/*linear feedback shift register of length L consisting of L stages numbered 0,1,......,L-1 each capable of storing one bit, 
the intial stage s0,s1,s2,.......,sL-1, stored in 0th,ist,2nd,.......,L-1th stage respecyively.0th stage is given the coeffnt. cl,1st stage the
coeffnt. cL-1,..........L-1th stage the coeffnt. c1 */



#include<stdio.h>
#include<conio.h>
#include<iostream.h>
#include<conio.h>
#include<string.h>
#include<math.h>
const int a1=300;
const int a2=300;
void encrypt();
void lfsr(int[a1],int,int);
void decrypt(void);
void pass(int[a2]);
int powr(int);
void main()
{
	//clrscr();
	int chk;
	do
	{
	printf("\n--------------------------------------------------------------------------------\n");
	printf("\n                                     *MAIN MENU*");
	printf("\n");
	printf("\t \t \t \n                          *PR0GRAMME TO DEMONSTRATE LFSR*");
	printf("\n--------------------------------------------------------------------------------\n");
	printf("\n Press 1 to ENCRYPT:");
	printf("\n Press 2 to DECRYPT:");
	printf("\n Press 3 to EXIT:");
	printf("\n Please enter your choice......");
	scanf("%d",&chk);
	printf("\n");
	switch(chk)
		{
		case 1:
		encrypt();
		break;
		case 2:
		decrypt();
		break;
		case 3:
		break;
		default:
		printf("\n You have enter a wrong choice!!!!!!!!\n");
		break;
		}
	}
	while(chk!=3);
}
void encrypt()
{
char pln[40];
int l,L,s,cnt=0,cip[200],i;
printf("\n Enter the length of the LFSR:");
scanf("%d",&L);
printf("\n Enter the plain  text:");
cin.getline(pln,'\40');
l=strlen(pln);
printf("\n The plain text in binary form is:");
for(int k=0;k<l;k++)
	{
	int rem=0,q=0,a[20],i=19,b,cn=1,j;
	b=pln[k];
	rem=b%2;
		while(q!=1)
		{
		rem=b%2;
		q=(b-rem)/2;
		b=q;
		a[i]=rem;
		i--;
		cn++;
		}
		a[i]=1;
		if(cn<8)
	{
	for(j=i-(8-cn);j<i;j++)
	 a[j]=0;
	 }
		for(j=i-(8-cn);j<20;j++)
		{
		cip[cnt]=a[j];
		cnt++;
		printf("%d",a[j]);
		}
	       //	printf(" ");
	}
printf("\n Length of the cipher text is:%d",cnt);
lfsr(cip,cnt,L);
}  //end of encrypt
void decrypt()
{
int cip[200],i,j,l,L,s[200],c[10],k;
int tmp=0,x;
printf("\n Enter the length of the cipher text:");
scanf("%d",&l);
printf("\n Enter the cipher text:");
for(i=0;i<l;i++)
	{
	scanf("%d",&cip[i]);
	}
printf("\n Enter the length of the LFSR:");
scanf("%d",&L);
for(j=0;j<l;j++)
	{
	s[j]=0;
	}
printf("\n Enter the initial state:");
for(i=0;i<L;i++)
	{
	scanf("%d",&s[i]);
	}
for(j=1;j<=L;j++)
	{
	printf("\n Enter the coeff. c(%d):",j);
	scanf("%d",&c[j]);
	}
for(j=0;j<l;j++)
	{
	if(j<L)
		{

		}
	else
		{
	for(i=1;i<=L;i++)
			{
			s[j]=(s[j]+(c[i]*s[j-i]))%2;
			}
		}
	}
for(i=0;i<l;i++)
	{
	cip[i]=(cip[i]+s[i])%2;
	//printf("%d",cip[i]);
	}
	k=8;
	j=0;
printf("\n The plain text is:");
for(i=1;i<=l;i=i+8)
	{
		do
		{
		x=powr(k);
		tmp=tmp+(x*cip[j]);
		k--;
		j++;
		}
		while(k>0);
		printf("%c",tmp);
		k=8;
		tmp=0;

	}
}     //end of decrypt function
void lfsr(int cip[a1],int cnt,int L)
{
int s[400],i,c[10],j,ci[400];
printf("\n");
for(j=0;j<=cnt;j++)
	{
	s[j]=0;
	}
printf("\n Enter the initial state:");
for(i=0;i<L;i++)
	{
	scanf("%d",&s[i]);
	}
for(j=1;j<=L;j++)
	{
	printf("\n Enter the coeff. c(%d):",j);
	scanf("%d",&c[j]);
	}
printf("\n");
printf("\n The LFSR sequence is:            ");
for(j=0;j<cnt;j++)
	{
	if(j<L)
		{
		printf("%d",s[j]);
		}
	else
		{
	for(i=1;i<=L;i++)
			{
			s[j]=(s[j]+(c[i]*s[j-i]))%2;
			}
		printf("%d",s[j]);
		}
	}
printf("\n");
printf("\n The cipher text is:            \t");
for(i=1;i<=cnt;i++)
	{
	ci[i-1]=(s[i-1]+cip[i-1])%2;
	printf("%d",(s[i-1]+cip[i-1])%2);
	if(i%8==0)
		{
		printf(" ");
		}
	}
printf("\n");
int chk;
printf("\n Press 1 to pass cipher text for decryption:");
printf("\n Press 2 for manual input of cipher text:");
scanf("%d",&chk);
if(chk==1)
	{
	pass(ci);
	}
else
	{
	decrypt();
	}
}
void pass(int cip[a2])
{
printf("\n");
int i,j,l,L,s[200],c[10],k=7;
int tmp=0,x;
printf("\n Enter the length of the cipher text:");
scanf("%d",&l);
printf("\n Enter the length of the LFSR:");
scanf("%d",&L);
for(j=0;j<l;j++)
	{
	s[j]=0;
	}
printf("\n");
printf("\n Enter the initial state:");
for(i=0;i<L;i++)
	{
	scanf("%d",&s[i]);
	}
for(j=1;j<=L;j++)
	{
	printf("\n Enter the coeff. c(%d):",j);
	scanf("%d",&c[j]);
	}
printf("\n");
for(j=0;j<l;j++)
	{
	if(j<L)
		{

		}
	else
		{
	for(i=1;i<=L;i++)
			{
			s[j]=(s[j]+(c[i]*s[j-i]))%2;
			}
		}
	}
for(i=0;i<l;i++)
	{
	cip[i]=(cip[i]+s[i])%2;
	}
	k=8;
	j=0;
printf("\n");
printf("\n The plain text is:");
for(i=1;i<=l;i=i+8)
	{
		do
		{
		x=powr(k);
		tmp=tmp+(x*cip[j]);
		k--;
		j++;
		}
		while(k>0);
		printf("%c",tmp);
		k=8;
		tmp=0;

	}
}//end of pass

int powr(int k)
{
int l=1,i;
for(i=1;i<k;i++)
	{
	l=(l*2);
	}
return l;
}