This work considers a network consisting of a server and a layer of relay nodes equipped with cache memories which aim to deliver content to end nodes that also have cache memories. The server and the end nodes consider the intermediate relay caches to be untrusted with the content. As a result, the server must design strategies to place content in relay caches not only to serve end users, but also to ensure that any a subset of them, even when colluding, cannot gain any information about the contents of the server database. The end users randomly connect to a subset of these untrusted caches at the beginning of the delivery phase via multicast links. For this network model, a coded caching scheme is developed by jointly optimizing the cache placement and delivery phases using secure regenerating codes. In addition, the scheme is extended to the setup of combination networks with untrusted relays, where the untrusted relays are connected to the end users via unicast links. The study highlights the benefits of cooperating with untrusted caches by designing the end users' caches to provide multicast opportunities in order to minimize the delivery load.