#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <string>
#include <fstream>

using namespace std;

int main(int argc, char** argv)
{
	if (argc < 2)
	{
		cout << "Error: First command line argument must be the output filename." << endl;
		return -1;
	}

	char* text = new char;
	char key[8] = { 1, 2, 3, 4, 5, 6, 7, 8 };
	char* encrypted = new char;

	printf("Enter the text to be encrypted: \n");

	int x = 0;
	scanf("%s", &text);

	for (int i = 0; i < strlen(text); i++) // This line causes the error
	{
		encrypted[i] = text[i] ^ key[x];
		if (x++>7) // Key is and 8 element array, so we need another counter to prevent array index out of bounds exception
		{
			x = 0;
		}
	}

	FILE* file = fopen(argv[1], "w");
	fprintf(file, "%s", encrypted);  //write to file

	return 0;
}

#include <iostream>
#include <fstream>
#include <string>
#include <array>

int main(int argc, char** argv)
{
    if (argc < 2)
    {
        std::cout << "Error: First command line argument must be the output filename.\n";
        return -1;
    }

    const std::array<char, 8> key = { 1, 2, 3, 4, 5, 6, 7, 8 };

    std::string text;
    std::cout << "Enter the text to be encrypted:\n> ";
    std::cin >> text;

    std::string encrypted;
    unsigned key_index = 0;

    for (auto ch : text)
        encrypted += ch ^ key[key_index++ % key.size()];

    std::ofstream out(argv[1]);
    out << encrypted;
}