Encrypt a message in C using file processing

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;
}

To answer the "how to read a full line" question: see this question & answer. — user707650
– user707650, Commented Dec 11, 2014 at 16:34

Spikatrix · Accepted Answer · 2017-03-23 03:36:46Z

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.

edited Mar 23, 2017 at 3:36

answered Dec 11, 2014 at 16:47

Spikatrix

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1 Comment

Eddy Mogollón Over a year ago

This was so easy, I figured it out before you told me. I just tried changing it and it worked, surprisingly hahaha. Thank you very much

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