♻️ Encode the challenge on-chain

You may probably ask why we encode the challenge in base64 on-chain
instead of of sending it already encoded to save some gas. This library
is opinionated and assumes that it is used in the context of Account
Abstraction. In this context, valuable information required to proceed
with the transaction correctly will be stored in the challenge field
meaning we need the challenge in clear to use later in the flow. That's
why we decided to encode it here.

src/WebAuthn.sol

@@ -1,14 +1,22 @@
 // SPDX-License-Identifier: MIT
 pragma solidity >=0.8.19 <0.9.0;
 
-// import { Base64 } from "../lib/solady/src/utils/Base64.sol";
+import { Base64 } from "../lib/solady/src/utils/Base64.sol";
 import { ECDSA256r1 } from "../lib/secp256r1-verify/src/ECDSA256r1.sol";
 
 error InvalidAuthenticatorData();
 error InvalidClientData();
 
-/// dev: this implementation assumes the caller check if User Presence (0x01) or User Verification (0x04) are set
+/// @title A library to verify ECDSA signature though WebAuthn on the secp256r1 curve
+/// @dev This implementation assumes the caller check if User Presence (0x01) or User Verification (0x04) are set
 library WebAuthn {
+    /// @notice Validate the webauthn data and generate the signature message needed to recover
+    /// @dev You may probably ask why we encode the challenge in base64 on-chain instead of
+    ///      of sending it already encoded to save some gas. This library is opiniated and
+    ///      it assumes that it is used in the context of Account Abstraction. In this context,
+    ///      valuable informations required to proceed the transaction will be stored in the challenge
+    ///      meaning we need the challenge in clear to use it later in the flow. That's why we decided to
+    ///      encode it here.
     function generateMessage(
         bytes1 authenticatorDataFlagMask,
         bytes calldata authenticatorData,
@@ -26,15 +34,15 @@ library WebAuthn {
                 revert InvalidAuthenticatorData();
             }
 
-            // TODO: Pass non-encode challenge? Convert the challenge to `bytes memory` and encode it to Base64
-            // bytes memory challengeEncoded = bytes(Base64.encode(abi.encodePacked(clientChallenge), true, true));
+            // Encode the client challenge in base64 and explicitly convert it to bytes
+            bytes memory challengeEncoded = bytes(Base64.encode(clientChallenge, true, true));
 
             // Extract the challenge from the client data and hash it
             bytes32 challengeHashed =
-                keccak256(clientData[clientChallengeOffset:(clientChallengeOffset + clientChallenge.length)]);
+                keccak256(clientData[clientChallengeOffset:(clientChallengeOffset + challengeEncoded.length)]);
 
-            // hash the encoded challenge and check both challenges are equal
-            if (keccak256(clientChallenge) != challengeHashed) {
+            // Hash the encoded challenge and check both challenges are equal
+            if (keccak256(challengeEncoded) != challengeHashed) {
                 revert InvalidClientData();
             }
 
@@ -43,6 +51,7 @@ library WebAuthn {
         }
     }
 
+    /// @notice Verify ECDSA signature though WebAuthn on the secp256r1 curve
     function verify(
         bytes1 authenticatorDataFlagMask,
         bytes calldata authenticatorData,
@@ -58,7 +67,6 @@ library WebAuthn {
         returns (bool)
     {
         unchecked {
-            // Verify the signature over sha256(authenticatorData || sha256(clientData))
             bytes32 message = generateMessage(
                 authenticatorDataFlagMask, authenticatorData, clientData, clientChallenge, clientChallengeOffset
             );

test/WebAuthn.t.sol

@@ -68,8 +68,8 @@ contract ContractTest is Test {
                 hex"65223a224e546f2d3161424547526e78786a6d6b61544865687972444e583369"
                 hex"7a6c7169316f776d4f643955474a30222c226f726967696e223a226874747073"
                 hex"3a2f2f66726573682e6c65646765722e636f6d222c2263726f73734f726967696e223a66616c73657d",
-                // clientChallengeBase64
-                hex"4e546f2d3161424547526e78786a6d6b61544865687972444e5833697a6c7169316f776d4f643955474a30",
+                // clientChallenge
+                hex"353a3ed5a0441919f1c639a46931de872ac3357de2ce5aa2d68c2639df54189d",
                 // clientChallengeOffset
                 0x24,
                 // r
@@ -95,8 +95,8 @@ contract ContractTest is Test {
             hex"65223a224e546f2d3161424547526e78786a6d6b61544865687972444e583369"
             hex"7a6c7169316f776d4f643955474a30222c226f726967696e223a226874747073"
             hex"3a2f2f66726573682e6c65646765722e636f6d222c2263726f73734f726967696e223a66616c73657d",
-            // clientChallengeBase64
-            hex"4e546f2d3161424547526e78786a6d6b61544865687972444e5833697a6c7169316f776d4f643955474a30",
+            // clientChallenge
+            hex"353a3ed5a0441919f1c639a46931de872ac3357de2ce5aa2d68c2639df54189d",
             // clientChallengeOffset
             0x24
         );