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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.8. Note: this package includes the NTRU encryption algorithms.

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package org.bouncycastle.crypto.paddings;

import java.security.SecureRandom;

import org.bouncycastle.crypto.InvalidCipherTextException;

/**
 * A padder that adds PKCS7/PKCS5 padding to a block.
 */
public class PKCS7Padding
    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 "PKCS7";
    }

    /**
     * add the pad bytes to the passed in block, returning the
     * number of bytes added.
     */
    public int addPadding(
        byte[]  in,
        int     inOff)
    {
        byte code = (byte)(in.length - inOff);

        while (inOff < in.length)
        {
            in[inOff] = code;
            inOff++;
        }

        return code;
    }

    /**
     * return the number of pad bytes present in the block.
     */
    public int padCount(byte[] in)
        throws InvalidCipherTextException
    {
        int count = in[in.length - 1] & 0xff;
        byte countAsbyte = (byte)count;

        // constant time version
        boolean failed = (count > in.length | count == 0);

        for (int i = 0; i < in.length; i++)
        {
            failed |= (in.length - i <= count) & (in[i] != countAsbyte);
        }

        if (failed)
        {
            throw new InvalidCipherTextException("pad block corrupted");
        }

        return count;
    }
}