C Exercises: Decrypt a previously encrypted file

Last update on August 08 2025 12:53:19 (UTC/GMT +8 hours)

14. Decrypt File

Write a program in C to decrypt a previously encrypted file.

 Assume that, the content of the file test.txt was  :                                                                       
������Ʉ�ӄۗ�������ɒ��ђn

Sample Solution:

C Code:

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

void main()
{
	char ch, fname[20];
	FILE *fpts, *fptt;
	
	printf("\n\n Decrypt a text file :\n");
	printf("--------------------------\n"); 	
	
	printf(" Input the name of file to decrypt : ");
	scanf("%s",fname);	
	
	fpts=fopen(fname, "w");
	if(fpts==NULL)
	{
		printf(" File does not exists or error in opening..!!");
		exit(7);
	}
	fptt=fopen("temp.txt", "r");
	if(fptt==NULL)
	{
		printf(" File does not exists or error in opening..!!");
		fclose(fpts);
		exit(9);
	}
	while(1)
	{
		ch=fgetc(fptt);
		if(ch==EOF)
		{
			break;
		}
		else
		{
			ch=ch-100;
			fputc(ch, fpts);
		}
	}
	printf(" The file %s decrypted successfully..!!\n\n",fname);
	fclose(fpts);
	fclose(fptt);
}

Sample Output:

 Decrypt a text file :                                                                                        
--------------------------                                                                                    
 Input the name of file to decrypt : test.txt                                                                 
 The file test.txt decrypted successfully..!!

After decrypt, the content of the file is : 
Welcome to w3resource.com.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to decrypt a file that was encrypted using a simple substitution cipher.
Write a C program to decrypt a file encrypted with XOR encryption by prompting the user for the key.
Write a C program to decrypt a file and then verify the decrypted content against a known checksum.
Write a C program to decrypt a file and restore its original formatting and line breaks.

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