m4c2m4a - Tap Tap m4c decrypter

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

typedef unsigned char u8;
#define HEADSIZE 256

#define BUFLEN (1<<16)
static u8 buf[BUFLEN];

static u8 key[8],head[HEADSIZE];
int m4c2m4a(const int argc, const char **argv){
    if(isatty(fileno(stdin))||isatty(fileno(stdout))){
        fprintf(stderr,"m4c2m4a <in.m4c >out.m4a\n");
        return 1;
    }

    int i,j,readlen;
    fread(head,1,HEADSIZE,stdin);

    //determine other than byte3
    memcpy(key,head,8);
    for(i=0;i<8;i++)key[i]^="\0\0\0?ftyp"[i];

    //determine byte3
    u8 *p=head;
    for(i=0;i<HEADSIZE/8;i++)
        for(j=i+1;j<HEADSIZE/8;j++)
            if(!memcmp(p+(i*8),p+(j*8),8))goto done;
    fprintf(stderr,"key find failed\n");
    return 2;

done:
    key[3]=p[j*8+3];
    fprintf(stderr,"key=");
    for(i=0;i<8;i++)fprintf(stderr,"%02x",key[i]);
    fprintf(stderr,"\n");

    //decrypt head
    for(i=0;i<HEADSIZE;i++)head[i]^=key[i%8];
    fwrite(head,1,HEADSIZE,stdout);

    //decrypt remaining data
    for(;readlen=fread(buf,1,BUFLEN,stdin);){
        for(i=0;i<readlen;i++)buf[i]^=key[i%8];
        fwrite(buf,1,readlen,stdout);
    }
    return 0;
}