|God Age Script|
1- Court scribes 'imported' from Baekje and Silla
2- Buddhism and its many sūtra commentaries
In the Kamakura (1185–1333) and Edo (1603-1868) periods, some Japanese scholars associated with the movement that wanted to separate Shintō from Buddhism, and chagrined at the idea of Japan owing its writing system to China and Korea, claimed that the Japanese already had a uniquely Japanese writing system before the introduction of the Chinese system. This putative writing system wasn't even human in origin, but had been gifted to the Japanese by no less than the Japanese gods!
These scholars claimed that the jindai moji had been created by the god Izanagi with his demiurgic naginata called the Amenonuhoko (天沼矛 "heavenly jewelled spear"), and that the divine script fell into disuse after Prince Shōtoku (聖徳太子, 572-622) replaced it with Chinese characters. However, the Yoshida (吉田) family was supposed to have secretly kept the knowledge from then on. This is why the existence of jindai moji came to be prominently accepted amongst Yoshida Shintō believers. (Yoshida Shintō was a brand of Neo-Confucian Shintō that arose during the late Muromachi period.)
The 'divine' writing system was variedly named as kamiyo no moji (script of the age of the gods), jindai moji (god age script), hifumi (after its first three syllables), etc. Several tablets, bamboo slips and secret scrolls were unearthed or produced as 'proof' of the existence of the script, but all were later analysed by professional linguists and revealed as forgeries.
Amongst the various legends and rumours associated with the ninja, one was that they used jindai moji as a secret script to carry undecipherable messages.
In your campaign game, you may obviously decide that the jindai moji is genuine and indeed of divine origin, and use it as a magical script for arcane and/or occult purposes.