feat(ChainlinkRNG): Add Chainlink RNG contract (WIP)

contracts/standard/rng/ChainlinkRNG.sol

@@ -0,0 +1,133 @@
+/**
+ *  @authors: [@MerlinEgalite]
+ *  @reviewers: []
+ *  @auditors: []
+ *  @bounties: []
+ *  @deployments: []
+ */
+pragma solidity ^0.6.6;
+
+import "@chainlink/contracts/src/v0.6/VRFConsumerBase.sol";
+import "./RNG.sol";
+
+
+interface IKlerosLiquid {
+    function passPhase() external;
+}
+
+
+/**
+ * @title Random Number Generator using Chainlink Verifiable Randomness Mechanism on Polygon
+ * @author Merlin Egalite - <egalite.merlin@gmail.com>
+ *
+ * @dev This contract implements the RNG standard and inherits from VRFConsumerBase to use Chainlink Verifiable Randomness Mechanism.
+ * @dev It allows to store the random number associated to the requests made.
+ * @dev Note that to make requests to Chainlink, the contract needs to be funded with some LINK.
+ * @dev Chainlink documentation: https://docs.chain.link/docs/chainlink-vrf/
+ * @dev For SECURITY CONSIDERATIONS, you might also have look to: https://github.com/smartcontractkit/chainlink/blob/master/evm-contracts/src/v0.6/VRFConsumerBase.sol
+ */
+contract ChainlinkRNG is RNG, VRFConsumerBase {
+
+    /* Storage */
+
+    IKlerosLiquid public kleros; // The address of Kleros Liquid.
+    bytes32 internal keyHash; // The key hash for the VRF Coordinator.
+    uint256 internal fee; // The amount of LINK to send with a request.
+    mapping(bytes32 => uint256) public randomNumber; // randomNumber[requestId] is the random number for the requestId, 0 otherwise.
+
+    /* Modifier */
+
+    modifier onlyByKleros() {
+        require(msg.sender == address(kleros), "ChainlinkRNG: not called by Kleros");
+        _;
+    }
+
+    /* Constructor */
+
+    /**
+     * @dev Constructs the ChainlinkRNG contract.
+     * @param _vrfCoordinator The address of VRFCoordinator contract.
+     * @param _link The address of LINK token contract.
+     * @param _kleros The address of Kleros Liquid's contract.
+     * @param _keyHash The key hash for the VRF Coordinator.
+     * @param _fee The amount of LINK to send with a request.
+     *
+     * @dev https://docs.chain.link/docs/link-token-contracts
+     */
+    constructor(
+        address _vrfCoordinator,
+        address _link,
+        IKlerosLiquid _kleros,
+        bytes32 _keyHash,
+        uint256 _fee
+    )
+        VRFConsumerBase(_vrfCoordinator, _link)
+        public
+    {
+        keyHash = _keyHash;
+        kleros = _kleros;
+        fee = _fee;
+    }
+
+    /* External */
+
+    /**
+     * @dev Withdraws all LINK tokens locked in this contract.
+     */
+    function withdrawLink() external onlyByKleros {
+        require(LINK.transfer(msg.sender, LINK.balanceOf(address(this))), "ChainlinkRNG: unable to transfer LINK tokens");
+    }
+
+    /**
+     * @dev Changes the `fee` storage variable.
+     * @param _newFee The new value for the `fee` storage variable.
+     */
+    function changeFee(uint256 _newFee) external onlyByKleros {
+        fee = _newFee;
+    }
+
+    /**
+     * @dev Changes the `kleros` storage variable.
+     * @param _newKleros The new value for the `kleros` storage variable.
+     */
+    function changeKleros(IKlerosLiquid _newKleros) external onlyByKleros {
+        kleros = _newKleros;
+    }
+
+    /**
+     * @dev Requests a random number.
+     * @dev The _seed parameter is vestigial, and is kept only for API
+     * @dev compatibility with older versions. It can't *hurt* to mix in some of
+     * @dev your own randomness, here, but it's not necessary because the VRF
+     * @dev oracle will mix the hash of the block containing your request into the
+     * @dev VRF seed it ultimately uses.
+     * @param _seed seed mixed into the input of the VRF.
+     * @return requestId unique ID for this request.
+     */
+    function requestRN(uint _seed) external onlyByKleros returns (bytes32 requestId) {
+        require(LINK.balanceOf(address(this)) >= fee, "ChainlinkRNG: not enough LINK to pay the fee");
+        return requestRandomness(keyHash, fee, _seed);
+    }
+
+    /**
+     * @dev Gets the random number associated to a `_requestId`.
+     * @param _requestId The request Id initially returned by requestRN.
+     * @return RN Random Number. If the number is not ready or has not been required it returns 0.
+     */
+    function getRN(bytes32 _requestId) external view returns (uint256 RN) {
+        return randomNumber[_requestId];
+    }
+
+    /* Internal */
+
+    /**
+     * @dev Stores the random number given by the VRF Coordinator and calls passPhase function on Kleros Liquid.
+     * @dev This is the callback function used by the VRF Coordinator.
+     * @param _requestId The request Id initially returned by requestRN.
+     * @param _randomness the VRF output.
+     */
+    function fulfillRandomness(bytes32 _requestId, uint256 _randomness) internal override {
+        randomNumber[_requestId] = _randomness;
+        IKlerosLiquid(kleros).passPhase();
+    }
+}

contracts/standard/rng/README.md

@@ -8,4 +8,6 @@ BlockhashRNGFallback.sol does the same as the previous contract but fallback to
 
 TrustedRNG.sol implements a random number generator based on a trusted third party.
 
-ConstantNG.sol implements a contracts always returning a specified number to ease testing.
+ConstantNG.sol implements a contract always returning a specified number to ease testing.
+
+Chainlink.sol implements the RNG standard using Chainlink's Verifiable Randomness Mechanism

package.json

@@ -55,6 +55,7 @@
   "dependencies": {
     "@kleros/kleros": "^0.1.2",
     "@realitio/realitio-contracts": "^2.0.5",
+    "@chainlink/contracts": "^0.1.7",
     "minimetoken": "^0.2.0",
     "openzeppelin-solidity": "^1.12.0",
     "web3-utils": "^1.2.1"