de.schlichtherle.crypto.io
Class CipherReadOnlyFile

java.lang.Object
  de.schlichtherle.io.rof.AbstractReadOnlyFile
      de.schlichtherle.io.rof.FilterReadOnlyFile
          de.schlichtherle.crypto.io.CipherReadOnlyFile

All Implemented Interfaces:

ReadOnlyFile

Direct Known Subclasses:

RaesReadOnlyFile

public abstract class CipherReadOnlyFile
extends FilterReadOnlyFile

A read only file for transparent random read access to an encrypted file.

The client must call init(SeekableBlockCipher, long, long) before it can actually read anything!

Note that this class implements its own virtual file pointer. Thus, if you would like to access the underlying ReadOnlyFile again after you have finished working with an instance of this class, you should synchronize their file pointers using the pattern as described in the base class FilterReadOnlyFile.

Since:

TrueZIP 6.0

Author:

Christian Schlichtherle

MAX_WINDOW_LEN

private static final int MAX_WINDOW_LEN

The maximum buffer length of the window to the encrypted file. This value has been adjusted to provide optimum performance at minimal size on a Windows XP computer - results may vary. Note that the actual size of the window is a multiple of the cipher's block size and may be smaller than the maximum window size.

See Also:

Constant Field Values

start

private long start

Start offset of the encrypted data.

length

private long length

The length of the encrypted data.

fp

private long fp

The virtual file pointer in the encrypted data. This is relative to the start.

windowOff

private long windowOff

The current offset in the encrypted file where the buffer window starts. This is always a multiple of the block size.

window

private byte[] window

The buffer window to the encrypted file. Note that this buffer contains encrypted data only. The actual size of the window is a multiple of the cipher's block size and may be slightly smaller than MAX_WINDOW_LEN.

cipher

private SeekableBlockCipher cipher

The seekable block cipher which allows random access.

blockOff

private long blockOff

The current offset in the encrypted file where the data starts that has been decrypted to the block. This is always a multiple of the block size.

block

private byte[] block

The block buffer to use for decryption of partial blocks. Note that this buffer contains decrypted data only.

closed

private boolean closed

Whether this read only file has been closed or not.

CipherReadOnlyFile

public CipherReadOnlyFile(ReadOnlyFile rof)

Creates a read only file for transparent random read access to an encrypted file. The client must call init(SeekableBlockCipher, long, long) before it can actually read anything!

Parameters:

rof - A read-only file. This may be null, but must be properly init before the call to init().

min

private static final long min(long a,
                              long b)

Returns the smaller parameter.

init

public void init(SeekableBlockCipher cipher,
                 long start,
                 long length)
          throws IOException

Initializes this cipher read only file - must be called before first read access!

Parameters:

start - The start offset of the encrypted data in this file.

length - The length of the encrypted data in this file.

Throws:

IOException - If this read only file has already been closed. This exception is not recoverable.

IllegalStateException - If this object has already been initialized. This exception is not recoverable.

NullPointerException - If FilterReadOnlyFile.rof is null or cipher is null. This exception is recoverable.

computeMac

protected byte[] computeMac(Mac mac)
                     throws IOException

Returns the authentication code of the encrypted data in this cipher read only file using the given Message Authentication Code (MAC) object. It is safe to call this method multiple times to detect if the file has been tampered with meanwhile.

Parameters:

mac - A properly initialized MAC object.

Throws:

IOException - On any I/O related issue.

length

public long length()
            throws IOException

Specified by:

length in interface ReadOnlyFile

Overrides:

length in class FilterReadOnlyFile

Throws:

IOException

getFilePointer

public long getFilePointer()
                    throws IOException

Specified by:

getFilePointer in interface ReadOnlyFile

Overrides:

getFilePointer in class FilterReadOnlyFile

Throws:

IOException

seek

public void seek(long fp)
          throws IOException

Description copied from interface: ReadOnlyFile

Sets the file pointer offset, measured from the beginning of this file, at which the next read occurs. Whether the offset may be set beyond the end of the file is up to the implementor. For example, the SimpleReadOnlyFile subclasses RandomAccessFile and passes "r" as a parameter to the superclass constructor. On the Windows platform, this implementation allows to seek past the end of file, but on the Linux platform it doesn't.

Specified by:

seek in interface ReadOnlyFile

Overrides:

seek in class FilterReadOnlyFile

Parameters:

fp - The offset position, measured in bytes from the beginning of the file, at which to set the file pointer.

Throws:

IOException - If pos is less than 0 or if an I/O error occurs.

read

public int read()
         throws IOException

Specified by:

read in interface ReadOnlyFile

Overrides:

read in class FilterReadOnlyFile

Throws:

IOException

read

public int read(byte[] buf,
                int off,
                int len)
         throws IOException

Description copied from interface: ReadOnlyFile

Reads up to len bytes of data from this read only file into the given array. This method blocks until at least one byte of input is available.

Specified by:

read in interface ReadOnlyFile

Overrides:

read in class FilterReadOnlyFile

Parameters:

buf - The buffer to fill with data.

off - The start offset of the data.

len - The maximum number of bytes to read.

Returns:

The total number of bytes read, or -1 if there is no more data because the end of the file has been reached.

Throws:

IOException - On any I/O related issue.

ensureInit

private final void ensureInit()
                       throws IOException

Ensures that this file is open and has been initialized.

Throws:

IOException - If the preconditions do not hold.

close

public void close()
           throws IOException

Closes this read only file and releases any resources associated with it. This method invalidates the state of this object, causing any subsequent calls to a public method to fail with an IOException.

Specified by:

close in interface ReadOnlyFile

Overrides:

close in class FilterReadOnlyFile

Throws:

IOException - If an I/O error occurs.

positionBlock

private void positionBlock()
                    throws IOException

Ensures that the block with the decrypted data for partial reading is positioned so that it contains the current virtual file pointer in the encrypted file.

Throws:

IOException - On any I/O related issue. The block is not moved in this case.

positionWindow

private void positionWindow()
                     throws IOException

Ensures that the window is positioned so that the block containing the current virtual file pointer in the encrypted file is entirely contained in it.

Throws:

IOException - On any I/O related issue. The window is invalidated in this case.