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

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

One DES cycle(total 16)

Slide 5

Slide 6

Slide 7 & 8

Slide 9

Initial permutation 16 cyclesInverse initial permutation

Combines permutation and substitution, including key

4

Permutation General idea: rearrange the characters

Example in class: Columnar transposition

Write the plain text in 5 columns

M A R Y L

A N D W V

U G A M E

I S T O N

I G H T X

Cipher text - read by column

mauii angsg rdath ywmot lvenx

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

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

S-Box Substitution, Choice

Start with 48 bits ( ) Divide them in 8 blocks, each long 6 bitsFor 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)

KeyR j 1

)81( iBi

Output from each block 4 bits

Inpu

t in

eac

h bl

ock

6 bi

ts

See the example on the next slide

8

1001

01

}010011{},,,,,{

5432

61

6543217

bbbb

bb

bbbbbbB

row 1

column 9

In block S7 choose row 1, column 9The output is 3, that is, 0011 binary (4 bits)

Example:The input to block S7 is 010011 (6 bits)

S-Box look-up table &Example

9

P- box Permutation

Bits Goes to position

1 - 8 9 17 23 31 13 28 2 18

9 – 16 24 16 30 6 26 20 10 1

17 – 24 8 14 25 3 4 29 11 19

25 - 32 32 12 22 7 5 27 15 21

Straight permutation: Each input bit is moved to a new position in the output

Rearrangement used in DES