RSA Algorithm > Java Program

System Security

RSA Algorithm > Java Program

RSA is a public key encryption algorithm and the name RSA derived from it's investors Rivest,Shamir,Adleman.RSA works on the principle that says it is too difficult to find the factor of large prime numbers.  

import java.io.*; 
import java.math.*; 
class rsa 
{ 
public static void main(String args[])throws IOException 
{ 

int q,p,n,pn,publickey=0,d=0,msg; 
double cipher,ptext; 
int check,check1; 
BufferedReader in=new BufferedReader(new InputStreamReader(System.in)); 
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);     
} 
} 

Output:
run:
ENTER NO
3
11
PUBLIC KEY :3
PRIVATE KEY :7
ENTER MESSAGE 
20
ENCRYPTED  :14.0
DECRYPTED  :20.0

BUILD SUCCESSFUL (total time: 11 seconds)