RC4 is a stream cipher that produces a keystream byte at each step. The algorithm consists of a self-modifying lookup table.

Suppose there are N key bytes, key[0],key[1],...,key[n-1]. Then the RC4 algorithm can be described as follows.

Initialize:

     for i = 0 to 255
          S[i] = i;
          K[i] = key[i (mod N)];
     next i

     j = 0;
     for j = 0 to 255
          j = (j + S[i] + K[i]) mod 256;
          swap(S[i], S[j]);
     next j

     i = j = 0;


For each keystream byte:

     i = (i + 1) mod 256;
     j = (j + S[i]) mod 256;
     swap(S[i], S[j]);
     t = (S[i] + S[j]) mod 256;
     keystreamByte = S[t];

Note: The first 256 keystream bytes must be discarded.