crc16.java

面向应用的智能安全代理平台和工具包是一个综合网络应用的安全共性需求而设计和实现的一个通用性的网络信息安全应用支撑平台

package au.net.aba.crypto.provider;

/*
 * $Id: CRC16.java,v 1.7 1998/10/26 01:46:28 leachbj Exp $
 * $Author: leachbj $
 *
 * Copyright (C) 1996-1998 Australian Business Access Pty Ltd.
 * All rights reserved.
 * 
 * Use, modification, copying and distribution of this software is subject the
 * terms and conditions of the ABA Public Licence. See the file
 * "PUBLIC_LICENCE" for additional information.
 *
 * If you have not received a copy of the Public Licence, you must destroy all
 * copies of this file immediately. 
 *
 * $Source: /aba/CVSROOT/jdk1.1/src/au.net.aba/crypto/provider/CRC16.java,v $
 * $Revision: 1.7 $
 * $Date: 1998/10/26 01:46:28 $
 * $State: Exp $
 */

import java.security.MessageDigest;

/**
 * A class that implements the CCITT CRC-16 checksumming algorithm, for the
 * reflected polynomial 0x8408. Note: if you want to do a zero value check
 * on some data and it's associated CRC, the CRC should be added after the
 * data, least significant byte first.
 */
public class CRC16 extends MessageDigest
{
	public final static String ident = "$Id: CRC16.java,v 1.7 1998/10/26 01:46:28 leachbj Exp $";

	//==================================
	// Private implementation
	//==================================

	private short			crc16;

	/**
	 * our CRC table 
	 */
	static short[]		crcTab;

	/**
	 * generate our lookup table - if we need. Note if we use a
	 * lookup table this poly is a reflected poly so the table
	 * lookup should be in the form:
	 *
	 *	accum = (short)(((accum >> 8) & 0xff)
	 *				^ crcTab[(accum ^ data) & 0xff]);
	 *
	 *	where accum is the accumulator for the digest, 
	 *	and data is the byte being processed.
	 */
	static {
		crcTab = new short[256];

		for (short i = 0; i < 256; i++)
		{
			crcTab[i] = crcCCITT(i, (short)0);
		}
	}
	//==================================
	// Constructors 
	//==================================

	/**
	 * This constructor is used to begin a new CRC-16 operation
	 */
	public CRC16()
	{
		super("CRC16");

		engineReset();
	}
	/**
	 * Compute a 16 bit CCITT CRC on the fly, we can either 
	 * use this to construct a table by setting the input
	 * CRC to zero, or accumulate values in the CRC value,
	 * passing it in each time.
	 */
	private static short crcCCITT(
		short	data,
		short	crc)
	{
		int	s;

		s = (data ^ crc) & 0xff;
		s = s ^ (s << 4);
		s = ((crc >> 8) & 0xff) ^ (s << 8) ^ (s << 3) ^ (s >>> 4);

		return (short)s;
	}
	/**
	 * return the CRC. Note this resets it.
	 *
	 * @return the crc16 as a byte array.
	 */
	protected synchronized byte[] engineDigest()
	{
		byte[]	digest;

		digest = new byte[2];

		digest[0] = (byte)(crc16 >>> 8);
		digest[1] = (byte)(crc16 & 0xff);

		engineReset();

		return digest;
	}
	/*
	 * MessageDigest methods.
	 */

	/**
	 * reset the CRC back to zero.
	 */
	protected void engineReset()
	{
		crc16 = 0;
	}
	/**
	 * update the CRC with an array of bytes.
	 *
	 * @param bytes the input array.
	 * @param offset the offset to start getting bytes from.
	 * @param length the number of bytes to be processed.
	 */
	protected synchronized void engineUpdate(
		byte[]	bytes,
		int	offset,
		int	length)
	{
		int i;

		for (i=0; i < length; i++)
		{
			crc16 = (short)(((crc16 >> 8) & 0xff)
				^ crcTab[(crc16 ^ bytes[offset + i]) & 0xff]);
		}
	}
	/**
	 * update the CRC with a single byte.
	 *
	 * @param b the byte of input.
	 */
	protected synchronized void engineUpdate(
		byte	b)
	{
		crc16 = (short)(((crc16 >> 8) & 0xff)
	 				^ crcTab[(crc16 ^ b) & 0xff]);
	}
}