PNG_CC/crc.c

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <sodium.h>
#include "CRCLib.h"
#include "crc.h"

const long png_signature[8] = { 0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a };
const long idat_signature = 1229209940;
const long iend_signature = 1229278788;
const int working = 1;

int check_file_header(char *addr) {
	int signature_match = 0;
	for( int i = 0; i < 8; i++ ) {
		if (addr[i] != png_signature[i]) {
			signature_match = 1;
		}
	}
	printf("Sig Match: %d\n", signature_match);
	return signature_match;

}


int check_header_length(unsigned char *addr, long offset) {
	unsigned int res = 0;
	for( int i = 0; i < 4; i++ ) {
		res |= addr[offset+i];
		if (i < 3) {
			res <<= 8;
		}
	}
	return res;
}

unsigned long first_idat(unsigned char *addr) {
	int idat_found = 0;
	unsigned long offset = 8;
	int jump_offset = 0;
	int header_type = 0;
	while(idat_found == 0) {
		jump_offset = check_header_length(addr, offset);
		header_type = check_header_length(addr, offset+4);
		if(header_type == idat_signature) {
			idat_found = 1;
		} else {
			offset = offset + jump_offset + 12;
		}
	}
	return offset;
}

int total_idat(unsigned char *addr) {
	int iend_found = 0;
	int found_idat = 0;
	unsigned long offset = 8;
	int jump_offset = 0;
	int header_type = 0;
	while(iend_found == 0) {
		jump_offset = check_header_length(addr, offset);
		header_type = check_header_length(addr, offset+4);
		if(header_type == iend_signature) {
			iend_found = 1;
		} else {
			if(header_type == idat_signature) {
				found_idat++;
			}
			offset = offset + jump_offset + 12;
		}
	}
	return found_idat;
}

int update_file_crc(unsigned char *addr, unsigned long offset , unsigned int crc_num) {
	int startCRC = 8 + offset + check_header_length(addr, offset);
	unsigned char new_crc;
	for(int i = 0; i < 4; i++) {
		new_crc = crc_num >> (8*(3-i)) & 0xFF;
		addr[startCRC+i] = new_crc;
	}
}

int change_idat_content(unsigned char *addr, char *message, int accuracy, unsigned long offset) {
	printf("Starting IDAT Tranform\n");
	if(accuracy > 4) {
		printf("Warning, accuracy cannot be larger than 4");
		return EXIT_FAILURE;
	}
	if(accuracy > 2) {
		printf("Notice, this could take a long time...");
	}
	if(total_idat(addr) < strlen(message)) {
		printf("Warning, message exceeds IDAT amount");
		return EXIT_FAILURE;
	}

	int idat_length = check_header_length(addr, offset);
	printf("IDAT: %d\n", idat_length);

	int prop_found = 0;
	long size = 1;
	long rounds = 0;
	unsigned int* idat_data = malloc(size * sizeof(unsigned int));
	for(int i = 0; i < idat_length; i++) {
		idat_data[i] = addr[i+offset+8];
		size++;
		int* new_idat_data = realloc(idat_data, size * sizeof(unsigned int));
	}
	unsigned int temp_idat_data[size];
	while(prop_found == 0) {
		for(int i = 0; i < idat_length; i++) {
			temp_idat_data[i] = idat_data[i];
		}
		int r = randombytes_uniform(5) + 1;
		int j = randombytes_uniform(idat_length);
		
		temp_idat_data[j] = (temp_idat_data[j] + r) % 255;
		unsigned char crc_check[size+4];
		unsigned int idat_header[] = { 0x49, 0x44, 0x41, 0x54 };
		for(int i = 0; i < 4; i++) {
			crc_check[i] = idat_header[i];
		}
		for(int i = 0; i < size; i++) {
			crc_check[i] = temp_idat_data[i+4];
		}
		unsigned int crcnum = crc(crc_check, idat_length);
		//printf("New CRC: %08X\n", crcnum);
		//printf("Test: %X\n", crcnum >> (8*3));
		unsigned int checked_crc = crcnum >> (8*3);
		rounds++;
		if(checked_crc == 61) {
			printf("Found %d in %d rounds\n", checked_crc ,rounds);
			printf("Full CRC: %08X\n", crcnum);
			printf("Original: %02X\n", idat_data[j]);
			printf("Change offset: %d to hex: %02X\n", j, temp_idat_data[j]);
			addr[offset+8+j] = temp_idat_data[j];
			update_file_crc(addr, offset, crcnum);
			prop_found = 1;
		}
	}

	return 0;
}

int create_cc_file(unsigned char *addr, unsigned long file_length) {
	FILE *fp;
	fp = fopen("png2.png", "w");

	if(fp == NULL) {
		return EXIT_FAILURE;
	}

	for(int i = 0; i < file_length; i++){
		fputc(addr[i], fp);
	}
	fclose(fp);
}

int main() {
	FILE *fp;
	unsigned int c;
	unsigned char* myArray = calloc(1000, sizeof(unsigned char));
	unsigned long i = 0;
	unsigned long offset = 0;
	char message[1];
	if(sodium_init() == -1) {
		return EXIT_FAILURE;
	}
	fp = fopen("./1.png", "rt");
	while((c = fgetc(fp)) != EOF) {
		myArray[i] = c;
		i++;
	}
	fclose(fp);
	offset = first_idat(myArray);
	change_idat_content(myArray, message, 2, offset);
	create_cc_file(myArray, i);
	//int crcnum = crc(myArray, 19);
	//printf("%08X\n", crcnum);
}