PNG_CC/crc.c

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

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

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

int first_idat(int *addr) {
	int idat_found = 0;
	int 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(int *addr) {
	int iend_found = 0;
	int found_idat = 0;
	int 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 change_idat_content(int *addr, char *message, int accuracy, int offset) {
	printf("Starting IDAT Tranform");
	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;
	}

	// Comment
	int i = total_idat(addr);
	printf("Total IDAT %d\n", i);

	int idat_length = check_header_length(addr, offset);
	int prop_found = 0;
	int size = 1;
	int* idat_data = malloc(size * sizeof(int));
	while(prop_found == 0) {
		for(i = 0; i < idat_length; i++) {
			idat_data[i] = addr[i+offset+8];
			size++;
			int* new_idat_data = realloc(idat_data, size * sizeof(int));
		}
		int r = randombytes_uniform(5) + 1;
		int j = randombytes_uniform(idat_length);
		
		idat_data[j] = (idat_data[j] + r) % 255;
		unsigned char crc_check[size+4];
		int idat_header[] = { 0x49, 0x44, 0x41, 0x54 };
		for(i = 0; i < 4; i++) {
			crc_check[i] = idat_header[i];
		}
		for(i = 0; i < size; i++) {
			crc_check[i] = idat_data[i+4];
		}
		int crcnum = crc(crc_check, idat_length);
		printf("New CRC: %08X\n", crcnum);
		printf("Test: %X\n", crcnum >> (8*3));
		prop_found = 1;
	}

	return 0;
}

int main() {
	FILE *fp;
	int c;
	int myArray[255] = {};
	int i = 0;
	int 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);
	//int crcnum = crc(myArray, 19);
	//printf("%08X\n", crcnum);
}