C Exercises: Encrypt a text file

Last update on August 08 2025 12:52:55 (UTC/GMT +8 hours)

13. Encrypt File

Write a program in C to encrypt a text file.

 Assume that, the content of the file test.txt is  :                                                                       
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Sample Solution:

C Code:

#include <stdio.h>
#include <stdlib.h>

void main()
{
	char fname[20], ch;
	FILE *fpts, *fptt;
	
	printf("\n\n Encrypt a text file :\n");
	printf("--------------------------\n"); 	
	
	printf(" Input the name of file to encrypt : ");
	scanf("%s",fname);	

	fpts=fopen(fname, "r");
	if(fpts==NULL)
	{
		printf(" File does not exists or error in opening..!!");
		exit(1);
	}
	fptt=fopen("temp.txt", "w");
	if(fptt==NULL)
	{
		printf(" Error in creation of file temp.txt ..!!");
		fclose(fpts);
		exit(2);
	}
	while(1)
	{
		ch=fgetc(fpts);
		if(ch==EOF)
		{
			break;
		}
		else
		{
			ch=ch+100;
			fputc(ch, fptt);
		}
	}
	fclose(fpts);
	fclose(fptt);
	fpts=fopen(fname, "w");
	if(fpts==NULL)
	{
		printf(" File does not exists or error in opening..!!");
		exit(3);
	}
	fptt=fopen("temp.txt", "r");
	if(fptt==NULL)
	{
		printf(" File does not exists or error in opening..!!");
		fclose(fpts);
		exit(4);
	}
	while(1)
	{
		ch=fgetc(fptt);
		if(ch==EOF)
		{
			break;
		}
		else
		{
			fputc(ch, fpts);
		}
	}
	printf(" File %s successfully encrypted ..!!\n\n", fname);
	fclose(fpts);
	fclose(fptt);
}

Sample Output:

 Encrypt a text file :                                                                                        
--------------------------                                                                                    
 Input the name of file to encrypt : test.txt                                                                 
 File test.txt successfully encrypted ..!!

If you read the file test.txt you will see the following :

������Ʉ�ӄۗ�������ɒ��ђn

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to encrypt a file using a simple substitution cipher and save the encrypted file.
Write a C program to encrypt a file using XOR encryption with a user-provided key.
Write a C program to encrypt the contents of a file line by line and then write the encrypted lines to a new file.
Write a C program to encrypt a file and then output the encrypted content to the console in hexadecimal format.

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C Programming Code Editor:

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