refactor: hash message and scope

Author: cedoor

packages/contracts/contracts/Semaphore.sol

@@ -3,7 +3,7 @@ pragma solidity 0.8.4;
 
 import "./interfaces/ISemaphore.sol";
 import "./interfaces/ISemaphoreVerifier.sol";
-import "./base/SemaphoreGroups.sol";
+import {SemaphoreGroups} from "./base/SemaphoreGroups.sol";
 
 /// @title Semaphore
 /// @dev This contract uses the Semaphore base contracts to provide a complete service
@@ -189,7 +189,7 @@ contract Semaphore is ISemaphore, SemaphoreGroups {
                 [proof[0], proof[1]],
                 [[proof[2], proof[3]], [proof[4], proof[5]]],
                 [proof[6], proof[7]],
-                [merkleTreeRoot, nullifier, message, scope]
+                [merkleTreeRoot, nullifier, _hash(message), _hash(scope)]
             )
         ) {
             revert Semaphore__InvalidProof();
@@ -199,4 +199,11 @@ contract Semaphore is ISemaphore, SemaphoreGroups {
 
         emit ProofVerified(groupId, merkleTreeRoot, nullifier, message, scope, proof);
     }
+
+    /// @dev Creates a keccak256 hash of a message compatible with the SNARK scalar modulus.
+    /// @param message: Message to be hashed.
+    /// @return Message digest.
+    function _hash(uint256 message) private pure returns (uint256) {
+        return uint256(keccak256(abi.encodePacked(message))) >> 8;
+    }
 }

packages/proof/package.json

@@ -30,6 +30,7 @@
         "access": "public"
     },
     "devDependencies": {
+        "@ethersproject/strings": "^5.7.0",
         "@rollup/plugin-commonjs": "^24.1.0",
         "@rollup/plugin-json": "^5.0.1",
         "@rollup/plugin-node-resolve": "^15.0.2",
@@ -46,6 +47,9 @@
         "@semaphore-protocol/identity": "3.15.2"
     },
     "dependencies": {
+        "@ethersproject/bignumber": "^5.7.0",
+        "@ethersproject/bytes": "^5.7.0",
+        "@ethersproject/keccak256": "^5.7.0",
         "@zk-kit/groth16": "0.5.0",
         "download": "^8.0.0",
         "tmp": "^0.2.1"

packages/proof/src/generate-proof.ts

@@ -1,9 +1,12 @@
+import { BigNumber } from "@ethersproject/bignumber"
+import { BytesLike, Hexable } from "@ethersproject/bytes"
 import { Group } from "@semaphore-protocol/group"
 import type { Identity } from "@semaphore-protocol/identity"
-import { prove } from "@zk-kit/groth16"
+import { NumericString, prove } from "@zk-kit/groth16"
 import getSnarkArtifacts from "./get-snark-artifacts.node"
+import hash from "./hash"
 import packProof from "./pack-proof"
-import { BigNumberish, SemaphoreProof, SnarkArtifacts } from "./types"
+import { SemaphoreProof, SnarkArtifacts } from "./types"
 
 /**
  * Generates a Semaphore proof.
@@ -18,8 +21,8 @@ import { BigNumberish, SemaphoreProof, SnarkArtifacts } from "./types"
 export default async function generateProof(
     identity: Identity,
     group: Group,
-    message: BigNumberish,
-    scope: BigNumberish,
+    message: BytesLike | Hexable | number | bigint,
+    scope: BytesLike | Hexable | number | bigint,
     treeDepth?: number,
     snarkArtifacts?: SnarkArtifacts
 ): Promise<SemaphoreProof> {
@@ -54,8 +57,8 @@ export default async function generateProof(
             treeDepth: merkleProofLength,
             treeIndices,
             treeSiblings,
-            scope,
-            message
+            scope: hash(scope),
+            message: hash(message)
         },
         snarkArtifacts.wasmFilePath,
         snarkArtifacts.zkeyFilePath
@@ -64,8 +67,8 @@ export default async function generateProof(
     return {
         treeRoot: publicSignals[0],
         nullifier: publicSignals[1],
-        message: publicSignals[2],
-        scope: publicSignals[3],
+        message: BigNumber.from(message).toString() as NumericString,
+        scope: BigNumber.from(scope).toString() as NumericString,
         proof: packProof(proof)
     }
 }

packages/proof/src/hash.ts

@@ -0,0 +1,16 @@
+import { BigNumber } from "@ethersproject/bignumber"
+import { BytesLike, Hexable, zeroPad } from "@ethersproject/bytes"
+import { keccak256 } from "@ethersproject/keccak256"
+import { NumericString } from "@zk-kit/groth16"
+
+/**
+ * Creates a keccak256 hash of a message compatible with the SNARK scalar modulus.
+ * @param message The message to be hashed.
+ * @returns The message digest.
+ */
+export default function hash(message: BytesLike | Hexable | number | bigint): NumericString {
+    message = BigNumber.from(message).toTwos(256).toHexString()
+    message = zeroPad(message, 32)
+
+    return (BigInt(keccak256(message)) >> BigInt(8)).toString() as NumericString
+}

packages/proof/src/index.test.ts

@@ -1,3 +1,4 @@
+import { formatBytes32String } from "@ethersproject/strings"
 import { Group } from "@semaphore-protocol/group"
 import { Identity } from "@semaphore-protocol/identity"
 import { getCurveFromName } from "ffjavascript"
@@ -10,8 +11,8 @@ import verifyProof from "./verify-proof"
 describe("Proof", () => {
     const treeDepth = 10
 
-    const message = 1
-    const scope = 2
+    const message = formatBytes32String("Hello world")
+    const scope = formatBytes32String("Scope")
 
     const identity = new Identity(42)
 

packages/proof/src/verify-proof.ts

@@ -2,6 +2,7 @@ import { verify } from "@zk-kit/groth16"
 import { SemaphoreProof } from "./types"
 import unpackProof from "./unpack-proof"
 import verificationKeys from "./verification-keys.json"
+import hash from "./hash"
 
 /**
  * Verifies a Semaphore proof.
@@ -21,7 +22,7 @@ export default function verifyProof({ treeRoot, nullifier, message, scope, proof
     }
 
     return verify(verificationKey, {
-        publicSignals: [treeRoot, nullifier, message, scope],
+        publicSignals: [treeRoot, nullifier, hash(message), hash(scope)],
         proof: unpackProof(proof)
     })
 }

yarn.lock

@@ -8499,6 +8499,10 @@ __metadata:
   version: 0.0.0-use.local
   resolution: "@semaphore-protocol/proof@workspace:packages/proof"
   dependencies:
+    "@ethersproject/bignumber": ^5.7.0
+    "@ethersproject/bytes": ^5.7.0
+    "@ethersproject/keccak256": ^5.7.0
+    "@ethersproject/strings": ^5.7.0
     "@rollup/plugin-commonjs": ^24.1.0
     "@rollup/plugin-json": ^5.0.1
     "@rollup/plugin-node-resolve": ^15.0.2