Encryption and Decryption using AES Algorithm in JAVA

Encryption and Decryption

Encryption is the process of converting plaintext to cipher-text (had to understand) by applying mathematical transformations.  These transformations are known as encryption algorithms and require an encryption key.

Decryption is the reverse process of getting back the original data from the cipher-text using a decryption key.  In Symmetric cryptology- The encryption key and the decryption key could be the same as in symmetric or secret key cryptography, The key can different as in asymmetric or public key cryptography. 

Types of cryptography

Symmetric cryptography uses the same secret (private) key to encrypt and decrypt its data

Symmetric requires that the secret key be known by the party encrypting the data and the party decrypting the data.

Asymmetric Cryptography uses both a public and private key.  Asymmetric allows for distribution of your public key to anyone. Using the key they can encrypt the data they want to send securely and then it can only be decoded by the person having the private key

Symmetric Algorithms

Common  Algorithms

DES (Data Encryption Standard)	Old Crypto algorithm. 64 Bits, 56 are effectively used, easy to break.3DES - derived from DES, works by cascading 3 instances of DES. This is slow
3DES	derived from DES, works by cascading 3 instances of DES. This is slow
AES (Advanced Encryption Standard)	Successor of DES. Accepts 128, 192, 256 bits. When in doubt use AES. This is supported after JDK 1.4 onwards

Symmetric key encryption/decryption uses a secret key during the process.   A String encrypted with a secret key cannot be decrypted using another secret key.   This is more effective if both the parties keep the key secret.

In our example we will be using a AES Algorithm with AES - 128 , AES has a fixed block size of 128 bits. We will be using a password based secret key. In the below table you will see the code comparison  of how encryption and decryption is easily done

Encryption Code	Decryption Code
Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");  SecretKeySpec secretKey = new SecretKeySpec(key, "AES"); cipher.init(Cipher.ENCRYPT_MODE, secretKey);  String encryptedString = Base64.encodeBase64String(cipher.doFinal(strToEncrypt.getBytes())); return encryptedString;	Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5PADDING"); SecretKeySpec secretKey = new SecretKeySpec(key, "AES"); cipher.init(Cipher.DECRYPT_MODE, secretKey); String decryptedString = new String(cipher.doFinal(Base64.decodeBase64(strToDecrypt))); return decryptedString;

Below is an example standalone java class which can generate a encrypted string, and decrypt a given string.  The secret-key used for encryption/decryption is "thisIsASecretKey"- converted to bytes.  The encrypted string at last encoded using Base64.encodeBase64String that gives an ascii string.  During the decryption, the ascii base64encoded string  is decoded using Base64.decodeBase64 and then decrypted using cipher.doFinal().

 

How to run the program

Encryption

java cipher.CipherUtils -encrypt abcd

String to Encrypt : abcd
Encrypted : 0ualDZ4X/FUDjmrqLWROyw==

Decryption

java cipher.CipherUtils -decrypt 0ualDZ4X/FUDjmrqLWROyw==

String To Decrypt : 0ualDZ4X/FUDjmrqLWROyw==
Decrypted : abcd

Code

package cipher;


import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

import org.apache.commons.cli.CommandLine;
import org.apache.commons.cli.CommandLineParser;
import org.apache.commons.cli.HelpFormatter;
import org.apache.commons.cli.Option;
import org.apache.commons.cli.Options;
import org.apache.commons.cli.PosixParser;
import org.apache.commons.codec.binary.Base64;

public class CipherUtils
{

    private static byte[] key = {
            0x74, 0x68, 0x69, 0x73, 0x49, 0x73, 0x41, 0x53, 0x65, 0x63, 0x72, 0x65, 0x74, 0x4b, 0x65, 0x79
    };//"thisIsASecretKey";

    public static String encrypt(String strToEncrypt)
    {
        try
        {
            Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");
            final SecretKeySpec secretKey = new SecretKeySpec(key, "AES");
            cipher.init(Cipher.ENCRYPT_MODE, secretKey);
            final String encryptedString = Base64.encodeBase64String(cipher.doFinal(strToEncrypt.getBytes()));
            return encryptedString;
        }
        catch (Exception e)
        {
           e.printStackTrace();
        }
        return null;

    }

    public static String decrypt(String strToDecrypt)
    {
        try
        {
            Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5PADDING");
            final SecretKeySpec secretKey = new SecretKeySpec(key, "AES");
            cipher.init(Cipher.DECRYPT_MODE, secretKey);
            final String decryptedString = new String(cipher.doFinal(Base64.decodeBase64(strToDecrypt)));
            return decryptedString;
        }
        catch (Exception e)
        {
          e.printStackTrace();

        }
        return null;
    }


    public static void main(String args[])
    {

        CommandLineParser parser = new PosixParser();
        Options options = new Options();
        Option help = new Option("help", "Display help");
        Option encrypt = new Option("encrypt", true, " - string to encrypt");
        Option decrypt = new Option("decrypt", true, " - string to decrypt");
        options.addOption(help);
        options.addOption(encrypt);
        options.addOption(decrypt);
        try
        {
            CommandLine cmd = parser.parse(options, args);
            if (cmd.hasOption("encrypt"))
            {
                final String strToEncrypt = cmd.getOptionValue("encrypt");
                final String encryptedStr = CipherUtils.encrypt(strToEncrypt.trim());
                System.out.println("String to Encrypt : " + strToEncrypt);
                System.out.println("Encrypted : " + encryptedStr);
            }
            else if (cmd.hasOption("decrypt"))
            {
                final String strToDecrypt = cmd.getOptionValue("decrypt");
                final String decryptedStr = CipherUtils.decrypt(strToDecrypt.trim());
                System.out.println("String To Decrypt : " + strToDecrypt);
                System.out.println("Decrypted : " + decryptedStr);
            }
            else
            {
                HelpFormatter formatter = new HelpFormatter();
                formatter.printHelp("[-h] [-encrypt ]", options);
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

    }
}

Dependencies: 

Required: commons-cli 1.2 and commons-codec 1.7

Eclipse Maven project to run this

https://www.dropbox.com/s/rg8tfz1mf592b92/cipher-project.zip