Use HTTPS encryption for creatives
Google is making a concerted effort to have HTTPS as the new default for all of ad serving, and almost all Ad Manager in-app ads are now automatically served through HTTPS (there may be minor discrepancies because of other factors that currently prevent HTTPS in a few situations).
Ad Manager can help you check creatives for SSL compatibility to ensure that your creatives are compliant. We strongly recommend all creatives to be fully SSL compatible to ensure these creatives are able to serve correctly on the latest mobile platforms.
For the IDFA or AdID macro and similar features, if any part of a creative is not secure, Ad Manager will reject it.