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DES in CNS-2

DES (Data Encryption Standard) is a symmetric-key cryptosystem developed by IBM in the 1970s, utilizing a 64-bit block size and a 56-bit key for encryption. It employs confusion and diffusion techniques through 16 rounds of transposition and substitution to secure data. The encryption process involves initial and inverse permutations, key scheduling, and the use of S-boxes for substitution, making it difficult for attackers to decipher the key or detect patterns.

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46 views6 pages

DES in CNS-2

DES (Data Encryption Standard) is a symmetric-key cryptosystem developed by IBM in the 1970s, utilizing a 64-bit block size and a 56-bit key for encryption. It employs confusion and diffusion techniques through 16 rounds of transposition and substitution to secure data. The encryption process involves initial and inverse permutations, key scheduling, and the use of S-boxes for substitution, making it difficult for attackers to decipher the key or detect patterns.

Uploaded by

santhoshr3669
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© © All Rights Reserved
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DES (Data Encryption Standard):

DES uses a 64-bit block for encryption and decryption.


Sixteen rounds of transposition and substitution are
performed to achieve DES.

HISTORY:
• DES was developed by IBM in the early 1970s.
• In 1973, NIST (National Institute of Standards and
Technology) requested proposals for a national
symmetric-key cryptosystem.
• IBM proposed a modification of their Lucifer cipher,
which was accepted as DES (Data Encryption
Standard).

DES uses two basic techniques of cryptography:


• Confusion
• Diffusion

CONFUSION:
• Each bit of the ciphertext depends on multiple parts of
the key.
• Makes it hard to find the relationship between the key
and ciphertext.
• Achieved using substitution (S-boxes) in DES.
• Prevents attackers from easily guessing the key.

DIFFUSION:
• Changing one bit in the plaintext affects many bits in the
ciphertext.
• Ensures that small changes in input create big
differences in output.
• Achieved using permutations and XOR operations in
DES.
• Makes it harder for attackers to detect patterns in
encryption.
Plaintext:
• The original message or data that needs to be
encrypted.
• In DES, plaintext is divided into 64-bit blocks for
encryption.

Key:
• DES uses a 56-bit secret key for encryption and
decryption.
• The same key is used for both encryption and
decryption (symmetric encryption).

Rounds:
DES performs 16 rounds of encryption.
Each round includes:
• Expansion
• X-OR
• Substitution
• Permutation
• Swaping
Working:
• 64-bit plaintext and 56-bit key as input.
• Initial Permutation (IP) rearranges plaintext.
• Key undergoes shifts to generate 16 subkeys.
• 16 rounds of encryption with expansion, XOR, S-Boxes,
permutation, and swapping.
• 32-bit swap after 16 rounds.
• Inverse Initial Permutation (IP⁻¹) gives 64-bit ciphertext.

Working:
• 32-bit left (L) and right ® halves from the previous round.
• Right half undergoes expansion/permutation (E table) to
48 bits.
• XOR operation with the subkey (K).
• Substitution using S-boxes (48 bits → 32 bits).
• Permutation (P) applied to scrambled output.
• XOR with the left half.
• Left half (L) and new right half ® form the next round
input.
• Key schedule: 28-bit halves (C and D) undergo left
shifts.
• Permutation Choice 2 (PC-2) generates the next subkey
(K).

S-Box:
DES uses 8 S-boxes to perform substitution.
Each S-box takes a 6-bit input and produces a 4-bit output.
The S-boxes introduce confusion into the encryption
process.

Evaluation Criteria for DES:


• Security
• Avalanche Effect
• Completeness
• Multiple DES (Double/Triple DES)

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