DES Algorithm

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DES Algorithm

• Data Encryption Standard

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DES Features

• Block cipher, 64 bits per block
• 64-bit key, with only 56 bits effective
• ECB mode and CBC mode

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DES Key Scheduling

• Specify a 64-bit key
• There are eight parity bits
• The key structure is in the next page

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(No Transcript)
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DES Key Scheduling

• Key permutation according to Permuted Choice
  1(PC-1)
• After permutation, the key length will be only 56
  bits
• 64 bits ? 56 bits? Why?
• The explanation is in the next page

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• Permuted Choice 1

Note The 8n-th bit is disappeared!!
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DES Key Scheduling

• We separate the 56-bit key to two 28-bit keys Lk
  and Rk
• Left rotate the Lk and Rk according to SRT
  (Subkey Rotation Table)
• After rotation, merge Lk and Rk
• Permutation according to Permuted Choice 2 (PC-2)
• We get a 48-bit subkey now

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DES Key Scheduling

• Repeat the operation 16 times to get 16 subkeys
• Use rotated Lk and Rk
• Why each subkey has length of only 48 bits
  instead of 56 bits?
• The answer is similar to PC-1

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Subkey Rotation Table
Permuted Choice 2
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DES Key Scheduling Flow Chart

• ltltlt is the operation of SRT
• We will call the subkeys K1, K2, , K16
  later

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Plaintext Preparation

• Cut the plaintext into 64-bit blocks
• Each 64-bit block should be permuted according to
  Initial Permutation Table (IP Table)
• Besides, we make an Inverse Initial Permutation
  Table (IP-1), which has effect opposite to IP

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IP Initial Permutation
IP-1 Inverse Initial Permutation
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DES Core Function

• L0 to L15, R0 to R15
• Feistel (F) function
• Inverse Initial Permutation

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Feistel Function

• Expansion using E-bit Selection Table
• Key Mixing XOR with subkeys
• Substitution S-box
• Permutation P-box

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Feistel Function - Expansion
E-Bit Selection Table

• Copy R0 to a buffer Rb
• Rb 32 bits ? 48 bits

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Feistel Function Key Mixing

• After the expansion operation, just XOR with
  subkey K1

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Feistel Function - Substitution

• 48-bit buffer becomes eight 6-bit blocks
• 8 S-boxes have output from 0 to 15 which can be
  represented with 4 bits
• How do S-boxes work? Wait and see
• 8 4-bit outputs can be merged, and store it in a
  32-bit buffer

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How do S-boxes Work?
Column
Row
Use the Row and Column number to find the
corresponding output number from the
S-box. Besides, the n-th block must use the n-th
S-box.
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S-Box 1 Substitution Box 1
S-Box 2 Substitution Box 2
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S-Box 3 Substitution Box 3
S-Box 4 Substitution Box 4
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S-Box 5 Substitution Box 5
S-Box 6 Substitution Box 6
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S-Box 7 Substitution Box 7
S-Box 8 Substitution Box 8
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Feistel Function - Permutation

• After the substitution, do permutation according
  to the P-box

P Permutation
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DES Core Function

• After one round of Feistel Function is completed,
  the buffer is XORed with L, becoming the next R
• Previous R becomes the next L
• Repeat the operations mentioned above 16 times

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DES Core Function

• When 16 rounds are finished, L and R are swapped
  and merged, then becomes a 64-bit pre-output
• Use IP-1 to become the final cipher output

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DES - Decryption

• Almost all operations are the same as those of
  encryption
• Only one is different use the subkeys in
  descending order

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DES Other Form

• Triple-DES quite literally. A Triple-DES key has
  length of 192 bits. It can be considered three
  64-bit DES key.
• DES40 a key is pre-processed to make it a 40-bit
  effective key

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DES - Modes

• ECB (Electronic Code Book)
• CBC (Cipher Block Chaining)

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DES Cryptanalysis