aticivural · May 20, 2019 13:14 · Dec 28, 2016
diff --git a/RsaExample.java b/RsaExample.java
@@ -0,0 +1,98 @@
+import javax.crypto.Cipher;
+import java.io.InputStream;
+import java.security.*;
+import java.util.Base64;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+public class RsaExample {
+    public static KeyPair generateKeyPair() throws Exception {
+        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
+        generator.initialize(2048, new SecureRandom());
+        KeyPair pair = generator.generateKeyPair();
+
+        return pair;
+    }
+
+    public static KeyPair getKeyPairFromKeyStore() throws Exception {
+        //Generated with:
+        //  keytool -genkeypair -alias mykey -storepass s3cr3t -keypass s3cr3t -keyalg RSA -keystore keystore.jks
+
+        InputStream ins = RsaExample.class.getResourceAsStream("/keystore.jks");
+
+        KeyStore keyStore = KeyStore.getInstance("JCEKS");
+        keyStore.load(ins, "s3cr3t".toCharArray());   //Keystore password
+        KeyStore.PasswordProtection keyPassword =       //Key password
+                new KeyStore.PasswordProtection("s3cr3t".toCharArray());
+
+        KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry) keyStore.getEntry("mykey", keyPassword);
+
+        java.security.cert.Certificate cert = keyStore.getCertificate("mykey");
+        PublicKey publicKey = cert.getPublicKey();
+        PrivateKey privateKey = privateKeyEntry.getPrivateKey();
+
+        return new KeyPair(publicKey, privateKey);
+    }
+
+    public static String encrypt(String plainText, PublicKey publicKey) throws Exception {
+        Cipher encryptCipher = Cipher.getInstance("RSA");
+        encryptCipher.init(Cipher.ENCRYPT_MODE, publicKey);
+
+        byte[] cipherText = encryptCipher.doFinal(plainText.getBytes(UTF_8));
+
+        return Base64.getEncoder().encodeToString(cipherText);
+    }
+
+    public static String decrypt(String cipherText, PrivateKey privateKey) throws Exception {
+        byte[] bytes = Base64.getDecoder().decode(cipherText);
+
+        Cipher decriptCipher = Cipher.getInstance("RSA");
+        decriptCipher.init(Cipher.DECRYPT_MODE, privateKey);
+
+        return new String(decriptCipher.doFinal(bytes), UTF_8);
+    }
+
+    public static String sign(String plainText, PrivateKey privateKey) throws Exception {
+        Signature privateSignature = Signature.getInstance("SHA256withRSA");
+        privateSignature.initSign(privateKey);
+        privateSignature.update(plainText.getBytes(UTF_8));
+
+        byte[] signature = privateSignature.sign();
+
+        return Base64.getEncoder().encodeToString(signature);
+    }
+
+    public static boolean verify(String plainText, String signature, PublicKey publicKey) throws Exception {
+        Signature publicSignature = Signature.getInstance("SHA256withRSA");
+        publicSignature.initVerify(publicKey);
+        publicSignature.update(plainText.getBytes(UTF_8));
+
+        byte[] signatureBytes = Base64.getDecoder().decode(signature);
+
+        return publicSignature.verify(signatureBytes);
+    }
+
+    public static void main(String... argv) throws Exception {
+        //First generate a public/private key pair
+        KeyPair pair = generateKeyPair();
+        //KeyPair pair = getKeyPairFromKeyStore();
+
+        //Our secret message
+        String message = "the answer to life the universe and everything";
+
+        //Encrypt the message
+        String cipherText = encrypt(message, pair.getPublic());
+
+        //Now decrypt it
+        String decipheredMessage = decrypt(cipherText, pair.getPrivate());
+
+        System.out.println(decipheredMessage);
+
+        //Let's sign our message
+        String signature = sign("foobar", pair.getPrivate());
+
+        //Let's check the signature
+        boolean isCorrect = verify("foobar", signature, pair.getPublic());
+        System.out.println("Signature correct: " + isCorrect);
+    }
+}