Centric Protocol Smart Contract Audit

# 1. Introduction
iosiro was commissioned by the [Centric Foundation](https://www.centric.com) to conduct a smart contract audit on the Centric Rise and Centric Swap (previously Centric Cash) smart contracts. The audit was performed between 03 April 2020 and 06 April 2020.

#### Update
A review was performed on 26 May 2021 of changes made to the contracts to allow for deployment on Binance Smart Chain (BSC).

This report is organized into the following sections.

* **[Section 2 - Executive Summary:](#section-2)** A high-level description of the findings of the audit.
* **[Section 3 - Audit Details:](#section-3)** A description of the scope and methodology of the audit.
* **[Section 4 - Design Specification:](#section-4)** An outline of the intended functionality of the smart contracts.
* **[Section 5 - Detailed Findings:](#section-5)** Detailed descriptions of the findings of the audit.

The information in this report should be used to better understand the risk exposure of the smart contracts, and as a guide to improving the security posture of the smart contracts by remediating issues identified. The results of this audit are only a reflection of the source code reviewed at the time of the audit and of the source code that was determined to be in-scope.

The purpose of this audit was to achieve the following:

* Identify potential security flaws.
* Ensure that the smart contracts functioned according to the documentation provided.  

Assessing the economics, game theory, or underlying business model of the platform were beyond the scope of this audit. Therefore, determining the viability of the deflationary currency or the effectiveness of the stabilizing mechanisms were beyond the scope of the audit.

Due to the unregulated nature and ease of transfer of cryptocurrencies, operations that store or interact with these assets are considered very high risk with regards to cyber attacks. As such, the highest level of security should be observed when interacting with these assets. This requires a forward-thinking approach, which takes into account the new and experimental nature of blockchain technologies. Strategies that should be used to encourage secure code development include:

* Security should be integrated into the development lifecycle and the level of perceived security should not be limited to a single code audit.
* Defensive programming should be employed to account for unforeseen circumstances.
* Current best practices should be followed where possible.

<a name="section-2"></a>
# 2. Executive Summary

This report presents the findings of an audit performed by iosiro on the Centric Rise and Centric Swap smart contracts. The Centric Foundation provided the [Centric Whitepaper](https://www.joincentric.com/docs/Centric_Whitepaper.pdf) (MD5=a8b3c2e33ef3e36bcc1667cbdd0190a5) as a reference for the audit.

#### Important Information for Users of the Centric System

Users of the Centric system should be explicitly informed about the following characteristics of the system.
- Admins are required to create Price Blocks. If an admin fails to create a Price Block for a point in time, the system will stop operating until a Price Block is created for that point in time.
- The only exchange rate governed by the system is the Centric Rise to Centric Swap exchange rate. This rate is programmatically increased based on a Growth Rate set by the admin each block. At a smart contract level, the exchange rate is in no way related to a USD exchange rate.
- The Price Factors that are used to establish the hourly increase in exchange rate, based on the Growth Rate, are calculated off-chain. This avoids unnecessarily implementing complex mathematical functionality on-chain; however, it means that there is the potential for Growth Rates and Price Factors to deviate from the algorithm defined in the whitepaper. As Growth Rates need to be locked before the first Price Block can be created, and cannot be modified after that point, users are recommended to verify that the Price Factors committed on-chain correspond to the Growth Rates according to the proposed algorithm.

#### Audit Results

At the conclusion of the audit, only informational level issues were open. These findings included security and functional recommendations that could be used to improve the system, either by enhancing the security model of the system or better adhering to best practices.

At a high level, the security posture of the smart contracts could be further strengthened by:

* Remediating the issues identified in this report and performing a review to ensure that the issues were correctly addressed.
* Performing additional audits at regular intervals, as security best practices, tools, and knowledge change over time. Additional audits over the course of the project's lifespan ensure the longevity of the codebase.
* Extending Centric's existing bug bounty program to encourage the responsible disclosure of security vulnerabilities in the smart contracts.

#### Audit Review

A review was performed on 14 April 2020 to verify changes made to the contracts based on the initial audit results, with the last commit at the time being [22a38fb](https://github.com/Centric-Org/contracts/commit/22a38fbebf82517f57d0b2004137a091ba7c53cd). The majority of informational issues opened were found to be addressed.

#### Tron to BSC Migration

A further review was performed on 26 May 2021 on updates made to the smart contracts to allow for deployment on BSC. No further issues were identified during the review.  

<a name="section-3"></a>
# 3. Audit Details
## 3.1 Scope
The source code considered in-scope for the assessment is described below. Code from all other files is considered to be out-of-scope. Out-of-scope code that interacts with in-scope code is assumed to function as intended and introduce no functional or security vulnerabilities for the purposes of this audit.

### 3.1.1 Centric Smart Contracts
**Project Name:** contracts<br/>
**Commits:** [4a86292](https://github.com/Centric-Org/contracts/tree/4a86292b19ce0e19dc917084e390c2a446424af4/contracts)<br/>
**Files:**  `Cash.sol`, `DateLib.sol`, `Rise.sol`, `RoundMath.sol`, `SafeMath.sol`, `TRC20.sol`, `helpers/Administrable.sol`, `helpers/Claimable.sol`

### 3.1.2 Centric Smart Contracts Review
**Project Name:** contracts<br/>
**Commits:** [22a38fb](https://github.com/Centric-Org/contracts/commit/22a38fbebf82517f57d0b2004137a091ba7c53cd)<br/>
**Files:**  `Cash.sol`, `DateLib.sol`, `Rise.sol`, `RoundMath.sol`, `SafeMath.sol`, `TRC20.sol`, `helpers/Administrable.sol`, `helpers/Claimable.sol`

### 3.1.3 Centric Smart Contracts BSC Migration
**Project Name:** contracts<br/>
**Commits:** [aae4b3e](https://github.com/Centric-Org/contracts/pull/8/commits/aae4b3e30f808bae89826589b2598d3d6157629e)<br/>
**Files:**  `Administrable.sol`, `BEP20.sol`, `CentricRise.sol`, `CentricSwap.sol`, `Claimable.sol`, `DateLib.sol`, `RoundMath.sol`, `SafeMath.sol`, `TRC20.sol`

## 3.2  Methodology

A variety of techniques, described below, were used to conduct the audit.

### 3.2.1 Code Review

The source code was manually inspected to identify potential security flaws. Code review is a useful approach for detecting security flaws, discrepancies between the specification and implementation, design improvements, and high risk areas of the system.

### 3.2.2 Dynamic Analysis

The contracts were compiled, deployed, and tested in a Ganache test environment, both manually and through the Truffle test suite provided. Manual analysis was used to confirm that the code operated at a functional level and to verify the exploitability of any potential security issues identified.

### 3.2.3 Automated Analysis

Tools were used to automatically detect the presence of several types of security vulnerabilities, including reentrancy, timestamp dependency bugs, and transaction-ordering dependency bugs. The static analysis results were manually analyzed to remove false-positive results. True positive results would be indicated in this report.

Static analysis was conducted using Slither, Securify, as well as MythX. Tools such as the Remix IDE, compilation output, and linters were also used to identify potential areas of concern.

## 3.3  Risk Ratings

Each issue identified during the audit has been assigned a risk rating. The rating is determined based on the criteria outlined below.

* **High Risk** - The issue could result in a loss of funds for the contract owner or system users.
* **Medium Risk** - The issue resulted in the code specification being implemented incorrectly.
* **Low Risk** - A best practice or design issue that could affect the security of the contract.
* **Informational** - A lapse in best practice or a suboptimal design pattern that has a minimal risk of affecting the security of the contract.
* **Closed** - The issue was identified during the audit and has since been addressed to a satisfactory level to remove the risk that it posed.

<a name="section-4"></a>
# 4. Design Specification

The following section outlines the intended functionality of the system at a high level. The specification is based on the implementation in the codebase and any perceived points of conflict should be highlighted with the auditing team to determine the source of the discrepancy.

## 4.1 Centric Rise
Centric Rise is a cryptocurrency with a deflationary supply.

#### BEP20 Token

Centric Rise should be represented by a BEP20-compliant token with the following values.

Field        | Value
------------ | -------------
Name         | Centric RISE
Symbol       | CNR
Decimals     | 8

#### Minting

A total of 1 billion tokens should be minted to an address controlled by the Centric Foundation during the creation of Centric Rise.

#### Exchanging

By using the functions in the Centric Rise contract, it should be possible to exchange Centric Rise for Centric Swap and vice versa, at an exchange rate determined by the Centric Rise contract. When a user exchanges Centric Rise for Centric Swap, an equivalent amount of Centric Rise is _quarantined_ in the Rise contract, according to a predetermined exchange rate set by the contract admins. When a user exchanges Centric Swap for Centric Rise, the Centric Swap amount is burned, and an equivalent amount of Centric Rise is sent from the quarantined amount in the Rise contract. As the algorithm should only ever output positive growth rates, there should not be any liquidity issues.  

#### Exchange Rate

The exchange rate of Centric Rise to Centric Swap should be stored in Price Blocks. The exchange rate should be calculated based on the following algorithm.

vn = vl * (1+r)^(1/t)

- _vn_ is the CNR price of the new Price Block.
- _vl_ is the CNR price of the previous Price Block.
- _r_ is the Growth Rate.
- _t_ is the number of hours in the month the Price Block references.

#### Price Blocks

Price Blocks should store the current, future, and past exchange rates of Centric Rise and Centric Swap. It should only be possible for an admin (i.e. Centric team) to create Price Blocks, and only once the Growth Factors have been locked. The admin will need to specify the Block Number and the Growth Rate when creating the Price Block. Once created, it should not be possible to modify or remove the Price Block. Each Price Block is referenced by a Block Number that corresponds to an hour in time, denoted in hours since Unix epoch. It should be possible to create any number of Price Blocks ahead of time. Price Blocks should be readable by any user. Each Price Block should contain:
* CNR price denominated in CNS.
* Monthly price growth rate, expressed as a percentage.
* CNR price change from the previous Price Block, expressed as a percentage.
* Created hour denoted in hours since Unix epoch.

#### Growth Rate

The Growth Rate is the rate at which the exchange rate of Centric Rise to Centric Swap increases over a one month period. Admins can call the `setPriceFactors(uint256 _growthRate, uint256[4] _priceFactors)` function to set the growth rate and price factors. Growth Rates and Price Factors need to be set before Price Blocks can be created, and can not be modified after that time. The Growth Rate should be more than 0% and less than 100%.

#### Price Factors

The Price Factors are passed into `setPriceFactors(uint256 _growthRate, uint256[4] _priceFactors)` function by an admin and rudimentary validation is performed in order to check that the values are within certain bounds to match the validation performed on the Growth Rate. The Price Factors are not calculated on-chain, but should be calculated by admins as:

_priceFactor[0] = (1+_growthRate/GROWTH_RATE_BASE)^(1/(28*24))
_priceFactor[1] = (1+_growthRate/GROWTH_RATE_BASE)^(1/(29*24))
_priceFactor[2] = (1+_growthRate/GROWTH_RATE_BASE)^(1/(30*24))
_priceFactor[3] = (1+_growthRate/GROWTH_RATE_BASE)^(1/(31*24))

#### Balancing

It should be possible for any account to burn Centric Rise held by the Rise contract, in excess of the required quarantined amount of Centric Rise. This should allow for the deflation of the currency to be tracked through the total supply.

#### Burn Lost Tokens

The contract owner should be able to burn any Centric Rise held by the Rise smart contract that is in excess of the required quarantined amount of Centric Rise.

## 4.2 Centric Swap

Centric Swap is intended to be traded on exchanges at market rates. It can be converted to Centric Rise and vice versa.

#### BEP20 Token

It should be represented by a BEP20 compliant token with the following values.

Field        | Value
------------ | -------------
Name         | Centric Swap
Symbol       | CNS
Decimals     | 8

#### Minting

Centric Swap tokens should only be minted by the Rise contract when a user quarantines Centric Rise. The amount of Centric Swap issued should be based on the Centric Rise to Centric Swap exchange rate in the Rise contract.  

#### Burning

When exchanging Centric Swap for Centric Rise, the Centric Swap amount should be burned from the total supply.

<a name="section-5"></a>
# 5. Detailed Findings
The following section details the findings of the audit.

## 5.1 High Risk

No high risk issues were present at the conclusion of the review.

## 5.2 Medium Risk

No medium risk issues were present at the conclusion of the review.

## 5.3 Low Risk

No low risk issues were present at the conclusion of the review.

## 5.4 Informational

### 5.4.3 Design Comments

Actions to improve the functionality and readability of the codebase are outlined below.

#### Use CNS instead of USD in Comments and Documentation

In the comments of Rise.sol, several references to the Centric Rise price being denominated in USD were made. For the sake of clarity and transparency, it is advised that these USD references be changed to CNS. No input of USD was made into the smart contract system, and the only exchange rate present in the smart contracts was between CNR and CNS. As this rate was calculated based on an algorithm that produced only increasing values, it could not appropriately model the real CNR:USD or CNS:USD exchange rate.

#### Secure Sensitive Functionality

Generally, the Centric system managed to minimize the use of trusted roles, and included assertions to minimize the potential for invalid or malicious values to be used.

However, the contract owner and admin roles still had the potential to disrupt the functioning of the system in the event of compromised or lost keys. This was a centralized, single point of failure risk. For example, admins were required to initiate the creation of Price Blocks. Losing this ability would mean a system-wide denial of service. The Centric Foundation indicated that their strategy would be to create a year's worth of Price Blocks at a time, in order to minimize the risk of accidentally reaching a state without a valid Price Block. However, this case could still be encountered.

The risk of a single rogue actor or a compromised private key of an owner or admin account can be mitigated by ensuring that a high standard of security is observed by sensitive accounts. For example, they should be multi-signature and signed by properly secured hardware-backed accounts.

##  5.5 Closed

### 5.5.1 Insufficient Unit Test Quality and Coverage (Informational)

Certain functionality was found to have insufficient unit test quality and coverage. To improve the maintainability of the code, and decrease the likelihood of introducing functional or security issues into the codebase, the test suite should be extended to cover the following:

- Rise.sol#L372-379,L403-404
- DateLib.sol#L75-80,L92-95

#### Review Update

Fixed in [22a38fb](https://github.com/Centric-Org/contracts/commit/22a38fbebf82517f57d0b2004137a091ba7c53cd). The test suite was extended to cover the previously uncovered functionality.

### 5.5.2 Design Comments (Informational)

Actions to improve the functionality and readability of the codebase are outlined below.

#### Validate Starting Price Block Time

When first calling the `doCreateBlock(uint256 _blockNumber, uint256 _growthRate)` function, the caller was required to pass a value in for `_blockNumber` that was greater than the current hour. This meant that only values in the future could be specified. However, there was no upper bound for the hour that could be specified, unnecessarily opening up the potential to accidentally set an incorrect value for the starting block.

It is recommended that validation be added to ensure that the `_blockNumber` is some reasonable amount of time in the future.

##### Review Update

Recommendation applied in [c72f535](https://github.com/Centric-Org/contracts/commit/c72f535a2bf5100becb2e3ca34e3619db587504e). `_blockNumber` is required to be within one year ahead of the current hour.

#### Code Style Improvements

The following changes are recommended to improve the readability of the codebase.
- Rise.sol#L35: `initialPrice` should be `INITIAL_PRICE` as it is a constant variable.
- Rise.sol#L330: `burnQuarantined()` should be `_burnQuarantined()` as it is an internal function.
- Rise.sol#L350: `createBlock(...)` should be `_createBlock(...)` as it is an internal function.

##### Review Update

Suggested changes applied in [ae3d93e](https://github.com/Centric-Org/contracts/commit/ae3d93e7a91d3df8fd8826e2a635ad2b9126f34c).

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