System Coin: TAI

ERC20 representation of the coins backed by system collateral

1. Summary

The Coin contract is the user-facing ERC20 token maintaining the accounting for external system coin balances. Most functions are standard for a token with changing supply, but it also has notable features such as the ability to approve transfers based on signed messages.

2. Contract Details & Functions

Variables

• name

• symbol

• version

• decimals

• changeData - if 1 governance can change the name and/or symbol and no one can use permit(); if different than 1 governance cannot change name or symbol anymore and permit() can be used

• totalSupply - total coin supply

• balanceOf(usr: address) - user balance

• allowance(src: address, dst: address) - approvals

• nonces(usr: address) - permit nonce

• wad - fixed point decimal with 18 decimals (for basic quantities, e.g. balances).

Functions

• mint(usr: address, amount: uint256) - mint coins to an address

• burn(usr: address, amount: uint256) - burn at an address

• push(usr: address, amount: uint256) - transfer

• pull(usr: address, amount: uint256)- transfer from

• move(src: address, dst: address, amount: uint256) - transfer from

• approve(usr: address, amount: uint256) - allow pulls and moves

• modifyParameters(parameter: bytes32, data: uint256) - modify the value of changeData

• setName(name_: string) - change the token's name if changeData is 1

• setSymbol(symbol_: string) - change the token's symbol if changeData is 1

• permit(holder: address, spender: address, nonce: uint256, expiry: uint256, allowed: bool, v: uint8, r: bytes32, s: bytes32) - approve by signature; only callable if changeData != 1

• transfer(dst: address, amount: uint256) - transfers coins from msg.sender to dst

3. Walkthrough

For the most part, coin.sol functions as a typical ERC20 token although it has a couple of core differences:

1. push, pull & move are aliases for transferFrom in the form of transferFrom(msg.sender, usr, amount) , transferFrom(usr, msg.sender, amount) & transferFrom(src, dst, amount) .

2. permit is a signature-based approval function. This allows an end-user to sign a message which can then be relayed by another party to submit their approval. This can be useful for applications in which the end-user does not need to hold ETH to pay for gas.

4. Gotchas (Potential Source of User Error)

Unlimited allowance is a relatively common practice. This could be something used to trick a user by a malicious contract into giving access to all their Coin. This is concerning in upgradeable contracts where the contract may appear innocent until upgraded to a malicious contract.

Coin is also susceptible to the known ERC20 race condition, but should not normally be an issue with unlimited approval. We recommend any users using the approval for a specific amount be aware of this particular issue and use caution when authorizing other contracts to perform transfers on their behalf.

There is a slight deviation in transferFrom functionality: If the src == msg.sender the function does not require approval first and treats it as a normal transfer from the msg.sender to the dst.

Built-in meta-transaction functionality of Coin

The Coin token provides offchain approval, which means that as an owner of an ETH address, you can sign a permission (using the permit() function) which basically grants allowance to another ETH address. The ETH address that you provide permission to can then take care of the execution of the transfer but has an allowance.

5. Failure Modes (Bounds on Operating Conditions & External Risk Factors)

• N/a