From 851432fe870be4218433703c0752cf1757dcdd60 Mon Sep 17 00:00:00 2001
From: Nguyen Duy Tiep <duytiep@gmail.com>
Date: Tue, 24 Oct 2017 20:39:11 +0800
Subject: [PATCH] RSA encryption

---
 ciphers/RSA.java | 62 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)
 create mode 100644 ciphers/RSA.java

diff --git a/ciphers/RSA.java b/ciphers/RSA.java
new file mode 100644
index 00000000..5baa61b0
--- /dev/null
+++ b/ciphers/RSA.java
@@ -0,0 +1,62 @@
+package ciphers;
+
+import java.math.BigInteger;
+import java.security.SecureRandom;
+
+/**
+ * Created by Nguyen Duy Tiep on 23-Oct-17.
+ */
+public class RSA {
+    private BigInteger modulus, privateKey, publicKey;
+
+    public RSA(int bits) {
+        generateKeys(bits);
+    }
+
+    public synchronized String encrypt(String message) {
+        return (new BigInteger(message.getBytes())).modPow(publicKey, modulus).toString();
+    }
+
+    public synchronized BigInteger encrypt(BigInteger message) {
+        return message.modPow(publicKey, modulus);
+    }
+
+    public synchronized String decrypt(String message) {
+        return new String((new BigInteger(message)).modPow(privateKey, modulus).toByteArray());
+    }
+
+    public synchronized BigInteger decrypt(BigInteger message) {
+        return message.modPow(privateKey, modulus);
+    }
+
+    /** Generate a new public and private key set. */
+    public synchronized void generateKeys(int bits) {
+        SecureRandom r = new SecureRandom();
+        BigInteger p = new BigInteger(bits / 2, 100, r);
+        BigInteger q = new BigInteger(bits / 2, 100, r);
+        modulus = p.multiply(q);
+
+        BigInteger m = (p.subtract(BigInteger.ONE)).multiply(q.subtract(BigInteger.ONE));
+
+        publicKey = new BigInteger("3");
+
+        while (m.gcd(publicKey).intValue() > 1) {
+            publicKey = publicKey.add(new BigInteger("2"));
+        }
+
+        privateKey = publicKey.modInverse(m);
+    }
+
+    /** Trivial test program. */
+    public static void main(String[] args) {
+        RSA rsa = new RSA(1024);
+
+        String text1 = "This is a message";
+        System.out.println("Plaintext: " + text1);
+
+        String ciphertext = rsa.encrypt(text1);
+        System.out.println("Ciphertext: " + ciphertext);
+
+        System.out.println("Plaintext: " + rsa.decrypt(ciphertext));
+    }
+}