package org.bouncycastle.crypto.paddings;

import java.security.SecureRandom;

import org.bouncycastle.crypto.InvalidCipherTextException;

A padder that adds Trailing-Bit-Compliment padding to a block.

This padding pads the block out with the compliment of the last bit
of the plain text.

/**
 * A padder that adds Trailing-Bit-Compliment padding to a block.
 * <p>
 * This padding pads the block out with the compliment of the last bit
 * of the plain text.
 * </p>
 */
public class TBCPadding
    implements BlockCipherPadding
{
    
Initialise the padder.
Params:
random – - a SecureRandom if available./**
     * Initialise the padder.
     *
     * @param random - a SecureRandom if available.
     */
    public void init(SecureRandom random)
        throws IllegalArgumentException
    {
        // nothing to do.
    }

    
Return the name of the algorithm the padder implements.
Returns:
the name of the algorithm the padder implements./**
     * Return the name of the algorithm the padder implements.
     *
     * @return the name of the algorithm the padder implements.
     */
    public String getPaddingName()
    {
        return "TBC";
    }

    
add the pad bytes to the passed in block, returning the
number of bytes added.

Note: this assumes that the last block of plain text is always 
passed to it inside in. i.e. if inOff is zero, indicating the
entire block is to be overwritten with padding the value of in
should be the same as the last block of plain text.

/**
     * add the pad bytes to the passed in block, returning the
     * number of bytes added.
     * <p>
     * Note: this assumes that the last block of plain text is always 
     * passed to it inside in. i.e. if inOff is zero, indicating the
     * entire block is to be overwritten with padding the value of in
     * should be the same as the last block of plain text.
     * </p>
     */
    public int addPadding(
        byte[]  in,
        int     inOff)
    {
        int     count = in.length - inOff;
        byte    code;
        
        if (inOff > 0)
        {
            code = (byte)((in[inOff - 1] & 0x01) == 0 ? 0xff : 0x00);
        }
        else
        {
            code = (byte)((in[in.length - 1] & 0x01) == 0 ? 0xff : 0x00);
        }
            
        while (inOff < in.length)
        {
            in[inOff] = code;
            inOff++;
        }

        return count;
    }

    
return the number of pad bytes present in the block.
/**
     * return the number of pad bytes present in the block.
     */
    public int padCount(byte[] in)
        throws InvalidCipherTextException
    {
        byte code = in[in.length - 1];

        int index = in.length - 1;
        while (index > 0 && in[index - 1] == code)
        {
            index--;
        }

        return in.length - index;
    }
}