feat: make nullifier not leak identity between polls

packages/circuits/circom/anon/pollJoining.circom

@@ -41,8 +41,11 @@ template PollJoining(stateTreeDepth) {
     signal input stateRoot;
     // The actual tree depth (might be <= stateTreeDepth) Used in BinaryMerkleRoot
     signal input actualStateTreeDepth;
+    // The poll id
+    signal input pollId;
 
-    var computedNullifier = PoseidonHasher(1)([privKey]);
+    // Compute the nullifier (hash of private key and poll id)
+    var computedNullifier = PoseidonHasher(2)([privKey, pollId]);
     nullifier === computedNullifier;
 
     // User private to public key
@@ -69,8 +72,4 @@ template PollJoining(stateTreeDepth) {
     // Check credits
     var isCreditsValid = SafeLessEqThan(N_BITS)([credits, stateLeaf[STATE_LEAF_VOICE_CREDIT_BALANCE_IDX]]);
     isCreditsValid === 1;
-
-    // Check nullifier
-    var hashedPrivKey = PoseidonHasher(1)([privKey]);
-    hashedPrivKey === nullifier;
 }

packages/circuits/circom/circuits.json

@@ -3,7 +3,7 @@
     "file": "./anon/pollJoining",
     "template": "PollJoining",
     "params": [10],
-    "pubs": ["nullifier", "credits", "stateRoot", "pollPubKey"]
+    "pubs": ["nullifier", "credits", "stateRoot", "pollPubKey", "pollId"]
   },
   "ProcessMessages_10-20-2_test": {
     "file": "./core/qv/processMessages",

packages/circuits/ts/__tests__/PollJoining.test.ts

@@ -26,6 +26,7 @@ describe("Poll Joining circuit", function test() {
     "credits",
     "stateRoot",
     "actualStateTreeDepth",
+    "pollId",
   ];
 
   let circuit: WitnessTester<PollJoiningCircuitInputs>;

packages/circuits/ts/types.ts

@@ -54,6 +54,7 @@ export interface IPollJoiningInputs {
   credits: bigint;
   stateRoot: bigint;
   actualStateTreeDepth: bigint;
+  pollId: bigint;
 }
 
 /**

packages/cli/ts/commands/joinPoll.ts

@@ -101,6 +101,7 @@ const generateAndVerifyProof = async (
  * @param credits Credits for voting
  * @param pollPrivKey Poll's private key for the poll joining
  * @param pollPubKey Poll's public key for the poll joining
+ * @param pollId Poll's id
  * @returns stringified circuit inputs
  */
 const joiningCircuitInputs = (
@@ -111,6 +112,7 @@ const joiningCircuitInputs = (
   credits: bigint,
   pollPrivKey: PrivKey,
   pollPubKey: PubKey,
+  pollId: bigint,
 ): IPollJoiningCircuitInputs => {
   // Get the state leaf on the index position
   const { signUpTree: stateTree, stateLeaves } = signUpData;
@@ -146,7 +148,7 @@ const joiningCircuitInputs = (
 
   // Create nullifier from private key
   const inputNullifier = BigInt(maciPrivKey.asCircuitInputs());
-  const nullifier = poseidon([inputNullifier]);
+  const nullifier = poseidon([inputNullifier, pollId]);
 
   // Get pll state tree's root
   const stateRoot = stateTree.root;
@@ -167,6 +169,7 @@ const joiningCircuitInputs = (
     credits,
     stateRoot,
     actualStateTreeDepth,
+    pollId,
   };
 
   return stringifyBigInts(circuitInputs) as unknown as IPollJoiningCircuitInputs;
@@ -208,19 +211,19 @@ export const joinPoll = async ({
     logError("Invalid MACI private key");
   }
 
+  if (pollId < 0) {
+    logError("Invalid poll id");
+  }
+
   const userMaciPrivKey = PrivKey.deserialize(privateKey);
   const userMaciPubKey = new Keypair(userMaciPrivKey).pubKey;
-  const nullifier = poseidon([BigInt(userMaciPrivKey.asCircuitInputs())]);
+  const nullifier = poseidon([BigInt(userMaciPrivKey.asCircuitInputs()), pollId]);
 
   // Create poll public key from poll private key
   const pollPrivKeyDeserialized = PrivKey.deserialize(pollPrivKey);
   const pollKeyPair = new Keypair(pollPrivKeyDeserialized);
   const pollPubKey = pollKeyPair.pubKey;
 
-  if (pollId < 0) {
-    logError("Invalid poll id");
-  }
-
   const maciContract = MACIFactory.connect(maciAddress, signer);
   const pollContracts = await maciContract.getPoll(pollId);
 
@@ -325,6 +328,7 @@ export const joinPoll = async ({
       loadedCreditBalance!,
       pollPrivKeyDeserialized,
       pollPubKey,
+      pollId,
     ) as unknown as CircuitInputs;
   }
 

packages/contracts/contracts/MACI.sol

@@ -220,7 +220,8 @@ contract MACI is IMACI, DomainObjs, Params, Utilities {
       _messageBatchSize,
       _coordinatorPubKey,
       extContracts,
-      emptyBallotRoots[voteOptionTreeDepth - 1]
+      emptyBallotRoots[voteOptionTreeDepth - 1],
+      pollId
     );
 
     address mp = messageProcessorFactory.deploy(_verifier, _vkRegistry, p, msg.sender, _mode);

packages/contracts/contracts/Poll.sol

@@ -93,6 +93,9 @@ contract Poll is Params, Utilities, SnarkCommon, IPoll {
   /// @notice Poll voting nullifier
   mapping(uint256 => bool) private pollNullifier;
 
+  /// @notice The Id of this poll
+  uint256 public immutable pollId;
+
   error VotingPeriodOver();
   error VotingPeriodNotOver();
   error PollAlreadyInit();
@@ -125,13 +128,15 @@ contract Poll is Params, Utilities, SnarkCommon, IPoll {
   /// @param _coordinatorPubKey The coordinator's public key
   /// @param _extContracts The external contracts
   /// @param _emptyBallotRoot The root of the empty ballot tree
+  /// @param _pollId The poll id
   constructor(
     uint256 _duration,
     TreeDepths memory _treeDepths,
     uint8 _messageBatchSize,
     PubKey memory _coordinatorPubKey,
     ExtContracts memory _extContracts,
-    uint256 _emptyBallotRoot
+    uint256 _emptyBallotRoot,
+    uint256 _pollId
   ) payable {
     // check that the coordinator public key is valid
     if (!CurveBabyJubJub.isOnCurve(_coordinatorPubKey.x, _coordinatorPubKey.y)) {
@@ -156,6 +161,8 @@ contract Poll is Params, Utilities, SnarkCommon, IPoll {
     deployTime = block.timestamp;
     // store the empty ballot root
     emptyBallotRoot = _emptyBallotRoot;
+    // store the poll id
+    pollId = _pollId;
   }
 
   /// @notice A modifier that causes the function to revert if the voting period is
@@ -350,13 +357,14 @@ contract Poll is Params, Utilities, SnarkCommon, IPoll {
     uint256 _index,
     PubKey calldata _pubKey
   ) public view returns (uint256[] memory publicInputs) {
-    publicInputs = new uint256[](5);
+    publicInputs = new uint256[](6);
 
     publicInputs[0] = _pubKey.x;
     publicInputs[1] = _pubKey.y;
     publicInputs[2] = _nullifier;
     publicInputs[3] = _voiceCreditBalance;
     publicInputs[4] = extContracts.maci.getStateRootOnIndexedSignUp(_index);
+    publicInputs[5] = pollId;
   }
 
   /// @inheritdoc IPoll

packages/contracts/contracts/PollFactory.sol

@@ -21,7 +21,8 @@
     uint8 _messageBatchSize,
     DomainObjs.PubKey calldata _coordinatorPubKey,
     Params.ExtContracts calldata _extContracts,
-    uint256 _emptyBallotRoot
+    uint256 _emptyBallotRoot,
+    uint256 _pollId
   ) public virtual returns (address pollAddr) {
     // deploy the poll
     Poll poll = new Poll(
@@ -30,7 +31,8 @@
       _messageBatchSize,
       _coordinatorPubKey,
       _extContracts,
-      _emptyBallotRoot
+      _emptyBallotRoot,
+      _pollId
     );
 
     // init Poll

packages/contracts/contracts/interfaces/IPollFactory.sol

@@ -14,13 +14,15 @@ interface IPollFactory {
   /// @param _coordinatorPubKey The coordinator's public key
   /// @param _extContracts The external contracts interface references
   /// @param _emptyBallotRoot The root of the empty ballot tree
+  /// @param _pollId The poll id
   /// @return The deployed Poll contract
   function deploy(
     uint256 _duration,
     Params.TreeDepths calldata _treeDepths,
     uint8 _messageBatchSize,
     DomainObjs.PubKey calldata _coordinatorPubKey,
     Params.ExtContracts calldata _extContracts,
-    uint256 _emptyBallotRoot
+    uint256 _emptyBallotRoot,
+    uint256 _pollId
   ) external returns (address);
 }

packages/contracts/tests/PollFactory.test.ts

@@ -52,6 +52,7 @@ describe("pollFactory", () => {
         coordinatorPubKey.asContractParam(),
         extContracts,
         emptyBallotRoot,
+        0n,
       );
       const receipt = await tx.wait();
       expect(receipt?.status).to.eq(1);

packages/core/ts/Poll.ts

@@ -142,6 +142,7 @@ export class Poll implements IPoll {
    * @param treeDepths - The depths of the trees used in the poll.
    * @param batchSizes - The sizes of the batches used in the poll.
    * @param maciStateRef - The reference to the MACI state.
+   * @param pollId - The poll id
    */
   constructor(
     pollEndTimestamp: bigint,
@@ -471,7 +472,7 @@ export class Poll implements IPoll {
 
     // Create nullifier from private key
     const inputNullifier = BigInt(maciPrivKey.asCircuitInputs());
-    const nullifier = poseidon([inputNullifier]);
+    const nullifier = poseidon([inputNullifier, this.pollId]);
 
     // Get pll state tree's root
     const stateRoot = this.stateTree!.root;
@@ -490,6 +491,7 @@ export class Poll implements IPoll {
       credits,
       stateRoot,
       actualStateTreeDepth,
+      pollId: this.pollId,
     };
 
     return stringifyBigInts(circuitInputs) as unknown as IPollJoiningCircuitInputs;

packages/core/ts/utils/types.ts

@@ -156,6 +156,7 @@ export interface IPollJoiningCircuitInputs {
   credits: string;
   stateRoot: string;
   actualStateTreeDepth: string;
+  pollId: string;
 }
 /**
  * An interface describing the circuit inputs to the ProcessMessage circuit