Converting symbols to lowercase letters in c - c

Question

I'm writing a program that encodes a file using xor and print the encrypted text into another file. It technically works, however the output contains several symbols rather than only lowercase characters. How would I tell the program to only print lowercase letters, and be able to decode it back?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *args[]){
FILE *inFile, *outFile, *keyFile;
int key_count = 0;
int encrypt_byte;
char key[1000];
inFile = fopen("input.txt", "r");
outFile = fopen("output.txt", "w");
keyFile = fopen("key.txt", "r");
while((encrypt_byte = fgetc(inFile)) !=EOF)
{
fputc(encrypt_byte ^ key[key_count], outFile); //XORs
key_count++;
if(key_count == strlen(key)) //Reset the counter
key_count = 0;
}
printf("Complete!");
fclose(inFile);
fclose(outFile);
fclose(keyFile);
return(0);
}
Here is the output I get:
ÕââÐå朶è”ó
I just want it to only use lowercase letters

Answer 1

You can't. You either XOR all data of the file, or you don't. XOR-ing will result in non-printable characters.
What you can do though, is first XOR it and then encode it base64.
To get your original text/data back, do the reverse.
See also How do I base64 encode (decode) in C?

Answer 2

use the function tolower()
here there is an example:
#include<stdio.h>
#include<ctype.h>
int main()
{
int counter=0;
char mychar;
char str[]="TeSt THis seNTeNce.\n";
while (str[counter])
{
mychar=str[counter];
putchar (tolower(mychar));
counter++;
}
return 0;
}