Issues with Caesar Cipher not decrypting correctly in C++ - c++

Question

I am aware that this question has been asked a few times, and I may have missed the question that answers my specific problem however I cannot seem to find one that gives me an answer that works for me.
When I am decrypting a Caesar Cipher it doesn't seem to wrap around correctly, my code seems to follow the specific mathmatics for the caesar cipher but it seems to return junk output when it's supposed to wrap around. My code is as follows, including a system I used to test the problem.
#include "main.h"
#include <QCoreApplication>
#include <QDebug>
String caesarCipher(QString in, int shift, bool decrypt)
/*
* Caesar shift is mathmatically represented as e = (q + s) mod 26
* Decryption is represented as d = (q - s) mod 26
* ROT13 is a caesar shift with 13 shift
*/
{
QString out;
if (!decrypt)
{
for (int i = 0; i < in.length(); ++i)
{
if (in[i] >= 'a' && in[i] <= 'z')
{
int q = (in[i].unicode() - 'a');
int e = (q + shift) % 26;
out += e + 'a';
}
else if (in[i] >= 'A' && in[i] <= 'Z')
{
int q = (in[i].unicode() - 'A');
int e = (q + shift) % 26;
out += e + 'A';
}
else
out += in[i];
}
return out;
}
else
{
for (int i = 0; i < in.length(); ++i)
{
if (in[i] >= 'a' && in[i] <= 'z')
{
int q = (in[i].unicode() - 'a');
int d = (q - shift) % 26;
int r = d + 'a';
out += r;
}
else if (in[i] >= 'A' && in[i] <= 'Z')
{
int q = (in[i].unicode() - 'A');
int d = (q - shift) % 26;
int r = d + 'A';
out += r;
}
else
out += in[i];
}
return out;
}
}
int main() // Testing
{
QString testString = "abcdefghijklmnopqrstuvwxyz";
QString upperTest = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const int shifting = 3;
qDebug() << "Test String: " << testString;
qDebug() << "Test String (Upper): " << upperTest;
{
QString e = caesarCipher(testString, shifting, false);
QString E = caesarCipher(upperTest, shifting, false);
QString d = caesarCipher(e, shifting, true);
QString D = caesarCipher(E, shifting, true);
qDebug() << "Shift amount: " << shifting;
qDebug() << "Encrypt (Lower): " << e;
qDebug() << "Encrypt (Upper): " << E;
qDebug() << "Decrypt (Lower): " << d;
qDebug() << "Decrypt (Upper): " << D;
}
return 0;
}
The expected result is
Test String: "abcdefghijklmnopqrstuvwxyz"
Test String (Upper): "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
Shift amount: 3
Encrypt (Lower): "defghijklmnopqrstuvwxyzabc"
Encrypt (Upper): "DEFGHIJKLMNOPQRSTUVWXYZABC"
Decrypt (Lower): "abcdefghijklmnopqrstuvwxyz"
Decrypt (Upper): "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
Press <RETURN> to close this window...
The result I get:
Test String: "abcdefghijklmnopqrstuvwxyz"
Test String (Upper): "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
Shift amount: 3
Encrypt (Lower): "defghijklmnopqrstuvwxyzabc"
Encrypt (Upper): "DEFGHIJKLMNOPQRSTUVWXYZABC"
Decrypt (Lower): "abcdefghijklmnopqrstuvw^_`"
Decrypt (Upper): "ABCDEFGHIJKLMNOPQRSTUVW>?#"
Press <RETURN> to close this window...
I have tried to move code around, change where the shift is removed, where the modulo is done and where the 'a' character is added
For reference the code was initially before I altered it for readability:
for (int i = 0; i < in.length(); ++i)
{
if (in[i] >= 'a' && in[i] <= 'z')
out.resultString += (((in[i].unicode() - 'a') - shift) % m) + 'a';
else if (in[i] >= 'A' && in[i] <= 'Z')
out.resultString += (((in[i].unicode() - 'A') - shift) % m) + 'A';
else
out.resultString += in[i];
}

Answer 1

The % operator can return negative results when used with negative numbers. In your case, when decrypting an 'a', q will be 0, d will be (-3 % 26), which can be -3.
The solution is to ensure the number is positive before calculating the remainder:
int d = (q - shift + 26) % 26;
Or, if the shift amount is unknown, or can be more than 25, check if d is negative and add 26 to it after your initial calculation.