Data Encryption Standard (DES) An example of secret key (i.e., symmetric) encryption system

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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.

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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
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One DES cycle (total 16)
Initial permutation 16 cycles Inverse initial
permutation
Combines permutation and substitution, including
key
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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
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Expansion permutation
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Permuted Choice
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S-Box Substitution, Choice
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S-Box look-up table Example
Example The input to block S7 is 010011 (6 bits)
In block S7 choose row 1, column 9 The output is
3, that is, 0011 binary (4 bits)
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P- box Permutation
Straight permutation Each input bit is moved to
a new position in the output
Rearrangement used in DES
Bits Goes to position Goes to position Goes to position Goes to position Goes to position Goes to position Goes to position 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