Introduction to stream cipher
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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

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Introduction to stream cipher

Introduction toStream Cipher

Sayed Mahdi Mohammad Hasanzadeh

[email protected]

Spring 2004


Introduction to stream cipher

Why we need to Cryptography ?


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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