diff --git a/modules/4337/contracts/experimental/verifiers/WebAuthnVerifier.sol b/modules/4337/contracts/experimental/verifiers/WebAuthnVerifier.sol
index defb50a3..79610234 100644
--- a/modules/4337/contracts/experimental/verifiers/WebAuthnVerifier.sol
+++ b/modules/4337/contracts/experimental/verifiers/WebAuthnVerifier.sol
@@ -128,11 +128,11 @@ contract WebAuthnVerifier is IWebAuthnVerifier, P256Wrapper {
             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)
+            for {let i := 250} lt(i, 251)
             {
-                i := add(i, 6)
+                i := sub(i, 6)
             } {
-                sixBitGroup := and(shr(sub(250, i), challenge), 0x3F)
+                sixBitGroup := and(shr(i, challenge), 0x3F)
                 mstore8(resultPtr, mload(add(tablePtr, sixBitGroup)))
                 resultPtr := add(resultPtr, 1)
             }