feat(backend): Initial handshake nonce pre-work

.changeset/better-vans-obey.md

@@ -0,0 +1,5 @@
+---
+'@clerk/backend': minor
+---
+
+Initial stub of the new handshake payload flow with nonce

packages/backend/src/api/endpoints/HandshakePayloadApi.ts

@@ -0,0 +1,18 @@
+import type { HandshakePayload } from '../resources/HandshakePayload';
+import { AbstractAPI } from './AbstractApi';
+
+const BASE_PATH = '/handshake_payload';
+
+type GetHandshakePayloadParams = {
+  nonce: string;
+};
+
+export class HandshakePayloadAPI extends AbstractAPI {
+  public async getHandshakePayload(queryParams: GetHandshakePayloadParams) {
+    return this.request<HandshakePayload>({
+      method: 'GET',
+      path: BASE_PATH,
+      queryParams,
+    });
+  }
+}

packages/backend/src/api/resources/HandshakePayload.ts

@@ -0,0 +1,15 @@
+export type HandshakePayloadJSON = {
+  nonce: string;
+  payload: string;
+};
+
+export class HandshakePayload {
+  constructor(
+    readonly nonce: string,
+    readonly payload: string,
+  ) {}
+
+  static fromJSON(data: HandshakePayloadJSON): HandshakePayload {
+    return new HandshakePayload(data.nonce, data.payload);
+  }
+}

packages/backend/src/constants.ts

@@ -21,6 +21,7 @@ const Cookies = {
   Handshake: '__clerk_handshake',
   DevBrowser: '__clerk_db_jwt',
   RedirectCount: '__clerk_redirect_count',
+  HandshakeNonce: '__clerk_handshake_nonce',
 } as const;
 
 const QueryParameters = {
@@ -33,6 +34,7 @@ const QueryParameters = {
   HandshakeHelp: '__clerk_help',
   LegacyDevBrowser: '__dev_session',
   HandshakeReason: '__clerk_hs_reason',
+  HandshakeNonce: Cookies.HandshakeNonce,
 } as const;
 
 const Headers = {