POPA ReconHistorical ninja manuals such as the Bansenshūkai (1676) and Shōninki (1681) describe several steganographic methods:
Historical ninja manuals such as the Bansenshūkai (1676) and Shōninki (1681) describe several steganographic methods: Invisible Ink (Aburidashi): • Messages were written with plant-based substances, such as mashed soybean juice or unfiltered sake. • The writing dried invisible and only became readable when the paper was heated (e.g., held near a flame). This allowed secret information to travel in plain sight as ordinary-looking documents. Encrypted and Folded Letters: • Using unusual or substituted kanji characters known only to the intended recipient. • Mixing real message text with du…
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