Java IO.pdf - Nguyen Dang Binh

10.6.2 CipherOutputStream
CipherOutputStream is a subclass of FilterOutputStream.
public class CipherOutputStream extends FilterOutputStream
Java I/O
Each CipherOutputStream object contains a Cipher object used to decrypt or encrypt all
data passed as arguments to the write() method before writing it to the underlying stream.
This Cipher object is set in the constructor. Like all filter stream constructors, this constructor
takes another input stream as an argument:
public CipherOutputStream(OutputStream out, Cipher c)
The Cipher object used here must be a properly initialized instance of
javax.crypto.Cipher, most likely returned by Cipher.getInstance(). The Cipher object
c should be initialized for encryption or decryption by calling init() before being passed to
the CipherOutputStream() constructor. There is also a protected constructor that might be
used by subclasses that want to implement their own, non-JCE-based encryption scheme:
protected CipherOutputStream(OutputStream out)
CipherOutputStream has all the usual methods of any output stream, like write(),
flush(), and close(). It overrides five of these methods to do its filtering:
public void write(int b) throws IOException
public void write(byte[] data) throws IOException
public void write(byte[] data, int offset, int length) throws IOException
public void flush() throws IOException
public void close() throws IOException
Clients use these methods the same way they use them in any output stream. Before the data
is written, the stream's cipher either decrypts or encrypts the data. Each of these five methods
makes the necessary adjustments to handle encrypted data. For example, the flush() method
(which is invoked by the close() method as well) calls doFinal() on the Cipher object to
make sure it has finished padding and encrypting all the data before it flushes the final data to
the underlying stream.
There are no new methods in CipherOutputStream not declared in the superclass. Anything
else you need to do, such as getting the cipher's initialization vector, must be handled by the
Cipher object.
Example 10.9 uses CipherOutputStream to decrypt files encrypted by the DigestEncryptor
of Example 10.8. A digest input stream chained to a file input stream checks the digest of the
ciphertext as it's read from the file. If the digest does not match, an error message is printed.
The file is still written into the output file, since—depending on the algorithm and mode
used—it may be partially legible, especially if the error does not occur until relatively late in
the encrypted data.
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