Java IO.pdf - Nguyen Dang Binh

Java I/O
A CipherInputStream object contains a Cipher object that's used to decrypt or encrypt all
data read from the underlying stream before passing it to the eventual source. This Cipher
object is set in the constructor. Like all filter stream constructors, this constructor takes
another input stream as an argument:
public CipherInputStream(InputStream in, Cipher c)
The Cipher object used here must be a properly initialized instance of
javax.crypto.Cipher, most likely returned by Cipher.getInstance(). This Cipher object
must also have been initialized for either encryption or decryption with init() before being
passed into the 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 CipherInputStream(InputStream in)
CipherInputStream overrides most methods declared in FilterInputStream. Each of these
makes the necessary adjustments to handle encrypted data. For example, skip() skips the
number of bytes after encryption or decryption, which is important if the ciphertext does not
have the same length as the plaintext. The available() method also returns the number of
bytes available after encryption or decryption. The markSupported() method returns false;
you cannot mark and reset a cipher input stream, even if the underlying class supports
marking and resetting. Allowing this would confuse many encryption algorithms. However,
you can make a cipher input stream the underlying stream of another class like
BufferedInputStream, which does support marking and resetting.
Strong encryption schemes have the distinct disadvantage that changing even a single bit in
the data can render the entire file unrecoverable gibberish. Therefore, it's useful to combine
encryption with a digest so you can tell whether a file has been modified. Example 10.8 uses
CipherInputStream to DES-encrypt a file named on the command line, but that's not all. The
ciphertext is also digested and the digest saved so corruption can be detected.
Example 10.8. DigestEncryptor
import java.io.*;
import java.security.*;
import java.security.spec.*;
import javax.crypto.*;
import javax.crypto.spec.*;
public class DigestEncryptor {
public static void main(String[] args) {
if (args.length != 2) {
System.err.println("Usage: java DigestEncryptor filename password");
return;
}
String filename = args[0];
String password = args[1];
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