content: /*************************** The CRY.ME project (2023) *************************************************
   *
   *  This file is part of the CRY.ME project (https://github.com/ANSSI-FR/cry-me).
   *  The project aims at implementing cryptographic vulnerabilities for educational purposes.
   *  Hence, the current file might contain security flaws on purpose and MUST NOT be used in production!
   *  Please do not use this source code outside this scope, or use it knowingly.
   *
   *  Many files come from the Android element (https://github.com/vector-im/element-android), the
   *  Matrix SDK (https://github.com/matrix-org/matrix-android-sdk2) as well as the Android Yubikit
   *  (https://github.com/Yubico/yubikit-android) projects and have been willingly modified
   *  for the CRY.ME project purposes. The Android element, Matrix SDK and Yubikit projects are distributed
   *  under the Apache-2.0 license, and so is the CRY.ME project.
   *
   ***************************  (END OF CRY.ME HEADER)   *************************************************/
  
  /*
   * Copyright (C) 2019 Yubico.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *       http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.yubico.yubikit.core.otp;
  
  /**
   * Utility methods for calculating and verifying the CRC13239 checksum used by YubiKeys.
   */
  public class ChecksumUtils {
      // When verifying a checksum the CRC_OK_RESIDUAL should be the remainder.
      private static final short CRC_OK_RESIDUAL = (short) 0xf0b8;
  
      /**
       * Calculate the CRC13239 checksum for a byte buffer.
       *
       * @param data   byte buffer to be checksummed.
       * @param length how much of the buffer should be checksummed
       * @return the calculated checksum
       */
      static public short calculateCrc(byte[] data, int length) {
          int crc = 0xffff;
  
          for (int index = 0; index < length; index++) {
              int i, j;
              crc ^= data[index] & 0xFF;
              for (i = 0; i < 8; i++) {
                  j = crc & 1;
                  crc >>= 1;
                  if (j == 1) {
                      crc ^= 0x8408;
                  }
              }
          }
  
          return (short) (crc & 0xFFFF);
      }
  
      /**
       * Verifies a checksum.
       *
       * @param data   the data, ending in the 2 byte CRC checksum to verify
       * @param length The length of the data, including the checksum at the end
       * @return true if the checksum is correct, false if not
       */
      static public boolean checkCrc(byte[] data, int length) {
          return calculateCrc(data, length) == CRC_OK_RESIDUAL;
      }
  
      private ChecksumUtils() {
          throw new IllegalStateException();
      }
  }
  