1. 1 Data Encryption Standard (DES) An example of secret key (i.e., symmetric) encryption system Note: These figures are meant to help illustrating a few points. (The goal is to save time in reproducing large tables & figures on the board and coping them in the notebooks.) There are other important points covered in class that are not given in the following slides. Please refer to the lectures and the text book for details.

2. 2 DES is a complex combination of substitution and transposition (CONFUSION and DIFFUSION) repeatedly applied, one on top of the other, for total of 16 cycles Cycle 1 Cycle 2 Cycle 16

3. 3 One DES cycle (total 16) Slide 5 Slide 6 Slide 7 & 8 Slide 9 Initial permutation 16 cycles Inverse initial permutation Combines permutation and substitution, including key

4. 4 Permutation General idea: rearrange the characters Example in class: Columnar transposition Write the plain text in 5 columns MARYL ANDWV UGAME ISTON IGHTX Cipher text - read by column mauii angsg rdath ywmot lvenx

5. 5 Expansion permutation General idea: rearrange the bits & repeat certain bits Pattern used in each cycle of DES to expand the right block from 32 bits to 48 bits

6. 6 Permuted Choice General idea: rearrange the bits & omit certain bits Pattern used in each cycle of DES to shorten the sifted key from 56 bits to 48 bits

7. 7 S-Box Substitution, Choice Start with 48 bits ( ) Divide them in 8 blocks, each long 6 bits For each block using the look-up table on the next slide produce 4 bits End result is 8 blocks, each long 4 bits (total 32 bits output from the S-box) Output from each block 4 bits Input in each block 6 bits See the example on the next slide

8. 8 row 1 column 9 In block S 7 choose row 1, column 9 The output is 3, that is, 0011 binary (4 bits) Example: The input to block S 7 is 010011 (6 bits) S-Box look-up table & Example

9. 9 P- box Permutation BitsGoes to position 1 - 891723311328218 9 – 1624163062620101 17 – 248142534291119 25 - 3232122275271521 Straight permutation: Each input bit is moved to a new position in the output Rearrangement used in DES