Reading a file and then rewriting it. Cryptography program in C - c

Question

this is a program that is giving me many headaches, but I am tackling it!
I want to create a program that basically reads a file called message.txt which would have some texts with a message, let's say: ''Hello I am a program'' and then encrypts that message and puts it in a file called encryptMessage.txt, additionally it will save the key used for the user in the file key.txt. Now this is what I have done so far.
That part is already finished. There's only one last thing that I need to do.
The file message.txt has more than one line. For example:
hello I like programming
this is a test
to see if this program
can read and encrypt many lines
I want the program to read all those lines and then encrypt them and save them in encryptMessage.txt, such as this (let's suppose the key is 3):
khoor L olnh surjudpplqj
wklv lv d whvw
wr vhh wklv surjudp
fdq uhdg dqg hgfu|sw pdq| olqhv
However, I do not know how to make it work. I know I need to use a loop and !feof function or something like that. But I sincerely do not know how to implement, do you have any idea? Thank you very much!
#include <stdio.h>
#include <ctype.h>
#define MAXSIZE 100
int main(void)
{
FILE *message;
FILE *encryptMessage;
FILE *key;
message = fopen("message.txt", "r");
encryptMessage = fopen("encryptMessage.txt", "w");
key = fopen("key.txt", "w");
if ((message == NULL) || (encryptMessage == NULL) || (key == NULL))
{
printf("Error reading file!!!\n");
return 1;
}
int userKey;
char sentence[MAXSIZE];
char q[MAXSIZE];
int i = 0;
printf("Input the text that you want to encrypt:\n> ");
fgets(sentence, 99, message);
// printf("\nThe string that you wrote is:\n%s\n\n", sentence);
printf("Input the key:\n");
scanf("%d", &userKey);
fprintf(key, "%d", userKey);
//printf("\nThe key that you selected is: %d\n\n", userKey);
for(i = 0; sentence[i] != '\0'; ++i)
{
if( ( isupper(sentence[i]) ) || ( islower(sentence[i]) ) )
{
q[i] = sentence[i] + (char)userKey;
}
else
{
q[i] = (sentence[i]);
}
}
q[i] = '\0';
printf("%s", q);
fprintf(encryptMessage, "%s", q);
fclose(encryptMessage);
fclose(key);
fclose(message);
return 0;
}

Answer 1

Here's how you can read entire file:
while(fgets(sentence, MAXSIZE - 1, message)) {
// do something with sentence
}
Here is modified source code: http://pastebin.com/KxAe9KcS