Java Cryptography - Decrypting Data

You can decrypt the encrypted data using the Cipher class of the javax.crypto package. Follow the steps given below to decrypt given data using Java.

Step 1: Create a KeyPairGenerator object

The KeyPairGenerator class provides getInstance() method which accepts a String variable representing the required key-generating algorithm and returns a KeyPairGenerator object that generates keys.

Create KeyPairGenerator object using the getInstance() method as shown below.

//Creating KeyPair generator object
KeyPairGenerator keyPairGen = KeyPairGenerator.getInstance("DSA");

Step 2: Initialize the KeyPairGenerator object

The KeyPairGenerator class provides a method named initialize() this method is used to initialize the key pair generator. This method accepts an integer value representing the key size.

Initialize the KeyPairGenerator object created in the previous step using the initialize() method as shown below.

//Initializing the KeyPairGenerator
keyPairGen.initialize(2048);

Step 3: Generate the KeyPairGenerator

You can generate the KeyPair using the generateKeyPair() method of the KeyPairGenerator class. Generate the key pair using this method as shown below.

//Generate the pair of keys
KeyPair pair = keyPairGen.generateKeyPair();

Step 4: Get the public key

You can get the public key from the generated KeyPair object using the getPublic() method as shown below.

//Getting the public key from the key pair
PublicKey publicKey = pair.getPublic();

Step 5: Create a Cipher object

The getInstance() method of Cipher class accepts a String variable representing the required transformation and returns a Cipher object that implements the given transformation.

Create the Cipher object using the getInstance() method as shown below.

//Creating a Cipher object
Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");

Step 6: Initialize the Cipher object

The init() method of the Cipher class accepts two parameters

• An integer parameter representing the operation mode (encrypt/decrypt)
• Key object representing the public key

Initialize the Cypher object using the init() method as shown below.

//Initializing a Cipher object
cipher.init(Cipher.ENCRYPT_MODE, publicKey);

Step 7: Add data to the Cipher object

The update() method of the Cipher class accepts a byte array representing the data to be encrypted and updates the current object with the data given.

Update the initialized Cipher object by passing the data to the update()method in the form of byte array as shown below.

//Adding data to the cipher
byte[] input = "Welcome to Tutorialspoint".getBytes();   
cipher.update(input);

Step 8: Encrypt the data

The doFinal() method of the Cipher class completes the encryption operation. Therefore, finish the encryption using this method as shown below.

//Encrypting the data
byte[] cipherText = cipher.doFinal();

Step 9: Initialize the Cipher object for decryption

To decrypt the cypher encrypted in the previous steps you need to initialize it for decryption.

Therefore, initialize the cipher object by passing the parameters Cipher.DECRYPT_MODE and PrivateKey object as shown below.

//Initializing the same cipher for decryption
cipher.init(Cipher.DECRYPT_MODE, pair.getPrivate());

Step 10: Decrypt the data

Finally, Decrypt the encrypted text using the doFinal() method as shown below.

//Decrypting the text
byte[] decipheredText = cipher.doFinal(cipherText);

Example

Following Java program accepts text from user, encrypts it using RSA algorithm and, prints the cipher of the given text, decrypts the cipher and prints the decrypted text again.

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.Signature;

import javax.crypto.Cipher;

public class CipherDecrypt {
   public static void main(String args[]) throws Exception{
    //Creating a Signature object
      Signature sign = Signature.getInstance("SHA256withRSA");
      
      //Creating KeyPair generator object
      KeyPairGenerator keyPairGen = KeyPairGenerator.getInstance("RSA");
      
      //Initializing the key pair generator
      keyPairGen.initialize(2048);
      
      //Generate the pair of keys
      KeyPair pair = keyPairGen.generateKeyPair();   
      
      //Getting the public key from the key pair
      PublicKey publicKey = pair.getPublic();  

      //Creating a Cipher object
      Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");

      //Initializing a Cipher object
      cipher.init(Cipher.ENCRYPT_MODE, publicKey);
   
      //Add data to the cipher
      byte[] input = "Welcome to Tutorialspoint".getBytes();   
      cipher.update(input);
   
      //encrypting the data
      byte[] cipherText = cipher.doFinal();  
      System.out.println( new String(cipherText, "UTF8"));

      //Initializing the same cipher for decryption
      cipher.init(Cipher.DECRYPT_MODE, pair.getPrivate());
      
      //Decrypting the text
      byte[] decipheredText = cipher.doFinal(cipherText);
      System.out.println(new String(decipheredText));
   }
}

Output

The above program generates the following output −

Encrypted Text:
]/[?F3?D?p
v?w?!?H???^?A??????P?u??FA?
?
???_?? ???_jMH-??>??OP?'?j?_?n`
?_??'`????o??_GL??g???g_f?????f|???LT?|?Vz_TDu#??\?<b,,?$C2???Bq?#?lDB`??g,^??K?_?v???`}
?;LX?a?_5e???#???_?6?/B&B_???^?__Ap^#_?q?IEh????_?,??*??]~_?_?D?
_y???lp??a?P_U{

Decrypted Text:
Welcome to Tutorialspoint