Bouch's Final Product :D

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>

using namespace std;

//function prototypes
char encipher(char key, char plain);
char decipher(char key, char cipher);
void pressEnterToContinue(void);

int main(int argc, char *argv[])
{
    //declarations
    string keyWord = string(argv[2]);
    char plainChar;
    char cipherChar;
    string inputFileName = argv[3];
    string outputFileName = argv[4];

    //opening input and output streams for files
    ifstream inStream;
    inStream.open(inputFileName);
    ofstream outStream;
    outStream.open(outputFileName);
    //test if fail to open file
    if (inStream.fail())
    {
        cout << "File nonexistent!" << endl;
        pressEnterToContinue();
    }

    //checks if the "encode" flag (-e) is used
    if (argv[1][0] == '-' && argv[1][1] == 'e')
    {
        //repeats getting new character, enciphering it, and dumping to output file.
        int i = 0;
        while (true)
        {
            i++;
            if (i == keyWord.length()) i = 0;
            inStream.get(plainChar);
            if (inStream.eof()) break;
            char ch = encipher(keyWord[i], plainChar);
            outStream << ch;
        }
    }


    //checks if the "decode" flag (-d) is used
    else if (argv[1][0] == '-' && argv[1][1] == 'd')
    {
        //repeats getting new character, enciphering it, and dumping to output file.
        int i = 0;
        while (true)
        {
            i++;
            if (i == keyWord.length()) i = 0;
            inStream.get(cipherChar);
            if (inStream.eof()) break;
            char ch = decipher(keyWord[i], cipherChar);
            outStream << ch;
        }
    }

    //if neither flag is used, throws incorrect input.
    else cout << "incorrect flag input. " << endl;

    //closing program
    inStream.close();
    outStream.close();
    cout << "Done!" << endl;
    pressEnterToContinue();
    return 0;
}

//Enciphers a character with a key and plain text.
//preconditions: char key, char plain
//postconditions: cipher
char encipher(char key, char plain)
{
    char cipher;
    if (plain == '\n') return plain;
    int offset = ' ';
    int alphabet = 95; // The ASCII printable alphabet is 95 characters.
    cipher = (((plain - offset) + (key - offset)) % alphabet) + offset;
    return cipher;
}

//Deciphers an enciphered character with a key and enciphered text.
//preconditions: char key, char cipher (enciphered text)
//postconditions: char plain (deciphered text)
char decipher(char key, char cipher)
{
    char plain;
    if (cipher == '\n') return cipher;
    int offset = ' ';
    int alphabet = 95; // The ASCII printable alphabet is 95 characters.
    plain = ((cipher - offset) - (key - offset));
    if (plain < 0) plain += alphabet;
    plain += offset;
    return plain;
}

//Ends program, no pre/post conditions.
void pressEnterToContinue(void)
{
    cout << "\nPress 'Enter' to continue... ";
    cin.ignore(1024, '/n');
    cin.clear();
    return;
}