This explanation shows the confusion property of S-DES in a
dramatic, graphical manner and explains the property concisely.
This is a graph of the probability that the 8th bit of a 10-bit SDES key is a “1” as a function of plaintext and ciphertext. It was generated by brute force, by trying every possible plaintext and every possible key. You would not want to do this with full DES. This is actually just a 32x32 piece of a 256x256 graph. There are some plaintext/ciphertext pairs that no key could possibly produce. In this case it was assumed that the probability of a “1” in the key bit would be 0.5.
The property
of confusion says that given information about the plaintext and ciphertext,
it is still very hard to find statistical information about the key used for
encryption. In other words, confusion
says that you won’t be able to make sense of this graph and come up with a formula
like P(x, y) = some kind of Fourier series or something.
Because the more sense you make of this graph the better cryptanalysis
attack you’ll be able to mount.
For a good background reading on S-DES and confusion see
Chapter 3 of Cryptography
and Network Security: Principles and Practice by William Stallings
This was created as part of the Cryptography Module
of NSF Award No. 0113627: "Increasing
Security Expertise in Aviation-oriented Computing Education: A Modular Approach",
at Embry-Riddle Aeronautical University in Prescott, Arizona.
