Windows C++ char not displaying properly, just some unknkown character displaying - c++

Question

I have written the code for RSA in C++ on Ubuntu. It was working fine on that, it's working fine on Windows Dev C++ as well, but it doesn't show the character properly.
Here is the code :
#include<iostream>
#include<stdlib.h> // for rand()
#include<math.h> // for floor function
#include<string.h>
using namespace std;
//function to check whether a number is prime or not
int check_prime(int number)
{
int count = 0;
for(int i = 2; i<number + 1; i++)
{
if(number%i == 0)
{
count++;
}
}
if(count>2)
{
return 0;
}
else
{
return 1;
}
}
//function to generate a random prime number
int generate_random_prime()
{
int temp;
while(1)
{
temp = rand() % 50;
if(check_prime(temp) == 1)
{
return temp;
}
}
}
int gcd(int a, int b)
{
int temp;
while(b != 0)
{
temp = b;
b = a%b;
a = temp;
}
return a;
}
// Extended Euclid GCD to find d such de congruent to 1
int extended_gcd(int a, int b)
{
int d, x, y, r, q;
if(b == 0)
{
d = a;
x = 1;
y = 0;
cout << "\n d= " << d << " x= " << x << " y= " << y << "\n";
}
int x2, x1, y2, y1;
x2 = 1;
x1 = 0;
y2 = 0;
y1 = 1;
while(b > 0)
{
q = floor(a / b);
r = a - q*b;
x = x2 - q*x1;
y = y2 - q*y1;
a = b;
b = r;
x2 = x1;
x1 = x;
y2 = y1;
y1 = y;
}
d = a;
x = x2;
y = y2;
return x2;
}
//returns a^b mod n using square and multiply method
int modular_exponentiation(int a, int b, int n)
{
if(a == 1)
{
return 0;
}
int c = 1;
for(int i = 1; i < b + 1; i++)
{
c = (c*a) % n;
}
return c;
}
//cipher text = (message^e) %n
int cipher_text(int m, int e, int n)
{
return modular_exponentiation(m, e, n);
}
//decrypted_text= (cipher^d)%n
int decrypt_cipher(int c, int d, int n)
{
return modular_exponentiation(c, d, n);
}
int main()
{
// generating two random prime p and q
int p = generate_random_prime();
int q = generate_random_prime();
cout << "Prime p : " << p << "and q : " << q << "\n";
int n = p*q;
cout << "n=p*q = " << n << "\n";
//calculating Euler Totient for prime p and q
int euler_phi = (p - 1)*(q - 1);
cout << "Euler totient is : " << euler_phi << "\n";
int d, e;
// calculating e such that 1<e<euler_phi and gcd(n,euler_phi)=1
while(1)
{
e = rand() % (euler_phi - 1 + 1) + 1;
if(gcd(euler_phi, e) == 1)
{
break;
}
}
cout << "e value is : " << e << "\n";
//calculating d such that ed congruent 1, ed=1
d = extended_gcd(e, euler_phi);
//d=5;
cout << "d value is : " << d << "\n";
//storing the message to be encrypted as char array and encrypting each char element
char message[20];
int cipher[20];
cout << "Enter the message to be encrypted : ";
cin >> message;
cout << "Message to be encrypted is : " << message << "\n";
int size = strlen(message);
//calculating cipher text c
for(int i = 0; i < size; i++)
{
cipher[i] = cipher_text(int(message[i]), e, n);
}
cout << "Cipher text is : ";
for(int i = 0; i < size; i++)
{
cout << cipher[i] << " ";
}
char message_decrypted[size];
//decrypting cipher text
for(int i = 0; i < size; i++)
{
message_decrypted[i] = decrypt_cipher(cipher[i], d, n);
}
cout << "\nDecrypted message is : ";
for(int i = 0; i < size; i++)
{
cout << message_decrypted[i];
}
cout << "\n";
return 0;
}
I have tried the code on DevC++ and using g++.
Check the images :
Image using g++ compiler
I need a way to print the char to be displayed properly.
I think that message_decrypted[i]=decrypt_cipher(cipher[i],d,n); needs to be changed to print the character properly in Devcpp
Here is the link to the code in online IDE where it works fine https://repl.it/#shubhamjohar/RSA

Answer 1

When your main routine invokes
decrypt_cipher(cipher[i], d, n);
cipher[0] is 386 as matching your output above. d is -179. And n is 697
The corresponding call into modular_exponentiation(a=386, b=-179, n=697) results in this for-loop getting skipped:
for (int i = 1; i<b + 1; i++) {
c = (c*a) % n;
}
Because i < (b + 1) evaluates to (1 < -178), which evaluates to false.
Therefore, your modular_exponentiation returns 1, which is an unprintable character.
Same applies for the subsequent calls to decrypt_cipher from main.
I don't know enough about the RSA algorithm to know if your implementation is correct. But when d is negative, that for-loop isn't going to do any loops.

Answer 2

Maybe it is incurred by the following expression in your program:
char message_decrypted[size];
There is some standard change related to this usage. please read the following page for more details.
https://www.geeksforgeeks.org/variable-length-arrays-in-c-and-c/
Or try to use something like new char[size] to allocate memory dynamically.