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java Read a file to multiple byte arrays?


Cipher
Cipher.DECRYPT_MODE
CipherInputStream
CipherOutputStream
java.io.FilterOutputStream
java.io.OutputStream
public static void main(String[] args) {
    encryptFile("exampleInput.txt", "exampleOutput.txt");
}

public static void encryptFile(String source, String sink) {
    FileInputStream fis = null;
    try {
        fis = new FileInputStream(source);
        CipherOutputStream cos = null;
        try {
            cos = new CipherOutputStream(new FileOutputStream(sink), getEncryptionCipher());
            IOUtils.copy(fis, cos);
        } finally {
            if (cos != null)
                cos.close();
        }
    } finally {
        if (fis != null)
            fis.close();
    }
}

private static Cipher getEncryptionCipher() {
    // Create AES cipher with whatever padding and other properties you want
    Cipher cipher = ... ;
    // Create AES secret key
    Key key = ... ;
    cipher.init(Cipher.ENCRYPT_MODE, key);
}

Moreover, this class catches all exceptions that are not thrown by its ancestor classes.

Edit: note that this can result in certain encryption exceptions being ignored, particularly BadPaddingException and IllegalBlockSizeException. This behavior can be found in the CipherOutputStream source code. (Granted, this source is from the OpenJDK, but it probably does the same thing in the Sun JDK.) Also, from the CipherOutputStream javadocs:

If you need to know the number of bytes that were copied, you can use IOUtils.copyLarge instead of IOUtils.copy if the file sizes exceed Integer.MAX_VALUE bytes (2 GB).

Look up cipher streams in Java. You can use them to encrypt/decrypt streams on the fly so you don't have to store the whole thing in memory. All you have to do is copy the regular FileInputStream for your source file to the CipherOutputStream that's wrapping your FileOutputStream for the encrypted sink file. IOUtils even conveniently contains a copy(InputStream, OutputStream) method to do this copy for you.

Take a look here for more info on cipher streams in Java.

The Java cipher streams may shove exceptions, especially BadPaddingExceptions under the table. You've been warned.

The bolded line here implies that the cryptographic exceptions are ignored, which they are. This may cause some unexpected behavior while trying to read an encrypted file, especially for block and/or padding encryption algorithms like AES. Make a mental note of this that you will get zero or partial output for the encrypted (or decrypted for CipherInputStream) file.

This will save you space because you won't need to store byte arrays of your own anymore. The only stored byte[] in this system is the internal byte[] of the Cipher, which will get cleared each time enough input is entered and an encrypted block is returned by Cipher.update, or on Cipher.doFinal when the CipherOutputStream is closed. However, you don't have to worry about any of this since it's all internal and everything is managed for you.

Note