This is simply for fun. This will not be used for any actual encryption. I'm only first year comp sci student and love cryptography.
This took a long time to get working. At approximately N = 18, it begins breaking down. It won't encrypt messages properly after that point. I'm not sure why. Any insights? I'd also appreciate any links you could provide me to tutorials or interesting reading about Cryptography.
import java.math.BigInteger;
import java.security.SecureRandom;
/**
* Cryptography.
*
* Generates public and private keys used in encryption and
* decryption
*
*/
public class RSA
{
private final static BigInteger one = new BigInteger("1");
private final static SecureRandom random = new SecureRandom();
// prime numbers
private BigInteger p;
private BigInteger q;
// modulus
private BigInteger n;
// totient
private BigInteger t;
// public key
private BigInteger e;
// private key
private BigInteger d;
private String cipherText;
/**
* Constructor for objects of class RSA
*/
public RSA(int N)
{
p = BigInteger.probablePrime(N/2, random);
q = BigInteger.probablePrime(N/2, random);
// initialising modulus
n = p.multiply(q);
// initialising t by euclid's totient function (p-1)(q-1)
t = (p.subtract(one)).multiply(q.subtract(one));
// initialising public key ~ 65537 is common public key
e = new BigInteger("65537");
}
public int generatePrivateKey()
{
d = e.modInverse(t);
return d.intValue();
}
public String encrypt(String plainText)
{
String encrypted = "";
int j = 0;
for(int i = 0; i < plainText.length(); i++){
char m = plainText.charAt(i);
BigInteger bi1 = BigInteger.valueOf(m);
BigInteger bi2 = bi1.modPow(e, n);
j = bi2.intValue();
m = (char) j;
encrypted += m;
}
cipherText = encrypted;
return encrypted;
}
public String decrypt()
{
String decrypted = "";
int j = 0;
for(int i = 0; i < cipherText.length(); i++){
char c = cipherText.charAt(i);
BigInteger bi1 = BigInteger.valueOf(c);
BigInteger bi2 = bi1.modPow(d, n);
j = bi2.intValue();
c = (char) j;
decrypted += c;
}
return decrypted;
}
}
