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package org.spongycastle.crypto.paddings;
import java.security.SecureRandom;
import org.spongycastle.crypto.InvalidCipherTextException;
/**
* 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.
*
* @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.
*
* @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.
* <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.
*/
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;
}
}
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