字符和字符不起作用的比较

Question

I have made a (very bad) attempt at a crackme.

This is the code:

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

int main()
{
    char passphrase[20];
    printf("Welcome to first_level.\n");
    printf("Hello. What's your passphrase?\n");

    fgets(passphrase,20,stdin);

    passphrase[strcspn(passphrase, "\n")] = 0;

    if(strlen(passphrase) != 10){
        // you lost
    } else
    {
        int counter = 0;
        for(int i = 0; i < 10; i++)
        {
            char index = i;
            char currentChar = passphrase[i];
            //printf(passphrase[i]);
            printf("---\nindex: %d\nchar: %c\n",index, currentChar);
            if(index == currentChar){
                //printf("ass\n");
                counter++;
            }
        }
        if(counter == 10)
        {
            printf("Congrats!\n");
            return 0;
        }

        printf("counter %d\n", counter);
    }

    printf("You lost!\n");
    return 0;
}

Now theoretically, the char comparison should work. Unfortunately, I believe that the characters are converted to int, and then compared.

With the magical printf before the comparison I noticed that if I printed the number (%d), the char would be >48, while when printing the character (%c -- like the code provided), the character number is printed correctly.

I am wondering how could I do this? I tried strcmp already but obviously it expects a string and not a char.

Answer 1

The C Standard mandates that the characters '0' , '1' , ..., and '9' be sequential and consecutive. So we know the value of '1' is 1 greater than the value of '0' (and similarly for the other digits).

Taking the above into consideration, we know that

'0' - '0' == 0;
'1' - '0' == 1;
....
'9' - '0' == 9;

Note that all of the above must work as I described, whether running on an ASCII based computer, or EBCDIC, or Klingon, or whatever.

So, to compare digits in character form ( '0' , ..., '9' ) with integer values ( 0 , ..., 9 ) simply subtract '0' from the char.

if (index == currentChar - '0') /* ... */;