[feat] Single signer verification functions for multisig

contracts/signature/MultiSigCheckable.sol

@@ -359,6 +359,23 @@ abstract contract MultiSigCheckable is WithAdmin, EIP712 {
         usedHashes[salt] = true;
     }
 
+    /**
+     @notice Validate signature of message
+     @param message The message to verify
+     @param expectedGroupId The expected group ID
+     @param multiSignature The signatures formatted as a multisig
+     */
+    function validateSignature(
+        bytes32 message,
+        uint64 expectedGroupId,
+        bytes memory multiSignature
+    ) internal view returns (address signer) {
+        require(multiSignature.length != 0, "MSC: multiSignature required");
+        (, bool result, address signer) = tryVerifySingleSigner(message, expectedGroupId, multiSignature);
+        require(result, "MSC: Invalid signature");
+        return signer;
+    }
+
     function verifyUniqueSaltWithQuorumId(
         bytes32 message,
         address expectedQuorumId,
@@ -368,7 +385,7 @@ abstract contract MultiSigCheckable is WithAdmin, EIP712 {
     ) internal virtual {
         require(multiSignature.length != 0, "MSC: multiSignature required");
         bytes32 digest = _hashTypedDataV4(message);
-        (bool result, address[] memory signers) = tryVerifyDigestWithAddress(digest, expectedGroupId, multiSignature);
+        (bool result, address[] memory signers) = tryVerifyDigestWithAddress(digest, expectedGroupId, multiSignature, true);
         require(result, "MSC: Invalid signature");
         require(!usedHashes[salt], "MSC: Message already used");
         require(
@@ -414,7 +431,8 @@ abstract contract MultiSigCheckable is WithAdmin, EIP712 {
         (result, ) = tryVerifyDigestWithAddress(
             digest,
             expectedGroupId,
-            multiSignature
+            multiSignature,
+            true
         );
     }
 
@@ -424,13 +442,15 @@ abstract contract MultiSigCheckable is WithAdmin, EIP712 {
      @param expectedGroupId The expected group ID
      @param multiSignature The signatures formatted as a multisig. Note that this
         format requires signatures to be sorted in the order of signers (as bytes)
+     @param checkForMinSigs If the minimum number of signatures check should be performed
      @return result Identifies success or failure
      @return signers Lis of signers.
      */
     function tryVerifyDigestWithAddress(
         bytes32 digest,
         uint64 expectedGroupId,
-        bytes memory multiSignature
+        bytes memory multiSignature,
+        bool checkForMinSigs
     ) internal view returns (bool result, address[] memory signers) {
         require(multiSignature.length != 0, "MSC: multiSignature required");
         MultiSigLib.Sig[] memory signatures = MultiSigLib.parseSig(
@@ -479,10 +499,11 @@ abstract contract MultiSigCheckable is WithAdmin, EIP712 {
             // This ensures there are no duplicate signers
             require(signers[i - 1] < _signer, "MSC: Sigs not sorted");
         }
-        require(
-            signatures.length >= q.minSignatures,
-            "MSC: not enough signatures"
-        );
+
+        if (checkForMinSigs) {
+            require(signatures.length >= q.minSignatures, "MSC: not enough signatures");
+        }
+
         return (true, signers);
     }
 
@@ -510,4 +531,34 @@ abstract contract MultiSigCheckable is WithAdmin, EIP712 {
         digest = _hashTypedDataV4(message);
         result = tryVerifyDigest(digest, expectedGroupId, multiSignature);
     }
+
+    /**
+     @notice Tries to verify a message hash
+        @dev example message;
+
+        bytes32 constant METHOD_SIG =
+            keccak256("WithdrawSigned(address token,address payee,uint256 amount,bytes32 salt)");
+        bytes32 message = keccak256(abi.encode(
+          METHOD_SIG,
+          token,
+          payee,
+          amount,
+          salt
+     @param message The message
+     @param expectedGroupId The expected group ID
+     @param multiSignature The signatures formatted as a multisig
+    */
+    function tryVerifySingleSigner(
+        bytes32 message,
+        uint64 expectedGroupId,
+        bytes memory multiSignature
+    ) internal view returns (bytes32 digest, bool result, address signer) {
+        digest = _hashTypedDataV4(message);
+        (result, signer) = tryVerifyDigestWithAddress(
+            digest,
+            expectedGroupId,
+            multiSignature,
+            false
+        );
+    }
 }