Code
From Tardis Wiki, the free Doctor Who reference
Messages could be written in code. (PROSE: Mystery Message) Skagra came to believe that The Worshipful and Ancient Law of Gallifrey was written in code, but the Doctor could not decode it and they came to the conclusion that it was in fact filled with meaningless gibberish, since it was actually a key disguised as a book rather than an actual book. (TV: Shada, WC: Shada)
