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This is a library do decode LoRaWan packets
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/*
 * AesCmac - AES CMAC implementation using SunSPOT crypto
 *
 * Authors:
 *      Juho Vaha-Herttua  
 *
 * Copyright (C) 2011  Aalto University
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 * 
 * Modifications by Romain Cambier 
 */
package be.romaincambier.lorawan;

import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.NoSuchAlgorithmException;
import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.ShortBufferException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

/**
 *
 * @author Romain Cambier
 */
public class AesCmac {

    private static final byte CONSTANT = (byte) 0x87;
    private static final int BLOCK_SIZE = 16;
    private static final IvParameterSpec ZERO_IV = new IvParameterSpec(new byte[16]);

    private int macLength;
    private Cipher aesCipher;

    private byte[] buffer;
    private int bufferCount;

    private byte[] k1;
    private byte[] k2;

    public AesCmac() throws NoSuchAlgorithmException {
        this(BLOCK_SIZE);
    }

    public AesCmac(int length) throws NoSuchAlgorithmException {
        if (length > BLOCK_SIZE) {
            throw new NoSuchAlgorithmException("AES CMAC maximum length is " + BLOCK_SIZE);
        }

        try {
            macLength = length;
            aesCipher = Cipher.getInstance("AES/CBC/NOPADDING");
            buffer = new byte[BLOCK_SIZE];
        } catch (NoSuchPaddingException ex) {
            throw new RuntimeException(ex);
        }
    }

    private byte[] doubleSubKey(byte[] k) {
        byte[] ret = new byte[k.length];

        boolean firstBitSet = ((k[0] & 0x80) != 0);
        for (int i = 0; i < k.length; i++) {
            ret[i] = (byte) (k[i] << 1);
            if (i + 1 < k.length && ((k[i + 1] & 0x80) != 0)) {
                ret[i] |= 0x01;
            }
        }
        if (firstBitSet) {
            ret[ret.length - 1] ^= CONSTANT;
        }
        return ret;
    }

    public final void init(Key key) throws InvalidKeyException, InvalidAlgorithmParameterException {
        if (!(key instanceof SecretKeySpec)) {
            throw new InvalidKeyException("Key is not of required type SecretKey.");
        }
        if (!((SecretKeySpec) key).getAlgorithm().equals("AES")) {
            throw new InvalidKeyException("Key is not an AES key.");
        }
        aesCipher.init(Cipher.ENCRYPT_MODE, key, ZERO_IV);
        byte[] k0 = new byte[BLOCK_SIZE];
        try {
            aesCipher.update(k0, 0, k0.length, k0, 0);
        } catch (ShortBufferException sbe) {
        }
        k1 = doubleSubKey(k0);
        k2 = doubleSubKey(k1);

        aesCipher.init(Cipher.ENCRYPT_MODE, key, ZERO_IV);
        bufferCount = 0;
    }

    public final void updateByte(byte b) {
        updateBlock(new byte[]{b});
    }

    public final void updateBlock(byte[] data) {
        int currentOffset = 0;

        if (data.length < BLOCK_SIZE - bufferCount) {
            System.arraycopy(data, 0, buffer, bufferCount, data.length);
            bufferCount += data.length;
            return;
        } else if (bufferCount > 0) {
            System.arraycopy(data, 0, buffer, bufferCount, BLOCK_SIZE - bufferCount);
            try {
                aesCipher.update(buffer, 0, BLOCK_SIZE, buffer, 0);
            } catch (ShortBufferException sbe) {
            }
            currentOffset += BLOCK_SIZE - bufferCount;
            bufferCount = 0;
        }
        while (currentOffset + BLOCK_SIZE < data.length) {
            try {
                aesCipher.update(data, currentOffset, BLOCK_SIZE, buffer, 0);
            } catch (ShortBufferException sbe) {
            }
            currentOffset += BLOCK_SIZE;
        }
        if (currentOffset != data.length) {
            System.arraycopy(data, currentOffset, buffer, 0, data.length - currentOffset);
            bufferCount = data.length - currentOffset;
        }
    }

    public final byte[] doFinal() {
        byte[] subKey = k1;
        if (bufferCount < BLOCK_SIZE) {
            buffer[bufferCount] = (byte) 0x80;
            for (int i = bufferCount + 1; i < BLOCK_SIZE; i++) {
                buffer[i] = (byte) 0x00;
            }
            subKey = k2;
        }
        for (int i = 0; i < BLOCK_SIZE; i++) {
            buffer[i] ^= subKey[i];
        }
        try {
            aesCipher.doFinal(buffer, 0, BLOCK_SIZE, buffer, 0);
        } catch (ShortBufferException | IllegalBlockSizeException | BadPaddingException ex) {
        }
        bufferCount = 0;

        byte[] mac = new byte[macLength];
        System.arraycopy(buffer, 0, mac, 0, macLength);
        return mac;
    }
}