Chapter: Cryptography and Network Security


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



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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