If we were to find sentient life outside of Earth, would they be friendly or not? This is the core of all SF involving aliens, and we have another take on it in Cryptic, by Jack McDevitt. It's a Hugo nominee from 1984. In it, the current director of the SETI project finds what looks like a signal stowed away without publicity by the previous director, and the story revolves around what that current head does to find out about the signal. Why was it hidden? Turns out it was double-hidden--it shows signs of encryption at its core. And there were similar ones from two separate but nearby stars. To the author, this indicates that they are at war.
Of course we know much more now about security. Even as the Cold War drew to an end, SF writers who wrote on human conflict focused much more on large states facing off, than on terror by small actors. Now we'd say those transmissions were just taking normal precautions. If aliens use even techniques we know about now, these, combined with the sheer alienness of the content, will render them impossible to decipher. And that would be normal, rational behavior. Oh well.