Japanese Cryptology From The 1500s To Meiji - Two-Part Codes

Two-Part Codes

Code books contain two lists --- one of code groups and one of plaintext letters, words, and phrases. Someone encoding a message looks up the words in the plaintext list and substitutes the corresponding code group. Obviously it is important for that person's sanity that the plaintext be in some sort of order so words can be looked up easily. Since the system is similar for decoding --- look up the code group and substitute the plaintext --- it is equally important to have the code groups in order as well. With a one-part code, both lists are in alphabetical (or numerical) order. This means that you can encode and decode using the same book.

It also makes it easier for the enemy to break the code because once they realize they are dealing with a one-part code, they can use known groups to draw conclusions about unknown groups. For example, if the enemy knows that aabbc is Antwerp and aabbz is available, they will know that aabbm cannot be Tokyo.

A two-part code mixes the lists, making the code stronger by avoiding the problem described above. The drawback is that you now need two books. One, for encoding, has the plaintext in order to make encoding easy and the other, for decoding, has the code groups in order. Hence the name "two-part" code. The increase in security usually outweighs the increase in size and extra security concerns. Antoine Rossignol invented the two-part code around 1650 or so. The idea could hardly be considered new or secret by the 20th century, so again it is surprising to see Japanese cryptographers taking so long to begin using a common cryptographic method.