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This is a drop in replacement for the auth0 java-jwt library (see https://github.com/auth0/java-jwt). This jar makes sure there are no external dependencies (e.g. fasterXml, Apacha Commons) needed. This is useful when deploying to an application server (e.g. tomcat with Alfreso or Pega).
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package org.bouncycastle.crypto.engines;

import org.bouncycastle.crypto.BlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.InvalidCipherTextException;
import org.bouncycastle.crypto.Wrapper;
import org.bouncycastle.crypto.params.KeyParameter;
import org.bouncycastle.crypto.params.ParametersWithIV;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.util.Arrays;

/**
 * an implementation of the AES Key Wrapper from the NIST Key Wrap
 * Specification as described in RFC 3394.
 * 
 * For further details see: http://www.ietf.org/rfc/rfc3394.txt
 * and  http://csrc.nist.gov/encryption/kms/key-wrap.pdf.
 */
public class RFC3394WrapEngine
    implements Wrapper
{
    private BlockCipher     engine;
    private boolean         wrapCipherMode;
    private KeyParameter    param;
    private boolean         forWrapping;

    private byte[]          iv = {
                              (byte)0xa6, (byte)0xa6, (byte)0xa6, (byte)0xa6,
                              (byte)0xa6, (byte)0xa6, (byte)0xa6, (byte)0xa6 };

    /**
     * Create a RFC 3394 WrapEngine specifying the encrypt for wrapping, decrypt for unwrapping.
     *
     * @param engine the block cipher to be used for wrapping.
     */
    public RFC3394WrapEngine(BlockCipher engine)
    {
        this(engine, false);
    }

    /**
     * Create a RFC 3394 WrapEngine specifying the direction for wrapping and unwrapping..
     *
     * @param engine the block cipher to be used for wrapping.
     * @param useReverseDirection true if engine should be used in decryption mode for wrapping, false otherwise.
     */
    public RFC3394WrapEngine(BlockCipher engine, boolean useReverseDirection)
    {
        this.engine = engine;
        this.wrapCipherMode = (useReverseDirection) ? false : true;
    }

    public void init(
        boolean             forWrapping,
        CipherParameters    param)
    {
        this.forWrapping = forWrapping;

        if (param instanceof ParametersWithRandom)
        {
            param = ((ParametersWithRandom) param).getParameters();
        }

        if (param instanceof KeyParameter)
        {
            this.param = (KeyParameter)param;
        }
        else if (param instanceof ParametersWithIV)
        {
            this.iv = ((ParametersWithIV)param).getIV();
            this.param = (KeyParameter)((ParametersWithIV) param).getParameters();
            if (this.iv.length != 8)
            {
               throw new IllegalArgumentException("IV not equal to 8");
            }
        }
    }

    public String getAlgorithmName()
    {
        return engine.getAlgorithmName();
    }

    public byte[] wrap(
        byte[]  in,
        int     inOff,
        int     inLen)
    {
        if (!forWrapping)
        {
            throw new IllegalStateException("not set for wrapping");
        }

        int     n = inLen / 8;

        if ((n * 8) != inLen)
        {
            throw new DataLengthException("wrap data must be a multiple of 8 bytes");
        }

        byte[]  block = new byte[inLen + iv.length];
        byte[]  buf = new byte[8 + iv.length];

        System.arraycopy(iv, 0, block, 0, iv.length);
        System.arraycopy(in, inOff, block, iv.length, inLen);

        engine.init(wrapCipherMode, param);

        for (int j = 0; j != 6; j++)
        {
            for (int i = 1; i <= n; i++)
            {
                System.arraycopy(block, 0, buf, 0, iv.length);
                System.arraycopy(block, 8 * i, buf, iv.length, 8);
                engine.processBlock(buf, 0, buf, 0);

                int t = n * j + i;
                for (int k = 1; t != 0; k++)
                {
                    byte    v = (byte)t;

                    buf[iv.length - k] ^= v;

                    t >>>= 8;
                }

                System.arraycopy(buf, 0, block, 0, 8);
                System.arraycopy(buf, 8, block, 8 * i, 8);
            }
        }

        return block;
    }

    public byte[] unwrap(
        byte[]  in,
        int     inOff,
        int     inLen)
        throws InvalidCipherTextException
    {
        if (forWrapping)
        {
            throw new IllegalStateException("not set for unwrapping");
        }

        int     n = inLen / 8;

        if ((n * 8) != inLen)
        {
            throw new InvalidCipherTextException("unwrap data must be a multiple of 8 bytes");
        }

        byte[]  block = new byte[inLen - iv.length];
        byte[]  a = new byte[iv.length];
        byte[]  buf = new byte[8 + iv.length];

        System.arraycopy(in, inOff, a, 0, iv.length);
        System.arraycopy(in, inOff + iv.length, block, 0, inLen - iv.length);

        engine.init(!wrapCipherMode, param);

        n = n - 1;

        for (int j = 5; j >= 0; j--)
        {
            for (int i = n; i >= 1; i--)
            {
                System.arraycopy(a, 0, buf, 0, iv.length);
                System.arraycopy(block, 8 * (i - 1), buf, iv.length, 8);

                int t = n * j + i;
                for (int k = 1; t != 0; k++)
                {
                    byte    v = (byte)t;

                    buf[iv.length - k] ^= v;

                    t >>>= 8;
                }

                engine.processBlock(buf, 0, buf, 0);
                System.arraycopy(buf, 0, a, 0, 8);
                System.arraycopy(buf, 8, block, 8 * (i - 1), 8);
            }
        }

        if (!Arrays.constantTimeAreEqual(a, iv))
        {
            throw new InvalidCipherTextException("checksum failed");
        }

        return block;
    }
}