An alternate SHA Interleave algorithm as implemented in the SRP distribution : SHA Secure Hash Algorithm « Security « Java Tutorial

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 * published by the Free Software Foundation; either version 2.1 of
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 * Lesser General Public License for more details.
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 */

import java.io.ByteArrayOutputStream;
import java.security.MessageDigest;
import java.security.MessageDigestSpi;
import java.security.NoSuchAlgorithmException;
import java.security.ProviderException; 

/** An alternate SHA Interleave algorithm as implemented in the SRP
 distribution. This version reverses the even and odd byte streams before
 performing the SHA digest.

 This product includes software developed by Tom Wu and Eugene
 Jhong for the SRP Distribution (http://srp.stanford.edu/srp/).

@author Scott.Stark@jboss.org
@version $Revision: 85945 $
*/
public class SHAReverseInterleave extends MessageDigestSpi
{
   private static final int SHA_HASH_LEN = 20;

   private ByteArrayOutputStream evenBytes;
   private ByteArrayOutputStream oddBytes;
   private int count;
   private boolean skipLeadingZeros;
   private MessageDigest sha;

   /** Creates a new instance of SHAReverseInterleave
    @exception ProviderException thrown if MessageDigest.getInstance("SHA")
    throws a NoSuchAlgorithmException.
    */
   public SHAReverseInterleave()
   {
      try
      {
         sha = MessageDigest.getInstance("SHA");
      }
      catch(NoSuchAlgorithmException e)
      {
         throw new ProviderException("Failed to obtain SHA MessageDigest");
      }
      evenBytes = new ByteArrayOutputStream();
      oddBytes = new ByteArrayOutputStream();
      engineReset();
   }

   protected int engineGetDigestLength()
   {
      return 2 * SHA_HASH_LEN;
   }

   /**
    * Completes the digest computation by performing final
    * operations such as padding. Once <code>engineDigest</code> has
    * been called, the engine should be reset (see
    * {@link #engineReset() engineReset}).
    * Resetting is the responsibility of the
    * engine implementor.
    *
    * @return the array of bytes for the resulting digest value.
    */
   protected byte[] engineDigest()
   {
      byte[] E = evenBytes.toByteArray();
      // If the count is odd, drop the first byte
      int length = E.length;
      if( count % 2 == 1 )
         length --;
      // Reverse the order of the even bytes
      byte[] tmp = new byte[length];
      for(int i = 0; i < length; i ++)
      {
         tmp[i] = E[E.length - i - 1];
         System.out.println("E["+i+"] = "+tmp[i]);
      }
      E = tmp;
      byte[] G = sha.digest(E);

      byte[] F = oddBytes.toByteArray();
      // Reverse the order of the even bytes
      tmp = new byte[F.length];
      for(int i = 0; i < F.length; i ++)
      {
         tmp[i] = F[F.length - i - 1];
         System.out.println("F["+i+"] = "+tmp[i]);
      }
      F = tmp;
      sha.reset();
      byte[] H = sha.digest(F);
      length = G.length + H.length;
      byte[] digest = new byte[length];
      for(int i = 0; i < G.length; ++i)
         digest[2 * i] = G[i];
      for(int i = 0; i < H.length; ++i)
         digest[2 * i + 1] = H[i];
      engineReset();
      return digest;
   }

   /**
    * Resets the digest for further use.
    */
   protected void engineReset()
   {
      skipLeadingZeros = true;
      count = 0;
      evenBytes.reset();
      oddBytes.reset();
      sha.reset();
   }

   /**
    * Updates the digest using the specified byte.
    *
    * @param input the byte to use for the update.
    */
   protected void engineUpdate(byte input)
   {
      if( skipLeadingZeros == true && input == 0 )
         return;
      skipLeadingZeros = false;
      if( count % 2 == 0 )
         evenBytes.write(input);
      else
         oddBytes.write(input);
      count ++;
   }

   /**
    * Updates the digest using the specified array of bytes,
    * starting at the specified offset.
    *
    * @param input the array of bytes to use for the update.
    * @param offset the offset to start from in the array of bytes.
    * @param len the input of bytes to use, starting at
    * <code>offset</code>.
    */
   protected void engineUpdate(byte[] input, int offset, int len)
   {
      for(int i = offset; i < offset+len; i ++)
         engineUpdate(input[i]);
   }

}