Simplistic C program to encrypt and decrypt a string [closed]

Question

I wanted to improve a crypter from internet. I found it poor and modified it.

Do you like it?

#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>

//Simple C program to encrypt and decrypt a string

#include <stdio.h>

int main()
{
  int scnt;
  int scnt2;
   int i, x;
   char str[512];
   printf("\nCrypter ver0.1\n");
   printf("\nYou need enter or create two keys for security\nWrite they in safe place\nTHIS PROGRAM CONTAINS BUGS");
   printf("\nPlease enter a string:\t");
   gets(str);

   printf("\nPlease choose following options:\n");
   printf("1 = Encrypt the string.\n");
   printf("2 = Decrypt the string.\n");
   scanf("%d", &x);
   printf("Create or Provide Exist Key:\n");
   scanf("%d", &scnt);
   printf("Create or Provide Exist Second Key:\n");
   scanf("%d", &scnt2);



   //using switch case statements
   switch(x)
   {
   case 1:
      for(i = 0; (i < 512 && str[i] != '\0'); i++)
        str[i] = str[i] + scnt - scnt2; //the key for encryption is 3 that is added to ASCII value
        printf("\nYou Encrypted String! You now in safe");
      printf("\nEncrypted string: %s\n", str);
      break;

   case 2:
      for(i = 0; (i < 512 && str[i] != '\0'); i++)
        str[i] = str[i] - scnt + scnt2; //the key for encryption is 3 that is subtracted to ASCII value
        printf("\nYou Decrypted String!\n");
      printf("\nDecrypted string: %s\n", str);
      break;

   default:
      printf("\nYou entered wrong number Select from 1 to 2\n");
   }
   return 0;
}

yes I know it contains so many bugs but I tested it on a long string and it worked (bugs are: 1. spaces don't work 2. in some keys it halts 3. unstable) but it decrypts fully without mistakes (tested around 50+ keys).
I made it so hard to hack even if you have source of program because my crypter uses 2 keys what makes it more hard to hack!

Answer 1

Use modern C

gets() has been dropped from the C standard library since C11 (12 years) and was even then considered poor practice.

Consider alternatives like using fgets() for all user input.

Test not needed

i < 512 is useless. For that to be false, prior gets() would have overrun str[] and that is undefined behavior (UB).

// for(i = 0; (i < 512 && str[i] != '\0'); i++)
for(i = 0; str[i]; i++)

Simplify

Code does either + scnt - scnt2 or - scnt + scnt2. Consider forming the difference once.

int diff = scnt - scnt2;
...
str[i] += diff;
...
str[i] -= diff;

Use auto-formatting

Early null character

str[i] + scnt - scnt2 may result in 0 and then the later printf("\nEncrypted string: %s\n", str); will not print the entire string.

Overflow

As str[i] + scnt - scnt2 may overflow or easily result in a value outside the char range. Re-code to prevent overflow and nicely handle out-of-range results. Perhaps a limit on scnt, scnt2 like code did with x?