feat(auth): add WebAuthn MFA credentials (#1628)

* feat(auth): add WebAuthn MFA credentials

* fix(auth): harden WebAuthn MFA review paths

* fix(auth): polish WebAuthn error handling

* fix(auth): handle duplicate WebAuthn credential races

* fix(auth): permit WebAuthn credential params

* fix(auth): trim WebAuthn registration controller cleanup

* fix(auth): tighten WebAuthn MFA handling

* fix(auth): pin WebAuthn relying party config

app/javascript/utils/webauthn.js

@@ -0,0 +1,85 @@
+function bufferToBase64url(buffer) {
+  const bytes = new Uint8Array(buffer);
+  const binary = String.fromCharCode(...bytes);
+
+  return btoa(binary)
+    .replace(/\+/g, "-")
+    .replace(/\//g, "_")
+    .replace(/=+$/g, "");
+}
+
+function base64urlToBuffer(value) {
+  const base64 = value.replace(/-/g, "+").replace(/_/g, "/");
+  const padded = base64.padEnd(
+    base64.length + ((4 - (base64.length % 4)) % 4),
+    "=",
+  );
+  const binary = atob(padded);
+  const bytes = new Uint8Array(binary.length);
+
+  for (let i = 0; i < binary.length; i += 1) {
+    bytes[i] = binary.charCodeAt(i);
+  }
+
+  return bytes.buffer;
+}
+
+export function prepareCredentialCreationOptions(options) {
+  options.challenge = base64urlToBuffer(options.challenge);
+  options.user.id = base64urlToBuffer(options.user.id);
+  options.excludeCredentials = (options.excludeCredentials || []).map(
+    (credential) => ({
+      ...credential,
+      id: base64urlToBuffer(credential.id),
+    }),
+  );
+
+  return options;
+}
+
+export function prepareCredentialRequestOptions(options) {
+  options.challenge = base64urlToBuffer(options.challenge);
+  options.allowCredentials = (options.allowCredentials || []).map(
+    (credential) => ({
+      ...credential,
+      id: base64urlToBuffer(credential.id),
+    }),
+  );
+
+  return options;
+}
+
+export function serializePublicKeyCredential(credential) {
+  const serialized = {
+    id: credential.id,
+    rawId: bufferToBase64url(credential.rawId),
+    type: credential.type,
+    authenticatorAttachment: credential.authenticatorAttachment,
+    clientExtensionResults: credential.getClientExtensionResults(),
+  };
+
+  if (credential.response.attestationObject) {
+    serialized.response = {
+      attestationObject: bufferToBase64url(
+        credential.response.attestationObject,
+      ),
+      clientDataJSON: bufferToBase64url(credential.response.clientDataJSON),
+      transports: credential.response.getTransports
+        ? credential.response.getTransports()
+        : [],
+    };
+  } else {
+    serialized.response = {
+      authenticatorData: bufferToBase64url(
+        credential.response.authenticatorData,
+      ),
+      clientDataJSON: bufferToBase64url(credential.response.clientDataJSON),
+      signature: bufferToBase64url(credential.response.signature),
+      userHandle: credential.response.userHandle
+        ? bufferToBase64url(credential.response.userHandle)
+        : null,
+    };
+  }
+
+  return serialized;
+}