In Shmuel Alef (20, 20-22) Yonatan arranges a whole scheme to secretly inform David whether or not it's safe for him to return. However, after the sign is given that it's not safe (20, 40), Yonatan ...
The string "כוזו במוכסז כוזו" is written on the back side of the parchment in many a doorpost scripture. According to the Ra"sh (hilchos m'zuza 18) וכן נהגו לכתוב באשכנז ובצרפת כנגד ה' אלקינו ה' ...
Are there any known uses of cryptography (or similar) in Jewish History?