Set of functions to read & write information (in this case a simple sentence) to a file in binary and 'encrypt' it (just adding 5 to each character) and writing it to a new user specified file.

13_19.cpp

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <cstring>
using namespace std;

void write(const string &input, string oname) {
	fstream output(oname, ios::out | ios::binary);
	if (output.is_open()) {
		output.write(input.c_str(), input.size());
		output.close();
	}
}

string read(const string &fname) {
	int size;
	vector<char> buffer;
	fstream input(fname, ios::in | ios::binary);
	if (input.is_open()) {
		input.seekg(0, ios::end);
		size = input.tellg();
		input.seekg(0, ios::beg);
		buffer.resize(size);
		input.read(&buffer.front(), size);
		input.close();
	}
	string result(buffer.begin(), buffer.end());
	return result;
}

void encrypt(string iname, string oname) {
	// read
	int size;
	vector<char> buffer;
	fstream input(iname, ios::in | ios::binary);
	if (input.is_open()) {
		// get size
		input.seekg(0, ios::end);
		size = input.tellg();
		input.seekg(0, ios::beg);
		// resize buffer to size
		buffer.resize(size);
		input.read(&buffer.front(), size);
		input.close();
	}

	// encrypt
	char* target = new char[buffer.size()];
	copy(buffer.begin(), buffer.end(), target);
	for (int i = 0; i < buffer.size(); i++)
		target[i] += 5;
	string result(target);

	// output
	write(result, oname);
}

int main () {

	string iname, oname;
	cout << "Input: ";
	cin >> iname;
	cout << "Output: ";
	cin >> oname;

	encrypt(iname, oname);

	return 0;
}