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# Introduction to Stream Cipher - PowerPoint PPT Presentation

Introduction to Stream Cipher. Sayed Mahdi Mohammad Hasanzadeh Hasanzadeh@Raymandcrypto.ir 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 toStream Cipher

Spring 2004

• Source  “Plain text” ABCD…

• Source coding  100101001010111…

• Encryption

• Channel coding

• Data Transfer

• 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

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 ,…

• Symmetric encryption

• Hash functions

• Message authentication codes

• Random number generators

• Public-key encryption

• Digital signatures

• Authentication and identification

• Key establishment, management, and certification

• 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

Pr(Pi=0) Pr(Pi=1) 1/2

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

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

• Randomness

• Provable security

• Bit rate

• Key length

• Complexity of algorithm

• Memory

• Resistant against every attack