Chapter: Cryptography and Network Security

Stream Ciphers

1. Stream Cipher Structure 2. Stream Cipher Properties

STREAM CIPHERS

 

·        process message bit by bit (as a stream)

 

·        have a pseudo random keystream

 

·        combined (XOR) with plaintext bit by bit

 

·        randomness of stream key completely destroys statistically properties in message

 

Ci = Mi XOR StreamKeyi

 

·        but must never reuse stream key

 

·        otherwise can recover messages (cf book cipher)

 

1. Stream Cipher Structure


 

 

2. Stream Cipher Properties

 

§  some design considerations are:

o     long period with no repetitions

 

o     statistically random

 

o     depends on large enough key

 

o     large linear complexity

 

§  properly designed, can be as secure as a block cipher with same size key

 

§  but usually simpler & faster


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