🐛 fix challenge offset constants

The constant value used as an offset to retrieve the
challenge from the webauthn response was only correct
for payload of type `webauthn.get`. This commit makes
the offset value dynamic to also handle `webauthn.create`
payloads. Both payloads are now correctly supported.

src/WebAuthn256r1.sol

@@ -3,7 +3,15 @@ pragma solidity >=0.8.19 <0.9.0;
 
 import { ECDSA256r1 } from "../lib/secp256r1-verify/src/ECDSA256r1.sol";
 import { Base64 } from "../lib/solady/src/utils/Base64.sol";
-import { UV_FLAG_MASK, OFFSET_CLIENT_CHALLENGE, OFFSET_FLAG } from "src/utils.sol";
+import {
+    UV_FLAG_MASK,
+    OFFSET_CLIENT_CHALLENGE_GET,
+    OFFSET_CLIENT_CHALLENGE_CREATE,
+    OFFSET_FLAG,
+    OFFSET_CLIENT_TYPE,
+    TYPE_GET_INDICATOR,
+    TYPE_CREATE_INDICATOR
+} from "src/utils.sol";
 
 /// @title WebAuthn256r1
 /// @notice A library to verify ECDSA signature though WebAuthn on the secp256r1 curve
@@ -76,12 +84,20 @@ library WebAuthn256r1 {
             // Encode the client challenge in base64 and explicitly convert it to bytes
             bytes memory challengeEncoded = bytes(Base64.encode(clientChallenge, true, true));
 
+            // Extract the client challenge offset based on the client type
+            // By checking the indicator we can determine if we need to use the offset for the get or create flow
+            // @dev: we don't need to check the overflow here as the EVM will automatically revert if
+            //       `OFFSET_CLIENT_TYPE` is out of bound.
+            uint256 clientChallengeOffset = clientData[OFFSET_CLIENT_TYPE] == TYPE_CREATE_INDICATOR
+                ? OFFSET_CLIENT_CHALLENGE_CREATE
+                : OFFSET_CLIENT_CHALLENGE_GET;
+
             // Extract the challenge from the client data and hash it
             // @dev: we don't need to check the overflow here as the EVM will automatically revert if
-            //       `OFFSET_CLIENT_CHALLENGE + challengeEncoded.length` overflow. This is because we will
+            //       `clientChallengeOffset + challengeEncoded.length` overflow. This is because we will
             //       try to access a chunk of memory by passing an end index lower than the start index
             bytes32 challengeHashed =
-                keccak256(clientData[OFFSET_CLIENT_CHALLENGE:(OFFSET_CLIENT_CHALLENGE + challengeEncoded.length)]);
+                keccak256(clientData[clientChallengeOffset:(clientChallengeOffset + challengeEncoded.length)]);
 
             // Hash the encoded challenge and check both challenges are equal
             if (keccak256(challengeEncoded) != challengeHashed) {

src/utils.sol

@@ -18,14 +18,32 @@ bytes1 constant UP_FLAG_MASK = 0x01;
 bytes1 constant UV_FLAG_MASK = 0x04;
 bytes1 constant BOTH_FLAG_MASK = 0x05;
 
-// The offset of the client challenge in the client data
-uint256 constant OFFSET_CLIENT_CHALLENGE = 0x24;
+// The challenge is stored in the client data field of the WebAuthn response.
+// The client data is a JSON object that contains a type, the challenge and the origin.
+// For passkeys, the type is always "webauthn.get" or "webauthn.create".
+// Ex: `{"type":"webauthn.create","challenge":<challenge>,"origin":"<origin>"}`
+// Ex: `{"type":"webauthn.get","challenge":<challenge>,"origin":"<origin>"}`
+//
+// The client data always starts with a constant value which is the same for both
+// the get and create flows. This constant value correspond to the beginning of
+// the JSON object which is: `{"type":"webauthn.`. The next byte allow to distinguish
+// between the get and create flows. It is either `g` (0x67) for get or `c` (0x63) for create.
+// The three constants located below can be used to know if a WebAuthn response is a get or create flow.
+uint256 constant OFFSET_CLIENT_TYPE = 0x12;
+bytes1 constant TYPE_GET_INDICATOR = 0x67;
+bytes1 constant TYPE_CREATE_INDICATOR = 0x63;
+// The offset of the client challenge for the get flow
+// Correspond to the constant value `{"type":"webauthn.get","challenge":`
+uint256 constant OFFSET_CLIENT_CHALLENGE_GET = 0x24;
+// The offset of the client challenge for the create flow
+// Correspond to the constant value `{"type":"webauthn.create","challenge":`
+uint256 constant OFFSET_CLIENT_CHALLENGE_CREATE = 0x27;
 // The offset where the credential ID length starts and its length
 uint256 constant OFFSET_CREDID_LENGTH = 0x35; // 53
 uint256 constant CREDID_LENGTH_LENGTH = 0x02;
 // The offset where the credential ID value starts
 uint256 constant OFFSET_CREDID = 0x37; // 55
 // The offset of the flag mask
 uint256 constant OFFSET_FLAG = 0x20; // 32
-
+// The length of the public key coordinates
 uint256 constant P256R1_PUBKEY_COORD_LENGTH = 0x20; // 32