Implementation error in the ARCFOUR cipher

[molnarg]

The RC4 cipher generates a constant byte as key stream after a while (~160 characters). The reason is that i and j are initialized to 0 every time arcfour_generate_stream() is called, but they should be part of the global state initialized to 0 only once.

Pseudocode of RC4 PRG algorithm:

i := 0
j := 0
while GeneratingOutput:
    i := (i + 1) mod 256
    j := (j + S[i]) mod 256
    swap values of S[i] and S[j]
    K := S[(S[i] + S[j]) mod 256]
    output K
endwhile

The implementation in this repo:

void arcfour_generate_stream(BYTE state[], BYTE out[], size_t len)
{
    int i, j;
    size_t idx;
    BYTE t;

    for (idx = 0, i = 0, j = 0; idx < len; ++idx)  {
        i = (i + 1) % 256;
        j = (j + state[i]) % 256;
        t = state[i];
        state[i] = state[j];
        state[j] = t;
        out[idx] = state[(state[i] + state[j]) % 256];
    }
}