#include<iostream.h>
#include<string.h>
#include<conio.h>
#include<stdio.h>
void encrypt(void);
void decrypt(void);
int gcd(int);
int invr(int);
int phi(int);
void main()
{
	clrscr();
	char chk;
	do
	{
	printf("\n \n MAIN MENU:");
	printf("\n");
	printf("\t \t \t \n PROGRAM TO DEMONSTRATE AFFINE CIPHER:");
	printf("\n");
	printf("\n Press 'e' to encrypt 'd' to decrypt 'x' to exit:");
	scanf("%c",&chk);
	printf("\n");
	switch(chk)
		{
		case 'd':
		decrypt();
		break;
		case 'e':
		encrypt();
		break;
		case 'x':
		break;
		default:
		break;
		}
		clrscr();
	}
	while(chk!='x');
}
void decrypt()
{
char cip[50];char* res;
int t,l,i=0,arr[50],flag=0,j=0,m=0;
printf("\n Enter the cipher text in upper case:");
cin.getline(cip,'\50');
l=strlen(cip);
res=new char[312*l+1];
printf("\n The possible plain texts are:");
		do
		{
		arr[j]=int(cip[j])-65;
		t=arr[j];
		//cout<<t;
		j++;
			for(int b=1;b<=26;b++)
			{
				if(phi(b)!=0)
				{
				for(i=1;i<=26;i++)
				{
					int z=t-i;
					if(z<0)
						{
						z=z+26;
						}
					int r=(invr(phi(b))*(z))%26;
					if(r<0)
						{
						int k=(r*(-1))%26;
						r=k;
						}
					//printf("(%d,%d,%d)%d ",phi(b),invr(phi(b)),i,r);
					res[m]=char(r+97);
					m++;
					//getche();
						}
				}
			}
		flag++;
		}while(flag<l);
int lm=0,k=1;
cout<<"\n";
for(i=0;i<=m;i++)
	{
	cout<<res[lm];
	lm=lm+312;
	if(lm>312*l)
		{
		lm=k;
		k++;
		cout<<"\t";
		//getche();
		}
	}
delete res;
getche();
}
void encrypt()
{
char pln[50];
int a,b,t,l,arr[50],i;
printf("\n Enter the key:");
scanf("%d%d",&a,&b);
gcd(a);
if(gcd(a)!=1)
	{
	printf("\n Message can be encrypted but cannot be decrypted back:");
	getche();
	}
else
	printf("\n Enter the plaintext in lower case:");
	cin.getline(pln,'\50');
	l=strlen(pln);
	printf("\n The encrypted message is:");
	for(i=0;i<l;i++)
		{
		arr[i]=int(pln[i]);
		t=arr[i]-97;
		int u=a*t+b;
		u=97+(u%26);
		cout<<char(u-32);
		}
		getche();
}
int gcd(int r0)
{

	int r1=26,q,r2;
	if(r0==0)
		{
		return r1;
		}
	else
	q=int(r1/r0);
	r2=r1-q*r0;
	while(r2!=0)
	{
	r1=r0;
	r0=r2;
	q=int(r1/r0);
	r2=r1-q*r0;
	}

	return (r0);
	getche();
}
int phi(int j)
{

	int q,r2,tmp,r1;
	r1=j;
	int r0=26;
	tmp=r0;
		q=int(tmp/j);
		r2=tmp-q*j;
		while(r2!=0)
		{
			tmp=j;
			j=r2;
			q=int(tmp/j);
			r2=tmp-q*j;
		}
			if(j==1)
			{
			return r1;
			}
			else{ return(0);}
}
int invr(int u)
{
int temp=1,inv,v=26;
//cout<<"\n";
do
	{
	inv=u*temp-1;
	temp++;
	}
	 while(inv%v!=0);
	 return (temp-1);
}