Fix internal server error when sending stories to unknown recipient.

service/src/main/java/org/whispersystems/textsecuregcm/controllers/MessageController.java

@@ -213,7 +213,19 @@ public class MessageController {
       if (!isStory) {
         OptionalAccess.verify(source.map(AuthenticatedAccount::getAccount), accessKey, destination);
       }
-      assert (destination.isPresent());
+
+      boolean needsSync = !isSyncMessage && source.isPresent() && source.get().getAccount().getEnabledDeviceCount() > 1;
+
+      // We return 200 when stories are sent to a non-existent account. Since story sends bypass OptionalAccess.verify
+      // we leak information about whether a destination UUID exists if we return any other code (e.g. 404) from
+      // these requests.
+      if (isStory && destination.isEmpty()) {
+        return Response.ok(new SendMessageResponse(needsSync)).build();
+      }
+
+      // if destination is empty we would either throw an exception in OptionalAccess.verify when isStory is false
+      // or else return a 200 response when isStory is true.
+      assert destination.isPresent();
 
       if (source.isPresent() && !isSyncMessage) {
         checkMessageRateLimit(source.get(), destination.get(), userAgent);
@@ -254,7 +266,6 @@ public class MessageController {
         }
       }
 
-      boolean needsSync = !isSyncMessage && source.isPresent() && source.get().getAccount().getEnabledDeviceCount() > 1;
       return Response.ok(new SendMessageResponse(needsSync)).build();
     } catch (NoSuchUserException e) {
       throw new WebApplicationException(Response.status(404).build());

service/src/test/java/org/whispersystems/textsecuregcm/controllers/MessageControllerTest.java

@@ -846,6 +846,23 @@ class MessageControllerTest {
     );
   }
 
+  @Test
+  void testSendStoryToUnknownAccount() throws Exception {
+    String accessBytes = Base64.getEncoder().encodeToString(UNIDENTIFIED_ACCESS_BYTES);
+    String json = jsonFixture("fixtures/current_message_single_device.json");
+    UUID unknownUUID = UUID.randomUUID();
+    IncomingMessageList list = SystemMapper.getMapper().readValue(json, IncomingMessageList.class);
+    Response response =
+        resources.getJerseyTest()
+            .target(String.format("/v1/messages/%s", unknownUUID))
+            .queryParam("story", "true")
+            .request()
+            .header(OptionalAccess.UNIDENTIFIED, accessBytes)
+            .put(Entity.entity(list, MediaType.APPLICATION_JSON_TYPE));
+
+    assertThat("200 masks unknown recipient", response.getStatus(), is(equalTo(200)));
+  }
+
   private void checkBadMultiRecipientResponse(Response response, int expectedCode) throws Exception {
     assertThat("Unexpected response", response.getStatus(), is(equalTo(expectedCode)));
     verify(messageSender, never()).sendMessage(any(), any(), any(), anyBoolean());