US08-01: Make the Trust Boundary Configurable and Authenticated (#96)

frontend/js/api.js

@@ -7,9 +7,28 @@ export const BASE = "/api/v1";
 // what makes it proof that the caller is this app and not another page.
 let csrfToken = null;
 
+// A deployment reachable through a proxy trades an operator secret for that cookie.
+// Kept per tab: sessionStorage dies with the tab, and the secret never enters a URL.
+const SECRET_KEY = "pp_access_secret";
+
+async function bootstrap(secret) {
+  return fetch(BASE + "/session", {
+    credentials: "same-origin",
+    headers: secret ? { "X-Access-Secret": secret } : {},
+  });
+}
+
 async function session() {
   if (csrfToken === null) {
-    const response = await fetch(BASE + "/session", { credentials: "same-origin" });
+    let response = await bootstrap(sessionStorage.getItem(SECRET_KEY));
+    if (response.status === 401) {
+      sessionStorage.removeItem(SECRET_KEY);
+      const secret = prompt("Access secret");
+      if (secret) {
+        response = await bootstrap(secret);
+        if (response.ok) sessionStorage.setItem(SECRET_KEY, secret);
+      }
+    }
     const body = await response.json().catch(() => null);
     csrfToken = (body && body.csrf_token) || null;
   }