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Introduction to Stream Cipher

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Sayed Mahdi Mohammad Hasanzadeh

Hasanzadeh@Raymandcrypto.ir

Spring 2004

Why we need to Cryptography ?

- 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

- Stream cipher: The encryption rule depends on the plaintext symbol’s position in the stream of plaintext symbols.

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