vignere decipher cpp

// program to implement vigenere cipher decryption

#include <iostream>
#include <vector>
#include <algorithm>
#include <fstream>
#include <sstream>

using namespace std;
// method that recives string as input and returns decrypted string as output
string decrypt(string original_msg, string key){

    // allchars is our keyspace
    string allChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    int all_chars_length = allChars.length();

    // converting our string to uppercase
    transform(original_msg.begin(), original_msg.end(), original_msg.begin(), ::toupper);

    // removing all numbers, punctuations
    original_msg.erase(remove_if(original_msg.begin(), original_msg.end(), [](char c) { return (!isalpha(c) && c!='\n' && c!='\r'); } ), original_msg.end());

    // converting key to uppercase
    transform(key.begin(), key.end(), key.begin(), ::toupper);
    int index = 0;
    string decrypted_msg = "";

    // iterating over the original message character by chracter and encrypting it and appending it to the new string
    for(int i=0; i< original_msg.length(); i++){
        char letter = original_msg[i];
        if(isalpha(letter)) {
            int position = (allChars.find(letter) - allChars.find(key[index]) + allChars.length()) % allChars.length();
            char e_letter = allChars[position];
            decrypted_msg += e_letter;
            index++;
            // reset index if it is greater that key length
            if(index >= key.length()){
                index %= key.length();
            }
        // case when \n or \r is encountered
        }else{
            decrypted_msg += letter;
            index = 0;
        }
    }

    cout<<decrypted_msg;
    return decrypted_msg;
}

// method that decrypts file
static void decrypt_file(string key){
    ifstream input_file;
    string file_name, data;
    cout<<"Enter File name : ";
    cin>>file_name;
    try {
        input_file.open(file_name);
        stringstream ss;
        ss << input_file.rdbuf();
        data = ss.str();
        cout<<"File read successufully!!"<<endl;
    } catch(exception e){
        cout<<e.what();
    }
    string decrypted_data = decrypt(data, key);
    cout<<"File data decrypted successfully!!"<<endl;
    ofstream output_file;
    try {
        output_file.open("C:\\Users\\MOHIT\\Desktop\\dss.txt");
        output_file << decrypted_data;
        output_file.close();
        cout<<"decrypted file stored on Desktop!!"<<endl;
    }catch(exception ee){
        cout<<ee.what();
    }


}

int main()
{
    cout<<"Program to implement Vignere Cipher"<<endl;
    cout<<"Enter the decryption key : ";
    string key;
    cin>>key;
    decrypt_file(key);
}

/*
Output for decrypting a file


Enter File name :F:/python_3.4/sample.txt
Enter File name : F:/python_3.4/sample.txt
File read successufully!!
File data decrypted successfully!!
decrypted file stored on Desktop!!


 */