diff --git a/contracts/contracts/KMSVerifier.sol b/contracts/contracts/KMSVerifier.sol
index 6ea689e8..9a2f695a 100644
--- a/contracts/contracts/KMSVerifier.sol
+++ b/contracts/contracts/KMSVerifier.sol
@@ -13,6 +13,26 @@ import "@openzeppelin/contracts/utils/Strings.sol";
 /// @notice This contract allows for the management of signers and provides methods to verify signatures
 /// @dev The contract uses OpenZeppelin's EIP712Upgradeable for cryptographic operations
 contract KMSVerifier is UUPSUpgradeable, Ownable2StepUpgradeable, EIP712Upgradeable {
+    /// @notice Returned if the KMS signer to add is already a signer.
+    error KMSAlreadySigner();
+
+    /// @notice Returned if the recovered KMS signer is not a valid KMS signer.
+    /// @param invalidSigner Address of the invalid signer.
+    error KMSInvalidSigner(address invalidSigner);
+
+    /// @notice Returned if the KMS signer to remove is not a signer.
+    error KMSNotASigner();
+
+    /// @notice Returned if the KMS signer to add is the null address.
+    error KMSSignerNull();
+
+    /// @notice Returned if the number of signatures is inferior to the threshold.
+    /// @param numSignatures Number of signatures.
+    error KMSSignatureThresholdNotReached(uint256 numSignatures);
+
+    /// @notice Returned if the number of signatures is equal to 0.
+    error KMSZeroSignature();
+
     struct DecryptionResult {
         address aclAddress;
         uint256[] handlesList;
@@ -113,9 +133,15 @@ contract KMSVerifier is UUPSUpgradeable, Ownable2StepUpgradeable, EIP712Upgradea
     /// @dev Only the owner can add a signer
     /// @param signer The address to be added as a signer
     function addSigner(address signer) public virtual onlyOwner {
-        require(signer != address(0), "KMSVerifier: Address is null");
+        if (signer == address(0)) {
+            revert KMSSignerNull();
+        }
+
         KMSVerifierStorage storage $ = _getKMSVerifierStorage();
-        require(!$.isSigner[signer], "KMSVerifier: Address is already a signer");
+        if ($.isSigner[signer]) {
+            revert KMSAlreadySigner();
+        }
+
         $.isSigner[signer] = true;
         $.signers.push(signer);
         applyThreshold();
@@ -158,7 +184,9 @@ contract KMSVerifier is UUPSUpgradeable, Ownable2StepUpgradeable, EIP712Upgradea
     /// @param signer The address to be removed from signers
     function removeSigner(address signer) public virtual onlyOwner {
         KMSVerifierStorage storage $ = _getKMSVerifierStorage();
-        require($.isSigner[signer], "KMSVerifier: Address is not a signer");
+        if (!$.isSigner[signer]) {
+            revert KMSNotASigner();
+        }
 
         // Remove signer from the mapping
         $.isSigner[signer] = false;
@@ -225,14 +253,24 @@ contract KMSVerifier is UUPSUpgradeable, Ownable2StepUpgradeable, EIP712Upgradea
     /// @return true if enough provided signatures are valid, false otherwise
     function verifySignaturesDigest(bytes32 digest, bytes[] memory signatures) internal virtual returns (bool) {
         uint256 numSignatures = signatures.length;
-        require(numSignatures > 0, "KmsVerifier: no signatures provided");
+
+        if (numSignatures == 0) {
+            revert KMSZeroSignature();
+        }
+
         uint256 threshold = getThreshold();
-        require(numSignatures >= threshold, "KmsVerifier: at least threshold number of signatures required");
+
+        if (numSignatures < threshold) {
+            revert KMSSignatureThresholdNotReached(numSignatures);
+        }
+
         address[] memory recoveredSigners = new address[](numSignatures);
         uint256 uniqueValidCount;
         for (uint256 i = 0; i < numSignatures; i++) {
             address signerRecovered = recoverSigner(digest, signatures[i]);
-            require(isSigner(signerRecovered), "KmsVerifier: Invalid KMS signer");
+            if (!isSigner(signerRecovered)) {
+                revert KMSInvalidSigner(signerRecovered);
+            }
             if (!tload(signerRecovered)) {
                 recoveredSigners[uniqueValidCount] = signerRecovered;
                 uniqueValidCount++;
diff --git a/contracts/test/kmsVerifier/kmsVerifier.ts b/contracts/test/kmsVerifier/kmsVerifier.ts
index 59bf260e..07870e00 100644
--- a/contracts/test/kmsVerifier/kmsVerifier.ts
+++ b/contracts/test/kmsVerifier/kmsVerifier.ts
@@ -42,8 +42,9 @@ describe('KMSVerifier', function () {
       expect(y).to.equal(true); // in this case, one signature still suffices to pass the decrypt (threshold is still 1)
 
       const kmsSignerDup = new ethers.Wallet(privKeySigner).connect(ethers.provider);
-      await expect(kmsVerifier.connect(deployer).addSigner(kmsSignerDup.address)).to.revertedWith(
-        'KMSVerifier: Address is already a signer',
+      await expect(kmsVerifier.connect(deployer).addSigner(kmsSignerDup.address)).to.revertedWithCustomError(
+        kmsVerifier,
+        'KMSAlreadySigner',
       ); // cannot add duplicated signer
       expect((await kmsVerifier.getSigners()).length).to.equal(2);
 
@@ -59,9 +60,9 @@ describe('KMSVerifier', function () {
 
       const tx5 = await contract.requestUint4({ gasLimit: 5_000_000 });
       await tx5.wait();
-      await expect(awaitAllDecryptionResults()).to.revertedWith(
-        'KmsVerifier: at least threshold number of signatures required',
-      ); // should revert because now we are below the threshold! (we receive only 1 signature but threshold is 2)
+      await expect(awaitAllDecryptionResults())
+        .to.revertedWithCustomError(kmsVerifier, 'KMSSignatureThresholdNotReached')
+        .withArgs(1n); // should revert because now we are below the threshold! (we receive only 1 signature but threshold is 2)
       const y2 = await contract.yUint4();
       expect(y2).to.equal(0);
 
@@ -90,7 +91,9 @@ describe('KMSVerifier', function () {
         contract2.requestMixedBytes256Trustless(encryptedAmount2.handles[0], encryptedAmount2.inputProof, {
           gasLimit: 5_000_000,
         }),
-      ).to.revertedWith('KmsVerifier: at least threshold number of signatures required'); // this should fail because in this case the InputVerifier received only one KMS signature (instead of at least 2);
+      )
+        .to.revertedWithCustomError(kmsVerifier, 'KMSSignatureThresholdNotReached')
+        .withArgs(1n); // should revert because now we are below the threshold! (we receive only 1 signature but threshold is 2)
 
       if (process.env.IS_COPROCESSOR === 'true') {
         // different format of inputProof for native