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Homekit Accessory Protocol for Java

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

import io.github.hapjava.server.impl.pairing.TypeLengthValueUtils.DecodeResult;
import java.math.BigInteger;

abstract class PairSetupRequest {

  private static final short VALUE_STAGE_1 = 1;
  private static final short VALUE_STAGE_2 = 3;
  private static final short VALUE_STAGE_3 = 5;

  public static PairSetupRequest of(byte[] content) throws Exception {
    DecodeResult d = TypeLengthValueUtils.decode(content);
    short stage = d.getByte(MessageType.STATE);
    switch (stage) {
      case VALUE_STAGE_1:
        return new Stage1Request();

      case VALUE_STAGE_2:
        return new Stage2Request(d);

      case VALUE_STAGE_3:
        return new Stage3Request(d);

      default:
        throw new Exception("Unknown pair process stage: " + stage);
    }
  }

  public abstract Stage getStage();

  public static class Stage1Request extends PairSetupRequest {
    @Override
    public Stage getStage() {
      return Stage.ONE;
    }
  }

  public static class Stage2Request extends PairSetupRequest {

    private final BigInteger a;
    private final BigInteger m1;

    public Stage2Request(DecodeResult d) {
      a = d.getBigInt(MessageType.PUBLIC_KEY);
      m1 = d.getBigInt(MessageType.PROOF);
    }

    public BigInteger getA() {
      return a;
    }

    public BigInteger getM1() {
      return m1;
    }

    @Override
    public Stage getStage() {
      return Stage.TWO;
    }
  }

  static class Stage3Request extends PairSetupRequest {

    private final byte[] messageData;
    private final byte[] authTagData;

    public Stage3Request(DecodeResult d) {
      messageData = new byte[d.getLength(MessageType.ENCRYPTED_DATA) - 16];
      authTagData = new byte[16];
      d.getBytes(MessageType.ENCRYPTED_DATA, messageData, 0);
      d.getBytes(MessageType.ENCRYPTED_DATA, authTagData, messageData.length);
    }

    public byte[] getMessageData() {
      return messageData;
    }

    public byte[] getAuthTagData() {
      return authTagData;
    }

    @Override
    public Stage getStage() {
      return Stage.THREE;
    }
  }
}