Code (cryptography) - Superencipherment

Superencipherment

In more recent practice, it became typical to encipher a message after first encoding it, so as to provide greater security by increasing the degree of difficulty for cryptanalysts. With a numerical code, this was commonly done with an "additive" - simply a long key number which was digit-by-digit added to the code groups, modulo 10. Unlike the codebooks, additives would be changed frequently. The famous Japanese Navy code, JN-25, was of this design, as were several of the (confusingly named) Royal Navy Cyphers used after WWI and into WWII.

One might wonder why a code would be used if it had to be enciphered to provide security. As well as providing security, a well designed code can also compress the message, and provide some degree of automatic error correction. For ciphers, the same degree of error correction has generally required use of computers.