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The Bouncy Castle Java APIs for the OpenPGP Protocol. The APIs are designed primarily to be used in conjunction with the BC LTS provider but may also be used with other providers providing cryptographic services.

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package org.bouncycastle.openpgp.operator;

import org.bouncycastle.openpgp.PGPException;

/**
 * Utility class that provides padding addition and removal for PGP session keys.
 */
public class PGPPad
{
    private PGPPad()
    {

    }

    public static byte[] padSessionData(byte[] sessionInfo)
    {
        return padSessionData(sessionInfo, true);
    }

    public static byte[] padSessionData(byte[] sessionInfo, boolean obfuscate)
    {
        int length = sessionInfo.length;
        int paddedLength = ((length >>> 3) + 1) << 3;

        if (obfuscate)
        {
            paddedLength = Math.max(40, paddedLength);
        }

        int padCount = paddedLength - length;
        byte padByte = (byte)padCount;

        byte[] result = new byte[paddedLength];
        System.arraycopy(sessionInfo, 0, result, 0, length);
        for (int i = length; i < paddedLength; ++i)
        {
            result[i] = padByte;
        }
        return result;
    }

    public static byte[] unpadSessionData(byte[] encoded)
        throws PGPException
    {
        int paddedLength = encoded.length;
        byte padByte = encoded[paddedLength - 1];
        int padCount = padByte & 0xFF;
        int length = paddedLength - padCount;
        int last = length - 1;

        int diff = 0;
        for (int i = 0; i < paddedLength; ++i)
        {
            int mask = (last - i) >> 31;
            diff |= (padByte ^ encoded[i]) & mask;
        }

        diff |= paddedLength & 7;
        diff |= (40 - paddedLength) >> 31;

        if (diff != 0)
        {
            throw new PGPException("bad padding found in session data");
        }

        byte[] result = new byte[length];
        System.arraycopy(encoded, 0, result, 0, length);
        return result;
    }
}