Introduction to Cryptography with Open-Source Software

172 Cryptography with Open-Source Software

S

1

S

1

S

1

S

2

S

2

S

2

S

3

S

3

S

3

S

4

S

4

S

4

P

laintext

Key

K

0

K

1

K

2

K

3

Permutation

Permutation

Permutation

Round 1Round 2Round 3

FIGURE 8.2: A substitution/permutation cipher.

Block ciphers and the data encryption st andard 173

substitution will result in at least half of the output bits changing. This is

in fact not merely desirable, but one of the design characteristics of a good

modern cryptosystem.

Notice how the number of bits that differ increases as the rounds advance.

The number of rounds required for a complete mixing must be enough ...