Verifying Signatures

With the ability to sign messages, we can now work on the smart contract. To recap, the smart contract will:

Hold funds (received during deployment).
Transfer funds, aka blessing, to users (if they provide a corresponding signature from a Grandmaster).
Promote a user to Grandmaster (if they provide a corresponding signature from a Grandmaster).

We will build a Grandmasters contract that supports the IGrandmasters interface.

Function	Description
`grandmasters(address user) -> (bool)`	Returns true if `user` is a grandmaster. Returns false otherwise.
`acceptInvite(bytes signature)`	Promotes `msg.sender` to grandmaster if the provided `signature` can be verified to be from a grandmaster. Reverts if `signature` is invalid.
`receiveBlessing(uint256 amount, bytes signature)`	Transfers `amount` of funds to the `msg.sender` if the provided `signature` can be verified to be from a grandmaster. Reverts if `signature` is invalid.

Like before, the signed messages to verify are as follows:

Instruction for smart contract to promote an address to be a Grandmaster.

keccak256(
    "\x19Ethereum Signed Message:\n32" 
    + keccak256("Invite{address}")
 )

address : Address of individual to be invited as a Grandmaster [20 bytes long]

Instruction for smart contract to transfer ETH to a recipient.

keccak256(
     "\x19Ethereum Signed Message:\n32" 
     + keccak256("Bless{address}{amount}{ctr}")
)

address : Address of recipient of blessing [20 bytes long]
amount : Amount of blessing (in wei) [32 bytes long]
ctr : Number of blessings the recipient has been given beforehand [4 bytes long].

Lastly, take note of these 2 pointers:

The contract's creator is by default, a Grandmaster.
The contract's constructor should be payable, so that the contract can receive funds.

Your Task

Complete Grandmasters.sol by implementing IGrandMasters.

Run tests in Questplay

Submit work in Questplay

The Grandmasters’ words are absolute. It is important for their followers to be able to discern them from lies.