What is DOMAIN_SEPARATOR in EIP712?

Question

Can someone please explain to me (with a practical example) the use of DOMAIN_SEPARATOR in EIP712 and how it ensures different signatures for the same messages by the same user in two different Dapps?

Answer 1

This is a value unique to each domain that is ‘mixed in’ the signature. It makes signatures from different domains incompatible because is designed to include bits of DApp unique information such as the name of the DApp, the intended validator contract address, the expected DApp domain name, etc.

Source:https://eips.ethereum.org/EIPS/eip-712

Answer 2

It is possible that two DApps come up with an identical structure like Transfer(address from,address to,uint256 amount) that should not be compatible. By introducing a domain separator the dApp developers are guaranteed that there can be no signature collision.

You can see a practical example of how Uniswap uses it in ERC721Permit.sol

   function DOMAIN_SEPARATOR() public view override returns (bytes32) {
        return
            keccak256(
                abi.encode(
                    // keccak256('EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)')
                    0x8b73c3c69bb8fe3d512ecc4cf759cc79239f7b179b0ffacaa9a75d522b39400f,
                    nameHash,
                    versionHash,
                    ChainId.get(),
                    address(this)
                )
            );
    }

DOMAIN_SEPARATOR returns a hash value (bytes32) and this is hash value is used in permit function

   function permit(
        address spender,
        uint256 tokenId,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external payable override {
        require(_blockTimestamp() <= deadline, 'Permit expired');
        // it is used to create another hash value to make it unique
        bytes32 digest = keccak256(
            abi.encodePacked(
                '\x19\x01',
                DOMAIN_SEPARATOR(),
                keccak256(abi.encode(PERMIT_TYPEHASH, spender, tokenId, _getAndIncrementNonce(tokenId), deadline))
            )
        );
        address owner = ownerOf(tokenId);
        require(spender != owner, 'ERC721Permit: approval to current owner');

        if (Address.isContract(owner)) {
            require(IERC1271(owner).isValidSignature(digest, abi.encodePacked(r, s, v)) == 0x1626ba7e, 'Unauthorized');
        } else {
            address recoveredAddress = ecrecover(digest, v, r, s);
            require(recoveredAddress != address(0), 'Invalid signature');
            require(recoveredAddress == owner, 'Unauthorized');
        }

        _approve(spender, tokenId);
    }