Encrypting text file with ASCII hex

Hello all,
I need your help. I need a very quick program on taking in a file filled with characters and writing into a file with each character's ASCII (hex representation). This is a very dirty code but it does work. The problem I am having is how to write into a new file. Please help. Thank you.

Code:

// This program takes a sample C++ file and encrypts
// it with the ASCII hex representation

#include <iostream>
#include <string>

using namespace std;

string lookup_hex(char blurb);

int main(){
	// creates a file pointer
	FILE * thisFile;	// file object to read from
	FILE * thatFile;	// file object to write from

	// opens file
	thisFile = fopen("test.txt", "r");
	thatFile = fopen("output.txt", "w");
	// initializes the character
	char c;

	while (thisFile != NULL){
		// takes in a character
		c = fgetc(thisFile);
		
		// evaluates character
		string hex_value = lookup_hex(c);
		
		// writes ANSI characters in new file
		fwrite(, thatFile);
	}

	return 0;
}

string lookup_hex(char blurb){
	char test[2];
	switch(blurb){
	case ' ':
		test[0] = '2';
		test[1] = '0';
	break;
	case '!':
		test[0] = '2';
		test[1] = '1';
	break;
	case '"':
		test[0] = '2';
		test[1] = '2';
	break;
	case '#':
		test[0] = '2';
		test[1] = '3';
	break;

	};

	return test;
}

Use the fstream class defined in, you guessed it, <fstream>


The hex lookup would be so much simpler if you did.
It would just be

fstream_object << std::hex << static_cast<int>(c);

But you would probably want to format it somehow so you could read it back in easily. If you want to, prepend an 'x' or '0x' to the number for easy parsing.