A FilterInputStream that encrypts or decrypts the data passing through
it. Typically, this stream would be used as a filter to read
an encrypted file. It can also be used to encrypt network communications
(although this would normally require use of a stream cipher, or a block
cipher in a stream-like mode such as CFB or OFB).
This class has a constructor that takes an input stream and a Cipher
as arguments. The cipher is used to encrypt or decrypt all data read
through the stream.
The data is encrypted or decrypted, depending on the initialisation
state of the Cipher. To get the encryption/decryption result bytes,
make one or more calls to the
read
methods. One
read
method, with no arguments,
returns the next result byte. The other
read
method returns
multiple result bytes at once.
For block ciphers, some buffering of the data received from the input
stream is done. The buffer length is calculated as follows:
buffer length = cipher.getInputBlockSize() +
(cipher.isPaddingBlockCipher() &&
cipher.getState() == Cipher.DECRYPT) ? 1 : 0
Each read operation will attempt to completely fill the buffer. The maximum
number of bytes that can remain unprocessed after each
read
call
is one less than the buffer length. In the case of a padding block cipher in
DECRYPT mode, this means that one full ciphertext block may remain
unprocessed, because it is necessary to read an extra byte in order to
determine whether this is the last block of the stream. (It would be
incorrect to process a block without making this check, since it may have
padding bytes that need to be stripped out.)
When EOF is reached for a padding/unpadding cipher in DECRYPT mode, the
the decryption result is unpadded. If EOF is encountered part-way through
a block, an
IllegalBlockSizeException is thrown.
When EOF is reached for a padding/unpadding cipher in ENCRYPT mode, the
last block is padded before encryption. If the cipher does not support
padding and the last block is incomplete, an
IllegalBlockSizeException
is thrown.
For stream ciphers, that is, ciphers capable of encrypting or decrypting
a byte at a time, no buffering is necessary.
Note: calling methods of a cipher while it is being used by a
CipherInputStream (apart from methods that have no side-effects,
like
getAlgorithm()
,
get*BlockSize()
, etc.)
will probably result in incorrect or unexpected output.
Copyright © 1997
Systemics Ltd on behalf of the
Cryptix Development Team.
All rights reserved.
$Revision: 1.10 $
available
public int available()
throws IOException
Returns the number of bytes that can be guaranteed to be read from this
input stream without blocking.
close
public void close()
throws IOException
Closes the input stream.
mark
public void mark(int readlimit)
Does nothing, since this class does not support mark/reset.
markSupported
public boolean markSupported()
Tests if this input stream supports the mark
and
reset
methods of InputStream, which it does not.
false
, since this class does not support the
mark
and reset
methods.
read
public int read()
throws IOException
Returns the next encrypted or decrypted byte, depending on the
cipher state.
- the next encrypted or decrypted byte, or -1 if the
last encrypted/decrypted byte was already returned.
java.security.Cipher.ENCRYPT
, java.security.Cipher.DECRYPT
, java.security.Cipher.getState
read
public int read(byte[] out,
int offset,
int length)
throws IOException
Fills up the specified bytes of the out array with the next
len encrypted or decrypted bytes (depending on the cipher state).
out
- the byte array into which the encrypted or decrypted
bytes will be read.offset
- the offset into out indicating where the
first encrypted or decrypted byte should be read.length
- the number of encrypted/decrypted bytes to read.
- the number of bytes read into out, or -1 if no
encrypted or decrypted bytes remained.
java.security.Cipher.ENCRYPT
, java.security.Cipher.DECRYPT
, java.security.Cipher.getState
reset
public void reset()
throws IOException
Always throws an IOException, since this class does not support mark/reset.
skip
public long skip(long n)
throws IOException
Skips over and discards n bytes of data from the
input stream. The skip
method may, for a variety of
reasons, end up skipping over some smaller number of bytes,
possibly 0. The actual number of bytes skipped is returned.
n
- the number of bytes to be skipped.
- the actual number of bytes skipped.