Initializing strings in C in Visual Studio - c

Question

I try to learn the XOR algorithm using C. I have found a great example on KyleBank's GitHub:
#include <stdio.h>
#include <string.h>
void encryptDecrypt(char *input, char *output) {
char key[] = {'K', 'C', 'Q'}; //Can be any chars, and any size array
int i;
for(i = 0; i < strlen(input); i++) {
output[i] = input[i] ^ key[i % (sizeof(key)/sizeof(char))];
}
}
int main (int argc, char *argv[]) {
char baseStr[] = "kylewbanks.com";
char encrypted[strlen(baseStr)];
encryptDecrypt(baseStr, encrypted);
printf("Encrypted:%s\n", encrypted);
char decrypted[strlen(baseStr)];
encryptDecrypt(encrypted, decrypted);
printf("Decrypted:%s\n", decrypted);
}
The above works well under Linux and gcc.
However, it does not compile in Visual Studio under Windows.
I am using build tools included in Visual Studio 2017.
What am I doing wrong?

Answer 1

Microsoft's compiler does not support C99 VLAs (see the note here). Array sizes must be a constant expression. The code is also broken because it fails accommodate and place a nul terminator in the output.
In this case, decrypted and encrypted might be declared thus:
char encrypted[sizeof(baseStr)] ;
...
char decrypted[sizeof(baseStr)] ;
And encryptDecrypt() modified thus:
void encryptDecrypt(char *input, char *output) {
...
output[i] = 0 ;
}
Finally the signed mismatch warning may be cleaned up by declaring i as type size_t.
On Windows of course you could always use MinGW/GCC if you want more modern C support. Or you could use C++ and std::string or std::vector containers if you want to stick with Microsoft's compiler.

Answer 2

Use malloc for dynamic memory allocation. Requires #include <stdlib.h>
char baseStr[] = "123";
char *encrypted = malloc(strlen(baseStr) + 1);
...
free(encrypted);
As mentioned before, you have to add 1 for the null-terminated character at the end.
The char* pointer is one piece of information, it shows where the string begins. But where does it end? strlen and other C functions have no idea where the string ends, so they go through all the characters until a '\0' character is encountered.
For efficiency, take strlen(input) out of the loop and calculate it only once:
void encryptDecrypt(char *input, char *output)
{
char key[] = { 'K', 'C', 'Q' };
int keysize = sizeof(key);
size_t i;
size_t len = strlen(input);
for(i = 0; i < len; i++)
output[i] = input[i] ^ key[i % keysize];
output[len] = 0; //will be same as output[i] = 0;
}
The function int main should return zero. Note that this method cannot be described as "encryption" by modern standards. You can call it "obfuscation".