凯撒密码和逆向文本程序的问题

Question

I'm trying to make a function that gets a string and a number and if the number is bigger the '0' so it will make the caesar cipher with the string and the number that the user entered. for example -> 'stack' and the number is '3' -> 'uvdfn'. if the number is '0' so it will reverse the string. for example - 'stack' -> 'kcats'

I don't know what is the issue with the code, i don't see anything wrong.

 #include <stdio.h> #include <stdlib.h> #include <string.h> void decryptText(char* encText, int n); #define STR_SIZE 50 int main(void) { char str[STR_SIZE]; int num = 0; printf("Please enter the string : "); fgets(str, STR_SIZE, stdin); printf("Please enter a number : "); scanf("%d", &num); decryptText(str, num); system("PAUSE"); return 0; } void decryptText(char* encText, int n) { int i = 0; int j = 0; char temp = 0; int strLen = strlen(encText); if (n > 0) { for (i = 0; i < strLen; i++) { if (*(encText + i) == ' ') { } else { if (*(encText + i) >= 'x') { *(encText + i) = (*(encText + i)) - 26; } *(encText + i) = (*(encText + i)) + n; } } printf("The array after the program deciphered it : \\n"); printf("%s", encText); } else if (n == 0) { for (i = 0; i < strLen; i++) { for (j = 0; j >= 0; j--) { temp = *(encText + i); *(encText + i) = *(encText + j); *(encText + i) = temp; } } printf("The array after the program cracked it : \\n"); printf("%s", encText); } }

Answer 1

if (*(encText + i) >= 'x')
    {
        *(encText + i) = (*(encText + i)) - 26;
    }

Should be

if (*(encText + i) + n > 'z')
{
    *(encText + i) = (*(encText + i)) - 26;
}

Answer 2

The error in your encoding part is the following snippet:

if (*(encText + i) >= 'x')
{
    *(encText + i) = (*(encText + i)) - 26;
}
*(encText + i) = (*(encText + i)) + n;

First of all you have to determine if you get lower- or uppercase input. For beginning we assume lowercase input only. In this snippet you firstly have to subtract an 'a' from the actual character, secondly add the chosen rotation number to the calculated value, thirdly calculate the modul of that and fourthly add an 'a' to the value.

char temp;
temp = *(encText + i);
temp -= 'a';
temp += n;
temp %= 26;
temp += 'a';
*(encText + i) = temp;

or in short:

*(encText + i) = (*(encText + i) - 'a' + n) % 26 + 'a';

BTW: Your cracking operation doesnt look very effective...