Encryption And Decryption Programs In Java

The Java Cipher ( javax.crypto.Cipher) class represents an encryption algorithm.The term Cipher is standard term for an encryption algorithm in the world of cryptography.That is why the Java class is called Cipher and not e.g. Encrypter / Decrypteror something else.You can use a Cipher instance to encrypt and decrypt data in Java.This Java Cipher tutorial will explain how the Cipher class of the Java Cryptography API works.Creating a CipherBefore you can use a Java Cipher you just create an instance of the Cipher class.You create a Cipher instance by calling its getInstance method with a parametertelling what type of encryption algorithm you want to use. Here is an example of creating a Java Cipherinstance:Cipher cipher = Cipher.getInstance('AES');This example creates a Cipher instance using the encryption algorithm called AES.Cipher ModesSome encryption algorithms can work in different modes. An encryption mode specifies details about howthe algorithm should encrypt data. Thus, the encryption mode impacts part of the encryption algorithm.The encryption modes can sometimes be used with multiple different encryption algorithms - like a techniquethat is appended to the core encryption algorithm. That is why the modes are thought of as separate fromthe encryption algorithms themselves, and rather 'add-ons' to the encryption algorithms.

Here aresome of the most wellknown cipher modes:. ECB - Electronic Codebook. CBC - Cipher Block Chaining. CFB - Cipher Feedback. OFB - Output Feedback. CTR - CounterWhen instantiating a cipher you can append its mode to the name of the encryption algorithm. For instance,to create an AES Cipher instance using Cipher Block Chaining (CBC) you use this code:Cipher cipher = Cipher.getInstance('AES/CBC/PKCS5Padding');Since Cipher Block Chaining requires a 'padding scheme' too, the padding scheme is appended in the end of theencryption algorithm name string.Please keep in mind that not all encryption algorithms and modes are supported by the default Java SDKcryptography provider.

You might need an external provider like Bouncy Castle installed to create yourdesired Cipher instance with the required mode and padding scheme.Initializing a CipherBefore you can use a Cipher instance you must initialize it. Initializing a Cipheris done by calling its init method. The init method takes two parameters:. Encryption / decryption cipher operation mode. Encryption / decryption key.Here is an example of initializing a Cipher instance in encryption mode:Key key =.

// get / create symmetric encryption keycipher.init(Cipher.ENCRYPTMODE, key);Here is an example of initializing a Cipher instance in decryption mode:Key key =. // get / create symmetric encryption keycipher.init(Cipher.DECRYPTMODE, key);Encrypting and Decrypting DataIn order encrypt or decrypt data with a Cipher instance you call one of these two methods:. update. doFinalThere are several overridden versions of both update and doFinal whichtakes different parameters. I will cover the most commonly used versions here.If you have to encrypt or decrypt a single block of data, just call the doFinal with thedata to encrypt or decrypt. Here is an encryption example:byte plainText = 'abcdefghijklmnopqrstuvwxyz'.getBytes('UTF-8');byte cipherText = cipher.doFinal(plainText);The code actually looks pretty much the same in case of decrypting data.

Just keep in mind that theCipher instance must be initialized into decryption mode. Here is how decrypting a single blockof cipher text looks:byte plainText = cipher.doFinal(cipherText);If you have to encrypt or decrypt multiple blocks of data, e.g. Multiple blocks from a large file, youcall the update once for each block of data, and finish with a call to doFinalwith the last data block. Here is an example of encrypting multiple blocks of data:byte data1 = 'abcdefghijklmnopqrstuvwxyz'.getBytes('UTF-8');byte data2 = 'zyxwvutsrqponmlkjihgfedcba'.getBytes('UTF-8');byte data3 = '012345'.getBytes('UTF-8');byte cipherText1 = cipher.update(data1);byte cipherText2 = cipher.update(data2);byte cipherText3 = cipher.doFinal(data3);The reason a call to doFinal is needed for the last block of data is, that some encryption algorithmsneed to pad the the data to fit a certain cipher block size (e.g.

An 8 byte boundary). But - we do not want topad the intermediate blocks of data encrypted. Hence the calls to update for intermediate blocksof data, and the call to doFinal for the last block of data.When decrypting multiple blocks of data you also call the Cipher update methodfor intermediate data blocks, and the doFinal method for the last block. Here is an exampleof decrypting multiple blocks of data with a Java Cipher instance:byte plainText1 = cipher.update(cipherText1);byte plainText2 = cipher.update(cipherText2);byte plainText3 = cipher.doFinal(cipherText3);Again, the Cipher instance must be initialized into decryption mode for this example to work.Encrypting / Decrypting Part of a Byte ArrayThe Java Cipher class encryption and decryption methods can encrypt or decrypt part of thedata stored in a byte array.

You simply pass an offset and length to the updateand / or doFinal method. Here is an example:int offset = 10;int length = 24;byte cipherText = cipher.doFinal(data, offset, length);This example will encrypt (or decrypt, depending on the initialization of the Cipher) frombyte with index 8 and 24 bytes forward.Encrypting / Decrypting Into an Existing Byte ArrayAll the encryption and decryption examples shown in this tutorial so far have been returning theencrypted or decrypted data in a new byte array. However, it is also possible to encrypt or decryptdata into an existing byte array. This can be useful to keep the number of created byte arrays down.You can encrypt or decrypt data into an existing byte array by passing the destination byte arrayas parameter to the update and / or doFinal method. Here is an example:int offset = 10;int length = 24;byte dest = new byte1024;cipher.doFinal(data, offset, length, dest);This example encrypts the data from the byte with index 10 and 24 bytes forwardinto the dest byte array from offset 0.

If you want to set a differentoffset for the dest byte array there is a version of update and doFinalwhich takes an offset parameter extra. Here is an example of calling the doFinal methodwith an offset into the dest array:int offset = 10;int length = 24;byte dest = new byte1024;int destOffset = 12cipher.doFinal(data, offset, length, dest, destOffset);Reusing a Cipher InstanceInitializing a Java Cipher instance is an expensive operation. Therefore it is a good idea toreuse Cipher instances.

Encryption In Java

Luckily, the Cipher class was designed with reuse in mind.When you call the doFinal method on a Cipher instance, the Cipherinstance is returned to the state it had just after initialization. The Cipher instance canthen be used to encrypt or decrypt more data again.Here is an example of reusing a Java Cipher instance:Cipher cipher = Cipher.getInstance('AES');Key key =. // get / create symmetric encryption keycipher.init(Cipher.ENCRYPTMODE, key);byte data1 = 'abcdefghijklmnopqrstuvwxyz'.getBytes('UTF-8');byte data2 = 'zyxwvutsrqponmlkjihgfedcba'.getBytes('UTF-8');byte cipherText1 = cipher.update(data1);byte cipherText2 = cipher.doFinal(data2);byte data3 = '012345'.getBytes('UTF-8');byte cipherText3 = cipher.doFinal(data3);First the Cipher instance is created and initialized, and then used to encrypt two blocks ofcoherent data. Notice the call to update and then doFinal for these two blocks of data.Now the Cipher instance can be used again to encrypt more data. This is done with thedoFinal call with the third data block. After this doFinal call you can encryptyet another block of data with the same Java Cipher instance.