/*
 * piggyback.c (c) 2005 by Torben H. Nielsen <torben@t-hoerup.dk>
 *
 * requires openssl-dev
 * compile with gcc -o piggyback -l crypto piggyback.c
 *   (perhaps with -O3)
 *
 * return values:
 *    0: success
 *    1: could not open file
 *    2: md5 mismatch
 */

/*****************************************************************
 * ToDo:
 *  V At detach, remove() outfile if data md5 mismatch is detected
 *  V detach - read options field
 *  - ReWrite commandline args handling
 *  - Compression
 *  - Encryption/Decryption
 */
 
 
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <openssl/md5.h>
#include <sys/stat.h>

#include "gzcompress.h"

#define CHUNK_SIZE 512


void printusage( void ) {
	
	printf("piggyback by hoerup\n");
	printf("Usage: piggyback [options] attach <carrier-file> <payload-file> <output-file>\n");
	printf("       piggyback detach <carrier-file> <output-file>\n\n");
	printf("Options:\n");
	printf("     --compress:    use gzcompression on payload data\n");
	printf("\n");
	
	exit(0);
}

int main(int argc, char *argv[]) {
	FILE *carrierfile, *payloadfile, *outfile;
	int carrieridx;
	
	unsigned char buf[CHUNK_SIZE];
	unsigned char md5sum[16];
	unsigned char md5string[33];
	unsigned char options[8];
	unsigned char tmpstr[40];
	unsigned char header[80];
	
	char *ptr;
	int num, bytes_extracted;
	int pointer, headerpointer;
	int i;
	MD5_CTX md5;
	
	if(  !( (argc ==4  && strcmp(argv[1],"attach") ) || (argc == 5 && strcmp(argv[1],"detach")) ) ) {
		printusage();
	}
	carrieridx = 2;
	
	//create output-file with 644 permissions
	umask(022);

	if ( strcmp(argv[1], "attach")==0 ) { //attach
		carrierfile = fopen(argv[carrieridx], "r");
		payloadfile = fopen(argv[carrieridx+1], "r");
		outfile = fopen(argv[carrieridx+2], "w");

		if (carrierfile == 0) {
			printf("Could not open carrier file (%s) for reading\n", argv[carrieridx]);
			return 1;
		}
		if (payloadfile == 0) {
			printf("Could not open payload file (%s) for reading\n", argv[carrieridx+1]);
			return 1;
		}
		if (outfile == 0) {
			printf("Could not open output file (%s) for writing\n", argv[carrieridx+2]);
			return 1;
		}
		
		for (i=0;i<8;i++)
			options[i] = 0;

		pointer=0;
		while (!feof(carrierfile)) {
			num = fread(buf, 1, CHUNK_SIZE, carrierfile);
			fwrite(buf, 1, num, outfile);
			pointer += num;
		}

		MD5_Init(&md5);

		while (!feof(payloadfile)) {
			num = fread(buf, 1, CHUNK_SIZE, payloadfile);
			fwrite(buf, 1, num, outfile);
			MD5_Update(&md5, buf, num);
		}

		MD5_Final(md5sum, &md5);

		for (i=0; i<16; i++) 
			fprintf(outfile, "%02x", md5sum[i]);

		sprintf(header, "%08x", pointer);

		for (i=0; i<8; i++) {
			sprintf(tmpstr, "%c", options[i]+48); //ascii(48)='0'
			strcat(header, tmpstr);
		}


		fprintf(outfile, header, 16);
		
		MD5_Init(&md5);		
		MD5_Update(&md5, header, 16);
		MD5_Final(md5sum, &md5);

		for (i=0; i<16; i++)
			fprintf(outfile, "%02x", md5sum[i]);

		fclose(carrierfile);
		fclose(payloadfile);
		fclose(outfile);
		
	} else { //detach
		carrierfile = fopen(argv[carrieridx], "r");
		outfile = fopen(argv[carrieridx+1], "w");


		if (carrierfile == 0) {
			printf("Could not open carrier file (%s) for reading\n", argv[carrieridx]);
			return 1;
		}

		if (outfile == 0) {
			printf("Could not open output file (%s) for writing\n", argv[carrieridx+1]);
			return 1;
		}
		//header size is 80 bytes
		fseek(carrierfile, -80, SEEK_END);
		headerpointer = ftell(carrierfile);
		fread(header, 1, 80, carrierfile);
		
		ptr = header + 32;
		strncpy(tmpstr, ptr, 16); 
		
		MD5_Init(&md5);
		MD5_Update(&md5, tmpstr, 16);
		MD5_Final(md5sum, &md5);
		md5string[0] = 0;

		for (i=0; i<16;i++) {
			sprintf(tmpstr, "%02x", md5sum[i]);
			strcat(md5string, tmpstr);
		}

		ptr += 16;
		strncpy(tmpstr, ptr, 32);
		tmpstr[32] = 0;
		
		if (strncmp(md5string, tmpstr, 32) != 0) {
			printf("Header MD5 checksum mismatch!\n");
			return 2;
		}

		// Read data-pointer
		ptr = header + 32;
		strncpy(tmpstr, ptr, 8);

		tmpstr[8] = 0; //make tmpstr a 0-terminated string

		pointer = strtol(tmpstr, (char **)NULL, 16);
		
		// Read Options fields
		ptr += 8;
		strncpy(options, ptr, 8);
		for (i=0; i<8; i++)
			options[i] -= 48;

		bytes_extracted = 0;
		fseek(carrierfile, pointer, SEEK_SET);
		MD5_Init( &md5 );
		num=CHUNK_SIZE;
		while ( num == CHUNK_SIZE)  {
			num = headerpointer -  pointer;
			num -= bytes_extracted;
			
			if ( num >= CHUNK_SIZE )
				num = CHUNK_SIZE;
			
			fread(buf, 1 , num, carrierfile);
			fwrite(buf, 1,	num, outfile);
			MD5_Update( &md5, buf, num);
			bytes_extracted += num;
		}
		
		fclose(carrierfile);
		fclose(outfile);
		
		MD5_Final(md5sum, &md5);
		md5string[0] = 0;

		for (i=0; i<16; i++) {
			sprintf(tmpstr, "%02x", md5sum[i]);
			strcat( md5string, tmpstr);
		}
		
		strncpy(tmpstr, header, 32);
		tmpstr[32] = 0;
		if (strncmp(md5string, tmpstr, 32) !=0) {
			printf("Payload MD5 checksum mismatch!\n");
			remove( argv[carrieridx+1] );
			return 2;
		}
	}
	
	
	return 0;
}