Lecture 8 of Computing Security 1 Course for 4th clas

Lecture 8 overview:
20. Fundamental of Block ciphers.
21. Data Encryption Standard DES (Part 1).
20. Fundamental of Block ciphers
The basic scheme of a block cipher is depicted as follows:
Figure (11): basic scheme of a block cipher.
The size of block is fixed in the given scheme. The choice of block size
does not directly affect to the strength of encryption scheme. The strength of
cipher depends up on the key length.
According to Shannon, there are two basic approaches to encryption:
Confusion encryption operation where the relationship between cleartext
and ciphertext is obscured (e.g. Ceaser cipher where main operation is
substitution), Diffusion encryption by spreading out the influence of one
cleartext letter over many ciphertext letters (e.g. message reversal
permutation).
Combining confusion with diffusion is a common practice for obtaining a
secure scheme is called Product cipher. Data Encryption Standard (DES)
is a good example of that. See figure (12) below:
Computing Security 1 Lecture 8
Page 2
Figure (12): Example of combining confusion with diffusion (product)
Feistel Cipher is not a specific scheme of block cipher. It is a design model
from which many different block ciphers are derived. DES is just one example
of a Feistel Cipher. A cryptographic system based on Feistel cipher structure
uses the same algorithm for both encryption and decryption. The encryption
process uses the Feistel structure consisting multiple rounds of processing of
the plaintext, each round consisting of a “substitution” step followed by a
permutation step.