Question

How to create different files to test whether it passes the serial number checking. (Hint: need a hex editor.)

please provide two files, one pass, one cannot pass

C code:
----------------------------------------------
#include <stdio.h>
#include <string.h>

int chkserial(char *s)
{
    char buffer[16];
    strcpy(buffer, s);

    return strcmp(buffer, "cs780880");
}

void fullversion()
{
    printf("Thanks for purchasing! Enjoy the full-version software!\n");
}

void trialversion()
{
    printf("This is a trial version. Please purchase the full version to enable all features!\n");
}

int main(int argc, char **argv){
    char buf[512];

    FILE *f = fopen(argv[1], "r");



    fread(buf, 512, 1, f);

    printf("buf: %s\n", buf);

    if (chkserial(buf) == 0)
        fullversion();
    else
        trialversion();

    return 0;
}

Answer 1

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

int chkserial(char *s)
{
char buffer[16];
strcpy(buffer, s);

return strcmp(buffer, "cs780880");
}

void fullversion()
{
printf("Thanks for purchasing! Enjoy the full-version software!\n");
}

void trialversion()
{
printf("This is a trial version. Please purchase the full version to enable all features!\n");
}

int main(int argc, char **argv){
char buf[512];

FILE *f = fopen(argv[1], "r");

fread(buf, 512, 1, f);

printf("buf: %s\n", buf);

if (chkserial(buf) == 0)//change 0 to 1 this will bypass the full version
fullversion();
else
trialversion();

return 0;
}

if you have any doubt then please ask me without any hesitation in the comment section below , if you like my answer then please thumbs up for the answer , before giving thumbs down please discuss the question it may possible that we may understand the question different way and we can edit and change the answers if you argue, thanks :)