The ciphers in this chapter are intricate, and yet in most cases they still seem to be lacking something compared to those we met in Chapter 2 on rotor machines. Some of my own designs in the section on the Quadibloc series of ciphers do attempt to recapture just that something lacking, with the result that they are extremely elaborate designs.
In DES, the basic cipher operation is in essence a degenerate form of the autokey. Each half of the block modifies the other half alternately with several repetitions.
Yet, the number of the repetitions alone at least seems to have the effect of concealing the key.
Also, the standard modes proposed for using DES all seem to be designed to avoid increasing its security, except for the removal of trivial vulnerabilities. It would seem that combining DES with some form of stream cipher, even a fairly poor one, would also remove those vulnerabilities at little cost, with additional benefits in security.
Also, it is time to note something about fashions in ciphers, adopted for reasons other than security. Although the reasons are not really quite so whimsical as the term fashion implies, as they are instead connected to the practicalities of using the ciphers in the real world.
Stream ciphers are usually viewed as sources of bits which appear random to be XORed with the plaintext. They are very poor relations to block ciphers at present.
Transposing the bytes of a message, perhaps between two encryptions in DES in ECB mode, would probably produce a highly secure cipher, especially if the transposition is both controlled by a secret key, and yet varies with each message. But this is an option seldom considered.
This is because for many applications it is important that occasional communications errors, even if they corrupt somewhat larger areas in a message because of the use of encryption, not totally obliterate the entire message. So, only those encryption modes and methods that have a limited level of error propagation are considered.
Some of the techniques we have seen in this chapter will now be explored in a number of conceptual designs, some of them perhaps a bit over-elaborate.
Table of Contents