Random Numbers
The "one-time pad" system is only cipher system that is totally secure. It uses random numbers to encode the plaintext. If the numbers are truly random and the encoder never reuses those numbers, the encoded message cannot be broken. Fortunately for cryptologists, random numbers are very difficult to come up with and creating, distributing, and managing pads for more than a handful of correspondents is beyond the capabilities of even most governments.
Using random numbers for cryptography was first done around 1917 for securing teleprinter communications. It proved unfeasible for the reasons mentioned above. By the mid-1920s however, the German government was using one-time pads for diplomatic correspondence. They had learned their lessons from World War I and were determined not to let it happen again.
Hara devised a system that used random numbers to superencipher Japanese army codes. Possibly because of the logistical difficulties inherent in the one-time pad system, Hara's system used tables of pseudo-random numbers. The encipherer had to indicate where in the table he (or much less likely at the time, she) did this by hiding the row and column headers from the table in the message.
This system is not new. Diplomats and armies started superenciphering with additives sometime during or soon after the First World War and by the 1920s it was common. German diplomats in Paris were using, shortly after the First World War, a codebook of 100,000 groups superenciphered twice from a book of 60,000 additive groups! It would be very surprising if after five to ten years of training with the Poles, Japanese Army cryptologists were not already familiar with superenciphering with additive tables.
Superencipherment is fairly strong. It can be, and was, broken, but it is very hard to do. With the exception of the one-time pad, which will keep its secrets until the end of time, any code or cipher can be broken. All that is required is sufficient material. All that can be expected of a code or cipher system is that by the time the enemy breaks it, the information in the message is no longer useful. This is just a cryptographic fact of life.
Hara's pseudo-random code system, like every additive system other than the one-time pad, can be broken. Eventually someone, somewhere will use overlapping parts of the additive charts. The first thing the cryptanalyst does is identify where in the message the starting point of the chart (the "indicator") is hidden --- this allows the messages that are enciphered with the same sections of the number charts to be lined up and the additives stripped off.
Read more about this topic: Japanese Cryptology From The 1500s To Meiji
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