C Program to Encrypt and Decrypt Files

In this article, you will learn and get code about how to encrypt or decrypt a file using C language. But before going through the program of encryption and decryption, let's first understand about what it actually means when encrypting or decrypting a file.

What Encryption or Decryption means ?

Data encryption means converting the original data into a form or code that can not be read or understand by any people (public). Because encrypted data can only be accessed by authorized person. Here authorized person means, a man who knows its decryption key (formula or password). Decryption key is a password or formula that is used to convert the cyphertext to plaintext or original text.

Note - Encrypted data is known as cyphertext, whereas unencrypted data is known as plaintext.

Encrypt or Decrypt a File in C

Encrypting a file means, we will convert the plaintext (original content of file) to cyphertext. So that our credential information stored in a file gets converted into cyphertext. Whereas decrypting a file means, getting our content back to original form.

Things to Do before Program

Here are the things that must have to do, before going through the program given below. Because, we have created a program to encrypt a file and then created another program to decrypt the same file. Then we must have to create a file in the same directory where program's source code is going to be saved.

For example, create a folder named c programs to the parent directory named Documents. And create a file named codescracker.txt with following content:

my usernamed is codescracker
my password is 1234@cc

Here is the snapshot of the c programs folder present in the parent directory of Documents:

As you can clearly see from the above snapshot, a file named codescracker.txt is available in the folder c programs. Now let's move on the program to encrypt the content of this file using C program given given below.

Program to Encrypt a File in C

To encrypt a file in C programming, you have to open that file and start reading the file character by character. At the time of reading, create some algorithm to encrypt the content of file. And place the content in a temporary file character by character. Finally copy the content of temporary file to original file as shown in the program given below:

#include<stdio.h>
#include<conio.h>
int main()
{
    char fname[20], ch;
    FILE *fps, *fpt;
    printf("Enter Filename: ");
    gets(fname);
    fps = fopen(fname, "r");
    if(fps == NULL)
        return 0;
    fpt = fopen("temp.txt", "w");
    if(fpt == NULL)
        return 0;
    ch = fgetc(fps);
    while(ch != EOF)
    {
        ch = ch+100;
        fputc(ch, fpt);
        ch = fgetc(fps);
    }
    fclose(fps);
    fclose(fpt);
    fps = fopen(fname, "w");
    if(fps == NULL)
        return 0;
    fpt = fopen("temp.txt", "r");
    if(fpt == NULL)
        return 0;
    ch = fgetc(fpt);
    while(ch != EOF)
    {
        ch = fputc(ch, fps);
        ch = fgetc(fpt);
    }
    fclose(fps);
    fclose(fpt);
    printf("\nFile %s Encrypted Successfully!", fname);
    getch();
    return 0;
}

This program was written under Code::Blocks IDE. Before running the above program, first save it by navigating File->Save file as... Menu (in Code::Blocks). Save the file into the same directory, where the file codescracker.txt is saved as done at early of this article, that is in c programs folder.

Now build and run the above program after saving it. Because, we have used w as file opening mode (for the file temp.txt), that is used to write. And if file does not exists, then a new file gets created with same name, that is temp.txt (in this case). Now here is the sample run of above program:

Now supply the name of file as codescracker.txt and press ENTER key to encrypt it. Here is the second snapshot of the same sample run:

Now your file codescracker.txt gets encrypted. Here is the snapshot of the folder c programs. You will see that a new file text.txt automatically gets created inside the same folder:

The content of file, codescracker.txt before encryption is:

And the content of same file, codescracker.txt after encryption is:

Algorithm used to Encrypt File in previous Program

This is the simple encryption key that was used in above program to encrypt the content of file, codescracker.txt:

ch = ch+100;

It means:

• To every character, we have added 100
• Adding 100 means, if first character of file codescracker.txt is A. And as the ASCII value of A is 65. Therefore adding 100 in it makes it 165, so a character whose ASCII value is 165, gets placed to temp.txt file as its first character
• And process for the second character of codescracker.txt file and put it to temp.txt file after applying the algorithm
• Continue the process for all the character
• Finally replace the content of codescracker.txt file with the content present in temp.txt file. That's it

Rest of the things included in above program are related to file handling, so to learn about file handling in detail, then refer to File Input/Output tutorial.

Now let's go through the program that decrypt the same file that are encrypted using the above program.

Program to Decrypt a File in C

The question is, write a program in C that decrypts the file encrypted using previous program. The answer to this question is given below:

#include<stdio.h>
#include<conio.h>
int main()
{
    char fname[20], ch;
    FILE *fps, *fpt;
    printf("Enter Filename: ");
    gets(fname);
    fps = fopen(fname, "w");
    if(fps == NULL)
        return 0;
    fpt = fopen("temp.txt", "r");
    if(fpt == NULL)
        return 0;
    ch = fgetc(fpt);
    while(ch != EOF)
    {
        ch = ch-100;
        fputc(ch, fps);
        ch = fgetc(fpt);
    }
    fclose(fps);
    fclose(fpt);
    printf("\nFile %s Decrypted Successfully!", fname);
    getch();
    return 0;
}

Here is the sample run of previous program (just above this sentence):

Now if you will see the file codescracker.txt, then it contains its original content that was:

my usernamed is codescracker
my password is 1234@cc

That is, if you will see your file's content, then you will found that your file's content will be decrypted i.e., you will get back your original content.

Note - You can use a lot of algorithm and technique (with your own) to encrypt and decrypt your file's content.

Same Program in Other Languages

• C++ Encrypt and Decrypt Files

C Online Test