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      1 package org.bouncycastle.crypto.paddings;
      2 
      3 import java.security.SecureRandom;
      4 
      5 import org.bouncycastle.crypto.InvalidCipherTextException;
      6 
      7 /**
      8  * A padder that adds Trailing-Bit-Compliment padding to a block.
      9  * <p>
     10  * This padding pads the block out with the compliment of the last bit
     11  * of the plain text.
     12  * </p>
     13  */
     14 public class TBCPadding
     15     implements BlockCipherPadding
     16 {
     17     /**
     18      * Initialise the padder.
     19      *
     20      * @param random - a SecureRandom if available.
     21      */
     22     public void init(SecureRandom random)
     23         throws IllegalArgumentException
     24     {
     25         // nothing to do.
     26     }
     27 
     28     /**
     29      * Return the name of the algorithm the padder implements.
     30      *
     31      * @return the name of the algorithm the padder implements.
     32      */
     33     public String getPaddingName()
     34     {
     35         return "TBC";
     36     }
     37 
     38     /**
     39      * add the pad bytes to the passed in block, returning the
     40      * number of bytes added.
     41      * <p>
     42      * Note: this assumes that the last block of plain text is always
     43      * passed to it inside in. i.e. if inOff is zero, indicating the
     44      * entire block is to be overwritten with padding the value of in
     45      * should be the same as the last block of plain text.
     46      * </p>
     47      */
     48     public int addPadding(
     49         byte[]  in,
     50         int     inOff)
     51     {
     52         int     count = in.length - inOff;
     53         byte    code;
     54 
     55         if (inOff > 0)
     56         {
     57             code = (byte)((in[inOff - 1] & 0x01) == 0 ? 0xff : 0x00);
     58         }
     59         else
     60         {
     61             code = (byte)((in[in.length - 1] & 0x01) == 0 ? 0xff : 0x00);
     62         }
     63 
     64         while (inOff < in.length)
     65         {
     66             in[inOff] = code;
     67             inOff++;
     68         }
     69 
     70         return count;
     71     }
     72 
     73     /**
     74      * return the number of pad bytes present in the block.
     75      */
     76     public int padCount(byte[] in)
     77         throws InvalidCipherTextException
     78     {
     79         byte code = in[in.length - 1];
     80 
     81         int index = in.length - 1;
     82         while (index > 0 && in[index - 1] == code)
     83         {
     84             index--;
     85         }
     86 
     87         return in.length - index;
     88     }
     89 }
     90