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This chapter deals with ancilliary functions, rather than encryption itself.

However, the idea of converting text from binary form to letters of the alphabet does suggest a way to complicate any attempt to analyze a cipher system. Since such a conversion unavoidably adds a small degree of redundancy to the message being encrypted, as a safety precaution the key used for encryption before conversion and the key used for encryption after conversion should be unrelated.

The various forms of encryption we have seen here can be organized into groups based on common properties: I once essayed to classify the ciphers of the first four chapters, ciphers other than public-key systems, by the following scheme, based on the kind of operations performed on the plaintext:

This form of classification only, however, addresses one dimension in which ciphers vary. Another classification has been mentioned in a previous section in the current chapter, which does include public-key systems:

Also, the first classification is based on what happens to the plaintext. But when stream ciphers are classified, while the classification takes into account what changes as successive letters or bits or blocks are enciphered, how the changes are determined was ignored. A classification of stream ciphers based on this dimension might look like this:

however, this classification is little more than a listing of methods commonly in use.

Another way of looking at the deficiency in the first classification that this one attempts to remedy is that the first classification is based on the operation performed directly on the plaintext to produce ciphertext.

But that operation may not be the principal difficulty in cracking a cipher.

For example, let us compare these two ciphers:

Thus, all the workings of a cipher need to be considered in classifying it.

I originally composed the classification of ciphers which appears first on this page to illustrate my own view of how cipher systems should be categorized, as a response to the classification entitled "A Cipher Taxonomy", by Terry Ritter. That illustrates another approach to the classification of different types of cipher.

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