diff --git a/modules/passkey/contracts/verifiers/WebAuthnVerifier.sol b/modules/passkey/contracts/verifiers/WebAuthnVerifier.sol
index 1b3c7a51..e4b3c792 100644
--- a/modules/passkey/contracts/verifiers/WebAuthnVerifier.sol
+++ b/modules/passkey/contracts/verifiers/WebAuthnVerifier.sol
@@ -3,7 +3,6 @@
 pragma solidity >=0.8.0;
 
 import {IP256Verifier, P256VerifierLib} from "./IP256Verifier.sol";
-import {Base64Url} from "../vendor/FCL/utils/Base64Url.sol";
 
 /**
  * @title WebAuthnConstants
@@ -89,6 +88,9 @@ interface IWebAuthnVerifier {
  */
 contract WebAuthnVerifier is IWebAuthnVerifier {
     IP256Verifier internal immutable P256_VERIFIER;
+    
+    string internal constant ENCODING_TABLE =
+        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
 
     constructor(IP256Verifier verifier) {
         P256_VERIFIER = verifier;
@@ -106,15 +108,43 @@ contract WebAuthnVerifier is IWebAuthnVerifier {
         bytes32 challenge,
         bytes calldata clientDataFields
     ) internal pure returns (bytes32 message) {
-        string memory encodedChallenge = Base64Url.encode(abi.encodePacked(challenge));
         /* solhint-disable quotes */
+        // AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA is placeholder for the encoded challenge
         bytes memory clientDataJson = abi.encodePacked(
             '{"type":"webauthn.get","challenge":"',
-            encodedChallenge,
+            "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
             '",',
             clientDataFields,
             "}"
         );
+
+        string memory table = ENCODING_TABLE;
+        
+        // solhint-disable-next-line no-inline-assembly
+        assembly {
+            // Skip first 32 bytes of the table containing the length
+            let tablePtr := add(table, 1)
+            // Skip first 36 bytes of the clientDataJson containing '{"type":"webauthn.get","challenge":"'
+            let resultPtr := add(clientDataJson, 68)
+
+            // Store group of 6 bits from the challenge to be encoded. Storing of 6 bits group can be removed but, kept here for readability.
+            let sixBitGroup
+
+            // Iterate over challenge in group of 6 bits, for each 6 bits lookup the ENCODING_TABLE, transform it and store it in the result
+            for {let i := 0} lt(i, 252)
+            {
+                i := add(i, 6)
+            } {
+                sixBitGroup := and(shr(sub(250, i), challenge), 0x3F)
+                mstore8(resultPtr, mload(add(tablePtr, sixBitGroup)))
+                resultPtr := add(resultPtr, 1)
+            }
+
+            // Load the remaining last 4 bits of challenge that are yet to be encoded and then shift left to add 2 bits at the end to make it a group of 6 bits.
+            sixBitGroup := shl(2, and(challenge, 0x0F))
+            mstore8(resultPtr, mload(add(tablePtr, sixBitGroup)))
+        }
+
         /* solhint-enable quotes */
         message = sha256(abi.encodePacked(authenticatorData, sha256(clientDataJson)));
     }