RC4.java

/* ====================================================================
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   contributor license agreements.  See the NOTICE file distributed with
   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
   the License.  You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
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   See the License for the specific language governing permissions and
   limitations under the License.
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package org.apache.poi.hssf.record.crypto;

import org.apache.poi.util.HexDump;

/**
 * Simple implementation of the alleged RC4 algorithm.
 *
 * Inspired by <A HREF="http://en.wikipedia.org/wiki/RC4">wikipedia's RC4 article</A>
 *
 * @author Josh Micich
 */
final class RC4 {

    private int _i, _j;
	private final byte[] _s = new byte[256];

	public RC4(byte[] key) {
		int key_length = key.length;

	    for (int i = 0; i < 256; i++)
	        _s[i] = (byte)i;

	    for (int i=0, j=0; i < 256; i++) {
	        byte temp;

	        j = (j + key[i % key_length] + _s[i]) & 255;
	        temp = _s[i];
	        _s[i] = _s[j];
	        _s[j] = temp;
	    }

	    _i = 0;
	    _j = 0;
	}

	public byte output() {
	    byte temp;
	    _i = (_i + 1) & 255;
	    _j = (_j + _s[_i]) & 255;

	    temp = _s[_i];
	    _s[_i] = _s[_j];
	    _s[_j] = temp;

	    return _s[(_s[_i] + _s[_j]) & 255];
	}

	public void encrypt(byte[] in) {
		for (int i = 0; i < in.length; i++) {
			in[i] = (byte) (in[i] ^ output());
		}
	}
	public void encrypt(byte[] in, int offset, int len) {
		int end = offset+len;
		for (int i = offset; i < end; i++) {
			in[i] = (byte) (in[i] ^ output());
		}

	}
	@Override
	public String toString() {
		StringBuffer sb = new StringBuffer();

		sb.append(getClass().getName()).append(" [");
		sb.append("i=").append(_i);
		sb.append(" j=").append(_j);
		sb.append("]");
		sb.append("\n");
		sb.append(HexDump.dump(_s, 0, 0));

		return sb.toString();
	}
}