Encrypt a message in C using file processing - c

Question

Glad to ask you again!
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. I don't know how to make the program read the file message.txt, show it into the screen and then encrypt it into the file. Any piece of advice? Thank you!
I was planning to use fscanf, but I can't use it because it's a line, not just a single string.
Please, if possible write the code yourself so I can compare it to what I have written so far. I always appreciate your feedback, thanks!
#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 ((encryptMessage == NULL) || (encryptMessage == NULL) || (encryptMessage == 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> "); // These two lines are a test to see if I was able to encrypt the message, but this is not necessary. It should directly read the file called message.txt.
fgets(sentence, 99, stdin);
// 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);
return 0;
}

Answer 1

To read a line from message.txt you need to use fgets function.
fgets(sentence, 99, stdin);
The above fgets(which you have in your code) reads from the stdin which is normally the keyboard. To make it read from the text file,use
fgets(sentence, MAX_SIZE, message);
Note the change in the second argument too. If you want to display whatever was scanned,uncomment the below line which you have in your code
//printf("\nThe string that you wrote is:\n%s\n\n", sentence);
Don't forget to close(using fclose) all the FILE pointers which you had opened(using fopen) after its use.