#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<ctype.h>
void main()
{
	FILE *f1;
	FILE *f2;
	FILE *f3;
	char ch,nc;
	int i,j,k,key,key2;


	clrscr();

	f1=fopen("pt.txt","r");
	f2=fopen("ct.txt","w");

	if(f1==NULL)
	{
		printf("Can't open one of the files. Exiting ...");
		exit(1);
	}
	printf("Enter the key: ");
	scanf("%d",&key);

	while(1)
	{
		ch=fgetc(f1);
		if(ch==EOF)
			break;

		nc=ch;
		if(isupper(ch))
		{
			nc =key;
			if(nc>90)
				nc-=26;
		}
		if(islower(ch))
		{
			nc =key;
			if(nc>122)
				nc-=26;
		}

		fputc(nc,f2);



	}

	fclose(f1);
	fclose(f2);
	printf("\n\nDecryping now ,,, \n\n");

	f2=fopen("ct.txt","r");
	f3=fopen("dt.txt","w ");
	ch='a';

	while(1)
	{
		ch=fgetc(f2);
		if(ch==EOF)
			break;

		nc=ch;
		if(isupper(ch))
		{
			nc-=key;
			if(nc<65)
				nc =26;
		}
		if(islower(ch))
		{
			nc-=key;
			if(nc<97)
				nc =26;
		}

		fputc(nc,f3);



	}





	printf("\nEnd of Program. Press any key to exit.");
	getch();
}