Many people hold the mistaken notion that the manuals of ninjutsu are written in a cipher, handed down orally so that the secrecy of their contents might be preserved. Sometimes the scrolls consist merely of lists of skills and tools, and this creates the illusion of some kind of code. But this interpretation is wrong, save for a few examples — such as information on the ingredients of certain explosives.
The meaning of the individual names may have been plain fact to anyone versed in the military terminology of the period in which a given scroll was composed. That we do not understand these names today does not mean they were equally obscure to the people concerned in the past. To attach the word kuden to certain skills makes the skills without it general knowledge — or at least intelligible to the people of a given school and age.
Most instructional manuals contain detailed information with a reference to further skills that are to be passed on orally. A ninja manual is almost always highly informative, but with a touch of hidden meaning, or a secret subtext. Everything considered too secret to be written down is likewise marked as kuden.
Ninjutsu itself was never secret!
To pay a local inhabitant for information was regarded as ninjutsu, just as was sending some man to observe the banners and identifying marks of an enemy army. Shinobi no jutsu is in essence a series of levels with loose boundaries between them. Some techniques were known to many students, others only to a chosen few. The closer you came to the head of a school's system, the closer you came to its deep secrets. Both the Shinobi Hiden and the Shōninki speak of sharing the deepest secrets with only one person — and by this is meant not merely a member of the family, but the one who shows the greatest aptitude for the work of the ninja.
"If you have children or grandchildren who are not good enough, or do not have the right mindset, then they should not receive the complete traditions of ninjutsu, and certain skills should be kept hidden from them. And to one chosen person alone should they be given in their fullness." Ninjutsu Ōgiden, 1586
Ninjutsu, however, was not always handed on to just one person. A whole generation of men from the province of Iga was hired to teach others the art of the ninja across most of the Japanese armies. About fifty ninja were hired in this way in each province, which gives hundreds, even thousands, of schools. How else could the demands and needs of an entire country have been met, if only one man of a single generation acquired the knowledge of ninjutsu?
In history we find no Japanese schools whose content was handed on in secrecy, or built solely on an oral tradition, with the exception of the Ōgiden manual mentioned above, where parts of the inner mechanisms are passed to only one person per generation. Only a small percentage of the teaching's content was kept secret in this way from its members. It must be understood, however, that certain parts of the teaching were meant to be understood only by the students of the school that had gathered those skills. In this sense, kuden may be divided into three kinds:
- [1] Skills that are known to most of the people engaged in military operations, and to those who concern themselves with shinobi no jutsu or are themselves shinobi soldiers. These are skills such as reconnaissance, topography and the arts of measurement — in short, the elements used by an entire army.
- [2] Skills whose effect is understood by everyone — explosives, say, or the techniques of night raids — but whose detailed particulars are not commonly known. The path to these skills, not their result, is the kuden, and it is known only to a select number of those engaged in this activity.
- [3] Skills that are a deep secret and are passed only among a few, very few, members across a school. The people who are the target of their effect are ignorant of their application — such as eavesdropping on conversations from within an enemy fortification without the agent being detected, techniques of infiltration and the making of sleeper agents, or the mystical elements kept for the more advanced students.
Kuden appears throughout Japanese culture, and this is one of the reasons for the loss of some of the finer details of the art of the ninja. All the teaching lineages have died out, and we no longer possess a living oral tradition with a verifiable background.
For the reader from the Western world it is best to see kuden as the oral transmission of information from teacher to student, meant either to keep a secret or to indicate something too complex to be written down.
On the sources: The period manuals cited above (Shinobi Hiden / Ninpiden, Shōninki, Ninjutsu Ōgiden) are available today in the modern English translations of Antony Cummins and Yoshie Minami. Japanese terms and names are given in the standard (Hepburn) romanization.
