Add getChallenge() to Storage contract

contracts/Proofs.sol

@@ -89,6 +89,10 @@ contract Proofs {
     return _getChallenge(_getPointer(id, proofPeriod));
   }
 
+  function _getChallenge(bytes32 id) internal view returns (bytes32) {
+    return _getChallenge(id, currentPeriod());
+  }
+
   function _isProofRequired(bytes32 id, uint256 proofPeriod)
     internal
     view

contracts/Storage.sol

@@ -64,6 +64,10 @@ contract Storage is Collateral, Marketplace, Proofs {
     return _isProofRequired(contractId);
   }
 
+  function getChallenge(bytes32 contractId) public view returns (bytes32) {
+    return _getChallenge(contractId);
+  }
+
   function submitProof(bytes32 contractId, bool proof) public {
     _submitProof(contractId, proof);
   }

contracts/TestProofs.sol

@@ -44,6 +44,10 @@ contract TestProofs is Proofs {
     return _isProofRequired(id);
   }
 
+  function getChallenge(bytes32 id) public view returns (bytes32) {
+    return _getChallenge(id);
+  }
+
   function submitProof(bytes32 id, bool proof) public {
     _submitProof(id, proof);
   }

test/Proofs.test.js

@@ -112,6 +112,23 @@ describe("Proofs", function () {
       }
     }
 
+    it("provides different challenges per period", async function () {
+      await waitUntilProofIsRequired(id)
+      const challenge1 = await proofs.getChallenge(id)
+      await waitUntilProofIsRequired(id)
+      const challenge2 = await proofs.getChallenge(id)
+      expect(challenge2).not.to.equal(challenge1)
+    })
+
+    it("provides different challenges per id", async function () {
+      const id2 = ethers.utils.randomBytes(32)
+      const id3 = ethers.utils.randomBytes(32)
+      const challenge1 = await proofs.getChallenge(id)
+      const challenge2 = await proofs.getChallenge(id2)
+      const challenge3 = await proofs.getChallenge(id3)
+      expect(challenge1 === challenge2 && challenge2 === challenge3).to.be.false
+    })
+
     it("submits a correct proof", async function () {
       await proofs.submitProof(id, true)
     })