Why does encryption code gives question mark while text contains the letters after s and key is 13?

Question

So, I am writing an encryption code. My code takes the word or any message and also asks user to enter a key. The end output is the encrypted message. For example:

Please enter the text you want to encrypt: hello
Enter the key: 4
The encrypted text is: lipps

But there is a problem. When I enter text that contains an s inside, it gives a question mark for the encryption:

Please enter the text you want to encrypt: ssss
Enter the key: 13
The encrypted text is: ����

This problem doesn't occur when I write other keys than 13, and if the letter is uppercase. This problem happens when the text contains any letter that comes after s (t, v, u, w, x, y, z) and when the key is 13.

Aforementioned code is:

#include <stdio.h>
#include <string.h>
    
int main(void) {
    int i;
    int key;
    char text[101], ch;
    printf("Please enter the text you want to encrypt: ");
    fgets(text, sizeof(text), stdin);
    printf("Enter the key: ");
    scanf("%i", &key);
    for(i = 0; text[i] != '\0'; ++i){
        ch = text[i];
            
        if(ch >= 'a' && ch <= 'z'){
            ch = ch + key;
                
            if(ch > 'z'){
                ch = ch - 'z' + 'a' - 1;
            }
                
            text[i] = ch;
        }
        else if(ch >= 'A' && ch <= 'Z'){
            ch = ch + key;
                
            if(ch > 'Z'){
                ch = ch - 'Z' + 'A' - 1;
            }
                
            text[i] = ch;
        }
    }
    printf("The encrypted text is: %s", text);
}

the problem seems to be that when the resulting character exceeds the current alphabet sequence ('a'...'z', or 'A'...'Z') then the code is not wrapping back to the beginning of that alphabet sequence — user3629249, Apr 04 '21 at 15:24
`for(i = 0; text[i] != '\0'; ++i)` --> `for(i = 0; (text[i] != '\0') && (text[i] != '\n'); ++i) ` — asio_guy, Apr 04 '21 at 16:26

score 2 · Answer 1 · answered Apr 04 '21 at 16:15

The problem lies in the line ch = ch + key; when you have values for ch and key whose sum is greater than that which can be stored in a char variable. For example, for the character, 's' (ASCII value 115) and a key of 13, the sum is 128 - which overflows an 8-bit signed char (max value 127) and results in a negative number.

The problem is much less likely to occur for uppercase characters (unless you have a very big value for key), as their ASCII values are significantly lower ('A' thru 'Z' are 65 … 90, whereas 'a' thru 'z' are 97 … 122).

To fix the issue, make the 'temporary' ch variable an int and cast it back to a char after all calculations are complete:

#include <stdio.h>
#include <string.h>

int main(void)
{
    int i, ch; // Use an int for our temporary "ch" variable
    int key;
    char text[101];
    printf("Please enter the text you want to encrypt: ");
    fgets(text, sizeof(text), stdin);
    printf("Enter the key: ");
    scanf("%i", &key);
    for (i = 0; text[i] != '\0'; ++i) {
        ch = text[i];
        if (ch >= 'a' && ch <= 'z') {
            ch = ch + key;
            if (ch > 'z') {
                ch = ch - 'z' + 'a' - 1;
            }
            text[i] = (char)ch; // Cast the int to a char to avoid compiler warnings
        }
        else if (ch >= 'A' && ch <= 'Z') {
            ch = ch + key;

            if (ch > 'Z') {
                ch = ch - 'Z' + 'A' - 1;
            }
            text[i] = (char)ch;
        }
    }
    printf("The encrypted text is: %s", text);
    return 0;
}

Why does encryption code gives question mark while text contains the letters after s and key is 13?

1 Answers1