gt:                                   byte[] destination, int destOffset, byte[] decodabet) {
context: // Portions copyright 2002, Google, Inc.
  //
  // 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.annahid.libs.artenus.security;
  
  // This code was converted from code at http://iharder.sourceforge.net/base64/
  // Lots of extraneous features were removed.
  /* The original code said:
   * <p>
   * I am placing this code in the Public Domain. Do with it as you will.
   * This software comes with no guarantees or warranties but with
   * plenty of well-wishing instead!
   * Please visit
   * <a href="http://iharder.net/xmlizable">http://iharder.net/xmlizable</a>
   * periodically to check for updates or to contribute improvements.
   * </p>
   *
   * @author Robert Harder
   * @author rharder@usa.net
   * @version 1.3
   */
  
  import com.annahid.libs.artenus.BuildConfig;
  
  /**
   * Base64 converter class. This code is not a complete MIME encoder;
   * it simply converts binary data to base64 data and back.
   * <p/>
   * <p>Note {@code CharBase64} is a GWT-compatible implementation of this
   * class.
   */
  @SuppressWarnings("UnusedDeclaration")
  public class Base64 {
      /**
       * Specify encoding (value is {@code true}).
       */
      public final static boolean ENCODE = true;
  
      /**
       * Specify decoding (value is {@code false}).
       */
      public final static boolean DECODE = false;
  
      /**
       * The equals sign (=) as a byte.
       */
      private final static byte EQUALS_SIGN = (byte) '=';
  
      /**
       * The new line character (\n) as a byte.
       */
      private final static byte NEW_LINE = (byte) '\n';
  
      /**
       * The 64 valid Base64 values.
       */
      private final static byte[] ALPHABET =
              {(byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E', (byte) 'F',
                      (byte) 'G', (byte) 'H', (byte) 'I', (byte) 'J', (byte) 'K',
                      (byte) 'L', (byte) 'M', (byte) 'N', (byte) 'O', (byte) 'P',
                      (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T', (byte) 'U',
                      (byte) 'V', (byte) 'W', (byte) 'X', (byte) 'Y', (byte) 'Z',
                      (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e',
                      (byte) 'f', (byte) 'g', (byte) 'h', (byte) 'i', (byte) 'j',
                      (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n', (byte) 'o',
                      (byte) 'p', (byte) 'q', (byte) 'r', (byte) 's', (byte) 't',
                      (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x', (byte) 'y',
                      (byte) 'z', (byte) '0', (byte) '1', (byte) '2', (byte) '3',
                      (byte) '4', (byte) '5', (byte) '6', (byte) '7', (byte) '8',
                      (byte) '9', (byte) '+', (byte) '/'};
  
      /**
       * The 64 valid web safe Base64 values.
       */
      private final static byte[] WEBSAFE_ALPHABET =
              {(byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E', (byte) 'F',
                      (byte) 'G', (byte) 'H', (byte) 'I', (byte) 'J', (byte) 'K',
                      (byte) 'L', (byte) 'M', (byte) 'N', (byte) 'O', (byte) 'P',
                      (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T', (byte) 'U',
                      (byte) 'V', (byte) 'W', (byte) 'X', (byte) 'Y', (byte) 'Z',
                      (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd', (byte) 'e',
                      (byte) 'f', (byte) 'g', (byte) 'h', (byte) 'i', (byte) 'j',
                      (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n', (byte) 'o',
                      (byte) 'p', (byte) 'q', (byte) 'r', (byte) 's', (byte) 't',
                      (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x', (byte) 'y',
                      (byte) 'z', (byte) '0', (byte) '1', (byte) '2', (byte) '3',
                      (byte) '4', (byte) '5', (byte) '6', (byte) '7', (byte) '8',
                      (byte) '9', (byte) '-', (byte) '_'};
  
      /**
       * Translates a Base64 value to either its 6-bit reconstruction value
       * or a negative number indicating some other meaning.
       */
      private final static byte[] DECODABET = {-9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal  0 -  8
              -5, -5, // Whitespace: Tab and Linefeed
              -9, -9, // Decimal 11 - 12
              -5, // Whitespace: Carriage Return
              -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 14 - 26
              -9, -9, -9, -9, -9, // Decimal 27 - 31
              -5, // Whitespace: Space
              -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 33 - 42
              62, // Plus sign at decimal 43
              -9, -9, -9, // Decimal 44 - 46
              63, // Slash at decimal 47
              52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // Numbers zero through nine
              -9, -9, -9, // Decimal 58 - 60
              -1, // Equals sign at decimal 61
              -9, -9, -9, // Decimal 62 - 64
              0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, // Letters 'A' through 'N'
              14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // Letters 'O' through 'Z'
              -9, -9, -9, -9, -9, -9, // Decimal 91 - 96
              26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, // Letters 'a' through 'm'
              39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // Letters 'n' through 'z'
              -9, -9, -9, -9, -9 // Decimal 123 - 127
          /*  ,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 128 - 139
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 140 - 152
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 153 - 165
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 166 - 178
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 179 - 191
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 192 - 204
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 205 - 217
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 218 - 230
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 231 - 243
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9         // Decimal 244 - 255 */
      };
  
      /**
       * The web safe decodabet
       */
      private final static byte[] WEBSAFE_DECODABET =
              {-9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal  0 -  8
                      -5, -5, // Whitespace: Tab and Linefeed
                      -9, -9, // Decimal 11 - 12
                      -5, // Whitespace: Carriage Return
                      -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 14 - 26
                      -9, -9, -9, -9, -9, // Decimal 27 - 31
                      -5, // Whitespace: Space
                      -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, -9, // Decimal 33 - 44
                      62, // Dash '-' sign at decimal 45
                      -9, -9, // Decimal 46-47
                      52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // Numbers zero through nine
                      -9, -9, -9, // Decimal 58 - 60
                      -1, // Equals sign at decimal 61
                      -9, -9, -9, // Decimal 62 - 64
                      0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, // Letters 'A' through 'N'
                      14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // Letters 'O' through 'Z'
                      -9, -9, -9, -9, // Decimal 91-94
                      63, // Underscore '_' at decimal 95
                      -9, // Decimal 96
                      26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, // Letters 'a' through 'm'
                      39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // Letters 'n' through 'z'
                      -9, -9, -9, -9, -9 // Decimal 123 - 127
          /*  ,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 128 - 139
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 140 - 152
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 153 - 165
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 166 - 178
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 179 - 191
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 192 - 204
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 205 - 217
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 218 - 230
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,     // Decimal 231 - 243
          -9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9,-9         // Decimal 244 - 255 */
              };
  
      // Indicates white space in encoding
      private final static byte WHITE_SPACE_ENC = -5;
      // Indicates equals sign in encoding
      private final static byte EQUALS_SIGN_ENC = -1;
  
      /**
       * Defeats instantiation.
       */
      private Base64() {
      }
  
      /* ********  E N C O D I N G   M E T H O D S  ******** */
  
      /**
       * Encodes up to three bytes of the array <var>source</var>
       * and writes the resulting four Base64 bytes to <var>destination</var>.
       * The source and destination arrays can be manipulated
       * anywhere along their length by specifying
       * <var>srcOffset</var> and <var>destOffset</var>.
       * This method does not check to make sure your arrays
       * are large enough to accommodate <var>srcOffset</var> + 3 for
       * the <var>source</var> array or <var>destOffset</var> + 4 for
       * the <var>destination</var> array.
       * The actual number of significant bytes in your array is
       * given by <var>numSigBytes</var>.
       *
       * @param source      the array to convert
       * @param srcOffset   the index where conversion begins
       * @param numSigBytes the number of significant bytes in your array
       * @param destination the array to hold the conversion
       * @param destOffset  the index where output will be put
       * @param alphabet    is the encoding alphabet
       * @return the <var>destination</var> array
       * @since 1.3
       */
      private static byte[] encode3to4(byte[] source, int srcOffset,
                                       int numSigBytes, byte[] destination, int destOffset, byte[] alphabet) {
          //           1         2         3
          // 01234567890123456789012345678901 Bit position
          // --------000000001111111122222222 Array position from threeBytes
          // --------|    ||    ||    ||    | Six bit groups to index alphabet
          //          >>18  >>12  >> 6  >> 0  Right shift necessary
          //                0x3f  0x3f  0x3f  Additional AND
  
          // Create buffer with zero-padding if there are only one or two
          // significant bytes passed in the array.
          // We have to shift left 24 in order to flush out the 1's that appear
          // when Java treats a value as negative that is cast from a byte to an int.
          int inBuff =
                  (numSigBytes > 0 ? ((source[srcOffset] << 24) >>> 8) : 0)
                          | (numSigBytes > 1 ? ((source[srcOffset + 1] << 24) >>> 16) : 0)
                          | (numSigBytes > 2 ? ((source[srcOffset + 2] << 24) >>> 24) : 0);
  
          switch (numSigBytes) {
              case 3:
                  destination[destOffset] = alphabet[(inBuff >>> 18)];
                  destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
                  destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f];
                  destination[destOffset + 3] = alphabet[(inBuff) & 0x3f];
                  return destination;
              case 2:
                  destination[destOffset] = alphabet[(inBuff >>> 18)];
                  destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
                  destination[destOffset + 2] = alphabet[(inBuff >>> 6) & 0x3f];
                  destination[destOffset + 3] = EQUALS_SIGN;
                  return destination;
              case 1:
                  destination[destOffset] = alphabet[(inBuff >>> 18)];
                  destination[destOffset + 1] = alphabet[(inBuff >>> 12) & 0x3f];
                  destination[destOffset + 2] = EQUALS_SIGN;
                  destination[destOffset + 3] = EQUALS_SIGN;
                  return destination;
              default:
                  return destination;
          } // end switch
      } // end encode3to4
  
      /**
       * Encodes a byte array into Base64 notation.
       * Equivalent to calling
       * {@code encodeBytes(source, 0, source.length)}
       *
       * @param source The data to convert
       * @since 1.4
       */
      public static String encode(byte[] source) {
          return encode(source, 0, source.length, ALPHABET, true);
      }
  
      /**
       * Encodes a byte array into web safe Base64 notation.
       *
       * @param source    The data to convert
       * @param doPadding is {@code true} to pad result with '=' chars
       *                  if it does not fall on 3 byte boundaries
       */
      public static String encodeWebSafe(byte[] source, boolean doPadding) {
          return encode(source, 0, source.length, WEBSAFE_ALPHABET, doPadding);
      }
  
      /**
       * Encodes a byte array into Base64 notation.
       *
       * @param source    the data to convert
       * @param off       offset in array where conversion should begin
       * @param len       length of data to convert
       * @param alphabet  the encoding alphabet
       * @param doPadding is {@code true} to pad result with '=' chars
       *                  if it does not fall on 3 byte boundaries
       * @since 1.4
       */
      public static String encode(byte[] source, int off, int len, byte[] alphabet,
                                  boolean doPadding) {
          byte[] outBuff = encode(source, off, len, alphabet, Integer.MAX_VALUE);
          int outLen = outBuff.length;
  
          // If doPadding is false, set length to truncate '='
          // padding characters
          while (!doPadding && outLen > 0) {
              if (outBuff[outLen - 1] != '=') {
                  break;
              }
              outLen -= 1;
          }
  
          return new String(outBuff, 0, outLen);
      }
  
      /**
       * Encodes a byte array into Base64 notation.
       *
       * @param source        the data to convert
       * @param off           offset in array where conversion should begin
       * @param len           length of data to convert
       * @param alphabet      is the encoding alphabet
       * @param maxLineLength maximum length of one line.
       * @return the BASE64-encoded byte array
       */
      public static byte[] encode(byte[] source, int off, int len, byte[] alphabet,
                                  int maxLineLength) {
          int lenDiv3 = (len + 2) / 3; // ceil(len / 3)
          int len43 = lenDiv3 * 4;
          byte[] outBuff = new byte[len43 // Main 4:3
                  + (len43 / maxLineLength)]; // New lines
  
          int d = 0;
          int e = 0;
          int len2 = len - 2;
          int lineLength = 0;
          for (; d < len2; d += 3, e += 4) {
  
              // The following block of code is the same as
              // encode3to4( source, d + off, 3, outBuff, e, alphabet );
              // but inlined for faster encoding (~20% improvement)
              int inBuff =
                      ((source[d + off] << 24) >>> 8)
                              | ((source[d + 1 + off] << 24) >>> 16)
                              | ((source[d + 2 + off] << 24) >>> 24);
              outBuff[e] = alphabet[(inBuff >>> 18)];
              outBuff[e + 1] = alphabet[(inBuff >>> 12) & 0x3f];
              outBuff[e + 2] = alphabet[(inBuff >>> 6) & 0x3f];
              outBuff[e + 3] = alphabet[(inBuff) & 0x3f];
  
              lineLength += 4;
              if (lineLength == maxLineLength) {
                  outBuff[e + 4] = NEW_LINE;
                  e++;
                  lineLength = 0;
              } // end if: end of line
          } // end for: each piece of array
  
          if (d < len) {
              encode3to4(source, d + off, len - d, outBuff, e, alphabet);
  
              lineLength += 4;
              if (lineLength == maxLineLength) {
                  // Add a last newline
                  outBuff[e + 4] = NEW_LINE;
                  e++;
              }
              e += 4;
          }
  
          if (BuildConfig.DEBUG && e != outBuff.length)
              throw new AssertionError();
  
          return outBuff;
      }
  
  
      /* ********  D E C O D I N G   M E T H O D S  ******** */
  
  
      /**
       * Decodes four bytes from array <var>source</var>
       * and writes the resulting bytes (up to three of them)
       * to <var>destination</var>.
       * The source and destination arrays can be manipulated
       * anywhere along their length by specifying
       * <var>srcOffset</var> and <var>destOffset</var>.
       * This method does not check to make sure your arrays
       * are large enough to accommodate <var>srcOffset</var> + 4 for
       * the <var>source</var> array or <var>destOffset</var> + 3 for
       * the <var>destination</var> array.
       * This method returns the actual number of bytes that
       * were converted from the Base64 encoding.
       *
       * @param source      the array to convert
       * @param srcOffset   the index where conversion begins
       * @param destination the array to hold the conversion
       * @param destOffset  the index where output will be put
       * @param decodabet   the decodabet for decoding Base64 content
       * @return the number of decoded bytes converted
       * @since 1.3
       */
      private static int decode4to3(byte[] source, int srcOffset,
  