Java I/O

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

CipherInputStream

CipherInputStream is a subclass of FilterInputStream ...