RC4 algorithm for microcontrollers

micro_rc4.c

#include "micro_rc4.h"

static char rc4_state[256];
static unsigned char rc4_i, rc4_j;

__attribute__((always_inline)) inline
static void rc4_swap(void)
{
    unsigned char rc4_tmp = rc4_state[rc4_i];
    rc4_state[rc4_i] = rc4_state[rc4_j];
    rc4_state[rc4_j] = rc4_tmp;
}

void rc4_init(const char *key, unsigned char len)
{
    rc4_i = 0;
    do rc4_state[rc4_i] = rc4_i;
    while (++rc4_i != 0);

    rc4_i = rc4_j = 0;
    do {
        rc4_j += rc4_state[rc4_i] + key[rc4_i % len];
        rc4_swap();
    } while (++rc4_i != 0);

    rc4_i = rc4_j = 0;
}

char rc4_next()
{
    rc4_i++;
    rc4_j += rc4_state[rc4_i];
    rc4_swap();
    return rc4_state[(rc4_state[rc4_i] + rc4_state[rc4_j]) & 255];
}

micro_rc4.h

#ifndef MICRO_RC4_H_DDCF63B50CD9256B03223D3F947427E50DE494A5
#define MICRO_RC4_H_DDCF63B50CD9256B03223D3F947427E50DE494A5

#include <limits.h>

#if CHAR_BIT != 8
#error CHAR_BIT != 8 not supported
#endif

void rc4_init(const char *key, unsigned char len);
char rc4_next();

#endif /* MICRO_RC4_H_DDCF63B50CD9256B03223D3F947427E50DE494A5 */