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ERC20 wrapper over the Dai Savings Rate
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README.md

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1 Chai = 1 Dai * Pot.chi

chai is an ERC20 token representing a claim on deposits in the DSR. It can be freely converted to and from dai: the amount of dai claimed by one chai is always increasing, at the Dai Savings Rate. Like any well-behaved token, a user's chai balance won't change unexpectedly, while the chai's value grows day by day.

chai is a very simple contract. Apart from the standard ERC20 functionality, it also implements the same permit off-chain approval as dai itself. You can also call dai(address usr) to check how many dai a given user's chai balance claims. The token has no owner or authority. That's all there is to it.

mainnet deployment

chai is live on the mainnet since December 1st at 0x06af07097c9eeb7fd685c692751d5c66db49c215

You can interact with the Chai contract at chai.money. The source for the ui is hosted here.

building and testing

This contract is built using dapptools, and follows the standard dapptools procedure for building and testing.

To compile:

$ make all

To run the tests:

$ make test

documentation

ERC20 functions

Chai.sol implements the standard ERC20 functions (balanceOf, allowance, approve, transfer, transferFrom).

Join

The chai contract is an ERC20 token where minting happens in a function called join, which converts a users Dai balance into Chai:

    function join(address dst, uint wad) external {
        uint chi = (now > pot.rho()) ? pot.drip() : pot.chi();
        uint pie = rdiv(wad, chi);
        balanceOf[dst] = add(balanceOf[dst], pie);
        totalSupply    = add(totalSupply, pie);

        daiToken.transferFrom(msg.sender, address(this), wad);
        daiJoin.join(address(this), wad);
        pot.join(pie);
        emit Transfer(address(0), dst, pie);
    }

Calling this function transfers wad Dai into the pot contract from msg.sender, granting the dst address a Chai balance representing their claim of Dai in the pot contract.

Exit

Chai is burned (converted into Dai) through a function called exit:

    function exit(address src, uint wad) public {
        require(balanceOf[src] >= wad, "chai/insufficient-balance");
        if (src != msg.sender && allowance[src][msg.sender] != uint(-1)) {
            require(allowance[src][msg.sender] >= wad, "chai/insufficient-allowance");
            allowance[src][msg.sender] = sub(allowance[src][msg.sender], wad);
        }
        balanceOf[src] = sub(balanceOf[src], wad);
        totalSupply    = sub(totalSupply, wad);

        uint chi = (now > pot.rho()) ? pot.drip() : pot.chi();
        pot.exit(wad);
        daiJoin.exit(msg.sender, rmul(chi, wad));
        emit Transfer(src, address(0), wad);
    }

A msg.sender with sufficient approval from the address src can call this function to decrease their Chai balance by wad and transfer its underlying Dai value to the msg.sender.

Draw

Since the wad argument to the exit function above is denominated in chai, the exact Dai transferred will be determined at the time the transaction is included in a block. If you want to ensure that a specific Dai amount must be transfered, you can use the draw function instead, which takes a dai denominated argument:

    // wad is denominated in dai
    function draw(address src, uint wad) external {
        uint chi = (now > pot.rho()) ? pot.drip() : pot.chi();
        // rounding up ensures usr gets at least wad dai
        exit(src, rdivup(wad, chi));
    }

Move

Similarly to draw, there is a transferFrom function with a dai denominated argument, ensuring that the receiving address will receive Chai worth at least wad dai:

    // like transferFrom but dai-denominated
    function move(address src, address dst, uint wad) external returns (bool) {
        uint chi = (now > pot.rho()) ? pot.drip() : pot.chi();
        // rounding up ensures dst gets at least wad dai
        return transferFrom(src, dst, rdivup(wad, chi));
    }

Permit

The permit method lets a user approve an address to spend on their behalf by signing a ERC712 typed message called permit.

A permit consists of the following components:

  • holder, the address granting the permission and the signer of the message
  • spender, the address to which permission is being granted or denied
  • nonce, for replay attack protection
  • allowed, whether the spenders permission is being granted or revoked

Messages are signed using the procedure described in ERC712, using the PERMIT_TYPEHASH and DOMAIN_SEPARATOR constants:

    bytes32 public constant DOMAIN_SEPARATOR = 0x0b50407de9fa158c2cba01a99633329490dfd22989a150c20e8c7b4c1fb0fcc3;
    // keccak256("Permit(address holder,address spender,uint256 nonce,uint256 expiry,bool allowed)"));
    bytes32 public constant PERMIT_TYPEHASH  = 0xea2aa0a1be11a07ed86d755c93467f4f82362b452371d1ba94d1715123511acb;

permit are processed by calling the permit method, which increments the holder nonce and approves the spender to spend on the behalf of holder if allowed is true, and revokes it otherwise:

    // --- Approve by signature ---
    function permit(address holder, address spender, uint256 nonce, uint256 expiry, bool allowed, uint8 v, bytes32 r, bytes32 s) external
    {
        bytes32 digest = keccak256(abi.encodePacked(
            "\x19\x01",
            DOMAIN_SEPARATOR,
            keccak256(abi.encode(PERMIT_TYPEHASH,
                                 holder,
                                 spender,
                                 nonce,
                                 expiry,
                                 allowed))));
        require(holder != address(0), "chai/invalid holder");
        require(holder == ecrecover(digest, v, r, s), "chai/invalid-permit");
        require(expiry == 0 || now <= expiry, "chai/permit-expired");
        require(nonce == nonces[holder]++, "chai/invalid-nonce");

        uint can = allowed ? uint(-1) : 0;
        allowance[holder][spender] = can;
        emit Approval(holder, spender, can);
    }
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