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;
  
  import java.io.ByteArrayOutputStream;
  import java.util.HashMap;
  import java.util.Map;
  
  /**
   * Methods for encoding and decoding Modhex encoded Strings.
   * <p>
   * See: <a href="https://developers.yubico.com/yubico-c/Manuals/modhex.1.htm">Modhex specification</a>.
   */
  public class Modhex {
      private final static char[] ALPHABET = "cbdefghijklnrtuv".toCharArray();
  
      private static final Map<Character, Integer> table = new HashMap<>();
  
      static {
          for (int i = 0; i < ALPHABET.length; i++) {
              table.put(ALPHABET[i], i);
          }
      }
  
      /**
       * Decodes Modhex encoded string.
       */
      public static byte[] decode(String modhex) {
          if (modhex.length() % 2 != 0) {
              throw new IllegalArgumentException("Input string length is not a multiple of 2");
          }
  
          byte byteValue = 0;
          char[] chars = modhex.toLowerCase().toCharArray();
          ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
  
          for (int i = 0; i < chars.length; i++) {
              // find hex code for each symbol
              Integer code = table.get(chars[i]);
              if (code == null) {
                  throw new IllegalArgumentException("Input string contains non-modhex character(s).");
              }
  
              // 2 symbols merged into 1 byte
              boolean shift = i % 2 == 0;
              if (shift) {
                  byteValue = (byte) (code << 4);
              } else {
                  byteValue |= code;
                  outputStream.write(byteValue);
              }
          }
          return outputStream.toByteArray();
      }
  
      /**
       * Encodes data to Modhex.
       */
      public static String encode(byte[] bytes) {
          StringBuilder output = new StringBuilder();
          for (byte b : bytes) {
              output.append(ALPHABET[(b >> 4) & 0xF]).append(ALPHABET[b & 0xF]);
          }
          return output.toString();
      }
  }
  