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MODULE 2

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SYMMETRIC CIPHERS: Traditional Block Cipher structure, Data encryption standard (DES), The AES Cipher.

(Text 1: Chapter 2: Section1, 2, Chapter 4: Section 2, 3, 4)

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Text Books:

  1. William Stallings , “Cryptography and Network Security Principles and Practice”, Pearson

Education Inc., 6th Edition, 2014, ISBN: 978-93-325-1877-3

2. Bruce Schneier, “Applied Cryptography Protocols, Algorithms, and Source code in C”, Wiley

Publications, 2nd Edition, ISBN: 9971-51-348-X.

Reference Books:

1. Cryptography and Network Security, Behrouz A. Forouzan, TMH, 2007.

2. Cryptography and Network Security, Atul Kahate, TMH, 2003.

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Feistel Cipher Structure

  • Horst Feistel devised the feistel cipher
    • based on concept of invertible product cipher
  • partitions input block into two halves
    • process through multiple rounds which
    • perform a substitution on left data half
    • based on round function of right half & subkey
    • then have permutation swapping halves

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Feistel Cipher Structure

  • Block size: larger block sizes mean greater security
  • Key Size: larger key size means greater security
  • Number of rounds: multiple rounds offer increasing security
  • Subkey generation algorithm: greater complexity will lead to greater difficulty of cryptanalysis.
  • Fast software encryption/decryption: the speed of execution of the algorithm becomes a concern

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Henric Johnson

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Feistel Cipher Decryption

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Simplified Data Encryption Standard

The encryption algorithm involves five functions:

  1. An initial permutation (IP);

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  • A complex function labeled fK , which involves both permutation and substitution operations and depends on a key input;

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  • A simple permutation function that switches (SW) the two halves of the data;

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  • The function fK again; And finally

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  • A permutation function that is the inverse of the initial permutation (IP –1).

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Algorithm

Key Generation

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circular left shift (LS-1), or rotation, separately on the first five bits and the second five bits.

K1 = (10100100)

(00001 11000)

(00001 11000)

perform a circular left shift of 2 bit positions on each pair of 5-bit strings

(00100 00011)

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K2 = (01000011)

(00100 00011)

S-DES Encryption

1 & 5) Initial and Final Permutations:

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The Function fK

output of the IP stage = (10111101) and F(1101, SK) = (1110)

(01011101)

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Conventional Encryption Algorithms

  • Data Encryption Standard (DES)
    • The most widely used encryption scheme
    • The algorithm is reffered to the Data Encryption Algorithm (DEA)
    • DES is a block cipher
    • The plaintext is processed in 64-bit blocks
    • The key is 56-bits in length

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Data Encryption Standard

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Initial 64 Bit Data

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Initial Permutation (IP)

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Inverse Initial Permutation (IP-1)

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Single Round of DES Algorithm

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Expansion Permutation (E)

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Permutation Function (P)

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Key Generation

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The Avalanche Effect

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The AES Cipher (Rijndael)

Dr. Joan Daemen and Dr. Vincent Rijmen

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  1. Feistel Structure is not used.
  2. Key Expansion provides Four distinct keywords(128 bits) which serve as round key for each round.
  3. Four Different stages in first 9 rounds: One Permutation and Three Substitution.
  4. The Structure is Quite simple.
  5. Only the AddRoundKey stage makes use of the key. For this reason, the cipher begins and ends with an AddRoundKey stage.
  6. Each stage is easily reversible.

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AddRoundKey Transformation

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AES Key Expansion

KeyExpansion([Key0 to Key15], [w0 to w43])

{

for(i= 0 to 3)

wi🡨Key4i +Key4i+1 + Key4i+2 + Key4i+3

for(i=4 to 43)

{

if(i mod 4 ≠ 0)

wi 🡨wi-1+ wi-4

else

{

t🡨Subword(Rotword(wi-1)) + Rcon[i/4]

wi🡨t + wi-4

}}

}

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