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

#define TRUE 1
#define FALSE 0
#define BLOCKS 20

struct avirus {
	char code[8];
	int bytes;
	unsigned long lowcrc;
	int crcs[BLOCKS];
} antivir = { "CAPRIO\xDF\0" };

main (int argc, char *argv[])
{
	int crc;
	char fname[25];
	char *buff, *buffp;		/* Pointers to data buffer */
	int j1, j2 = 0, j3;
	int found = FALSE;		/* TRUE if patch code found */
	int bsize;				/* Size of CRC block */
	int rsize;				/* Amount of data read into buffer */
	int sector = 0, offset;		/* Position of patch code */
	long fl;				/* File length */
	FILE *fn;

	/* Open executable file for reading and writing */
	if (argc == 2)
		strcpy(fname, argv[1]);
	else {
		printf("Filename: ");
		gets(fname);
	}

	if ((fn = fopen(fname, "rb+")) == NULL) {
		printf("File not found\n" );
		exit(1);
	}

	/* Query user for size of CRC block */
	fl = filelength(fileno(fn));
	do {
		printf ("\nFile size is %ld, max blocks is %d\n", fl, BLOCKS);
		printf ("Lower limit on bytes per block is %d\n", fl/BLOCKS+1);
		printf ("\nEnter number of bytes per block: ");
		scanf ("%d", &bsize);
	} while ((bsize < fl/BLOCKS+1) || (bsize > fl));

	/* Find the string that precedes the patch area */
	buff = calloc(1024, sizeof(char));
	rsize = fread(buff, sizeof(char), 512, fn);

	while (!eof(fileno(fn)) && !found) {
		sector++;
		rsize = fread(&buff[512], sizeof(char), 512, fn);
		for (offset=0; offset < 512; offset++)
			if (strncmp(&buff[offset], antivir.code, 
				strlen(antivir.code)) == 0) {
			       found = TRUE;
			       break;
			}
		memcpy(buff, &buff[512], 512);
	}

	/* Search the last sector read if patch area not yet found */
	if (!found) {
		sector++;
		for (offset=0; offset < rsize - strlen(antivir.code); offset++)
			if (strncmp(&buff[offset+512], antivir.code, 
						strlen(antivir.code)) == 0) {
				found = TRUE;
				break;
			}
	}

	if (found)
		printf("Inserting antivirus data...\n\n");
	else {
		printf("\nPatch area not found in file\n");
		printf("No antivirus data inserted\n");
		exit(1);
	}

	/* Calculate CRC values and write them into the executable */
	free(buff);
	buff = calloc(bsize, sizeof(char));
	rewind(fn);

	do {
		rsize = fread (buff, sizeof(char), bsize, fn);
		buffp = buff;
		crc = 0;
		for (j1 = 0; j1 < rsize; j1++)
			crc = crc_update (crc, *(buffp++));
		crc = crc_finish(crc);
		antivir.crcs[j2++] = crc;
		printf ("CRC for block %d was %04x\n", j2, crc);
	} while (rsize == bsize);

	antivir.bytes = bsize;
	antivir.lowcrc = (sector-1) * 512 + offset;
	fseek (fn, antivir.lowcrc, SEEK_SET);
	fwrite(&antivir, sizeof(struct avirus), 1, fn);
	fclose(fn);
}

crc_update(int crcval, char crc_char)
{
	long tmp;
	int  j1;

	tmp = ((long)crcval << 8) + crc_char;
	for (j1 = 0; j1 < 8; j1++) {
		tmp = tmp << 1;
		if(tmp & 0x01000000)
			tmp = tmp ^ 0x01800500;
	}
	return((tmp & 0x00ffff00) >> 8);
}

crc_finish(int crc)
{
	return(crc_update(crc_update(crc,'\0'), '\0'));
}