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SUMMARY: INNER | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
java.lang.Object | +--java.io.InputStream | +--java.io.FilterInputStream | +--xjava.security.CipherInputStream
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 : 0Each 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 $
java.security.Cipher
,
java.security.Cipher#getInputBlockSize
,
java.security.Cipher#getOutputBlockSize
,
java.security.CipherOutputStream
Fields inherited from class java.io.FilterInputStream |
in |
Constructor Summary | |
CipherInputStream(java.io.InputStream is,
Cipher cipher)
Constructs an input stream using a cipher that must be initialised for either encryption or decryption, that is, a cipher whose state is either ENCRYPT or
DECRYPT . |
Method Summary | |
int |
available()
Returns the number of bytes that can be guaranteed to be read from this input stream without blocking. |
void |
close()
Closes the input stream. |
void |
mark(int readlimit)
Does nothing, since this class does not support mark/reset. |
boolean |
markSupported()
Tests if this input stream supports the mark and
reset methods of InputStream, which it does not. |
int |
read()
Returns the next encrypted or decrypted byte, depending on the cipher state. |
int |
read(byte[] out,
int offset,
int length)
Fills up the specified bytes of the out array with the next len encrypted or decrypted bytes (depending on the cipher state). |
void |
reset()
Always throws an IOException, since this class does not support mark/reset. |
long |
skip(long n)
Skips over and discards n bytes of data from the input stream. |
Methods inherited from class java.io.FilterInputStream |
read |
Methods inherited from class java.lang.Object |
|
Constructor Detail |
public CipherInputStream(java.io.InputStream is, Cipher cipher)
ENCRYPT
or
DECRYPT
.in
- the input stream.cipher
- an initialised cipher.NullPointerException
- if is == null || cipher == nulljava.lang.IllegalStateException
- if cipher.getState() == UNINITIALIZEDjava.security.Cipher
Method Detail |
public int read(byte[] out, int offset, int length) throws java.io.IOException
read
in class java.io.FilterInputStream
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.java.io.IOException
- if an error occurs.java.security.Cipher#ENCRYPT
,
java.security.Cipher#DECRYPT
,
java.security.Cipher#getState
public int read() throws java.io.IOException
read
in class java.io.FilterInputStream
java.io.IOException
- if an error occurs.java.security.Cipher#ENCRYPT
,
java.security.Cipher#DECRYPT
,
java.security.Cipher#getState
public long skip(long n) throws java.io.IOException
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.skip
in class java.io.FilterInputStream
n
- the number of bytes to be skipped.java.io.IOException
- if an I/O error occurs.public int available() throws java.io.IOException
available
in class java.io.FilterInputStream
java.io.IOException
- if an I/O error occurs.public void close() throws java.io.IOException
close
in class java.io.FilterInputStream
java.io.IOException
- if an error occurs.public void mark(int readlimit)
mark
in class java.io.FilterInputStream
public void reset() throws java.io.IOException
reset
in class java.io.FilterInputStream
public boolean markSupported()
mark
and
reset
methods of InputStream, which it does not.markSupported
in class java.io.FilterInputStream
false
, since this class does not support the
mark
and reset
methods.
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