Need assistance with Vigenere Cipher program - c++

Question

I am helping a friend create a program that encrypts/decrypts messages using the "Vigenere Cipher." I was unsure what this was, so I did my own research and think I have it figured out.
My code runs fine from a syntax perspective. But, from a logic perspective, it does not work. From my understanding, when I encrypt a message with a key, if I decrypt the encrypted message using the same key, it should give me the original message. Mine does not. From my debugging attempts, I think the issue lies somewhere in my decrypting algorithm, but could be entirely wrong.
Here is my code:
#include <iostream>
#include<stdio.h>
#include<string.h>
#include<iostream>
#include<string>
using namespace std;
int main(){
//initializing functions to be used
int giveInfo();
void encrypt(string message, string key);
void decrypt(string message, string key);
void newKey(string key);
string keyInput();
string messageInput();
int userChoice();
giveInfo();
//loop so that the user can decrypt/encrypt multiple messages
int counter = 1;
int userCounter;
while (counter == 1){
int choice = userChoice();
if(choice == 1){
string inputMessage = messageInput();
string inputKey = keyInput();
encrypt(inputMessage, inputKey);
}
else{
string inputMessage = messageInput();
string inputKey = keyInput();
decrypt(inputMessage, inputKey);
}
cout << "Would you like to decrypt/encrypt another message? (1 = yes, 2 = no)";
cin >> userCounter;
counter = userCounter;
system("CLS");
}
return 0;
}
//gives the user a basic description of cypher and what they need to input
int giveInfo(){
cout << "\nThe Vigenere Cypher is a polyalphabetic encryption/decryption method. It utilizes a 'key' (provided by the user, \nany word of any length) to determine which letters will replace others. This means in order to decrypt a message,\n one will need the key the person who encrypted the messafe used, ensuring a secure encryption. To use this program, \nyou will need to enter your message (this will be converted into all capitol letters) and a key which you would like to use. Do not use any spaces in your message.\n\n\n";
return 0;
}
string messageInput(){
//message place holder
string userMessage;
//asking for message
cout << "What is the message you would like to encrypt/decrypt?\n";
cin >> userMessage;
return userMessage;
}
string keyInput(){
//key place holder
string userKey;
//asking for key
cout << "What is the key you would like to use?\n";
cin >> userKey;
return userKey;
}
void decrypt(string message, string key){
//generating new key to match message length
int x = message.size();
for (int i = 0; ; i++)
{
if (x == i)
i = 0;
if (key.size() == message.size())
break;
key.push_back(key[i]);
}
string orig_text;
for (int i = 0 ; i < message.size(); i++)
{
// converting in range 0-25
int x = (message[i] - key[i] + 26) %26;
// convert into alphabets(ASCII)
x += 'A';
orig_text.push_back(x);
}
cout << "\n\nEncrypted Code: " + message+ "\n";
cout << "Key: " + key+ "\n";
cout << "Decrypted message: ";
cout << orig_text + "\n";
}
//takes user input (message to be encyrpted and key to be used) as arguments and returns encrypted
void encrypt(string message, string key){
string cipher_text;
//generating new key to match message length
int x = message.size();
for (int i = 0; ; i++)
{
if (x == i)
i = 0;
if (key.size() == message.size())
break;
key.push_back(key[i]);
}
for (int i = 0; i < message.size(); i++)
{
// converting in range 0-25
int x = (message[i] + key[i]) %26;
// convert into alphabets(ASCII)
x += 'A';
cipher_text.push_back(x);
}
cout << "\n\nOriginal message: " + message+ "\n";
cout << "Key: " + key+ "\n";
cout << "Encrypted message: ";
cout << cipher_text + "\n";
}
int userChoice(){
int choice;
cout << "Would you like to encrypt a message or decrypt a message? (1 =
encrypt, 2 = decrypt)\n";
cin >> choice;
return choice;
}
Any help?

Answer 1

Since your intention is to use an alphabet with a length of 26, you need to make sure your input is properly normalized prior to performing any encryption/decryption operations.
I would suggest ensuring that user input for the message & key is converted to upper case.
e.g. utilizing toupper: for(char &c : inputMessage) c = toupper(c)
Could you provide your input & output? Your code works just fine.
Original Message: ATTACKATDAWN
Key: LEMONLEMONLE
Encrypted Message: LXFOPVEFRNHR
Encrypted Code: LXFOPVEFRNHR
Key: LEMONLEMONLE
Decrypted Message: ATTACKATDAWN