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Introduction to Stream Cipher. Sayed Mahdi Mohammad Hasanzadeh [email protected] Spring 2004. Why we need to Cryptography ?. Data transfer algorithm. Source  “ Plain text” ABCD… Source coding  100101001010111… Encryption Channel coding Data Transfer.

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introduction to stream cipher
Introduction toStream Cipher

Sayed Mahdi Mohammad Hasanzadeh

[email protected]

Spring 2004

data transfer algorithm
Data transfer algorithm
  • Source  “Plain text” ABCD…
  • Source coding  100101001010111…
  • Encryption
  • Channel coding
  • Data Transfer
why we need to cryptography
Why we need to Cryptography
  • We have redundancy in every language Pr(A)  Pr(B)  Pr(C)  … Pr(Z)
  • Plain text redundancy leakage to Source Coding => Pr(1)  Pr(0)  ½
  • Attacker guesses some of the bites from sequence
  • Then attacker guesses the plain text
goals in cryptography
Goals in cryptography

1) Cryptography remove the redundancy from sequence

Pr(1)=Pr(0)=1/2

2) Cryptography is a mapping in the sequence space

3)Cryptography provide security in some cases: data integrity, Authentication and identification ,…

tools in cryptography
Tools in Cryptography
  • Symmetric encryption
  • Hash functions
  • Message authentication codes
  • Random number generators
  • Public-key encryption
  • Digital signatures
  • Authentication and identification
  • Key establishment, management, and certification
symmetric encryption
Symmetric Encryption
  • There are basically two ways to make a symmetric cipher:
    • Stream cipher: The encryption rule depends on the plaintext symbol’s position in the stream of plaintext symbols.
      • e.g.: Vigenere, RC4, A5
    • Block cipher: Encrypt several plaintext symbols at once in a block.
      • e.g.: DES, AES, Twofish, RC6
stream cipher
Stream Cipher

Pr(Pi=0) Pr(Pi=1) 1/2

Pr(Ci=0) =Pr(Ci=1) =1/2

Pr(ai=0) =Pr(ai=1) =1/2

generator properties
Generator Properties
  • Randomness
  • Provable security
  • Bit rate
  • Key length
  • Complexity of algorithm
  • Memory
  • Resistant against every attack
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