I vaguely recall one in the Venus Prime series by Arthur C Clarke. I did a quick search for you -- it was a Playfair cipher in Volume 2 of the series. I recall

I vaguely recall one in the Venus Prime series by Arthur C Clarke. I did a quick search for you -- it was a Playfair cipher in Volume 2 of the series. I recall it even encouraged the reader to crack it themselves with some hints! Hope you enjoy :)