Assignment7.java

import java.io.*; import java.math.*; 
class Assignmnet7
{
public static void main(String args[])throws IOException
{
int q,p,n,pn,publickey=0,d=0,msg; double cipher,ptext;
int check,check1;
DataInputStream in=new DataInputStream(System.in); 

System.out.println("*******************************");

System.out.println("Assignment No : 7");
System.out.println("*******************************");

System.out.println("ENTER NO"); p=Integer.parseInt(in.readLine()); q=Integer.parseInt(in.readLine());
check=prime(p); check1=prime(q); if(check!=1||check1!=1)
{
System.exit(0);
}
n=p*q;
pn=(p-1)*(q-1); for(int e=2;e<pn;e++)
{
if(gcd(e,pn)==1)
{
publickey=e; System.out.println("PUBLIC KEY :"+e); break;
}
}
for(int i=0;i<pn;i++)
{
d=i; 
if(((d*publickey)%pn)==1) 
   break;
}
System.out.println("PRIVATE KEY :"+d); 
System.out.println("ENTER MESSAGE "); 
msg=Integer.parseInt(in.readLine()); 
cipher=Math.pow(msg,publickey); 
cipher=cipher%n;
System.out.println("ENCRYPTED :"+cipher);
ptext=Math.pow(cipher,d);
ptext=ptext%n; System.out.println("DECRYPTED :"+ptext);
}
static int prime(int a)
{
int flag=0;
for(int i=2;i<a;i++)
{
if(a%i==0)
{
System.out.println(a+" is not a Prime Number"); flag = 1;
return 0;
}
}
if(flag==0) return 1;
return 1;
}
static int gcd(int number1, int number2)
{
if(number2 == 0){ return number1;
}
return gcd(number2, number1%number2);
}
}