This program utilizes the command line to encrypt / decrypt text files using the Vigenëre cipher - commonly referred to as the Caesar cipher.

caesar.cpp

// caesar.cpp ~ 6 December 2012
// SAMUEL ROTH  ~  ECCS 1611 
// This program utilizes the command line to encrypt / decrypt text files using
// the VigenËre cipher - commonly referred to as the Caesar cipher. 

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

using namespace std;

// FUNCTION PROTOTYPE // 
void pressEnterToContinue ( void );
 
// Functions pertaining to the ciphering process are held within this class.
class Vigenere {
	
	public:

		// Calling for the keyword that will be used to encrypt/decrypt.
		string key;
		
		// Vingenere( string key )
		// Input:  the keyword for the cipher, as submitted by the user.
		// Output: A 'standardized' keyword, so it can work with ASCII operations.
		// Notes:  This program doesn't interact with the user. It simplifies the
		//         operation (seen below) of accurately encrypting and decrypting
		//         the text file in question.
		Vigenere( string key ) {

			for( int i = 0; i < key.size(); ++i ) {
				if( key[i] >= 'A' && key[i] <= 'Z')
					this->key += key[i];
				else if( key[i] >= 'a' && key[i] <= 'z' )
					this->key += key[i] + 'A' - 'a';
			}
		}
		
		// string  encipher( string text )
		// Input:  The name of the text file to be encrypted.
		// Output: An encrypted version of the text file, under the file name
		//         that was specified by the user. 
		// Notes:  This process is done entirely with ASCII. Each character
		//         has a corresponding ASCII number that can be manipulated
		//         with simple math (addition and subtraction), etc.
		string encipher( string text ) {
			string out;
 
			for( int i = 0, j = 0; i < text.length(); ++i ) {
				char c = text[i];
				if(c >= 'a' && c <= 'z')
					c += 'A' - 'a';
				else if( c < 'A' || c > 'Z' ) {
					if (c == '.')
						out += '&';
					else if (c == ',')
						out += '%';
					else if (c == ' ')
						out += '#';
					continue;
				}

				out += (c + key[j] - 2*'A') % 26 + 'A'; 
				j = (j + 1) % key.length();
			}
 
		return out;
		}
		
		// string  decipher( string text )
		// Input:  The name of the text file to be decrypted.
		// Output: A decrypted version of the text file, under the file name
		//         that was specified by the user. 
		// Notes:  This process is done entirely with ASCII. Each character
		//         has a corresponding ASCII number that can be manipulated
		//         with simple math (addition and subtraction), etc.
		string decipher( string text ) {

			string out;
			string space = " ";
 
			for( int i = 0, j = 0; i < text.length(); ++i ) {
				char c = text[i];
 
				if(c >= 'a' && c <= 'z')
					c += 'A' - 'a';
				else if(c < 'A' || c > 'Z') {
					if (c == '&')
						out += ". ";
					if (c == '%')
						out += ", ";
					if (c == '#')
						out += space;
					continue;
				}
 
				out += (c - key[j] + 26) % 26 + 'A'; 
				j = (j + 1) % key.length();
			}
 
		return out;
		}
};

int main( int argc, char *argv[] ) {

	// Because this function is operated through the command line,
	// the user must begin with the proper parameters, as follows:
	//
	//   mp5 -e InputFileName.txt OutputFileName.txt
 	//
	// When the parameters are properly called for, the function
	// can perform properly.

	// 1. Opening the input/output streams to access external files.
	ifstream inputStream;
	ofstream outputStream;

	// 2. Establishing which parameters represent which text files.
    string original = argv[2];
	string result = argv[3];

	// 3. Calling for the keyword, to be specified by the user.
    string key;

	// Gathering the contents of the input file (the original).
	ifstream in( original );
	string contents((std::istreambuf_iterator<char>(in)), 
    istreambuf_iterator<char>());

	// Prompting user for the keyword, then storing it.
	cout << "Please enter the keyword: ";
    cin >> key;

	// Converting the keyword to manipulatable format (ASCII).
	Vigenere cipher(key);

	// Determining whether the input text file is to be encrypted or decrypted.
	if ( argv[1][1] == 'e' ) {
		string enciphered = cipher.encipher(contents); // Encrypting.
		outputStream.open (result); // Opening specified output file (or creating it.)
		outputStream << enciphered; // Writing results in this file.
		outputStream.close(); // Closing this file.
	} else if ( argv[1][1] == 'd' ) {
		string deciphered = cipher.decipher(contents); // Decrypting.
		outputStream.open (result); // Opening specified output file (or creating it.)
		outputStream << deciphered; // Writing results in this file.
		outputStream.close(); // Closing this file.
	} else {
		// In the event that the user makes a typing error...
		cout << "Invalid parameters. Please try again." << endl;
		pressEnterToContinue();
	}

	// Confirming to the user that the encryption/decryption has been completed.
	if ( argv[1][1] == 'e' )
		cout << "The text file has been encrypted.";
	else
		cout << "The text file has been decrypted.";

	// Allowing the user to continue at his or her own discretion.
    pressEnterToContinue();

	return 0;
}

// pressEnterToContinue( void )
// Input:  N/A
// Output: Text, prompting the user to continue at his/her own accord.
// Notes:  This action can really be completed with any key.
void pressEnterToContinue ( void ) {
	cout << endl << "Press enter to continue . . .";
	cin.ignore();
	cin.get();
	return;
}