Cryptography Reference
In-Depth Information
firmed the federal use of the cipher until 1999, when it was finally replaced by the
Advanced Encryption Standard (AES)
.
3.1.1 Confusion and Diffusion
Before we start with the details of DES, it is instructive to look at primitive op-
erations which can be applied in order to achieve strong encryption. According to
the famous information theorist Claude Shannon, there are two primitive operations
with which strong encryption algorithms can be built:
1.
Confusion
is an encryption operation where the relationship between key and
ciphertext is obscured. Today, a common element for achieving confusion is sub-
stitution, which is found in both DES and AES.
2.
Diffusion
is an encryption operation where the influence of one plaintext symbol
is spread over many ciphertext symbols with the goal of hiding statistical proper-
ties of the plaintext. A simple diffusion element is the bit permutation, which is
used frequently within DES. AES uses the more advanced Mixcolumn operation.
Ciphers which only perform confusion, such as the Shift Cipher (cf. Sect. 1.4.3)
or the World War II encryption machine Enigma, are not secure. Neither are ci-
phers which only perform diffusion. However, through the concatenation of such
operations, a strong cipher can be built. The idea of concatenating several encryp-
tion operation was also proposed by Shannon. Such ciphers are known as
product
ciphers
. All of today's block ciphers are product ciphers as they consist of rounds
which are applied repeatedly to the data (Fig. 3.1).
Fig. 3.1
Principle of an
N
round product cipher, where each round performs a confusion and
diffusion operation
Modern block ciphers possess excellent diffusion properties. On a cipher level
this means that changing of one bit of plaintext results
on average
in the change of
