Source for gnu.javax.crypto.mode.ICM

   1: /* ICM.java --
   2:    Copyright (C) 2001, 2002, 2003, 2006 Free Software Foundation, Inc.
   3: 
   4: This file is a part of GNU Classpath.
   5: 
   6: GNU Classpath is free software; you can redistribute it and/or modify
   7: it under the terms of the GNU General Public License as published by
   8: the Free Software Foundation; either version 2 of the License, or (at
   9: your option) any later version.
  10: 
  11: GNU Classpath is distributed in the hope that it will be useful, but
  12: WITHOUT ANY WARRANTY; without even the implied warranty of
  13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14: General Public License for more details.
  15: 
  16: You should have received a copy of the GNU General Public License
  17: along with GNU Classpath; if not, write to the Free Software
  18: Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
  19: USA
  20: 
  21: Linking this library statically or dynamically with other modules is
  22: making a combined work based on this library.  Thus, the terms and
  23: conditions of the GNU General Public License cover the whole
  24: combination.
  25: 
  26: As a special exception, the copyright holders of this library give you
  27: permission to link this library with independent modules to produce an
  28: executable, regardless of the license terms of these independent
  29: modules, and to copy and distribute the resulting executable under
  30: terms of your choice, provided that you also meet, for each linked
  31: independent module, the terms and conditions of the license of that
  32: module.  An independent module is a module which is not derived from
  33: or based on this library.  If you modify this library, you may extend
  34: this exception to your version of the library, but you are not
  35: obligated to do so.  If you do not wish to do so, delete this
  36: exception statement from your version.  */
  37: 
  38: 
  39: package gnu.javax.crypto.mode;
  40: 
  41: import gnu.java.security.Registry;
  42: import gnu.javax.crypto.cipher.IBlockCipher;
  43: 
  44: import java.math.BigInteger;
  45: 
  46: /**
  47:  * An implementation of <i>David McGrew</i> Integer Counter Mode (ICM) as an
  48:  * {@link IMode}.
  49:  * <p>
  50:  * ICM is a way to define a pseudorandom keystream generator using a block
  51:  * cipher. The keystream can be used for additive encryption, key derivation, or
  52:  * any other application requiring pseudorandom data. In the case of this class,
  53:  * it is used as additive encryption, XOR-ing the keystream with the input text
  54:  * --for both encryption and decryption.
  55:  * <p>
  56:  * In ICM, the keystream is logically broken into segments. Each segment is
  57:  * identified with a segment index, and the segments have equal lengths. This
  58:  * segmentation makes ICM especially appropriate for securing packet-based
  59:  * protocols. ICM also allows a variety of configurations based, among other
  60:  * things, on two parameters: the <i>block index length</i> and the <i>segment
  61:  * index length</i>. A constraint on those two values exists: The sum of
  62:  * <i>segment index length</i> and <i>block index length</i> <b>must not</b>
  63:  * half the <i>block size</i> of the underlying cipher. This requirement
  64:  * protects the ICM keystream generator from potentially failing to be
  65:  * pseudorandom.
  66:  * <p>
  67:  * For simplicity, this implementation, fixes these two values to the following:
  68:  * <ul>
  69:  * <li>block index length: is half the underlying cipher block size, and</li>
  70:  * <li>segment index length: is zero.</li>
  71:  * </ul>
  72:  * <p>
  73:  * For a 128-bit block cipher, the above values imply a maximum keystream length
  74:  * of 295,147,905,179,352,825,856 octets, since in ICM, each segment must not
  75:  * exceed the value
  76:  * <code>(256 ^ <i>block index length</i>) * <i>block length</i></code>
  77:  * octets.
  78:  * <p>
  79:  * Finally, for this implementation of the ICM, the IV placeholder will be used
  80:  * to pass the value of the <i>Offset</i> in the keystream segment.
  81:  * <p>
  82:  * References:
  83:  * <ol>
  84:  * <li><a
  85:  * href="http://www.ietf.org/internet-drafts/draft-mcgrew-saag-icm-00.txt">
  86:  * Integer Counter Mode</a>, David A. McGrew.</li>
  87:  * </ol>
  88:  */
  89: public class ICM
  90:     extends BaseMode
  91:     implements Cloneable
  92: {
  93:   /** The integer value 256 as a BigInteger. */
  94:   private static final BigInteger TWO_FIFTY_SIX = new BigInteger("256");
  95:   /** Maximum number of blocks per segment. */
  96:   private BigInteger maxBlocksPerSegment;
  97:   /** A work constant. */
  98:   private BigInteger counterRange;
  99:   /** The initial counter for a given keystream segment. */
 100:   private BigInteger C0;
 101:   /** The index of the next block for a given keystream segment. */
 102:   private BigInteger blockNdx;
 103: 
 104:   /**
 105:    * Trivial package-private constructor for use by the Factory class.
 106:    *
 107:    * @param underlyingCipher the underlying cipher implementation.
 108:    * @param cipherBlockSize the underlying cipher block size to use.
 109:    */
 110:   ICM(IBlockCipher underlyingCipher, int cipherBlockSize)
 111:   {
 112:     super(Registry.ICM_MODE, underlyingCipher, cipherBlockSize);
 113:   }
 114: 
 115:   /**
 116:    * Private constructor for cloning purposes.
 117:    *
 118:    * @param that the instance to clone.
 119:    */
 120:   private ICM(ICM that)
 121:   {
 122:     this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
 123:   }
 124: 
 125:   public Object clone()
 126:   {
 127:     return new ICM(this);
 128:   }
 129: 
 130:   public void setup()
 131:   {
 132:     if (modeBlockSize != cipherBlockSize)
 133:       throw new IllegalArgumentException();
 134:     counterRange = TWO_FIFTY_SIX.pow(cipherBlockSize);
 135:     maxBlocksPerSegment = TWO_FIFTY_SIX.pow(cipherBlockSize / 2);
 136:     BigInteger r = new BigInteger(1, iv);
 137:     C0 = maxBlocksPerSegment.add(r).modPow(BigInteger.ONE, counterRange);
 138:     blockNdx = BigInteger.ZERO;
 139:   }
 140: 
 141:   public void teardown()
 142:   {
 143:     counterRange = null;
 144:     maxBlocksPerSegment = null;
 145:     C0 = null;
 146:     blockNdx = null;
 147:   }
 148: 
 149:   public void encryptBlock(byte[] in, int i, byte[] out, int o)
 150:   {
 151:     icm(in, i, out, o);
 152:   }
 153: 
 154:   public void decryptBlock(byte[] in, int i, byte[] out, int o)
 155:   {
 156:     icm(in, i, out, o);
 157:   }
 158: 
 159:   private void icm(byte[] in, int inOffset, byte[] out, int outOffset)
 160:   {
 161:     if (blockNdx.compareTo(maxBlocksPerSegment) >= 0)
 162:       throw new RuntimeException("Maximum blocks for segment reached");
 163:     BigInteger Ci = C0.add(blockNdx).modPow(BigInteger.ONE, counterRange);
 164:     byte[] result = Ci.toByteArray();
 165:     int limit = result.length;
 166:     int ndx = 0;
 167:     if (limit < cipherBlockSize)
 168:       {
 169:         byte[] data = new byte[cipherBlockSize];
 170:         System.arraycopy(result, 0, data, cipherBlockSize - limit, limit);
 171:         result = data;
 172:       }
 173:     else if (limit > cipherBlockSize)
 174:       ndx = limit - cipherBlockSize;
 175: 
 176:     cipher.encryptBlock(result, ndx, result, ndx);
 177:     blockNdx = blockNdx.add(BigInteger.ONE); // increment blockNdx
 178:     for (int i = 0; i < modeBlockSize; i++) // xor result with input block
 179:       out[outOffset++] = (byte)(in[inOffset++] ^ result[ndx++]);
 180:   }
 181: }