A path to a file containing the <code>hmac_key</code>. If <code>null</code>, a key will be generated automatically on first start.If non-<code>null</code>, this option overrides any <code>hmac_key</code> specified in <a href="/nixpkgs/option/services.invidious.settings"><a href="/nixpkgs/option/services.invidious.settings" rel="nofollow">services.invidious.settings</a></a> or via <a href="/nixpkgs/option/services.invidious.extraSettingsFile"><a href="/nixpkgs/option/services.invidious.extraSettingsFile" rel="nofollow">services.invidious.extraSettingsFile</a></a>.