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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.

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package org.bouncycastle.crypto.paddings;

import java.security.SecureRandom;

import org.bouncycastle.crypto.InvalidCipherTextException;

/**
 * Block cipher padders are expected to conform to this interface
 */
public interface BlockCipherPadding
{
    /**
     * Initialise the padder.
     *
     * @param random the source of randomness for the padding, if required.
     */
    public void init(SecureRandom random)
        throws IllegalArgumentException;

    /**
     * Return the name of the algorithm the cipher implements.
     *
     * @return the name of the algorithm the cipher implements.
     */
    public String getPaddingName();

    /**
     * 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. The reason
     * for this is that some modes such as "trailing bit compliment"
     * base the padding on the last byte of plain text.
     * 
     */
    public int addPadding(byte[] in, int inOff);

    /**
     * return the number of pad bytes present in the block.
     * @exception InvalidCipherTextException if the padding is badly formed
     * or invalid.
     */
    public int padCount(byte[] in)
        throws InvalidCipherTextException;
}