Introduction to Stream Cipher

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# Introduction to Stream Cipher - PowerPoint PPT Presentation

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 toStream Cipher

[email protected]

Spring 2004

Data transfer algorithm
• Source  “Plain text” ABCD…
• Source coding  100101001010111…
• Encryption
• Channel coding
• Data Transfer
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

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
• 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
• 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

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