GATE (Graduate Aptitude Test in Engineering) Computer Science & IT (CS) Computer Networks-Network Security - Public Key and Private Key Cryptography Study Material (Page 5 of 9)

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Design of Block Cipher

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Number of Rounds

  • More number of rounds, difficult for cryptanalytic attack.
  • Even if function F is easy to break, a greater number of rounds make cipher strength high.

Function F Design

  • F adds confusion. F must be difficult to evaluate through linear equations.
  • F must produce avalanche effect. One-bit change produces change in several bits. F must follow bit independence criteria. Bit i is flipped for any i, j, k then output bit j and k should be changed independently.

Key Generation

  • Generate one subkey for each round.
  • Subkeys should be derived in such way that individual subkeys cannot be det…

… (12 more words) …

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DES (Data Encryption Standard)

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  • DES was issued in 1977 by the National Bureau of Standards Now National Institute of Standards and Technology (NIST)
  • In Data Encryption Algorithm, Data is encrypted in the block of 64 bits and key length is 56 bits. The output is of 64 bits.
  • For Decryption, the same keys are used in reverse.

DES Encryption

DES Encryption

Overview of Encryption Process

  • 64-bit plaintext passes through an Initial Permutation (IP) . The input bits are rearranged and permuted input is generated.
  • Then 16 rounds are performed with same function involving permutation and substitution.
  • The output is of 64 bits. The left and right halves are swa…

… (93 more equations, 35 figures) …

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