need help on debugging my c++ programming (text ascii manupulation) - c++

Question

I recently made a C++ program which encrypts texts based-on the vigenere cipher technique.
The encryption part works fine I think, but the decryption function doesn't seem to output the correct answer in some cases. I was hoping if anyone could have a look at code and tell me what's wrong with it.
CMD dialog:
Enter the key for encrytion:
magic
Enter message No. 1:
I love C programming
Message encrypted:
u rwxq i rdomzcymovi
Decrypted:
i love c pXogramming
Somehow it outputted "X" instead of "r" in this case.............
Here are the codes:
Secret.h:
#ifndef CPP_TUTORIALS_SECRET_H
#define CPP_TUTORIALS_SECRET_H
#include <iostream>
using namespace std;
class Secret {
private:
string message;
string key;
bool toEncrypt;
bool toDecrypt;
public:
bool isToDecrypt() const {
return toDecrypt;
}
Secret(const string &message = "", const string &key = "", bool toEncrypt = false);
~Secret();
void setToDecrypt(bool toDecrypt);
void encrypt();
void decrypt();
void display();
};
#endif //CPP_TUTORIALS_SECRET_H
Secret.cpp
#include "Secret.h"
Secret::Secret(const string &message, const string &key, bool toEncrypt) {
Secret::message = message;
Secret::key = key;
Secret::toEncrypt = toEncrypt;
}
Secret::~Secret(){
}
void Secret::setToDecrypt(bool toDecrypt) {
Secret::toDecrypt = toDecrypt;
}
void Secret::display() {
cout << message << endl;
}
void Secret::encrypt() {
if(toEncrypt) {
int keyAscii[key.length()];
int count = 0;
for(unsigned int i = 0; i < key.length(); i++) {
keyAscii[i] = key.at(i);
}
for(unsigned int i = 0; i < message.length(); i++) {
if (message.at(i) > 64 && message.at(i) < 91) {
message.at(i) = (char)((message.at(i) - 65 + (keyAscii[count] - 97)) % 26 + 97);
}
else if (message.at(i) > 96 && message.at(i) < 123) {
message.at(i) = (char)((message.at(i) - 97 + (keyAscii[count] - 97)) % 26 + 97);
}
else{
message.at(i) = message.at(i);
}
count++;
if(count == key.length()) {
count = 0;
}
}
}
}
void Secret::decrypt() {
if(toDecrypt) {
int keyAscii[key.length()];
int count = 0;
for(unsigned int i = 0; i < key.length(); i++) {
keyAscii[i] = key.at(i);
}
for(unsigned int i = 0; i < message.length(); i++) {
if (message.at(i) > 96 && message.at(i) < 123) {
message.at(i) = (char)((message.at(i) - 97) % 26 - (keyAscii[count] - 97) + 97);
}
else {
message.at(i) = message.at(i);
}
count++;
if (count == key.length()) {
count = 0;
}
}
}
}
main.cpp
#include <limits>
#include "Secret.h"
void calcMsgAmount(int &pArraySize);
void inputKey(string &key);
void encryptMsg(Secret &secret, string &msg, const string &key, bool toEncrypt, int index);
int main() {
int arraySize;
string key;
string msg;
calcMsgAmount(arraySize);
inputKey(key);
Secret secrets[arraySize];
for(int i = 0; i < arraySize; i++){
encryptMsg(secrets[i], msg, key, true, i);
}
cout << endl << "Message encrypted: " << endl;
for(Secret i: secrets){
i.display();
i.setToDecrypt(true);
if(i.isToDecrypt()){
i.decrypt();
cout << endl << "Decrypted: " << endl;
i.display();
}
cout << endl << endl;
}
return 0;
}
void calcMsgAmount(int &pArraySize) {
cout << "Enter the amount of messages you want to input: " << endl;
while(!(cin >> pArraySize)){
cout << endl << "There's something really wrong with your input, please enter again." << endl;
cout << "Enter the amount of messages you want to input: " << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
}
void inputKey(string &key){
cout << "Enter the key for encrytion: " << endl;
cin.ignore();
getline(cin, key);
}
void encryptMsg(Secret &secret, string &msg, const string &key, bool toEncrypt, int index){
cout << "Enter message No. " << index + 1 << ": " << endl;
getline(cin, msg);
secret = Secret(msg, key, toEncrypt);
secret.encrypt();
}
Many thanks

Answer 1

accroding to wikipedia the decrypt routine of Vigenere Cipher is
(C - K) mod 26
what you wrote in Secret::decrypt() is
message.at(i) = (char)((message.at(i) - 97) % 26 - (keyAscii[count] - 97) + 97);
which is C % 26 - K.
I changed the line to
message.at(i) = (char)((26 + (message.at(i) - 97) - (keyAscii[count] - 97)) % 26 + 97);
it seems to be correct. I haven't really understood why the first 26 is necessary, but without it the code doesn't work (something with % and negative numbers)
P.S. as for debugging part, you might have noticed that whong letters appear as wrong capital letters which have smaller ascii code, so you decrypt routine has negative numbers in it. After which you check your code against Wikipedia :)

Answer 2

The problem comes in the decrypt from two problems:
the first is that the calculation should be symetric from the the encryp, i.e. the modulo 26 for message - key (as already pointed out by effenok)
the second is that in the decrypt you can at char level, message-key can be negative, which might not give you the expected results modulo 26 (for example -2 % 26 is -2 so that you'll be out of the alphabetic range that you expect). The easy trick is to add 26 before doing the modulo, which makes sure it's done on a positive number.
Here slightly more compact functions, making the difference between uppercase and lowercase :
void Secret::encrypt() {
if(toEncrypt) {
for(unsigned int i = 0; i < message.length(); i++) {
if(isalpha(message[i])){
char start = isupper(message[i]) ? 'A' : 'a';
message[i] = (message[i] - start + (tolower(key[i%key.length()]) - 'a') + 26) % 26 + start;
}
}
}
}
void Secret::decrypt() {
if(toDecrypt) {
for(unsigned int i = 0; i < message.length(); i++) {
if (isalpha(message[i]) ){
char start = isupper(message[i]) ? 'A':'a';
message[i] = (message[i] - start - (tolower(key[i%key.length()]) - 'a') + 26) % 26 + start;
}
}
}
}