Computer Security and Cryptography

8.13 RC4

Designed in 1987 by Ronald Rivest of RSA Data Security, RC4 is a member of the suite of encipherment algorithms available in the Secure Socket Layer (Chapter 18). It provides security for wireless communications in Wired Equivalent Privacy (WEP), a protocol for wireless local area networks as defined in the IEEE 802.11b Standard.

RC4 generates a pseudorandom key stream consisting of a sequence of (8-bit) bytes. RC4 was a trade secret until 1994 and its name is still regarded as proprietary. RC4 has two components:

KSA – a key scheduling algorithm, which loads a key register with a permutation on integers 0 to 255; the key length varies from 40 to 128 bits;

PRGA – a pseudorandom number generator producing one 8-bit byte of key on each call of the generator.

8.13.1 The RC4 Algorithm

Key Scheduling Algorithm (KSA)

1. Input

   L bytes of key

   k(1) = (k(0), …, k(L − 1)) with

2. Initialization

   for i := 0 to 255 do

   S[i] := i;

3. Generation

   j := 0

   for i := 0 to 255 do

   j := (j + S[i] + k(i(modulo L))) (modulo 256);

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

TABLE 8.10 Key Register Cell Swapping in RC4

Example 8.9

If k = (4, 11, 18, 25, 32,…), the first six steps in KSA are given in Table 8.10. The contents of cells in the key register change; for example, the contents of S[4] undergo three changes during the KSA ...