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Homekit Accessory Protocol for Java

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package io.github.hapjava.server.impl.crypto;

import org.bouncycastle.crypto.macs.Poly1305;
import org.bouncycastle.crypto.params.KeyParameter;
import org.bouncycastle.util.Pack;

class PolyKeyCreator {

  public static byte[] create(KeyParameter macKey, byte[] additionalData, byte[] ciphertext) {
    Poly1305 poly = new Poly1305();
    poly.init(macKey);

    if (additionalData != null) {
      poly.update(additionalData, 0, additionalData.length);
      if (additionalData.length % 16 != 0) {
        int round = 16 - (additionalData.length % 16);
        poly.update(new byte[round], 0, round);
      }
    }

    poly.update(ciphertext, 0, ciphertext.length);
    if (ciphertext.length % 16 != 0) {
      int round = 16 - (ciphertext.length % 16);
      poly.update(new byte[round], 0, round);
    }

    // additional data length
    byte[] additionalDataLength;
    if (additionalData != null) {
      additionalDataLength = Pack.longToLittleEndian(additionalData.length);
    } else {
      additionalDataLength = new byte[8];
    }
    poly.update(additionalDataLength, 0, 8);
    byte[] ciphertextLength = Pack.longToLittleEndian(ciphertext.length);
    poly.update(ciphertextLength, 0, 8);

    byte[] calculatedMAC = new byte[poly.getMacSize()];
    poly.doFinal(calculatedMAC, 0);
    return calculatedMAC;
  }
}