Untitled

#include <iostream>
#include <string>
using namespace std;

// Function to convert a string to uppercase
// This function changes each lowercase letter to uppercase manually.
string convertToUpperCase(string inputText) {
    for (int i = 0; i < inputText.size(); i++) {
        if (inputText[i] >= 'a' && inputText[i] <= 'z') {
            inputText[i] = inputText[i] - 'a' + 'A'; // Manual conversion from lowercase to uppercase
        }
    }
    return inputText;
}

// Function to expand the key to match the length of the input message.
// The key is repeated until its length equals the message length.
string extendKey(string message, string keyword) {
    string extendedKey = keyword; // Start with the original key
    int messageLength = message.size();
    int keyLength = keyword.size();

    // Repeat the keyword to make it match the message length
    for (int i = 0; i < messageLength - keyLength; i++) {
        extendedKey += keyword[i % keyLength]; // Adding characters cyclically from the keyword
    }
    return extendedKey;
}

// Encrypt the input message using the extended key with a basic Vigenère cipher
string encryptMessage(string message, string extendedKey) {
    string encryptedMessage = "";

    // Loop through each character in the message and apply encryption formula
    for (int i = 0; i < message.size(); i++) {
        char encryptedChar = (message[i] - 'A' + extendedKey[i] - 'A') % 26 + 'A'; // Encrypt using Vigenère cipher
        encryptedMessage += encryptedChar; // Add the encrypted character to the result
    }
    return encryptedMessage;
}

// Decrypt the encrypted message using the extended key to retrieve the original message
string decryptMessage(string encryptedMessage, string extendedKey) {
    string decryptedMessage = "";

    // Loop through the encrypted message and apply the decryption formula
    for (int i = 0; i < encryptedMessage.size(); i++) {
        char decryptedChar = (encryptedMessage[i] - extendedKey[i] + 26) % 26 + 'A'; // Decrypt using Vigenère cipher
        decryptedMessage += decryptedChar; // Add the decrypted character to the result
    }
    return decryptedMessage;
}

// Main function to demonstrate the encryption and decryption process
int main() {
    string inputMessage = "HELLOVIGENERE"; // Original message to encrypt
    string keyword = "KEYWORD"; // Key used for encryption

    // Convert message and key to uppercase (for consistency)
    inputMessage = convertToUpperCase(inputMessage);
    keyword = convertToUpperCase(keyword);

    // Generate an extended key that matches the length of the message
    string extendedKey = extendKey(inputMessage, keyword);

    // Encrypt the message
    string encryptedMessage = encryptMessage(inputMessage, extendedKey);
    cout << "Encrypted Message: " << encryptedMessage << endl;

    // Decrypt the message back to the original text
    string decryptedMessage = decryptMessage(encryptedMessage, extendedKey);
    cout << "Decrypted Message: " << decryptedMessage << endl;

    return 0;
}