Java IO.pdf - Nguyen Dang Binh

Java I/O
getIV() returns null if the algorithm doesn't use initialization vectors or if the initialization
vector isn't yet set.
10.6 Cipher Streams
The Cipher class is the engine that powers encryption. Chapter 10 and Example 10.7 showed
how this class could be used to encrypt and decrypt data read from a stream. The
javax.crypto package also provides CipherInputStream and CipherOutputStream filter
streams that use a Cipher object to encrypt or decrypt data passed through the stream. Like
DigestInputStream and DigestOutputStream, they aren't a great deal of use in themselves.
However, you can chain them in the middle of several other streams. For example, if you
chain a GZIPOutputStream to a CipherOutputStream that is chained to a
FileOutputStream, you can compress, encrypt and write to a file, all with a single call to
write(). This is shown in Figure 10.3. Similarly, you might read from a URL with the input
stream returned by openStream(), decrypt the data read with a CipherInputStream, then
check the decrypted data with a MessageDigestInputStream, then finally pass it all into an
InputStreamReader for conversion from ISO Latin-1 to Unicode. On the other side of the
connection, a web server could read a file from its hard drive, write the file onto a socket with
an output stream, calculate a digest with a DigestOutputStream, and encrypt the file with a
CipherOutputStream.
Figure 10.3. The CipherOutputStream in the middle of a chain of filters
10.6.1 CipherInputStream
CipherInputStream is a subclass of FilterInputStream.
public class CipherInputStream extends FilterInputStream
CipherInputStream has all the usual methods of any input stream, like read(), skip(), and
close(). It overrides seven of these methods to do its filtering:
public int read() throws IOException
public int read(byte[] data) throws IOException
public int read(byte[] data, int offset, int length) throws IOException
public long skip(long n) throws IOException
public int available() throws IOException
public void close() throws IOException
public boolean markSupported()
These methods are all invoked much as they would be for any other input stream. However,
as the data is read, the stream's Cipher object either decrypts or encrypts the data. (Assuming
your program wants to work with unencrypted data, as is most commonly the case, the cipher
input stream will decrypt the data.)
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