); // Make sure to use a recent one
75    require(verifier.verifyProof(_proof, [uint256(_root), uint256(_nullifierHash), uint256(_recipient), uint256(_relayer), _fee, _refund]), "Invalid withdraw proof");
76
77    nullifierHashes[_nullifierHash] = true;
78    _processWithdraw(_recipient, _relayer, _fee, _refund);
79    emit Withdrawal(_recipient, _nullifierHash, _relayer, _fee);
80  }
81
82  /** @dev this function is defined in a child contract */
83  function _processWithdraw(address payable _recipient, address payable _relayer, uint256 _fee, uint256 _refund) internal;
84
85  /** @dev whether a note is already spent */
86  function isSpent(bytes32 _nullifierHash) public view returns(bool) {
87    return nullifierHashes[_nullifierHash];
88  }
89
90  /** @dev whether an array of notes is already spent */
91  function isSpentArray(bytes32[] calldata _nullifierHashes) external view returns(bool[] memory spent) {
92    spent = new bool[](_nullifierHashes.length);
93    for(uint i = 0; i < _nullifierHashes.length; i++) {
94      if (isSpent(_nullifierHashes[i])) {
95        spent[i] = true;
96      }
97    }
98  }
99
100  /**
101    @dev allow operator to update SNARK verification keys. This is needed to update keys after the final trusted setup ceremony is held.
102    After that operator rights are supposed to be transferred to zero address
103  */
104  function updateVerifier(address _newVerifier) external onlyOperator {
105    verifier = IVerifier(_newVerifier);
106  }
107
108  /** @dev operator can change his address */
109  function changeOperator(address _newOperator) external onlyOperator {
110    operator = _newOperator;
111  }
112}
113
114// File: contracts/ETHTornado.sol
115
116// https://tornado.cash
117/*
118* d888888P                                           dP              a88888b.                   dP
119*    88                                              88             d8'   `88                   88
120*    88    .d8888b. 88d888b. 88d888b. .d8888b. .d888b88 .d8888b.    88        .d8888b. .d8888b. 88d888b.
121*    88    88'  `88 88'  `88 88'  `88 88'  `88 88'  `88 88'  `88    88        88'  `88 Y8ooooo. 88'  `88
122*    88    88.  .88 88       88    88 88.  .88 88.  .88 88.  .88 dP Y8.   .88 88.  .88       88 88    88
123*    dP    `88888P' dP       dP    dP `88888P8 `88888P8 `88888P' 88  Y88888P' `88888P8 `88888P' dP    dP
124* ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
125*/
126
127pragma solidity ^0.5.8;
128
129
130contract TornadoCash_eth is Tornado {
131  constructor(
132    IVerifier _verifier,
133    uint256 _denomination,
134    uint32 _merkleTreeHeight,
135    address _operator
136  ) Tornado(_verifier, _denomination, _merkleTreeHeight, _operator) public {
137  }
138
139  function _processDeposit() internal {
140    require(msg.value == denomination, "Please send `mixDenomination` ETH along with transaction");
141  }
142
143  function _processWithdraw(address payable _recipient, address payable _relayer, uint256 _fee, uint256 _refund) internal {
144    // sanity checks
145    require(msg.value == 0, "Message value is supposed to be zero for ETH instance");
146    require(_refund == 0, "Refund value is supposed to be zero for ETH instance");
147
148    (bool success, ) = _recipient.call.value(denomination - _fee)("");
149    require(success, "payment to _recipient did not go thru");
150    if (_fee > 0) {
151      (success, ) = _relayer.call.value(_fee)("");
152      require(success, "payment to _relayer did not go thru");
153    }

JavaScript Tests

Language
Files
Lines
Blanks
Comments
Testing Code
Deployed Code
Complexity
JavaScript
3
1169
154
66
949
461
20

Tests to Code: 949 / 461 = 206 %