rc4

Makefile

rc4 : rc4.c
	gcc -funsigned-char -o $@ $^

rc4.c

/*
 * build with: gcc -funsigned-char -o test test.c
 */

#include <stdio.h>
#include <string.h>

#define rc4(d,l,t,m) \
{char s[256]="";int i=-1,j=0,x;for(;++i<256;)s[i]=i;for(i=-1;++i<256;){x=s[i];s[i]=s[j=(j+s[i]+d[i%l])%256];s[j]=x;}i=0;j=0;while(m--){i++;i&=255;x=s[i];s[i]=s[j=(j+x)%256];s[j]=x;*t++^=s[(s[i]+s[j])%256];}}

int main(void)
{
        char key[] = "abcdefghijklmnop";
        char plaintext[]="now is the time for all good men";
        char *t=plaintext;
        int m=strlen(plaintext);
        int n=m;
        printf("plaintext: \"%s\"\n", plaintext);
        rc4(key, strlen(key), t, m);
        printf("plaintext: \"%s\"\n", plaintext);
        t=plaintext;
        rc4(key, strlen(key), t, n);
        printf("plaintext: \"%s\"\n", plaintext);
        return 0;
}