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Encryption and decryption are fundamental requirements of every secure-aware application, therefore the Java platform provides strong support for encryption and decryption through its Java Cryptographic Extension (JCE) framework which implements the standard cryptographic algorithms such as AES, DES, DESede and RSA. This tutorial shows you how to basically encrypt and decrypt files using the Advanced Encryption Standard (AES) algorithm. AES is a symmetric-key algorithm that uses the same key for both encryption and decryption of data.

1. Basic Steps

Here are the general steps to encrypt/decrypt a file in Java:

• Create a Key from a given byte array for a given algorithm.
• Get an instance of Cipher class for a given algorithm transformation. See document of the Cipher class for more information regarding supported algorithms and transformations.
• Initialize the Cipher with an appropriate mode (encrypt or decrypt) and the given Key.
• Invoke doFinal(input_bytes) method of the Cipher class to perform encryption or decryption on the input_bytes, which returns an encrypted or decrypted byte array.
• Read an input file to a byte array and write the encrypted/decrypted byte array to an output file accordingly.

Now, let’s see some real examples.

2. The CryptoUtils class

Here’s a utility class that provides two utility methods, one for encrypt a file and another for decrypt a file:Both the methods encrypt() and decrypt() accepts a key, an input file and an output file as parameters, and throws a CryptoException which is a custom exception written as below:This custom exception eliminates the messy throws clause, thus make the caller invoking those methods without catching a lengthy list of original exceptions.

3. The CryptoUtilsTest class

The following code is written for a test class that tests the CryptoUtils class above:This test program simply encrypts a text file, and then decrypts the encrypted file. Note that the key used for encryption and decryption here is a string “Mary has one cat”;

4. Note about key size

The AES algorithm requires that the key size must be 16 bytes (or 128 bit). So if you provide a key whose size is not equal to 16 bytes, a java.security.InvalidKeyException will be thrown. In case your key is longer, you should consider using a padding mechanism that transforms the key into a form in which its size is multiples of 16 bytes. See the Cipher class Javadoc for more details.

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Creating and managing keys is an important part of the cryptographic process. Symmetric algorithms require the creation of a key and an initialization vector (IV). The key must be kept secret from anyone who should not decrypt your data. The IV does not have to be secret, but should be changed for each session. Asymmetric algorithms require the creation of a public key and a private key. The public key can be made public to anyone, while the private key must known only by the party who will decrypt the data encrypted with the public key. This section describes how to generate and manage keys for both symmetric and asymmetric algorithms.

Symmetric Keys

The symmetric encryption classes supplied by the .NET Framework require a key and a new initialization vector (IV) to encrypt and decrypt data. Whenever you create a new instance of one of the managed symmetric cryptographic classes using the parameterless constructor, a new key and IV are automatically created. Anyone that you allow to decrypt your data must possess the same key and IV and use the same algorithm. Generally, a new key and IV should be created for every session, and neither the key nor IV should be stored for use in a later session.

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To communicate a symmetric key and IV to a remote party, you would usually encrypt the symmetric key by using asymmetric encryption. Sending the key across an insecure network without encrypting it is unsafe, because anyone who intercepts the key and IV can then decrypt your data. For more information about exchanging data by using encryption, see Creating a Cryptographic Scheme.

The following example shows the creation of a new instance of the TripleDESCryptoServiceProvider class that implements the TripleDES algorithm.

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When the previous code is executed, a new key and IV are generated and placed in the Key and IV properties, respectively.

Sometimes you might need to generate multiple keys. In this situation, you can create a new instance of a class that implements a symmetric algorithm and then create a new key and IV by calling the GenerateKey and GenerateIV methods. The following code example illustrates how to create new keys and IVs after a new instance of the symmetric cryptographic class has been made.

When the previous code is executed, a key and IV are generated when the new instance of TripleDESCryptoServiceProvider is made. Another key and IV are created when the GenerateKey and GenerateIV methods are called.

Asymmetric Keys

The .NET Framework provides the RSACryptoServiceProvider and DSACryptoServiceProvider classes for asymmetric encryption. These classes create a public/private key pair when you use the parameterless constructor to create a new instance. Asymmetric keys can be either stored for use in multiple sessions or generated for one session only. While the public key can be made generally available, the private key should be closely guarded.

A public/private key pair is generated whenever a new instance of an asymmetric algorithm class is created. After a new instance of the class is created, the key information can be extracted using one of two methods:

• The ToXmlString method, which returns an XML representation of the key information.

• The ExportParameters method, which returns an RSAParameters structure that holds the key information.

Both methods accept a Boolean value that indicates whether to return only the public key information or to return both the public-key and the private-key information. An RSACryptoServiceProvider class can be initialized to the value of an RSAParameters structure by using the ImportParameters method.

Asymmetric private keys should never be stored verbatim or in plain text on the local computer. If you need to store a private key, you should use a key container. For more on how to store a private key in a key container, see How to: Store Asymmetric Keys in a Key Container.

The following code example creates a new instance of the RSACryptoServiceProvider class, creating a public/private key pair, and saves the public key information to an RSAParameters structure.

See also