Transcript Lecture #2 - Universitetet i Bergen

Introduction to Stream Cipher
Sayed Mahdi Mohammad Hasanzadeh
[email protected]
Spring 2004
Why we need to
Cryptography ?
Data transfer algorithm
1)
2)
3)
4)
5)
Source  “Plain text” ABCD…
Source coding  100101001010111…
Encryption
Channel coding
Data Transfer
Why we need to Cryptography
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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
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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
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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(ai=0) =Pr(ai=1) =1/2
Pr(Ci=0) =Pr(Ci=1)
=1/2
Generator Properties
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Randomness
Provable security
Bit rate
Key length
Complexity of algorithm
Memory
Resistant against every attack