Chapter: Cryptography and Network Security

RC4

1. RC4 Key Schedule 2. RC4 Encryption 3 RC4 Security

RC4

 

o   a proprietary cipher owned by RSA DSI

 

o   another Ron Rivest design, simple but effective

 

o   variable key size, byte-oriented stream cipher

 

o   widely used (web SSL/TLS, wireless WEP)

 

o   key forms random permutation of all 8-bit values

 

o   uses that permutation to scramble input info processed a byte at a time

 

1. RC4 Key Schedule

 

·        starts with an array S of numbers: 0..255

 

·        use key to well and truly shuffle

 

·        S forms internal state of the cipher

 

for i = 0 to 255 do S[i] = i

 

T[i] = K[i mod keylen])

 

j = 0

 

for i = 0 to 255 do

 

j = (j + S[i] + T[i]) (mod 256)

swap (S[i], S[j])

 

2. RC4 Encryption

 

·        encryption continues shuffling array values

 

·        sum of shuffled pair selects "stream key" value from permutation

 

·        XOR S[t] with next byte of message to en/decrypt

 

i = j = 0

 

for each message byte Mi

 

i = (i + 1) (mod 256)

 

j = (j + S[i]) (mod 256) swap(S[i], S[j])

 

t = (S[i] + S[j]) (mod 256)

 

Ci = Mi XOR S[t]

 


 

3 RC4 Security

 

·        claimed secure against known attacks ü have some analyses, none practical

 

·        result is very non-linear

 

·        since RC4 is a stream cipher, must never reuse a key

 

·        have a concern with WEP, but due to key handling rather than RC4 itself.


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