Java IO.pdf - Nguyen Dang Binh

Figure 10.2. Encrypting data
Java I/O
Example 10.6 is a simple program that reads a filename and a password from the command
line and encrypts the file with DES. The key is generated from the bytes of the password in a
fairly predictable and insecure fashion. The cipher is initialized for encryption with the DES
algorithm in CBC mode with PKCS5Padding and a random initialization vector. The
initialization vector and its length are written at the start of the encrypted file so they'll be
conveniently available for decryption.
Data is read from the file in 64-byte blocks. This happens to be an integral multiple of the
eight-byte block size used by DES, but that's not necessary. The Cipher object buffers as
necessary to handle nonintegral multiples of the block size. Each block of data is fed into the
update() method to be encrypted. update() returns either encrypted data or null if it
doesn't have enough data to fill out a block. If it returns the encrypted data, that's written into
the output file. When no more input data remains, the cipher's doFinal() method is invoked
to pad and flush any remaining data. Then both input and output files are closed.
Example 10.6. FileEncryptor
import java.io.*;
import java.security.*;
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