RSA implementation using java

Question

I am implementing RSA in java I have encountered a code which is given below it is showing plaintext in numeric form after decrypting the plaintext, but I want it in simple english which I entered. I don't want to use the java api.

TestRsa.Java

import java.io.IOException;
import java.math.BigInteger;
import java.util.Random;

public class TestRsa {

    private BigInteger p, q;
    private BigInteger n;
    private BigInteger PhiN;
    private BigInteger e, d;

    public TestRsa() {
        initialize();
    }

    public void initialize() {
        int SIZE = 512;
        /* Step 1: Select two large prime numbers. Say p and q. */
        p = new BigInteger(SIZE, 15, new Random());
        q = new BigInteger(SIZE, 15, new Random());
        /* Step 2: Calculate n = p.q */
        n = p.multiply(q);
        /* Step 3: Calculate ø(n) = (p - 1).(q - 1) */
        PhiN = p.subtract(BigInteger.valueOf(1));
        PhiN = PhiN.multiply(q.subtract(BigInteger.valueOf(1)));
        /* Step 4: Find e such that gcd(e, ø(n)) = 1 ; 1 < e < ø(n) */
        do {
            e = new BigInteger(2 * SIZE, new Random());
        } while ((e.compareTo(PhiN) != 1)
                || (e.gcd(PhiN).compareTo(BigInteger.valueOf(1)) != 0));
        /* Step 5: Calculate d such that e.d = 1 (mod ø(n)) */
        d = e.modInverse(PhiN);
    }

    public BigInteger encrypt(BigInteger plaintext) {
        return plaintext.modPow(e, n);
    }

    public BigInteger decrypt(BigInteger ciphertext) {
        return ciphertext.modPow(d, n);
    }

    public static void main(String[] args) throws IOException {
        TestRsa app = new TestRsa();
        int plaintext;
        System.out.println("Enter any character : ");
        plaintext = System.in.read();
        BigInteger bplaintext, bciphertext;
        bplaintext = BigInteger.valueOf((long) plaintext);
        bciphertext = app.encrypt(bplaintext);
        System.out.println("Plaintext : " + bplaintext.toString());
        System.out.println("Ciphertext : " + bciphertext.toString());
        bplaintext = app.decrypt(bciphertext);
        System.out.println("After Decryption Plaintext : "
                + bplaintext.toString());
    }
}

-1 for posting code that is more than 50% blank lines. If you want people to read your code, it's up to you to make it readable. — user207421
– user207421, Commented Jul 31, 2014 at 6:18

Elton da Costa · Accepted Answer · 2014-05-21 22:55:25Z

My RSA Class:

package com.infovale.cripto;

import java.io.UnsupportedEncodingException;
import java.math.BigInteger;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Arrays;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;

public class RSA {


static String kPublic = "";
static String kPrivate = "";

public RSA()
{

}


public String Encrypt(String plain) throws NoSuchAlgorithmException,
        NoSuchPaddingException, InvalidKeyException,
        IllegalBlockSizeException, BadPaddingException {

    String encrypted;
    byte[] encryptedBytes;      

    KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
    kpg.initialize(1024);
    KeyPair kp = kpg.genKeyPair();

    PublicKey publicKey = kp.getPublic();
    PrivateKey privateKey = kp.getPrivate();

    byte[] publicKeyBytes = publicKey.getEncoded();
    byte[] privateKeyBytes = privateKey.getEncoded();

    kPublic = bytesToString(publicKeyBytes);
    kPrivate = bytesToString(privateKeyBytes);

    Cipher cipher = Cipher.getInstance("RSA");
    cipher.init(Cipher.ENCRYPT_MODE, publicKey);
    encryptedBytes = cipher.doFinal(plain.getBytes());

    encrypted = bytesToString(encryptedBytes);
    return encrypted;

}

public String Decrypt(String result) throws NoSuchAlgorithmException,
        NoSuchPaddingException, InvalidKeyException,
        IllegalBlockSizeException, BadPaddingException {

    byte[] decryptedBytes;

    byte[] byteKeyPrivate = stringToBytes(kPrivate);

    KeyFactory kf = KeyFactory.getInstance("RSA");

    PrivateKey privateKey = null;
    try {

        privateKey = kf.generatePrivate(new PKCS8EncodedKeySpec(byteKeyPrivate));

    } catch (InvalidKeySpecException e) {
        e.printStackTrace();
    }

    String decrypted;

    Cipher cipher = Cipher.getInstance("RSA");
    cipher.init(Cipher.DECRYPT_MODE, privateKey);
    decryptedBytes = cipher.doFinal(stringToBytes(result));
    decrypted = new String(decryptedBytes);
    return decrypted;

}

public String bytesToString(byte[] b) {
    byte[] b2 = new byte[b.length + 1];
    b2[0] = 1;
    System.arraycopy(b, 0, b2, 1, b.length);
    return new BigInteger(b2).toString(36);
}

public byte[] stringToBytes(String s) {
    byte[] b2 = new BigInteger(s, 36).toByteArray();
    return Arrays.copyOfRange(b2, 1, b2.length);
}
}

Nogard · Accepted Answer · 2013-09-29 11:43:21Z

1

try these lines:

System.out.println("Plaintext : " +  new String(bplaintext.toByteArray() ) );
System.out.println("Ciphertext : " +  new String(bciphertext.toByteArray() ) );
bplaintext = app.decrypt(bciphertext);
        System.out.println("After Decryption Plaintext : " + new String(bplaintext.toByteArray() ) );

edited Sep 29, 2013 at 11:43

Nogard

1,7872 gold badges18 silver badges21 bronze badges

answered Sep 29, 2013 at 11:15

manish

111 bronze badge

Comments

Subhadeep Ray · Accepted Answer · 2019-09-23 12:35:51Z

I edited Elton Da Costa's class a little bit adding that feature to convert Public key from String to PublicKey class. Because the user of this class might ultimately want to give out the public key to a third party for encryption of the message and keep the private key with himself for decryption.

            package com.custom.util;

            import java.math.BigInteger;
            import java.security.InvalidKeyException;
            import java.security.KeyFactory;
            import java.security.KeyPair;
            import java.security.KeyPairGenerator;
            import java.security.NoSuchAlgorithmException;
            import java.security.PrivateKey;
            import java.security.PublicKey;
            import java.security.spec.InvalidKeySpecException;
            import java.security.spec.PKCS8EncodedKeySpec;
            import java.security.spec.X509EncodedKeySpec;
            import java.util.Arrays;

            import javax.crypto.BadPaddingException;
            import javax.crypto.Cipher;
            import javax.crypto.IllegalBlockSizeException;
            import javax.crypto.NoSuchPaddingException;

            public class RSA {

                static String kPublic = "";
                static String kPrivate = "";

                public RSA() {

                }

                public class KeyPairStrings {
                    private String pubKey;
                    private String privKey;

                    public String getPubKey() {
                        return pubKey;
                    }

                    public String getPrivKey() {
                        return privKey;
                    }

                    @Override
                    public String toString() {
                        return "KeyPairStrings [pubKey=" + pubKey + ", privKey=" + privKey + "]";
                    }



                }

            public KeyPairStrings getKeyPair() throws NoSuchAlgorithmException{

                KeyPairStrings keyPairStrings = new RSA.KeyPairStrings();

                KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
                kpg.initialize(1024);
                KeyPair kp = kpg.genKeyPair();

                PublicKey publicKey = kp.getPublic();
                PrivateKey privateKey = kp.getPrivate();

                byte[] publicKeyBytes = publicKey.getEncoded();
                byte[] privateKeyBytes = privateKey.getEncoded();

                keyPairStrings.pubKey = bytesToString(publicKeyBytes);
                keyPairStrings.privKey = bytesToString(privateKeyBytes);

                return keyPairStrings ;
            }


            public String encrypt(String text , String pubKey) throws NoSuchAlgorithmException, NoSuchPaddingException, IllegalBlockSizeException, BadPaddingException, InvalidKeyException{

                byte[] pubKeyByte = stringToBytes(pubKey) ;

                KeyFactory kf = KeyFactory.getInstance("RSA");

                PublicKey publicKey = null;
                try {

                    publicKey = kf.generatePublic(new X509EncodedKeySpec(pubKeyByte));

                } catch (InvalidKeySpecException e) {
                    e.printStackTrace();
                }

                Cipher cipher = Cipher.getInstance("RSA");
                cipher.init(Cipher.ENCRYPT_MODE, publicKey );
                byte[] encryptedBytes = cipher.doFinal(text.getBytes());

                String encrypted = bytesToString(encryptedBytes);

                return encrypted;
            }

            public String decrypt(String encryptedText , String privKey) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException{

                byte[] privKeyByte = stringToBytes(privKey) ;

                KeyFactory kf = KeyFactory.getInstance("RSA");

                PrivateKey privateKey = null;
                try {

                    privateKey = kf.generatePrivate(new PKCS8EncodedKeySpec(privKeyByte));

                } catch (InvalidKeySpecException e) {
                    e.printStackTrace();
                }

                String decrypted;

                Cipher cipher = Cipher.getInstance("RSA");
                cipher.init(Cipher.DECRYPT_MODE, privateKey);
                byte[] decryptedBytes = cipher.doFinal(stringToBytes(encryptedText));
                decrypted = new String(decryptedBytes);
                return decrypted;
            }


            public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeyException, NoSuchPaddingException, IllegalBlockSizeException, BadPaddingException {

                String text = "hello" ;

                RSA rsa = new RSA();

                KeyPairStrings keyPairStrings =  rsa.getKeyPair() ;

                System.out.println(keyPairStrings);

                String encrypted  = rsa.encrypt(text, keyPairStrings.getPubKey());

                System.out.println("encrypted : "+encrypted);   

                String textResult = rsa.decrypt(encrypted, keyPairStrings.getPrivKey());

                System.out.println("textResult : "+textResult);

            }



                public static String bytesToString(byte[] b) {
                    byte[] b2 = new byte[b.length + 1];
                    b2[0] = 1;
                    System.arraycopy(b, 0, b2, 1, b.length);
                    return new BigInteger(b2).toString(36);
                }

                public byte[] stringToBytes(String s) {
                    byte[] b2 = new BigInteger(s, 36).toByteArray();
                    return Arrays.copyOfRange(b2, 1, b2.length);
                }
            }

Eliott · Accepted Answer · 2019-10-04 06:08:59Z

0

Instead of this (which only reads the first character):

int plaintext;
plaintext = System.in.read();
bplaintext = BigInteger.valueOf((long) plaintext);

Use this (to read the string):

byte[] plaintext;
plaintext = new Scanner(System.in).nextLine().getBytes();
bplaintext = new BigInteger(plaintext);

Then add this to the end (to convert the decrypted BigInteger back to a string)

System.out.println("Original String: " + new String(bplaintext.toByteArray()));

answered Oct 4, 2019 at 6:08

Eliott

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