Introduction to Cryptography with Open-Source Software

192 Cryptography with Open-Source Software

Key

Key

L

i

L

i+1

R

i

R

i+1

Expansion perm.

S-b ox substitution

P-box pe rmutation

Shift Shift

Compression perm.

f

FIGURE 8.14: One round of DES.

Block ciphers and the data encryption st andard 193

For example: bit 58 in the plaintext is shifted to position 1; bit 50 in the

plaintext is shifted to pos itio n 2; bit 42 is shifted to position 3, and so on;

and conversely: bit 1 in the plaintext is shifted to position 40; bit 2 in the

plaintext is shifted to positio n 8; bit 3 is shifted to position 48, and so on.

The final permutation is simply the inverse of this; so bit 1 goes to position

58; bit 2 goes to position 50; bit ...